Steel Composite Beam (LRFD)

How it calculates

The Steel Composite Beam (LRFD, AISC 360-22) calculator covers both the pre-composite construction stage and the final composite stage in a single template. Two independent load sets, two summary panels, and two sets of deflection criteria are maintained so you can track construction-phase adequacy and in-service performance together.

Pre-composite stage checks

Before the concrete slab gains strength, the steel beam alone resists construction loads. The calculator applies AISC 360-22 Chapter F and G to the bare steel section:

• Flexural capacity: utilization = Mu_precomp / (phi × Mp_steel) ≤ 1.0 (or reduced for FLB/WLB/LTB as applicable)
• Shear capacity: utilization = Vu_precomp / (phi × 0.6 Fy Aw Cv1) ≤ 1.0
• Pre-composite deflections: instantaneous and DL+(LL or SL) deflection ratios compared to user-specified limits

A steel anchor ductility check per the prescriptive procedure is also included to confirm stud ductility requirements are met before relying on uniform shear distribution.

Section classification (AISC 360-22 B4)

Steel section flanges and webs are classified as compact, noncompact, or slender. For composite beams the compression flange is braced by the concrete deck in the composite stage; classification primarily affects the pre-composite flexural capacity and the post-composite negative moment region.

Composite flexural capacity - AISC 360-22 Chapter I3

Positive bending capacity uses the plastic stress distribution method per Chapter I3. The effective concrete flange width is computed per Section I3.1.1a. The calculator determines the governing compression force as the lesser of:

• Maximum tension force in the steel beam = As × Fy
• Maximum compression force in the slab = 0.85 f'c × Ac

For partial composite action, the compression force at the interface equals the total nominal shear strength of the stud anchors sumrV'n. The plastic neutral axis location is determined and the plastic moment resistance is calculated:

utilization = Mu_comp / (phi × Mp_composite) ≤ 1.0 where phi = 0.90

Negative bending uses the non-composite bare steel section properties (concrete in tension is ignored).

Stud anchor design - AISC 360-22 Section I8.2a

Individual stud anchor strength includes geometry, rib, and position reduction factors Rg and Rp. The required total shear force at the interface is determined from the governing compression force. Required stud count is computed and compared to provided studs. Detailing checks include minimum and maximum stud length, maximum longitudinal and transverse spacing per Section J3.4, and maximum diameter per Section I8.1.

Shear capacity - AISC 360-22 Chapter G (composite stage)

utilization = Vu_comp / (phi × 0.6 Fy Aw Cv1) ≤ 1.0 with phi = 1.0 for qualifying W-shapes.

Composite section properties

The lower-bound elastic moment of inertia ILB is computed per AISC 360-22 Commentary I3.2 for deflection calculations. This accounts for partial composite action and provides a conservative estimate of composite stiffness under service loads.

Deflection checks - composite stage

Three serviceability criteria are tracked:

• Instantaneous composite deflection - compared to L/n or absolute limit
• Long-term composite deflection - includes creep under sustained loads; typically L/240
• Simplified DL+(LL or SL) composite deflection - combined loading for total deflection

utilization = delta / delta_allow ≤ 1.0 for each criterion. A precamber recommendation is output based on dead load deflection.

Inputs summary

Steel section and yield strength Fy; composite action percentage; span lengths, support types, tributary width; concrete slab thickness, strength f'c, density, lightweight factor, cover; metal deck rib height, top width, bottom width, rib spacing, deck self-weight; stud diameter and ultimate strength Fu; pre-composite dead loads; composite dead and live loads; independent deflection limit criteria for pre-composite and composite stages.

Outputs summary

Pre-composite summary: moment and shear utilization ratios, pre-composite deflection ratios, ductility check. Composite summary: plastic moment resistance and demand, shear capacity and demand, total stud shear demand versus provided, individual stud strength, lower-bound ILB, composite deflection ratios, and precamber. All support reactions are formatted for automatic load linking to connected column and footing calculations.