Definitions

A

Allowable Stress Design (ASD)

A design approach in which stresses under service (unfactored) loads must stay below a code-specified allowable stress — typically the material's yield strength divided by a factor of safety. Used in wood design (NDS) and as an alternate method in steel (AISC). Compare with LRFD.

Axial Force

A force acting along the longitudinal axis of a member, producing either tension (pulling) or compression (pushing). Columns carry primarily axial compression; hangers and diagonal braces carry axial tension or compression depending on load direction.

Axial Stress (σ)

Stress due to axial force: σ = P / A, where P is the axial load and A is the cross-sectional area. When the load acts through the centroid, axial stress is uniform across the section. Units: ksi (kip per in²).

Amplification Factor

A multiplier applied to first-order analysis results to approximate second-order (P-Δ and P-δ) effects. AISC uses B1 (member sway) and B2 (story sway) factors. When B2 exceeds 1.5, a rigorous second-order analysis is required instead.

Arch

A curved structural element that transfers loads primarily through axial compression along its curved axis, minimizing bending. The horizontal thrust at the supports must be resisted by tie rods or buttresses. Arches are efficient for long spans — parabolic arches carry uniform loads in pure compression.

Accidental Torsion

Code-required additional torsional moment applied to each floor diaphragm even if the structure is symmetric. ASCE 7 requires displacing the center of mass by 5% of the building dimension perpendicular to the earthquake direction. Accounts for uncertainty in mass distribution and ground motion.

ACI 318

The American Concrete Institute's Building Code Requirements for Structural Concrete. The governing US code for design of reinforced and prestressed concrete structures. Updated on a 3-year cycle (current: ACI 318-19).

Aggregate

The granular material (sand, gravel, crushed stone) mixed with cement and water to form concrete. Aggregate makes up 60–80% of concrete by volume. Maximum aggregate size affects workability, cover requirements, and spacing of reinforcing bars.

Anchor Rod

A threaded steel rod embedded in or attached to a concrete foundation to secure a steel column base plate. Common diameters: 3/4" to 2". Anchor rods resist uplift tension from overturning and shear from lateral loads. Designed per ACI 318 Appendix D for concrete breakout, pullout, and steel failure.

B

Base Shear (V)

The total lateral force at the base of a structure due to seismic or wind loading. For equivalent lateral force (ELF) seismic design: V = CsW, where Cs is the seismic response coefficient and W is the effective seismic weight of the building.

Beam

A structural member designed to carry loads perpendicular to its longitudinal axis, primarily through bending and shear. Beams are typically horizontal, spanning between columns, walls, or other beams. They transfer load to their supports.

Bearing Wall

A wall that carries gravity loads from floors or roofs above and transfers them to the foundation below. Removing a bearing wall without replacing the load path — typically with a beam and new columns — is a structural alteration that requires engineering.

Bending Moment (M)

The internal moment at a cross-section that resists rotation caused by transverse loads. Numerically equal to the sum of all moments (forces × distances) of everything to one side of the section. Units: kip·ft or kip·in. Bending moment causes one side of the beam to go into tension and the other into compression.

Braced Frame

A lateral system in which diagonal steel members (X-braces, chevron/inverted-V, or single diagonal) carry lateral forces through axial tension and compression. Stiffer and more efficient than moment frames for controlling drift, but the diagonals can interfere with openings.

Buckling

A sudden loss of stability in which a slender member deflects laterally under axial compression — even before reaching the material's yield stress. The theoretical load for a pin-ended column is Euler's formula: Pcr = π²EI / L². Buckling is governed by geometry and stiffness, not just material strength.

Bearing

The transfer of compressive force from one element to another at their contact surface. Bearing stress equals force divided by the contact area: fp = P / A_bearing. Bearing checks appear at beam supports on walls, base plates on concrete, and bolts on plate edges.

Bearing Capacity

The maximum pressure a soil can sustain without shear failure or excessive settlement. Allowable bearing capacity (qa) is the ultimate capacity divided by a factor of safety (typically 2.5–3.0). Foundation size is often governed by bearing capacity: A_footing ≥ P / qa.

Block Shear

A connection failure mode in which a block of material tears out along a combination of tension and shear planes through the bolt holes. The capacity is the sum of shear rupture on one plane and tension rupture (or yield) on the perpendicular plane. Often governs coped beam connections and gusset plates.

Bolt

A threaded steel fastener used to connect structural members. Common grades: A325 (Fy = 92 ksi) and A490 (Fy = 130 ksi). Bolts resist loads through shear across the shank or through tension along the axis. Pretensioned (slip-critical) bolts clamp plates together so friction carries the load before the bolt ever bears.

Boundary Element

The reinforced edge or end zone of a shear wall or diaphragm, designed to resist the concentrated compression and tension forces from overturning. In concrete shear walls, boundary elements have closely spaced ties for confinement when compressive demands are high.

Balanced Section

A reinforced concrete cross-section in which the steel reaches yield strain and the concrete reaches its crushing strain (0.003) simultaneously. The balanced reinforcement ratio marks the boundary between under-reinforced (ductile) and over-reinforced (brittle) behavior. ACI 318 requires sections to be under-reinforced.

Base Isolation

A seismic protection strategy that decouples a building from ground motion by placing flexible bearings (lead-rubber or friction pendulum) between the foundation and the superstructure. Lengthens the building's period and dramatically reduces accelerations and forces in the structure above.

Base Plate

A steel plate welded to the bottom of a column, resting on a concrete foundation with grout between. The base plate spreads the column load over a large enough area of concrete. Sized so that bearing pressure on the concrete does not exceed the allowable: fp = Pu / A1 ≤ φ(0.85f'c).

Brittle Failure

A sudden fracture with little or no warning deformation — the opposite of ductile behavior. Unreinforced concrete in tension, over-reinforced beams, and net-section fracture of bolted connections are examples. Structural codes penalize brittle failure modes with lower resistance factors (φ = 0.75 vs 0.90) to provide extra safety margin.

C

Cantilever

A beam fixed at one end and free at the other. All equilibrium is provided by the fixed support, which must supply both a vertical reaction and a resisting moment. For a uniform load w over length L, the maximum moment at the fixed end is wL² / 2.

Chord

The perimeter member of a diaphragm (floor or roof plate) that resists the bending the diaphragm experiences as it spans between shear walls or frames. Chords act in axial tension or compression — analogous to the flanges of a beam. Often a spandrel beam or the top plate of a wood-framed wall.

Collector (Drag Strut)

A member that collects lateral forces from the diaphragm and delivers them to the vertical lateral resisting elements (shear walls or frames). Collectors are in the plane of the diaphragm and carry axial tension or compression.

Column

A vertical compression member that transfers axial load from beams and slabs above down to the foundation. Columns must be designed for both compressive strength and buckling stability. In steel, the compression capacity decreases with increasing slenderness ratio KL/r.

Combined Loading

A condition in which a member carries simultaneous axial force and bending moment — common in columns subjected to both gravity and lateral loads. Design uses an interaction equation: Pu/φPn + Mu/φMn ≤ 1.0 (simplified form). Neither component alone governs; their combined effect does.

Composite Beam

A steel beam acting together with a concrete slab above it through shear studs welded to the beam's top flange. The slab carries compression; the steel carries tension. This shared action uses the materials where each is strongest, giving greater strength and stiffness than either alone.

Compression

A state of stress in which material is being pushed or squeezed, shortening the member. Concrete and masonry are strong in compression but brittle. Steel is strong in both compression and tension, but slender steel members can buckle under compressive load.

Continuity

A beam is continuous if it spans over one or more intermediate supports without a moment release. Continuous beams have lower midspan moments and more efficient material use than simply supported beams of the same span, but they develop hogging (negative) moments over the supports.

Camber

An intentional upward curvature built into a beam or truss to offset the anticipated dead-load deflection. The beam is fabricated or erected with a slight crown so that under full dead load it appears level. Typical camber: 75–80% of the calculated dead-load deflection.

Centroid

The geometric center of a cross-section — the point where the area is equally distributed. For symmetric sections, the centroid lies on every axis of symmetry. Bending stress is zero at the centroid (neutral axis) and maximum at the extreme fibers.

Cladding

The non-structural exterior skin of a building — curtain walls, metal panels, precast concrete panels, or masonry veneer. Cladding must resist wind and seismic forces acting normal to its surface and accommodate story drift without cracking or falling. It attaches to the structure at discrete connections.

Coefficient of Thermal Expansion (α)

The change in length per unit length per degree of temperature change. For steel: α ≈ 6.5 × 10⁻⁶ /°F. Thermal movement = αLΔT. Expansion joints or flexible connections are needed when runs exceed about 200–300 ft without relief.

Concrete

A composite material of cement, water, aggregate, and often admixtures that hardens into a stone-like mass. Strong in compression (f'c typically 3–8 ksi) but weak in tension (~10% of f'c). Almost always reinforced with steel bars or prestressing tendons. The most widely used structural material on Earth.

Connection

The physical joint between two or more structural members, transferring forces from one to the next. Connections can be bolted, welded, or both. Their behavior — whether they transmit moment (rigid), allow rotation (pinned), or something in between (semi-rigid) — fundamentally affects how the structure responds to load.

Cope

A notch cut from the flange (and sometimes web) of a beam at its end, allowing the web to connect to a supporting beam or column without the flange interfering. Coped beams require block shear, flexural rupture, and local buckling checks at the reduced section.

Crippling

A localized failure of a beam's web under concentrated forces (reactions or point loads) where the web crushes or folds. AISC requires web crippling checks at bearing points. Bearing stiffeners are added when the web alone cannot resist the concentrated force.

Cross-Bracing

An X-shaped arrangement of diagonal members within a structural bay, forming a braced frame. Under lateral load, one diagonal goes into tension while the other buckles (or is designed to go slack). Cross-bracing is simple, stiff, and economical but blocks the bay opening.

Curvature (κ)

The rate of change of slope along a bent member: κ = M / EI. Higher moment or lower stiffness produces greater curvature. Integrating curvature along the beam gives the slope; integrating slope gives the deflection. Curvature is what bending physically looks like at a cross-section.

