Composite Design Calculator

Steel-concrete composite member design per AISC 360 Chapter I. Composite beams with headed stud shear connectors and lower-bound moment of inertia. Educational use only.

This page documents the scope, inputs, outputs, and computational approach of the Composite Design Calculator on steelcalculator.app. The interactive calculator runs in your browser; this documentation ensures the page is useful even without JavaScript.

What this tool is for

Determining the flexural capacity of composite steel beams with concrete slab per AISC 360 Chapter I.
Computing effective slab width, number of shear studs, and partial composite ratio.
Estimating lower-bound moment of inertia for deflection checks under service loads.

What this tool is not for

It does not handle composite columns (encased or filled) or composite deck-slab design.
It does not check construction-stage loading (unshored condition) with non-composite section properties.
It does not account for fire rating, vibration serviceability, or long-term creep effects.

Key concepts this page covers

plastic stress distribution method for composite beams
effective slab width per AISC I3.1a
shear stud strength (Qn) and layout requirements
partial composite action and lower-bound moment of inertia

Inputs and outputs

Typical inputs: steel beam section, slab thickness, concrete strength f'c, stud diameter and height, deck profile (rib height and spacing), beam span and spacing, and applied loads.

Typical outputs: full composite moment capacity phi-Mn, partial composite ratio, number of studs required, effective slab width, lower-bound moment of inertia ILB, and deflection under service loads.

Computation approach

The calculator uses the plastic stress distribution method from AISC 360 Section I3.2a. The compressive force in the slab is limited by the lesser of: concrete crushing (0.85 f'c times effective slab area), steel yielding (As times Fy), or the total shear stud capacity (N times Qn). The plastic neutral axis is located by force equilibrium, and the nominal moment is computed from the internal force couple. For partial composite, the stud capacity limits the compression flange force and the PNA shifts into the steel section.

Frequently Asked Questions

What is partial composite action? Partial composite action means the number of shear connectors is less than required for full composite action, so the horizontal shear transfer between the steel beam and concrete slab is limited by the stud capacity rather than by yielding or crushing. AISC allows a minimum of 25% composite action. Partial composite reduces the moment capacity but also reduces the number of studs needed, which can be more economical when full capacity is not required.

How is the lower-bound moment of inertia calculated? The lower-bound moment of inertia ILB is used for deflection calculations of partially composite beams. It accounts for the slip at the steel-concrete interface by interpolating between the non-composite moment of inertia (steel section alone) and the fully composite moment of inertia, based on the ratio of actual stud capacity to full composite capacity. AISC provides the formula in Commentary Section I3.2.

What determines the effective slab width? AISC 360 Section I3.1a defines the effective width of the concrete slab flange as the minimum of: beam span divided by 8 (each side), half the beam spacing, or the distance to the slab edge. This limits the slab width assumed to act compositely with the beam and prevents overestimating the concrete compression flange for widely spaced beams.

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.