Composite Members

The concrete slab can carry a large portion of positive-moment compression after composite action develops, but the steel top flange remains part of the steel section. It contributes to construction-stage strength/stability, steel-section properties, local behavior, stud attachment, negative-moment behavior, and the final composite force equilibrium. It must remain part of the modeled and detailed steel section.

Without adequate shear transfer, the slab and steel beam can slip relative to one another and do not develop the intended composite action. Headed stud anchors or other permitted connectors transfer longitudinal interface shear so the two materials deform compatibly within the design model.

Before the slab gains sufficient strength, the bare steel beam commonly carries the wet concrete, steel deck/formwork, beam self-weight, and construction loads. After composite action is established, the composite section carries the applicable later loads. Construction-stage LTB, deflection, ponding, deck support, and erection bracing can govern even when the final composite beam is strong.

Temporary shores can reduce construction-stage steel-beam demand and change the stress history before composite action develops. The analysis must model which loads are carried by the bare steel section and which are carried after composite action; do not simply assign all loads to the final section without considering the construction sequence.

For common building composite-beam cases, the effective slab width is limited by span, spacing to adjacent beams, and slab edges according to the adopted composite-design provision. Determine the contribution on each side of the beam and sum the permitted widths; edge beams can have unequal effective widths on the two sides.

For a common positive-moment composite beam, the PNA may lie in the concrete slab, steel top flange, or steel web depending on slab compression capacity, steel yield force, and connector strength. The assumed force blocks must satisfy equilibrium before a moment arm is calculated.

For the simple solid-slab teaching case, compare the maximum concrete compression force with the steel yield force:

If $C_c\ge P_y$ and full composite action can be developed, only part of the slab depth is required for compression and the PNA can lie in the slab. If $C_c<P_y$, part of the steel section must be in compression and the PNA moves into the steel.

The nominal strength of a headed stud depends on stud area/material and concrete/deck conditions under the adopted AISC composite provisions. Metal-deck rib orientation and geometry can reduce connector effectiveness. Do not assign an arbitrary constant force per stud independent of stud diameter, concrete, deck, and connection geometry.

If the provided connector strength from the relevant zero-moment point to maximum moment is sufficient to develop the full-composite force, the full-composite plastic model may be used subject to all other provisions. If less shear transfer is provided, the member is partially composite and flexural strength/service behavior must be evaluated using the governing partial-composite provisions.

Stud strength and concrete participation depend on deck rib orientation, rib geometry, stud position, number of studs per rib, and the applicable reduction factors/limitations. A solid-slab example is not automatically transferable to a ribbed-deck system.

Over interior supports, the slab concrete is generally cracked in tension for flexural strength modeling, while permitted longitudinal reinforcing steel can participate with the structural steel section. Compression-flange stability, local buckling, connection behavior, shear connectors, and cracking/serviceability require separate evaluation from the positive-moment slab-compression case.

Composite columns include structural steel shapes encased in reinforced concrete and concrete-filled HSS. Their strength and stability depend on component areas/materials, section slenderness, confinement/local buckling, effective stiffness, length, load eccentricity, and the specific AISC composite-column provisions.