Cantilever Method

An approximate hand-analysis technique for multi-story moment frames under lateral load. Assumes inflection points at mid-height of columns and midspan of beams, and that axial forces in columns are proportional to their distance from the frame centroid. Quick check on computer output.

Capacity Design

A seismic design philosophy that designates specific elements (fuses) to yield and dissipate energy while protecting all other elements with enough strength to remain elastic. In a moment frame, beams are the fuses; columns and connections are designed stronger (strong-column-weak-beam).

CLT (Cross-Laminated Timber)

An engineered wood panel made of layers of dimensional lumber glued at alternating 90° angles. CLT panels act as structural walls and floors, offering fire resistance, sustainability, and rapid construction. Designed per NDS and the CLT chapter of AWC's Special Design Provisions for Wind and Seismic.

Coefficient of Friction (μ)

The ratio of friction force to normal force between two surfaces in contact. Used in design of slip-critical bolted connections (μ = 0.35 for Class A surfaces), concrete-on-concrete construction joints (μ = 0.6–1.0), and base plate sliding checks.

Column Stability Factor (Cp)

A reduction factor in NDS wood design that accounts for buckling of a wood compression member. Analogous to the AISC column curve. Depends on slenderness ratio, modulus of elasticity, and reference compression strength: Fc' = Fc × Cp × (other adjustment factors).

Compatibility

The requirement that deformations in a structure be physically consistent — connected members must deform together at shared points. In indeterminate structures, compatibility equations supplement equilibrium to solve for unknown forces. Also called kinematic consistency.

Compressive Strength (f'c)

The specified 28-day compressive strength of concrete, measured by crushing standard 6×12 in. cylinders. Common values: 3,000–8,000 psi for buildings, up to 14,000+ psi for high-rise columns. f'c appears in nearly every concrete design equation and governs mix design.

Concrete Cover

The minimum clear distance between the surface of the concrete and the nearest reinforcing bar. Cover protects rebar from corrosion and fire. ACI 318 §20.6 specifies minimums: 1.5 in. for beams/columns, 3 in. for concrete cast against earth. Insufficient cover is one of the most common durability failures.

Confinement

Closely spaced transverse reinforcement (ties, hoops, or spirals) that restrains the concrete core against lateral expansion under high compressive stress. Confined concrete has higher strength and much greater ductility than unconfined concrete. Required in seismic column plastic hinge zones and shear wall boundary elements.

Conjugate Beam

An analysis method for finding slopes and deflections by replacing the real beam's support conditions with conjugate supports and loading the conjugate beam with the M/EI diagram. The shear in the conjugate beam equals the slope in the real beam; the moment equals the deflection.

Construction Load

Temporary loads from equipment, stored materials, and workers during building erection. Can govern the design of members not yet fully braced or composite. ASCE 37 provides guidance on construction load magnitudes. Particularly critical for post-tensioned slabs before tendons are stressed.

Continuity Plate

A horizontal stiffener welded inside a column at the level of a beam flange in a moment connection. Transfers the concentrated beam flange force across the column web, preventing web crippling and local flange bending. Required when the column web and flanges cannot resist the beam flange forces alone.

Couple

Two parallel forces of equal magnitude acting in opposite directions, separated by a perpendicular distance d. A couple produces pure moment (M = F × d) with no net translational force. Bending in a beam cross-section is resisted by an internal couple between the compression and tension resultants.

Crack Control

Detailing requirements in reinforced concrete to limit crack widths under service loads, preventing corrosion and maintaining appearance. ACI 318 §24.3 limits bar spacing based on stress and cover. Crack width is not calculated directly; instead, reinforcement is distributed to keep cracks tight.

Creep

The time-dependent increase in strain under sustained stress. Concrete creep can double or triple the initial elastic deflection of a beam over years. Affects long-term deflections, prestress losses, and column shortening in tall buildings. Creep coefficient depends on loading age, humidity, and member geometry.

D

Dead Load (D)

The weight of all permanently attached building components: structural framing, concrete fill, metal deck, ceilings, flooring, mechanical/electrical/plumbing systems, and partitions. Dead loads are constant and known with relatively high confidence. Typical office floor DL: 50–80 psf depending on the system.

Deflection (Δ)

The vertical displacement of a beam or member under load. Code limits deflection to prevent cracking of non-structural elements and maintain user comfort — for example, L/360 under live load for floors (where L is the span). Deflection is a serviceability check, not a strength check.

Design Load

The load or combination of loads used to size a structural member. Under LRFD, design loads include load factors (e.g., 1.2D + 1.6L); under ASD, design loads are unfactored service-level loads. The controlling combination is the one producing the largest demand on the member.

Diaphragm

A horizontal structural element — typically a floor slab, metal deck, or wood-sheathed roof — that acts as a large plate to collect and distribute lateral forces to the vertical lateral resisting system (shear walls or frames). Think of it as a flat beam spanning between shear walls.

Distributed Load (w)

A load spread continuously over a member's length, expressed in force per unit length (kip/ft or kN/m). A uniform distributed load has constant intensity; non-uniform loads vary along the span. Most floor beams and roof rafters carry distributed loads from the tributary area of deck above them.

Drift

The lateral displacement of a floor or roof level due to wind or seismic loading. Story drift ratio is drift divided by story height. Code limits (ASCE 7 Table 12.12-1) prevent structural damage, non-structural damage (cladding, partitions), and instability effects like P-delta.

Ductility

The ability of a material or structure to sustain large inelastic deformations before fracturing. Steel is highly ductile; unreinforced concrete and masonry are brittle. Ductility is critical in seismic design: ductile systems dissipate earthquake energy through controlled yielding rather than sudden failure.

Damping

The dissipation of vibration energy over time through friction, material hysteresis, and non-structural element deformation. Structural analysis typically assumes 5% critical damping for steel and concrete buildings. Damping reduces peak dynamic response — without it, a building would oscillate indefinitely after an earthquake.

Deck

Short for metal deck — corrugated steel sheet serving as formwork for concrete slabs and, once composite, as part of the structural floor. Also acts as a diaphragm to transfer lateral forces. Common types: roof deck (no concrete), composite floor deck (concrete fill), and form deck (non-composite support only).

Development Length

The minimum embedment length required for a reinforcing bar to develop its full yield strength through bond with the surrounding concrete. If the bar is too short, it pulls out before yielding. Development length depends on bar size, concrete strength, cover, and spacing. See ACI 318 §25.4.

Dowel

A short reinforcing bar or smooth rod embedded across a construction joint to transfer shear between two concrete elements — for example, between a slab and a wall or between a footing and a column pedestal. Dowels provide continuity where concrete is placed at different times.

Dynamic Load

A load that varies with time and induces inertial effects in the structure — including seismic ground motion, wind gusts, impact, machinery vibration, and blast. Dynamic loads require consideration of the structure's natural period, damping, and mass distribution, unlike static loads which are applied slowly.

Deflection Amplification Factor (Cd)

A multiplier in ASCE 7 seismic design that converts elastic story drift (from reduced seismic forces) to the expected inelastic drift: δx = Cd × δxe / Ie. Values range from 1.25 to 5.5 depending on the lateral system type. Used to check story drift limits.

Demand-Capacity Ratio (DCR)

The ratio of factored demand (applied load effect) to available capacity for a member or connection. A DCR ≤ 1.0 means the member passes. DCR is a universal metric used across all material types and limit states to quickly assess how close a member is to its limit.

Density

Mass per unit volume of a material. Steel: 490 lb/ft³. Normal-weight concrete: 150 lb/ft³. Wood: 25–45 lb/ft³ depending on species. Density determines dead load and seismic weight. Lightweight concrete (110 lb/ft³) reduces both dead load and seismic demands but has lower modulus of elasticity.

Dimensional Lumber

Solid-sawn wood members used in light-frame construction, typically 2× or 4× nominal sizes (actual dimensions are smaller: a 2×10 is 1.5×9.25 in.). Graded for strength by visual inspection or machine stress-rating. Span tables and NDS design values are based on species, grade, and size.

Doubler Plate

A steel plate welded to the web of a column within the panel zone (the region of the column web between beam flanges at a moment connection). Added when the column web alone cannot resist the high shear forces generated by the beam flange couple in a moment connection.

Ductile Failure

A failure mode preceded by large, visible deformation that provides warning before collapse. Steel yielding in tension and flexure are ductile failure modes. Structural codes prefer ductile modes because they allow load redistribution and warn occupants. The ductile mode is designed to occur before any brittle mode.

Duration of Load Factor (CD)

A multiplier in NDS wood design that increases allowable stresses for short-duration loads. Wood can sustain higher stresses briefly without damage. Typical values: CD = 0.90 for permanent (dead) load, 1.0 for 10-year (occupancy), 1.15 for 2-month (snow), 1.6 for 10-minute (wind/seismic).

E

Eccentricity

The distance between a load's line of action and the centroid (or shear center) of a cross-section. An eccentric axial load creates both axial stress and bending moment at the section. Eccentricity in plan (asymmetric stiffness vs. mass) causes torsion in a building under lateral loads.

Elastic

A material or structural behavior is elastic if it returns to its original shape when load is removed. No permanent deformation occurs. In steel, behavior is elastic up to the yield stress (Fy); above that, the material deforms plastically. Elastic analysis assumes all materials remain in the elastic range.

Elastic Modulus (E)

The ratio of stress to strain in the elastic range: E = σ / ε. Measures a material's stiffness. For structural steel: E = 29,000 ksi. For concrete: roughly 57,000√f'c (psi units). Also called Young's Modulus. Higher E → less deformation under load.

Equilibrium

The condition in which the sum of all forces and the sum of all moments acting on a body equal zero: ΣFx = 0, ΣFy = 0, ΣM = 0. This is the foundation of structural analysis — every free body diagram, reaction calculation, and internal force check relies on it.

Euler Buckling Load (Pcr)

The theoretical axial load at which an ideal, pin-ended column first buckles: Pcr = π²EI / L². Named after Leonhard Euler (1757). Real columns buckle at lower loads due to initial imperfections and residual stresses. The effective length KL replaces L for other end conditions.

