Composite Beam Flexural Strength (AISC 360-22 I3)
The moment a composite steel-and-slab beam can take.
Example
You enter
- Steel area As (in²) 8
- Steel yield Fy (ksi) 50
- Steel depth d (in) 16
- Slab thickness (in) 4
- Effective slab width be (in) 90
- Slab f'c (ksi) 4
You get
- C (kip) 400
- A (in) 1.307
- Phi mn (kip-ft) 340
Details, formula, and sources
PNA-in-slab composite moment: C = As Fy, a = C/(0.85 f'c be), Mn = C(d/2 + tslab - a/2), phi Mn = 0.90 Mn. As 8.0, Fy 50, d 16, slab 4, be 90, f'c 4 -> C 400 kip, a 1.31 in, phi Mn 340 kip-ft; a narrow be 24 pushes a past the slab and flags PNA-in-steel. The engineer of record governs.
C = As Fy; a = C / (0.85 f'c be) [require a <= tslab]; Mn = C(d/2 + tslab - a/2); phi Mn = 0.90 Mn.
AISC 360-22 §I3.2a composite beam flexural strength with the plastic neutral axis in the slab, by name.
The plastic-stress-distribution composite moment with the PNA in the slab is published in AISC 360-22 §I3; the arithmetic is public.
Estimate. AHJ and licensed professional govern.
Field names used by the API: as_in2, fy_ksi, d_in, tslab_in, be_in, fc_ksi, c_kip, a_in, phi_mn_kipft
- Compression block C = As Fy, a = C/(0.85 f'c be)AISC 360-22 I3.2a
- Moment Mn = C(d/2 + tslab - a/2), phi Mn = 0.90 MnAISC 360-22 I3.2a
- PNA-in-slab only valid while a <= slab thickness; else the general method appliesscope of this tile