G-04 — serviceability

strong enough isn't always stiff enough.

serviceability checker

check deflection and drift against code limits.

Serviceability uses unfactored loads to protect function, not safety — slide span, stiffness, and drift to see which limit governs.

beam deflection
Span L30 ft
w svc1.0 klf
E (ksi)
I (in⁴)300
Limit
story drift
Story h12 ft
Δs0.25 in
Limit
—
Δ_actual (in)
—  vs  L/— limit
—
drift ratio
beam deflection
story drift
live equation
set values above to see the live calculation
Δ = Vertical displacement under service loads
L/360 = Code limit for live load deflection
δ = Horizontal displacement between adjacent floors
h = Floor-to-floor height for drift ratio
explained
Serviceability checks use unfactored (service) loads — they protect function, not safety. A beam can be strong enough to never break but still deflect enough to crack ceiling tiles or make occupants uneasy. Drift limits protect cladding and partitions from racking damage during wind or seismic events.
key concepts
overviewDeflection and drift limits protect function, not safety

Serviceability limit states protect function, not safety. Beam deflection limits (L/240, L/360) prevent cracking of finishes and visible sag. Story drift limits (H/400, H/600) protect cladding and partitions under lateral loads. These checks use SERVICE-level loads (unfactored), not ultimate. A beam that easily passes strength may fail deflection — serviceability often governs.

strength vs. serviceabilityA beam can be strong enough but deflect too much

Strength checks ask “will it break?” Serviceability checks ask “will it work?” A beam can have plenty of strength but deflect so much that floor finishes crack, doors don’t close, or occupants feel uncomfortable. Serviceability uses unfactored (service-level) loads — the loads you’d actually see on a normal day — not the amplified loads used for strength.

why serviceability often governsLong-span beams and slender frames frequently fail deflection first

For long-span beams and lightweight floor systems, deflection almost always controls over strength. A W16×26 might easily pass the moment check for a 30-ft span but deflect L/200 — well beyond the L/360 limit for floors supporting plaster ceilings. Similarly, a slender moment frame might have adequate member strength but drift H/250 under wind — causing partition cracking and cladding damage.

common deflection limitsL/360 for floors, L/240 for total load, L/480 for sensitive elements

IBC/ASCE 7 deflection limits (span L): L/360 — floor beams under live load (most common). L/240 — floor beams under total load (dead + live). L/180 — roof beams under total load. L/480 — beams supporting sensitive equipment or glass. For a 30-ft beam, L/360 = 1.0 inch maximum. The key formula for uniform load: Δ = 5wL⁴/(384EI).

common drift limitsH/400 for wind, H/600 for sensitive cladding systems

Story drift limits control lateral system design: H/400 — typical for wind. H/600 — sensitive cladding or partition systems. ASCE 7 Table 12.12-1 gives seismic drift limits by Risk Category (typically 0.020hsx for most buildings). For a 12-ft story, H/400 = 0.36 inches. Drift is checked at each story, not just the roof — the worst story governs.