Effective Length

The equivalent pin-ended column length KL used in buckling calculations. It accounts for real end conditions: a fixed-fixed column with actual length L behaves like a pin-pin column of length 0.5L. Determined by the K-factor, which depends on the rotational restraint at each end and whether the frame is braced or unbraced.

Effective Width

The width of a concrete slab that participates in composite action with the supporting beam. Not the full slab width — shear lag reduces participation far from the beam. AISC limits effective width to the lesser of: beam span / 4, center-to-center beam spacing, or 8× slab thickness on each side.

Embed Plate

A steel plate with headed studs or reinforcing bars welded to its back face, cast into concrete to provide an attachment point. Embed plates transfer tension, shear, or moment between steel members and concrete elements. Design checks include stud pullout, concrete breakout, and plate bending.

Envelope (Load Envelope)

The bounding diagram showing the maximum and minimum values of shear, moment, or deflection at every point along a member, considering all applicable load combinations. For continuous beams, pattern loading creates different critical sections; the envelope captures the worst case everywhere.

Edge Distance

The distance from the center of a bolt hole to the nearest edge of the connected plate. Minimum edge distances prevent tearout failure and are specified in AISC Table J3.4 (typically 1× to 1.75× bolt diameter). Bearing strength at bolt holes is directly proportional to clear distance to the edge.

Effective Throat

The shortest distance from the root of a fillet weld to the face of the weld, measuring through the weld cross-section. For a standard 45° fillet weld: te = 0.707 × leg size. Weld strength is calculated using the effective throat dimension, not the leg size.

Equivalent Lateral Force (ELF)

The simplest code method for seismic design (ASCE 7 §12.8). Replaces dynamic earthquake loading with a set of static horizontal forces applied at each floor level. Total base shear V = Cs × W is distributed vertically in proportion to each floor's weight and height. Valid for regular, low-to-moderate height structures.

F

Factor of Safety (FS)

In ASD, the ratio of nominal capacity to applied load — a buffer against uncertainty in loads, materials, and construction. A factor of safety of 1.67 means the member can carry 1.67× the design load before reaching the allowable limit. LRFD uses separate load and resistance factors instead of a single FS.

Fixed Support

A support that prevents all translation and rotation. In 2D, it provides three reactions: horizontal force, vertical force, and moment. A column base can be designed as fixed (moment connection to foundation) or pinned (anchor bolts that allow rotation). Fixed supports make a beam statically indeterminate.

Flange

The horizontal plates at the top and bottom of an I-shaped (W-shape) section. Flanges are the primary bending elements — they carry the largest normal stresses because they are farthest from the neutral axis. Wide flanges provide better lateral stability and greater moment of inertia for a given depth.

Force

A push or pull acting on a body, characterized by magnitude, direction, and point of application (a vector). In structural engineering, common forces include gravity (weight, downward), wind (lateral surface pressure), seismic (lateral inertial), and soil/fluid pressure (lateral against walls).

Foundation

The structural element that transfers all building loads to the soil or rock below. Types include spread footings (isolated pads under columns), strip footings (continuous under walls), mat foundations (a single slab under the entire building), and deep foundations (piles, drilled piers) for weak or deep bearing soils.

Free Body Diagram (FBD)

A sketch of a body isolated from its surroundings, with all external forces and moments shown as arrows. The essential first step in any structural analysis. Applied loads, support reactions, and internal forces at cut sections all appear on an FBD. No FBD = no valid equilibrium calculation.

Fatigue

Progressive fracture of a material under repeated cyclic loading at stresses below the static yield strength. Each load cycle propagates microscopic cracks until sudden fracture. Relevant for crane runway beams, bridge girders, and connections subjected to many load reversals. AISC Appendix 3 provides fatigue design categories.

Fillet Weld

A triangular-profile weld deposited in the corner formed by two surfaces at roughly 90°. The most common structural weld type. Strength is based on the effective throat dimension (0.707 × leg size for 45° fillet) and the weld metal tensile strength (typically E70, FEXX = 70 ksi): φRn = 0.75 × 0.6 × FEXX × te × L.

Fireproofing

Material applied to structural steel to delay heating during a fire, maintaining the member's strength long enough for occupant evacuation. Common types: spray-applied fire-resistive material (SFRM), intumescent paint, and concrete encasement. Required fire ratings (1–4 hours) depend on building type, height, and occupancy.

Flexure

Bending — the structural behavior in which a member curves under transverse load, developing internal bending moment. Flexural stress varies linearly from the neutral axis: σ = My / I. Beams are designed primarily for flexure. Flexural capacity depends on section properties, material strength, and lateral bracing.

Footing

A spread foundation element, typically reinforced concrete, that distributes a column or wall load over a larger area of soil. Isolated (pad) footings support single columns; strip (continuous) footings support walls. Sized so that bearing pressure (P/A) does not exceed the soil's allowable bearing capacity.

Frequency (f)

The number of vibration cycles per second, in Hertz (Hz). Natural frequency: f = 1 / T where T is the natural period. Floor vibration is a serviceability concern — sensitive floors (offices, labs) should have a first natural frequency above 6–9 Hz to avoid perceptible vibration from footfall.

Flood Load

Hydrostatic and hydrodynamic forces on structures in flood-prone areas, per ASCE 7 Chapter 5 and ASCE 24. Includes buoyancy, lateral hydrostatic pressure, wave action, and debris impact. Structures in coastal high-hazard zones (V-zones) must be elevated on open foundations that allow water to pass beneath.

Fracture Toughness

A material property measuring resistance to crack propagation. Important for steel connections subject to high constraint, thick plates, or cold temperatures — conditions that promote brittle fracture. AISC requires Charpy V-Notch (CVN) impact testing for thick flanges and demand-critical welds in seismic applications.

G

Girder

A primary beam that supports secondary beams (joists or floor beams) rather than directly carrying deck or slab. In a typical steel floor system, floor beams span between girders, which in turn span between columns. Girders carry concentrated loads (reactions from floor beams) rather than uniform loads.

Gravity Load

Any vertical downward load due to weight: dead load (D), live load (L), snow load (S), and roof live load (Lr). Gravity loads are the primary loads in most building structures and drive the design of nearly all horizontal members (beams, girders) and vertical members (columns).

Girt

A horizontal steel member spanning between columns on the exterior of a building to support wall cladding. Girts carry wind load from the wall panels to the columns. Typically light C-shapes or Z-shapes at 4–6 ft spacing. The roof equivalent is a purlin.

Grade (Steel)

The specification defining a steel's mechanical properties. Common grades: ASTM A992 (W-shapes, Fy=50 ksi), A500 (HSS, Fy=46 ksi), A36 (plates/angles, Fy=36 ksi), A572 Gr.50 (plates, Fy=50 ksi). The grade determines the yield and ultimate strengths used in design.

Grout

A fluid cementitious mixture used to fill gaps and voids. Non-shrink grout fills the space between steel base plates and concrete foundations, ensuring uniform bearing. In masonry construction, grout fills the cells of concrete masonry units (CMU) around reinforcing bars.

Gusset Plate

A steel plate used to connect bracing members, beams, and columns at a single joint. Gusset plates transfer axial forces from braces into the frame through bolts or welds. Designed for the Whitmore section (effective width in tension), buckling, and block shear.

Gage

The transverse spacing between rows of bolts on a member, measured perpendicular to the member's axis. Standard gage dimensions for W-shapes are tabulated in the AISC Manual Part 1. Gage must satisfy minimum bolt spacing requirements (typically 2.67× bolt diameter) and edge distance requirements.

Galvanizing

A corrosion-protection coating of zinc applied to structural steel by hot-dipping in molten zinc. Common for exposed steel, outdoor connections, and elements in humid environments. Adds 2-5 mils per side of thickness that must be accounted for in bolt hole sizing. ASTM A123 governs structural galvanizing.

Glulam (Glued Laminated Timber)

An engineered wood product made by gluing multiple layers of dimensional lumber together to create larger beams, columns, and arches. Stronger and more consistent than solid-sawn timber. Available in custom sizes and curved shapes. Design per NDS with published design values per AITC or manufacturer.

H

Hinge (Pin Connection)

A connection that transfers forces but releases moment — it cannot resist rotation. A true pin allows connected members to rotate freely relative to each other, so the bending moment at that point is zero. Hinges are used in analysis to simplify indeterminate structures and in design to avoid transmitting moment through a connection.

Headed Stud

A short steel stud with a forged head, welded to a beam's top flange, that projects into the concrete slab to create composite action. The stud resists horizontal shear between the steel beam and concrete. Capacity per stud depends on stud diameter, concrete strength, and deck geometry.

Hold-Down

A hardware connector that anchors a shear wall's end post (chord) to the foundation or to the floor below, resisting the tension force from overturning. Common in wood-framed and cold-formed steel shear walls. Hold-down capacity must exceed the net uplift demand from the overturning moment minus resisting dead load.

Hollow Structural Section (HSS)

A closed steel tube section — round, square, or rectangular. HSS members are efficient in compression (high r for a given weight) and torsion (closed cross-section). Commonly used for columns, braces, and exposed architectural members. Connections require special detailing because you cannot access the inside.

Hooke's Law

The linear relationship between stress and strain in the elastic range: σ = Eε. Valid until the material yields. It is the foundation of elastic structural analysis — every deflection, reaction, and force distribution in a linear analysis relies on Hooke's Law holding true for all members.

Heat-Affected Zone (HAZ)

The region of base metal adjacent to a weld that has been heated enough to alter its microstructure and mechanical properties but not melted. The HAZ can have reduced toughness and is a common location for weld-related cracking. Controlled through preheat, interpass temperature, and weld procedure qualification.

Hoop

A closed-loop transverse reinforcing bar enclosing longitudinal bars in a concrete column or beam, with seismic hooks at both ends (135° minimum bend). Hoops provide confinement and shear resistance in plastic hinge regions. Distinguished from ties by the hook requirement — ties have 90° hooks that can open under large deformations.

I

I-Section (W-Shape)

The most common structural steel cross-section, shaped like the letter I: two horizontal flanges connected by a vertical web. Material is concentrated far from the neutral axis (in the flanges), making it highly efficient in bending. Designated as W(depth)×(weight/ft), e.g., W18×35.

