A-02 — forces through buildings

Forces Travel Through Buildings

load types

where loads come from — and how they're modeled

Snow lands on the roof and acts straight down. The roof deck spans to roof beams, then frames into columns. The load travels down floor by floor until it reaches the foundation and dissipates into the earth below.

p = 50 psf W = 20 ft D = 20 ft AREA LOAD (pressure) p = force per unit area units: psf (lb/ft²) or kPa (metric)
w = 2.0 k/ft LINE LOAD (distributed) w = force per unit length units: kips/ft (k/ft) or lb/ft (plf) Floor or roof framing loads a beam along its full length. w = area load x tributary width.
p = 50 psf w = 0.50 k/ft R = 5.0 k column reaction per support
Area Load p = 50 psf
Tributary Width trib. width = 10 ft
Span span = 20 ft
p x trib. / 1000 = w 50 psf x 10 ft / 1000 = 0.50 k/ft w x L / 2 = R 0.50 k/ft x 20 ft / 2 = 5.0 kips Column reaction: 5.0 kips
Load path: the continuous route a force follows from its point of application to the foundation. No gaps allowed.
Gravity path: deck → beams → girders → columns → footings → soil. Lateral path: wall pressure → diaphragm → shear walls/frames → foundation → soil. Both must be complete and continuous.
key concepts
overview Every load must find a continuous path to the ground

Every load applied to a building must find a path to the ground. Understanding load paths is the foundation of structural thinking.

every force needs a path No force disappears — trace it or the structure fails

No force disappears — it transfers from element to element until it reaches the ground. If any link in the chain is missing, the structure fails at that point. This is the load path concept, and it's the single most important idea in structural engineering. Before sizing any member, trace the load path. If you can't draw it continuously from source to foundation, something is wrong.

the two load paths Gravity path down, lateral path sideways then down

Buildings have two distinct load paths that must both be complete. The gravity path carries vertical loads downward: roof deck → joists → beams → girders → columns → footings → soil. The lateral path carries horizontal loads (wind, seismic) sideways then down: wall/facade → diaphragm (floor plate) → shear walls or braced frames → foundation → soil. Every building needs both.