Composite Beam Effective Flange Width
Composite Beam Effective Flange Width
In composite beam construction, a concrete slab is connected to the top flange of a steel beam with shear studs so the two act together as a single, much stiffer and stronger section than the steel beam alone. Not all of the slab actually participates at full strength — only a limited "effective width" on either side of the beam is assumed to work compositely, because stress in the slab decreases the farther it is from the beam centerline (a phenomenon called shear lag).Overestimating the effective width overstates both the stiffness and the moment capacity of the composite section, which can leave a beam under-designed for deflection or strength if the simplification is applied carelessly; codes address this by capping the effective width at the lesser of a fraction of the span or the actual tributary spacing to neighboring beams.
The effective flange width is code-limited to the smaller of a span-based limit and the physical beam spacing, b_eff = min(L/4, s_beam). where L is the beam span, s_beam is the center-to-center spacing to the adjacent beam, and b_eff is the resulting effective flange width used in the composite section calculation.
Inputs
L := 8 m=8 m
s_beam := 2 m=2 m
The code effective width limit is the smaller of one-quarter of the span (to reflect shear lag over long spans) and the actual beam spacing (since the slab cannot contribute beyond the tributary width to the next beam).
b_eff := min(L/4, s_beam)=2 m
Results
In this example the span-based limit (L/4 = 2 m) and the spacing limit (2 m) happen to coincide, so the full tributary slab width is available to act compositely — a favorable case. When beam spacing is wide relative to span, the span-based limit typically governs and caps the usable slab width well below the physical spacing, directly limiting the composite section's stiffness and capacity. Because effective width feeds directly into both deflection and moment capacity calculations, it is one of the first parameters checked when composite framing is being laid out.
Assumptions & limitations
- Effective width formula shown is a simplified span/4 vs. spacing rule; specific codes (AISC, Eurocode) have their own detailed limits.
- Full composite action (adequate shear stud capacity) between slab and beam is assumed.
- Beam is assumed simply supported with the slab in compression over its full effective width.
- Edge beams (with slab on only one side) require a modified effective width, not covered here.