Inflection Point

The location along a beam where the bending moment is zero and the curvature reverses sign — changing from sagging (positive moment, tension at bottom) to hogging (negative moment, tension at top). In a continuous beam, inflection points occur between the midspan maximum and the support maximum.

Internal Force

The force or moment acting within a cross-section of a structural member, found by making an imaginary cut and summing forces on one side. The three internal forces in 2D are: axial force (N), shear force (V), and bending moment (M). These are what shear diagrams and moment diagrams plot.

Impact Load

A dynamic load applied suddenly — a dropped weight, a vehicle collision, or a crane picking up a load. Impact amplifies the static force through a dynamic amplification factor that depends on the drop height and the stiffness of the receiving structure. Crane impact factors per ASCE 7: 25% vertical, 20% lateral.

Interaction Equation

A formula that checks the combined effect of two or more simultaneous demands against a member's capacity. The AISC H1-1 interaction equation for combined axial and bending: when Pu/φPn ≥ 0.2, use Pu/φPn + (8/9)(Mu/φMn) ≤ 1.0. The member passes only if the combined ratio stays at or below 1.0.

Importance Factor (Ie)

A multiplier in ASCE 7 that adjusts design loads based on a structure's risk category. Essential facilities (hospitals, fire stations) get Ie = 1.5 for wind and 1.25 for seismic, increasing design forces. Standard occupancy buildings use Ie = 1.0. Risk categories are assigned based on occupancy and failure consequence.

Influence Line

A diagram showing how a response quantity (reaction, shear, moment, or deflection at a specific point) varies as a unit load moves across the structure. Used to determine where to place live loads for maximum effect. The area under the influence line multiplied by the load intensity gives the response value.

Irregularity

A geometric or stiffness configuration in a building that deviates from simple, regular layouts. ASCE 7 Tables 12.3-1 and 12.3-2 define horizontal irregularities (torsional, reentrant corner, diaphragm discontinuity) and vertical irregularities (soft story, weight, geometric). Irregular buildings face stricter analysis requirements and lower R-factors.

J

Joist

A closely spaced secondary framing member that directly supports the floor deck or roof sheathing. In steel construction, open-web steel joists (bar joists) span between girders and support metal deck. In wood construction, dimensional lumber joists typically span 12–16 ft at 12", 16", or 24" on center.

K

K-Factor (Effective Length Factor)

A coefficient that adjusts column length for the actual end conditions. The effective length KL replaces L in buckling calculations. Theoretical values: pin-pin K=1.0, fixed-fixed K=0.5, fixed-free (flagpole) K=2.0, fixed-pin K=0.7. In practice, braced frames use K near 1.0; unbraced frames can exceed 1.0.

Kip

A unit of force equal to 1,000 pounds (1 kip = 1 klb). The standard unit for structural loads and reactions in US practice. Moments are expressed in kip-feet (kip·ft) or kip-inches (kip·in); stresses in ksi (kip per in²). 1 kip = 4.448 kN.

Knee Brace

A short diagonal member connecting a column to a beam near their intersection, stiffening the joint and providing partial moment resistance without a full moment connection. Common in industrial steel buildings. The knee brace creates a truss action that reduces the beam's effective span for lateral loads.

L

Lateral Bracing

Members or connections that prevent the compression flange of a beam from buckling sideways (lateral-torsional buckling). Bracing is provided by the floor deck, perpendicular framing, or dedicated brace members. The unbraced length Lb determines the beam's available flexural strength in AISC design.

Lateral Load

A horizontal force applied to a structure: wind pressure on exterior surfaces, seismic inertial forces from the building's own mass, or soil/fluid pressure on below-grade walls. Lateral loads must be transferred through the diaphragm to the lateral system and down to the foundation.

Lateral System

The structural system that resists horizontal forces and controls lateral drift. Every building must have a complete, continuous lateral system. Common types: moment frames (beam-column bending), braced frames (diagonal member axial forces), and shear walls (in-plane wall shear). Many buildings use a combination.

Lateral-Torsional Buckling (LTB)

A buckling mode in which the compression flange of a beam displaces laterally and the section twists, reducing bending capacity below Mp. It occurs when the unbraced length Lb exceeds a limiting value. Prevented by lateral bracing at the compression flange, or by using sections with high torsional stiffness.

Live Load (L)

Variable loads from occupancy and use — people, furniture, equipment, and movable partitions. Specified by code based on occupancy type. Per ASCE 7-22 Table 4.3-1: offices = 50 psf, corridors above first floor = 80 psf, lobbies = 100 psf, retail = 100 psf, residential = 40 psf.

Load Combination

A codified set of simultaneous loads used for design. ASCE 7 §2.3 LRFD combinations include: 1.4D, 1.2D + 1.6L + 0.5(Lr or S), 1.2D + 1.6(Lr or S) + (L or 0.5W), and others with wind and seismic. The combination producing the maximum demand controls.

Load Path

The continuous chain of structural members through which forces travel from their source to the foundation. A gravity load path runs: deck → floor beams → girders → columns → footings → soil. The load path must be complete and uninterrupted; any gap in the chain is a structural deficiency.

LRFD (Load and Resistance Factor Design)

The standard US design method: factored loads (demand) must not exceed factored resistance (capacity) — φRn ≥ ΣγiQi. Load factors (γ) amplify loads to account for uncertainty in their magnitude. Resistance factors (φ ≤ 1.0) reduce nominal capacity to account for material variability and fabrication tolerances.

Lap Splice

A method of joining two reinforcing bars by overlapping them side by side for a specified length so that bond with the surrounding concrete transfers force from one bar to the other. Splice length depends on bar size, concrete strength, and confinement. Class B splices (the most common) require 1.3× the development length.

Lateral Drift

The total horizontal displacement of a building at a given level under lateral loads, measured from the base. Distinct from story drift, which is the relative displacement between adjacent floors. Overall drift limits (H/400 to H/600) protect against perception of motion and global P-delta instability.

Link Beam

A short beam segment between two openings in a coupled shear wall, designed to yield in shear or flexure during a seismic event, dissipating energy while connecting the wall piers. In eccentrically braced frames (EBFs), the link is the segment of beam between brace connections, designed as a structural fuse.

Local Buckling

Buckling of an individual plate element (flange or web) of a cross-section rather than the member as a whole. Occurs when the width-to-thickness ratio exceeds code limits. AISC classifies sections as compact, noncompact, or slender based on these ratios. Compact sections can develop their full plastic moment before local buckling.

Long-Span

A structural system or member spanning distances greater than typical framing — generally over 40–60 ft. Long-span systems include trusses, castellated beams, plate girders, post-tensioned concrete, and cable systems. Design is often controlled by deflection, vibration, or erection stability rather than pure strength.

Limit State

A condition at which a structure or member ceases to fulfill its intended function. Strength limit states involve collapse or fracture (flexural yielding, buckling, rupture). Serviceability limit states involve functional impairment (excessive deflection, vibration, cracking). Every design check is a limit state check.

LVL (Laminated Veneer Lumber)

An engineered wood product made by bonding thin wood veneers together with the grain running in the same direction. Stronger and stiffer than dimensional lumber of the same size. Commonly used for beams, headers, and rim boards in light-frame construction. Design values are manufacturer-specific.

M

Mechanism

A structural condition in which enough hinges (real or plastic) have formed to allow the structure to move freely — it can no longer carry additional load and will collapse. In plastic analysis, the goal is to find what load causes the structure to form a mechanism. Preventing mechanisms prematurely is central to structural integrity.

Metal Deck

Corrugated cold-formed steel sheets used to form floors and roofs. For composite floors, concrete is cast on top of the deck and shear studs bond it to the beam. The deck also acts as a structural diaphragm, transferring lateral loads to the framing. Common profiles: 1.5" roof deck (non-composite), 2" and 3" composite deck.

Moment (M)

A force multiplied by a perpendicular distance, producing rotation about a point: M = F × d. In structural engineering, bending moments make beams flex; torsional moments make members twist; overturning moments threaten a wall's stability. Units: kip·ft or kip·in.

Moment Diagram

A diagram plotting the internal bending moment along a member's length. Key relationships: the slope of the moment diagram equals the shear force (dM/dx = V); the area under the shear diagram equals the change in moment. Maximum moment occurs where the shear diagram crosses zero.

Moment Frame

A lateral system in which beams and columns are connected with moment-resisting joints, allowing the frame to resist lateral forces through bending without diagonal bracing. Moment frames offer architectural flexibility (no bracing in the bay) but are more flexible than braced frames, leading to greater story drift.

Moment of Inertia (I)

A cross-section property measuring the distribution of area about a neutral axis: I = ∫ y² dA. Higher I → greater bending stiffness and strength. Units: in⁴. For a rectangle: I = bh³ / 12. Wide-flange sections concentrate area in the flanges to maximize I for a given weight.

Mass

The quantity of matter in a body, determining its inertial resistance to acceleration: F = ma. In seismic design, mass is critical — the base shear is proportional to the building's seismic weight (mass × gravity). Heavier buildings attract more seismic force. Mass is concentrated at floor levels for dynamic analysis.

Masonry

Construction of individual units (concrete block, brick, or stone) bonded with mortar. Structural masonry walls can serve as bearing walls and/or shear walls. Reinforced masonry (grouted cells with rebar) is designed similarly to reinforced concrete. Governed by TMS 402 (Building Code Requirements for Masonry Structures).

Modulus of Rupture (fr)

The tensile stress at which unreinforced concrete cracks in bending: fr = 7.5√f'c (psi units). It is not a true material property but a useful design parameter. Used to determine the cracking moment of a concrete section: Mcr = fr × Ig / yt. Much lower than the compressive strength.

Moment Connection

A beam-to-column connection designed and detailed to transfer the full bending moment (as well as shear and axial force) between the connected members. Required in moment frames for lateral resistance. Achieved through welded flanges, bolted end plates, or extended end-plate configurations.

Moment Redistribution

The transfer of bending moment from overstressed sections to less-stressed sections in a continuous, ductile beam as plastic hinges form. ACI 318 permits up to 20% redistribution of negative moments at supports (if certain ductility conditions are met), allowing more economical reinforcement layouts.

Moment Splice

A field connection in a beam or column designed to transfer the full bending moment across the splice. Typically achieved with bolted flange plates or complete-joint-penetration (CJP) groove welds. Located away from regions of maximum moment when possible. Required when members are too long to ship in one piece.

Mill Tolerance

The allowable variation in cross-section dimensions from the nominal values published in the AISC Manual. ASTM A6 specifies tolerances for depth, flange width, web thickness, and out-of-straightness. Engineers typically use nominal dimensions for design; fabricators account for mill tolerances in shop drawings and fit-up.

Modal Analysis

A dynamic analysis method that decomposes a structure's response into independent natural vibration modes, each with its own period, mode shape, and participation factor. Modal responses are combined (SRSS or CQC) to estimate total seismic demands. Required by ASCE 7 when the equivalent lateral force method is not permitted.

Mode Shape

The deformed shape of a structure vibrating freely at one of its natural frequencies. The first mode is typically a simple sway; higher modes involve more complex patterns with reversals. Mode shapes are normalized and used as weighting functions in modal response spectrum analysis.

Modular Ratio (n)

The ratio of elastic moduli of two materials in a composite section: n = Es / Ec (typically 8–10 for steel-concrete). Used to transform a composite cross-section into an equivalent homogeneous section for stress analysis. For long-term loads, an effective n accounting for concrete creep (2n or 3n) is used.

Moisture Content (MC)

The weight of water in wood expressed as a percentage of the oven-dry wood weight. Green lumber may exceed 30% MC; kiln-dried lumber is typically 12–19%. Wood shrinks and swells with MC changes. NDS design values are tabulated for 19% MC (sawn lumber) and must be adjusted with a wet service factor for higher MC.

N

Neutral Axis

The line through a cross-section where bending stress is zero. On one side of the neutral axis bending creates compression; on the other, tension. For a homogeneous, symmetric section, the neutral axis passes through the centroid. In a composite beam, it shifts toward the concrete due to the higher stiffness of the slab.

Node

In truss analysis, a joint where two or more members connect. Truss members are assumed to carry only axial forces between nodes (no bending), so loads must be applied at nodes. In finite element analysis (FEM), nodes are discrete points where displacements, forces, and stresses are computed.

Natural Period (T)

The time in seconds for one complete cycle of free vibration of a structure. Taller, more flexible buildings have longer periods. ASCE 7 approximate formula: Ta = Ct × hn^x where hn is building height. The period determines the spectral acceleration used in seismic design — it is the single most important dynamic property of a building.

Net Section

The cross-sectional area remaining after deducting bolt holes and other reductions. Tension capacity is checked at the net section: φPn = 0.75 × Fu × Ae, where Ae accounts for shear lag. Net section fracture often governs over gross section yielding in bolted tension connections because Fu/Fy > 1 but Ae/Ag < 1.

Nominal Strength (Rn)

The theoretical capacity of a member or connection calculated using nominal material properties (Fy, Fu, f'c) and code-specified equations — before applying any resistance factor (φ) or safety factor (Ω). Design strength = φRn (LRFD). Allowable strength = Rn/Ω (ASD).

NDS (National Design Specification)

The governing US standard for wood structural design, published by the American Wood Council (AWC). Covers sawn lumber, glulam, structural composite lumber, and connections. Design values are adjusted by a series of multiplicative factors (CD, CM, Ct, CF, etc.) to account for conditions of use.

Notch

A square cut or reduction in the cross-section of a wood member, typically at a bearing support. Notching significantly reduces shear capacity at the notch location. NDS limits the depth and location of notches — they are prohibited on the tension side of beams at midspan and must not exceed 1/4 of the member depth at supports.

Notional Load

A small lateral force applied to each story of a frame to account for the effect of initial out-of-plumb in columns during stability analysis. AISC requires notional loads equal to 0.2% of the gravity load at each level (Ni = 0.002Yi) when using the direct analysis method.

O

Overturning Moment

The tendency of a lateral force to tip or rotate a structure about its base. For a shear wall, the overturning moment equals the lateral force times the wall height. It is resisted by the building's gravity loads (restoring moment) and, when gravity is insufficient, by tension hold-down anchors at the wall ends.

Out-of-Plane

Forces or displacements acting perpendicular to the primary plane of a wall, slab, or frame. A masonry wall resists wind load out-of-plane (like a vertically spanning beam) while resisting lateral seismic forces in-plane (as a shear wall). Out-of-plane design often governs slender walls and unreinforced partitions.

Outrigger

A stiff horizontal structure (typically a truss or wall) connecting a central core to perimeter columns in a tall building. The outrigger engages the perimeter columns as a couple to resist overturning, dramatically increasing the building's effective lever arm. Used in supertall structures to control drift and reduce core moments.

Over-Reinforced

A reinforced concrete section with more tension steel than the balanced amount, causing the concrete to crush before the steel yields. This is a brittle failure mode. ACI 318 prevents it by limiting the net tensile strain in the steel to at least 0.004 at nominal strength and penalizing with a lower φ factor below 0.005.

Overstrength Factor (Ω₀)

A multiplier in ASCE 7 seismic design used to estimate the maximum force that can be delivered by yielding elements to their connections and adjacent members. Applied to protect non-ductile elements (columns, foundations, collectors) from forces exceeding the code-level design values. Typical values: 2.0–3.0.

P

P-Delta Effect (P-Δ)

A second-order effect in which lateral displacement (Δ) under axial load (P) creates an additional moment equal to P×Δ, which causes further displacement, which creates more moment — a destabilizing amplification. Significant in tall, slender structures. AISC requires amplification factors (B1, B2) to account for P-delta in combined loading.

Pin Support

A support that restrains translation in all directions but allows rotation. In 2D it provides two reactions: horizontal and vertical. A simply supported beam has a pin at one end and a roller at the other — this combination is statically determinate and requires no moment resistance from the supports.

Plastic Hinge

A region in a ductile member where the full plastic moment Mp has been reached, causing the section to rotate at constant moment under further loading. Unlike a real hinge, a plastic hinge still transmits moment (equal to Mp). Formation of enough plastic hinges to create a mechanism defines collapse in plastic analysis.

Plastic Moment (Mp)

The bending moment at which the entire cross-section has yielded: Mp = Fy × Z, where Z is the plastic section modulus. Mp exceeds the elastic yield moment My = Fy × S by the shape factor (Z/S). For a W-shape, the shape factor is typically 1.1–1.2, meaning the section can carry 10–20% more than first yield.

Point Load (Concentrated Load)

A load applied at a single location, treated as acting at a point. In reality all loads are distributed, but loads from columns, beams, or posts on a supporting member are modeled as point loads. For a simply supported beam with a midspan point load P: maximum moment = PL / 4.

Poisson's Ratio (ν)

The ratio of lateral strain to axial strain under uniaxial stress: ν = −ε_lateral / ε_axial. For structural steel: ν ≈ 0.3. When a bar is stretched axially, it contracts laterally; Poisson's ratio quantifies how much. Relevant in biaxial stress states and for computing shear modulus: G = E / 2(1+ν).

Pedestal

A short, stocky reinforced concrete compression member (height ≤ 3× least dimension) that transfers load from a steel column base plate to the footing below. Pedestals raise the base plate above the floor slab, protecting it from moisture and allowing anchor bolt access. Designed for bearing and compression.

Pier

A deep foundation element, typically a large-diameter drilled concrete shaft (also called a caisson or drilled shaft), extending to bearing soil or rock. Piers transfer load through end bearing, side friction, or both. Common where shallow soils cannot support spread footings. Diameters range from 18 in to 10+ ft.

Pile

A slender deep foundation element driven into the soil by impact or vibration. Materials include steel H-piles, steel pipe piles, precast concrete, and timber. Piles develop capacity through end bearing on a hard stratum, skin friction along their length, or both. Pile groups require consideration of group effects and settlement.

Plate Girder

A deep built-up beam fabricated by welding steel plates together — typically two flange plates and a web plate. Used when rolled W-shapes are not deep or strong enough, particularly for long spans, transfer beams, and bridge girders. The web may require transverse stiffeners to prevent shear buckling.

Post-Tensioning (PT)

A method of prestressing concrete in which high-strength steel tendons are tensioned after the concrete has hardened, compressing the concrete to offset future tensile stresses from loads. Allows longer spans, thinner slabs, and crack control. Common in parking structures, high-rise floor slabs, and bridges.

Propped Cantilever

A beam with a fixed support at one end and a simple support (pin or roller) at the other. It is statically indeterminate to the first degree. Under uniform load w, the fixed-end moment is wL²/8 and the maximum positive moment is 9wL²/128. More efficient than a simple beam of the same span.

Punching Shear

A shear failure mode in flat slabs where a column punches through the slab along a critical perimeter around the column. The critical section is located at d/2 from the column face. ACI 318 §22.6 limits the punching shear stress to 4√f'c (psi). Often controlled by adding shear reinforcement (studrails) or thickening the slab (drop panels).

Purlin

A horizontal member spanning between roof trusses or rafters to support the roof deck or sheathing. Purlins carry gravity loads (dead, live, snow) and wind uplift down to the primary framing. In metal buildings, purlins are typically cold-formed C- or Z-shapes at 4–5 ft spacing.

Participation Factor

In modal analysis, a measure of how much a given vibration mode contributes to the total seismic response. Higher participation means the mode is more significant. The first translational mode typically has the largest participation factor. ASCE 7 requires enough modes to capture at least 90% of the participating mass.

Peak Ground Acceleration (PGA)

The maximum horizontal acceleration of the ground surface during an earthquake, expressed as a fraction of gravity (g). PGA characterizes the intensity of ground shaking at a site. ASCE 7 maps provide spectral accelerations at short periods (Ss) and 1-second period (S1), which are related to PGA.

Performance-Based Design

An advanced design approach where the structure is engineered to achieve specific performance objectives (operational, immediate occupancy, life safety, collapse prevention) under defined hazard levels. Goes beyond code-minimum prescriptive rules. Common for tall buildings, hospitals, and critical facilities using nonlinear analysis.

Pitch

The center-to-center spacing between bolts along the line of force in a connection. Minimum pitch per AISC is 2.67× bolt diameter (commonly rounded to 3 in. for 3/4" and 7/8" bolts). Pitch affects net section area, bearing capacity, and connection length.

Plastic Section Modulus (Z)

A cross-section property used to compute the plastic moment: Mp = Fy × Z. Z equals the sum of first moments of area above and below the plastic neutral axis (equal-area axis). Always greater than the elastic section modulus S. For a rectangle: Z = bh²/4 vs S = bh²/6.

Ponding

Progressive accumulation of rainwater on a flat roof when deflection under the water's weight allows more water to collect, causing more deflection — a potentially unstable feedback loop. AISC Appendix 2 and IBC require a ponding stability check. Adequate roof slope (≥ 1/4 in./ft) or stiff enough framing prevents ponding instability.

Portal Method

An approximate hand-analysis technique for lateral load analysis of multi-story frames. Assumes inflection points at midspan of beams and mid-height of columns, and that interior columns carry twice the shear of exterior columns. Quick verification of computer results for moment frames.

Preheat

Heating the base metal to a specified temperature (typically 150–400°F) before welding to slow the cooling rate, reduce residual stresses, and prevent hydrogen-induced cracking in the heat-affected zone. Required for thick plates, high-strength steels, and high-restraint joints. Specified in AWS D1.1 Table 3.2.

Prescriptive Design

A design approach using pre-established rules, tables, and details (e.g., IRC for residential wood framing) instead of engineering calculations. Applicable within strict limitations on building size, geometry, and loading. Exceeding prescriptive limits triggers engineered design per IBC and the applicable material code.

Q

Q-Factor (Reliability Factor)

In structural reliability theory, a measure of the probability of failure used to calibrate load and resistance factors. In everyday US practice, the term rarely appears — but the concept underpins LRFD: load and resistance factors were calibrated to achieve a target reliability index (β ≈ 2.6 for members, ≈ 4.0 for connections) based on statistical distributions of loads and strengths.

R

Radius of Gyration (r)

A cross-section property that measures how far area is distributed from an axis: r = √(I / A). It appears in the slenderness ratio KL/r, which governs column buckling. A larger radius of gyration means greater resistance to buckling. For a W-shape, ry (weak axis) is always smaller than rx and typically controls column design.

Reaction

The force or moment exerted by a support on a structure in response to applied loads. Reactions are found by applying equilibrium to the free body diagram of the entire structure. For a simply supported beam with uniform load w and span L: each end reaction = wL / 2.

Redundancy

Having more load-carrying capacity than the strict minimum required for equilibrium. A statically indeterminate structure has redundant members or reactions — if one element fails, load can redistribute to others. Redundancy improves robustness and is explicitly rewarded in seismic design (lower seismic demands for highly redundant systems).

Reinforced Concrete

Concrete with embedded steel reinforcing bars (rebar) that compensate for concrete's near-zero tensile strength. Concrete carries compression; steel carries tension. Flexural members (beams, slabs) place rebar on the tension side. Columns have longitudinal bars for compression capacity plus ties to confine the concrete and prevent bar buckling.

Roller Support

A support that restrains movement in one direction (typically vertical) while allowing translation horizontally and rotation. It provides one reaction force, perpendicular to the rolling surface. A simply supported beam has a roller at one end to allow thermal expansion and rotation without generating horizontal thrust at the supports.

Roof Live Load (Lr)

The live load applied to roofs during maintenance and construction activities. Per ASCE 7-22 §4.8, the minimum is 20 psf for flat roofs, reduced for large tributary areas and steep slopes. Roof live load is distinct from snow load; both must be considered separately and in combination.

Rebar

Steel reinforcing bars embedded in concrete to carry tensile forces. Common grades: Grade 60 (Fy = 60 ksi) and Grade 80. Bars are designated by diameter in eighths of an inch: a #8 bar is 1" diameter (area = 0.79 in²). Rebar is placed where tension develops — the bottom of simple-span beams, the top over supports, and around columns.

Resistance Factor (φ)

A factor less than or equal to 1.0 applied to nominal strength to account for variability in material properties, fabrication, and analysis accuracy. In LRFD: φRn ≥ Ru. Typical values: φ = 0.90 for flexure and tension yielding, 0.75 for tension fracture and bolts, 0.65 for bearing on concrete.

Response Spectrum

A plot of peak structural response (acceleration, velocity, or displacement) versus natural period for a given earthquake ground motion and damping ratio. The design response spectrum in ASCE 7 defines the spectral acceleration Sa(T) used to calculate seismic base shear. It is the bridge between ground motion and structural demand.

Rigid Connection

A connection that transfers moment between members with negligible relative rotation — the connected members rotate together. Achieved through welded flanges, stiffened bolted end plates, or other configurations that develop the beam's plastic moment. Essential for moment frames to resist lateral loads through frame action.

Rigidity

The stiffness of a lateral-resisting element (shear wall or frame) used to distribute lateral forces. Lateral load is distributed to parallel walls or frames in proportion to their relative rigidities. A wall twice as rigid as its neighbor attracts twice the force. For a cantilever wall: rigidity ∝ 1 / (h³/3EI + h/GA).

Rain Load (R)

The load from accumulated rainwater on a roof that does not drain due to ponding or blocked drains. ASCE 7 §8.3 requires design for rain load based on the hydraulic head: R = 5.2(ds + dh) where ds is the static head (depth to drain) and dh is the hydraulic head from flow to the secondary drainage. Applied independently of other roof loads.

Reduced Beam Section (RBS)

A seismic connection detail (also called a "dog-bone") in which a portion of the beam flanges is trimmed near the beam-column joint to force the plastic hinge to form in the reduced section, away from the more vulnerable welded joint. Prequalified in AISC 358 for special and intermediate moment frames.

Reentrant Corner

A horizontal irregularity (ASCE 7 Table 12.3-1, Type 2) where the plan shape of a building has an interior corner, such as an L, T, or U shape, and the projection beyond the corner exceeds 15% of the building dimension. Creates stress concentrations at the corner and requires collectors and reinforced diaphragm connections.

Relaxation

The time-dependent decrease in stress in a prestressing tendon held at constant strain. Steel relaxation reduces the effective prestress force over time. Typical relaxation loss: 2–3% for low-relaxation strand. Must be included in the total prestress loss calculation alongside elastic shortening, creep, and shrinkage.

Residual Stress

Stress locked into a steel member from the manufacturing process — differential cooling after hot rolling or welding. Residual stresses can reach 30–50% of Fy and cause parts of the cross-section to yield early under applied load, reducing the effective column buckling strength. The AISC column curve accounts for this.

Response Modification Factor (R)

A factor in ASCE 7 seismic design that reduces elastic spectral forces to design-level forces, accounting for a system's expected ductility and overstrength. Higher R means lower design forces but greater detailing requirements. Examples: R = 8 for special moment frames, R = 3.25 for ordinary braced frames.

Resultant

The single force (or force-and-moment) that produces the same effect as a system of forces. Finding the resultant is the first step in many equilibrium problems. For a uniform distributed load w over length L, the resultant is a single force wL acting at the midpoint of the loaded length.

Return Period

The average recurrence interval for a given level of hazard (wind speed, earthquake ground motion, flood level). A 700-year return period for seismic design (ASCE 7) means a 7% probability of exceedance in 50 years. Longer return periods correspond to rarer, more severe events and are used for essential facilities.

S

Section Modulus (S)

An elastic cross-section property equal to S = I / c, where c is the distance from the neutral axis to the extreme fiber. Bending stress at the extreme fiber: σ = M / S. Higher S → lower stress for the same moment. The plastic section modulus Z (used for Mp) is always larger than S for the same section.

Seismic Load (E)

Inertial forces induced in a structure when the ground accelerates during an earthquake. Proportional to the building's seismic weight and the design spectral acceleration. Determined from hazard maps, site class, and building period per ASCE 7 Chapters 11–12. Seismic load is the basis for both base shear and vertical distribution of forces over the building height.

Serviceability

A design criterion ensuring adequate performance under service (everyday) loads, independent of strength. Serviceability checks include deflection limits (L/360, L/240), story drift limits, floor vibration, and crack widths in concrete. A member can pass all strength checks and still fail serviceability — particularly in long-span or sensitive structures.

Shear (V)

The internal force acting perpendicular to a member's longitudinal axis, produced by transverse loads. Shear is the algebraic sum of all transverse forces on one side of a cut section. Maximum shear in a simply supported beam occurs at the supports; for a cantilever, at the fixed end.

Shear Diagram

A diagram plotting the internal shear force along a member's length. Key relationships: the slope of the shear diagram equals the distributed load intensity (dV/dx = −w); point loads cause vertical jumps in the diagram; the area under the shear diagram between two points equals the change in bending moment over that segment.

Shear Wall

A vertical wall element that resists lateral forces through in-plane shear. The wall acts as a vertical cantilever from the foundation. Shear walls can be reinforced concrete, reinforced masonry, or wood-framed with structural panel sheathing. They must be designed for shear, overturning, and sliding.

Simply Supported Beam

A beam with a pin at one end and a roller at the other, free to rotate at both supports. Moments at both ends are zero; the beam develops maximum moment at some interior point. For a uniform load w over span L: maximum moment = wL² / 8 at midspan. The simplest and most common beam model.

Slenderness Ratio (KL/r)

The ratio of a column's effective length (KL) to its radius of gyration (r = √(I/A)). Higher slenderness → lower critical buckling stress → lower available compressive strength. AISC limits slenderness to 200 for compression members. Wide columns (large r) or short columns (small L) resist buckling more effectively.

Snow Load (S)

The load applied to roofs from accumulated snow, determined per ASCE 7 Chapter 7. The flat-roof snow load is pf = 0.7 × Ce × Ct × Is × pg, where pg is the ground snow load from maps, Ce is exposure factor, Ct is thermal factor, and Is is importance factor. Drift loads on lower roofs must also be checked.

Span

The distance between supports for a beam, or between lateral brace points for a column. Longer spans require deeper sections to control deflection and bending stress. The relationship between span and required depth is roughly linear for deflection control, but required section modulus scales with L² for a uniform load.

Statically Determinate

A structure in which all reactions and internal forces can be found using the three equilibrium equations alone (ΣFx = 0, ΣFy = 0, ΣM = 0). A simply supported beam is statically determinate. These structures are straightforward to analyze but have no load redistribution capacity — removing any support causes immediate collapse.

Statically Indeterminate

A structure with more reactions or members than needed for equilibrium alone. Extra unknowns require additional equations from compatibility (deformation consistency). Most real structures are indeterminate. They benefit from load redistribution and redundancy but require more complex analysis (slope-deflection, moment distribution, or computer methods).

Stiffness

The resistance of a structure or member to deformation under load: k = F / Δ. For a simply supported beam with midspan point load: k = 48EI / L³. Stiffness depends on material (E), cross-section (I), length (L), and boundary conditions. Lateral stiffness of a story determines how loads are shared between parallel lateral-resisting elements.

Story Drift

The lateral displacement of a floor relative to the floor immediately below it, due to wind or seismic loading. Calculated as amplified elastic drift (δx = Cd × δxe / Ie in seismic). ASCE 7 Table 12.12-1 limits story drift ratios to protect non-structural elements, cladding, and occupants, and to prevent P-delta instability.

Strain (ε)

Deformation per unit length: ε = δ / L. Dimensionless — often expressed as in/in or %. In the elastic range, strain and stress are related by Young's Modulus: ε = σ / E. Steel yields at approximately ε_y = Fy / E = 50 / 29,000 ≈ 0.0017. Strain is what materials actually "feel"; stress is the force intensity causing it.

Strength

The ability of a material or member to resist applied stress without failure. For steel, the yield strength (Fy) marks the onset of permanent deformation; the ultimate strength (Fu) is the maximum stress before fracture. In member design, nominal strength (Rn) is reduced by a resistance factor φ to get the design strength φRn.

Stress (σ, τ)

Internal force intensity per unit area: σ = F / A. Normal stress (σ) acts perpendicular to the cross-section face and is associated with tension, compression, and bending. Shear stress (τ) acts parallel to the face and is associated with transverse shear and torsion. Units: ksi (kip/in²) or psi (lb/in²).

Sag Rod

A tension rod running between purlins or girts at mid-span, preventing them from sagging or rolling under the component of gravity load parallel to the roof slope. Sag rods are light (often 5/8" or 3/4" diameter rods) but essential for keeping secondary framing properly aligned.

Second-Order Effects

Changes in internal forces and moments caused by the structure's deformed geometry — primarily P-Δ (gravity loads acting through story drift) and P-δ (axial load acting through member curvature). First-order analysis ignores these; second-order analysis (or amplification factors) accounts for them. Required by AISC for stability design.

Shear Center

The point through which a transverse load must pass to produce bending without twisting. For doubly symmetric sections (W-shapes), the shear center coincides with the centroid. For channels and angles, it lies outside the cross-section. Loading away from the shear center induces torsion.

Shear Connection

A beam-to-column or beam-to-beam connection designed to transfer only shear (vertical reaction), not moment. The connection allows relative rotation at the joint. Common types: single-angle, double-angle, shear tab (single plate), and seated connections. Also called a simple or pinned connection.

Shear Flow (q)

Shear force per unit length along a section: q = VQ / I. Important for the design of built-up sections (determining fastener spacing between flange plates and web), composite beams (stud spacing), and diaphragms (deck-to-beam weld pattern). Units: kip/in or lb/in.

Shear Key

A notch, groove, or projection cast into a concrete surface to mechanically interlock two elements and transfer shear across the joint — for example, between a footing and a retaining wall, or between precast panels. Shear keys supplement friction and dowels at construction joints.

Shear Stud

See Headed Stud. A short steel connector welded to a beam's top flange that projects into the concrete slab, providing the mechanical bond needed for composite action. Stud capacity is governed by concrete pullout, stud shear, and deck geometry. Spacing is determined by the required total shear transfer along the beam.

Slab

A flat, horizontal concrete plate that spans between beams, walls, or columns. Slabs distribute floor loads to the supporting framing. Types include one-way slabs (span in one direction), two-way slabs (span in both directions), flat plates (no beams), flat slabs (with drop panels), and waffle slabs (two-way joist system).

Splice

A connection joining two segments of the same member end-to-end. Beam splices transfer moment, shear, and sometimes axial force. Column splices transfer compression (typically by direct bearing on milled surfaces) plus any tension and moment demands. Field splices are needed when members exceed shippable lengths (~60 ft for truck, ~80 ft for rail).

Stability

The ability of a structure or member to maintain its equilibrium configuration under load. Instability manifests as buckling (columns, flanges, webs), overturning (walls, buildings), or progressive collapse. AISC Chapter C requires a stability analysis that accounts for initial imperfections, residual stresses, and second-order effects.

Seismic Weight (W)

The total effective weight of a building used to calculate seismic base shear: V = Cs × W. Includes all dead load, permanent equipment, a portion of storage live load (25%), and partitions (≥ 10 psf). Does not include typical floor live loads. Concentrated at each floor level for vertical distribution.

Shape Factor

The ratio of plastic section modulus to elastic section modulus: Z/S. Indicates how much additional moment capacity exists between first yield and full plastification. For a W-shape: typically 1.1–1.2. For a rectangle: 1.5. For a circle: 1.7. Higher shape factors mean more reserve strength beyond first yield.

Shear Lag

The phenomenon where stress is not uniformly distributed across a member's cross-section at a bolted or welded connection because only part of the section is directly connected. Reduces effective net area by a shear lag factor U. A W-shape connected only through its flanges has U less than 1.0 because the web trails behind in picking up stress.

Sheathing

Panel material (plywood, OSB, or structural fiberboard) fastened to wood or cold-formed steel framing to create walls, floors, and roofs. Structural sheathing provides diaphragm and shear wall action. Capacity depends on panel thickness, nail size, nail spacing, and framing member spacing. Governed by AWC SDPWS.

Shim

A thin steel plate placed between structural members during erection to fill gaps and achieve proper fit-up, alignment, or elevation. Shims in column splices (bearing connections) must be designed for the loads passing through them. Finger shims allow field adjustment without complete disassembly.

Site Class

A soil classification (A through F) in ASCE 7 that characterizes the ground conditions at a building site for seismic design. Based on the average shear wave velocity, SPT blow count, or undrained shear strength in the upper 100 ft. Site class modifies the spectral accelerations — soft soils (Class D/E) amplify ground motion.

Soft Story

A vertical irregularity (ASCE 7 Table 12.3-2) where a story's lateral stiffness is less than 70% of the story above or 80% of the average of the three stories above. Soft stories concentrate drift and damage in a single level — a common collapse mechanism in earthquakes. Triggers stricter analysis requirements or prohibition for certain systems.

Spalling

The breaking away of surface concrete, exposing aggregate or reinforcing steel. Caused by corrosion of embedded rebar (expansion cracks the cover), fire exposure (moisture in concrete turns to steam), or freeze-thaw cycles. Spalling reduces cross-section area and exposes reinforcement to further deterioration.

Spectral Acceleration (Sa)

The peak acceleration experienced by a single-degree-of-freedom oscillator of a given natural period subjected to a specific ground motion. The design response spectrum in ASCE 7 plots Sa vs. period T to define seismic demand. Sa at short periods (SDS) and 1-second (SD1) are the key design parameters.

Stagger

Offsetting bolt holes in adjacent rows so they are not aligned across the width of a connected member. Staggering increases the net section area compared to bolts in a single line, because the potential failure path must travel diagonally. Net width is calculated using the s²/4g rule per AISC §B4.3b.

Stiffener

A plate welded to a beam or column web to increase local resistance to concentrated loads, prevent web buckling, or transfer forces. Types include bearing stiffeners (at reactions), transverse stiffeners (for plate girder web shear buckling), and continuity plates (at moment connections).

Stirrup

A U-shaped or closed-loop transverse reinforcing bar in a concrete beam, spaced along the beam's length to resist diagonal tension (shear). Stirrup contribution to shear strength: Vs = Av × fyt × d / s where s is the stirrup spacing. Required wherever factored shear exceeds the concrete contribution Vc.

Strain Hardening

The increase in stress beyond the yield plateau in the stress-strain curve of steel. After yielding at Fy, steel strain-hardens until reaching the ultimate tensile strength Fu. Strain hardening provides additional reserve capacity but is not relied upon in standard design. It does explain why actual member strengths exceed calculated values.

Stress-Strain Curve

A plot of engineering stress (σ = P/A₀) vs. engineering strain (ε = δ/L₀) from a uniaxial tension test. For structural steel, the curve shows a linear elastic region (slope = E), a yield plateau at Fy, strain hardening up to Fu, and necking before fracture. The curve defines all key material properties used in design.

Superposition

The principle that the response of a linear elastic structure to multiple loads equals the sum of responses to each load applied individually. Valid only in the elastic range. Allows engineers to analyze each load case separately and combine results. Does not apply when second-order (P-delta) effects are significant.

T

Tension

A state of stress in which material is being pulled or stretched, lengthening the member. Steel performs excellently in tension — its design tension capacity is limited by yielding (φPn = 0.90 × Fy × Ag) and by net section fracture through holes (φPn = 0.75 × Fu × Ae). Concrete and unreinforced masonry cannot reliably carry tension.

Tributary Area

The floor or roof area assigned to a specific structural member for load calculation. An interior column in a uniform grid with bay dimensions b₁ × b₂ carries a tributary area of b₁ × b₂ (the area of one full bay). The column load equals the floor pressure (psf) multiplied by tributary area, summed over all floors above.

Tributary Width

The floor width assigned to a beam: half the distance to each adjacent parallel beam. A beam at the center of two 8 ft spacings has a tributary width of 8 ft. Line load on the beam = area load (psf) × tributary width (ft) = lb/ft or kip/ft. This is how area loads become the line loads used in beam analysis.

Truss

A structural system composed of straight members connected at nodes (pins), arranged in triangles. All members carry only axial forces — no bending. The triangular geometry makes trusses rigid and highly efficient for spanning long distances with light weight. Analysis uses the method of joints or method of sections combined with equilibrium.

Tendon

A high-strength steel strand or bar used in prestressed and post-tensioned concrete. Tendons are stressed to 75–80% of their ultimate tensile strength (fpu ≈ 270 ksi for strand), compressing the concrete. The compression offsets future tensile stresses from service loads, allowing longer spans and thinner members.

Thermal Load

Forces and stresses induced by temperature changes. A restrained member heated by ΔT develops a compressive stress σ = EαΔT. Buildings accommodate thermal movement through expansion joints, slotted connections, and flexible details. Long buildings without joints can develop large forces in the framing.

Tie

A tension member — typically a rod, bar, or cable — that holds two points together. Ties appear in many contexts: column ties (transverse rebar confining concrete), wind ties (connecting roof framing to walls), and tension ties in strut-and-tie models for deep beams and disturbed regions in concrete.

Tilt-Up

A construction method in which reinforced concrete wall panels are cast flat on the floor slab and tilted up into position by crane. Tilt-up panels serve as bearing walls and/or shear walls. Common in warehouses, industrial buildings, and big-box retail. Panels must be designed for both in-service loads and the temporary stresses during lifting.

Torsion

Twisting of a member about its longitudinal axis, caused by loads applied eccentric to the shear center. Torsion produces shear stress in the cross-section. Closed sections (HSS, pipes) resist torsion efficiently; open sections (W-shapes, channels) are much weaker in torsion and tend to warp. Designed per ACI 318 §22.7 or AISC Chapter H.

Transfer Beam

A beam or girder that supports one or more columns from above, redirecting their loads to different column locations below. Transfer beams are often very deep and heavily loaded. Common when upper-floor column grids do not align with lower-floor or parking layouts. Design must address shear, flexure, and deflection carefully.

Transformed Section

An equivalent cross-section used to analyze composite members (steel-concrete) by converting one material's area into an equivalent area of the other using the modular ratio n = E_steel / E_concrete. The transformed section has uniform material properties, so standard bending formulas (σ = My/I) can be applied.

Torsional Constant (J)

A cross-section property that governs resistance to uniform (St. Venant) torsion: T = GJθ/L. For closed sections (HSS): J is large (high torsional stiffness). For open sections (W-shapes): J is small, computed as the sum of bt³/3 for each plate element. Low J → susceptibility to lateral-torsional buckling.

Torsional Irregularity

A horizontal irregularity (ASCE 7 Table 12.3-1, Type 1) where the maximum story drift at one end of a floor exceeds 1.2 times the average drift. Caused by eccentric mass or stiffness. Extreme torsional irregularity (ratio > 1.4) is prohibited for certain seismic design categories and system types.

Tuned Mass Damper (TMD)

A passive vibration control device consisting of a mass, spring, and dashpot mounted near the top of a tall building, tuned to oscillate out of phase with the building's dominant mode. Reduces wind-induced accelerations for occupant comfort. Examples: Taipei 101 (730-ton pendulum), Citicorp Center.

U

Ultimate Strength (Fu)

The maximum stress a material sustains before fracture. For A992 structural steel: Fu = 65 ksi (vs. Fy = 50 ksi for yield). Fu governs the design of net-section tension checks, bolt bearing, and weld strengths — connections where localized fracture, not general yielding, controls capacity.

Uniform Load

A distributed load of constant intensity over the full length of a member (w, in kip/ft). For a simply supported beam: maximum moment at midspan = wL² / 8; end reactions = wL / 2; midspan deflection = 5wL⁴ / 384EI. The uniform load model applies to most floor beams loaded by tributary area.

Unbraced Length (Lb)

The distance between points of lateral support for the compression flange of a beam. Controls the available flexural strength: if Lb ≤ Lp, the beam can reach its full plastic moment Mp; if Lb exceeds Lr, lateral-torsional buckling governs and capacity drops significantly. Bracing the compression flange at closer intervals increases strength.

Uplift

A net upward force on a structural element. Sources include wind suction on roofs, seismic overturning on shear wall chords, and buoyancy from groundwater on below-grade structures. Uplift must be resisted by the weight of the structure (dead load) and mechanical anchorage (hold-downs, anchor bolts, piles in tension).

Under-Reinforced

A reinforced concrete section with less tension steel than the balanced amount, ensuring the steel yields well before the concrete crushes. This provides ductile behavior with visible cracking and deflection as warning before failure. ACI 318 requires all flexural members to be under-reinforced by limiting maximum reinforcement ratios.

Unit Weight (γ)

The weight per unit volume of a material, equal to density times gravitational acceleration. Common values: structural steel γ = 490 pcf, normal-weight concrete γ = 150 pcf, timber γ = 35 pcf (average), soil γ = 100–130 pcf. Used to calculate dead loads and earth pressures.

V

Vibration

Oscillatory motion of a structure or member. Floor vibration from footfall is a common serviceability concern in steel-framed buildings with long spans or light weight. Acceptable vibration depends on occupancy — offices tolerate more than hospitals or labs. Design guidelines (AISC Design Guide 11) limit peak acceleration to 0.5% g for offices.

Virtual Work

An energy method for calculating deflections: apply a virtual unit load at the point and direction of the desired displacement, then integrate the product of real and virtual internal forces over the structure. For a beam: Δ = ∫(Mm/EI)dx where M is the real moment and m is the virtual moment. Powerful for indeterminate structures.

Von Mises Stress

An equivalent uniaxial stress computed from the full stress tensor, used to predict yielding under complex (multi-axial) loading: σ_vm = √(σx² − σxσy + σy² + 3τxy²). A material yields when the Von Mises stress reaches Fy. Used in FEA results to evaluate stress concentrations at connections, gusset plates, and stiffeners.

Vertical Distribution

The method of distributing total seismic base shear V to each floor level as individual story forces Fx. Per ASCE 7 §12.8.3: Fx = CvxV where Cvx depends on each floor's weight and height raised to an exponent k (k=1 for short-period buildings, k=2 for long-period, interpolated between). Higher floors get larger forces.

W

W-Shape (Wide Flange)

The standard structural steel I-section with parallel, wider-than-web flanges. Designated W(nominal depth)×(weight/ft) — e.g., W18×35 is nominally 18" deep and weighs 35 lb/ft. The wide flanges provide good moment of inertia, ease of connection, and lateral stability. AISC publishes section properties (I, S, Z, r) for all standard W-shapes in the Steel Construction Manual.

Web

The vertical plate connecting the top and bottom flanges of an I-shaped section. The web primarily resists shear. It is typically thin (high depth-to-thickness ratio), so it may require transverse stiffeners at concentrated load or reaction points to prevent web crippling or local buckling.

Wind Load (W)

Pressure and suction forces on a building's exterior due to moving air, per ASCE 7 Chapters 26–30. Depends on basic wind speed (from maps), exposure category, building height, topographic effects, and gust factors. Creates both global lateral forces on the main wind force resisting system (MWFRS) and local pressure on cladding/components (C&C).

Weld

A metallurgical bond joining steel elements by melting filler metal (and sometimes base metal) together. Common structural weld types: fillet welds (triangular profile in a corner) and groove welds (filling a prepared gap between plates). Welds are inspected visually and, for critical joints, by ultrasonic testing (UT).

Welded Connection

A structural connection using welds instead of (or in addition to) bolts. Complete-joint-penetration (CJP) groove welds develop the full strength of the connected material. Fillet welds transfer force through throat shear. Welded connections allow cleaner geometry but require qualified welders and inspection.

Working Load

The actual service-level (unfactored) load applied to a structure — the load you'd expect to see on any given day. ASD checks that working loads produce stresses below allowable limits. LRFD factors working loads upward for strength checks but uses them unfactored for serviceability. Also called service load.

Warping Constant (Cw)

A cross-section property that governs resistance to warping torsion (non-uniform torsion) in open sections like W-shapes and channels. Warping torsion develops longitudinal flange stresses and is significant in beams loaded away from the shear center. Appears in the lateral-torsional buckling equations alongside J.

Water-Cement Ratio (w/c)

The ratio by weight of water to cement in a concrete mix. Lower w/c produces stronger, more durable concrete but is harder to place: 0.40 for high-strength, 0.45–0.50 for typical structural concrete, 0.60+ for mass concrete. ACI 318 Table 19.3.2 sets maximum w/c for durability based on exposure class.

Web Sidesway Buckling

A buckling failure mode in which the tension flange of a beam remains restrained while the compression flange displaces laterally, causing the web to buckle in a sidesway pattern. Checked at points of concentrated load or reaction on the compression flange. AISC §J10.4 provides the limit state equations.

Wet Service Factor (CM)

An NDS adjustment factor that reduces wood design values when the in-service moisture content exceeds reference conditions (19% for sawn lumber, 16% for glulam). Wood loses strength and stiffness when wet. CM values range from 0.7 to 1.0 depending on the property (bending, compression, modulus of elasticity).

Whitney Stress Block

A rectangular stress distribution assumed in ACI 318 ultimate strength design of concrete sections, replacing the actual parabolic compression stress profile. Uniform stress of 0.85f'c over a depth a = β₁c (where c is the neutral axis depth and β₁ = 0.85 for f'c ≤ 4000 psi). Simplifies flexural capacity calculations.

X

X-Bracing

A lateral bracing configuration in which two diagonal members cross to form an X within a structural bay. Under lateral load, one diagonal acts in tension while the other goes into compression. In seismic design, only the tension brace is typically counted for strength (the compression brace may buckle), though both contribute to stiffness.

Y

Yield Strength (Fy)

The stress at which steel first undergoes permanent (plastic) deformation and the stress-strain curve plateaus. For ASTM A992 steel (standard for wide-flange shapes): Fy = 50 ksi, Fu = 65 ksi. Fy is the primary material strength used in LRFD steel design for flexure, compression, and tension yielding limit states.

Young's Modulus

See Elastic Modulus (E). Named after Thomas Young (1807). For structural steel: E = 29,000 ksi. For normal-weight concrete: approximately E = 57,000√f'c (psi units). Young's Modulus controls elastic deflection, buckling loads, and vibration frequency — all stiffness-dependent behavior.

Z

Zero-Force Member

A truss member that carries no axial force under a given loading. Identified by inspection at joints: if two non-collinear members meet at an unloaded joint, both are zero-force members; if three members meet with two collinear and the joint unloaded, the third is zero-force. Removing them doesn't affect equilibrium, but they may provide stability or carry force under other load cases.