Commercial Building Inspector B2Floor ConstructionHard
A commercial building inspector is reviewing the structural plans for a new three-story building in a high seismic area. The floor system consists of structural steel beams and girders with a composite concrete deck. According to AISC 360, as referenced by the IBC, which type of diaphragm is typically assumed for a cast-in-place concrete slab on metal deck in a steel-framed building?
- AFlexible diaphragm
- BSemi-rigid diaphragm
- CNon-structural diaphragm
- DRigid diaphragm
Show answer & explanationAnswer & explanation
Correct answer: D. Rigid diaphragm
A cast-in-place concrete slab on metal deck, properly connected to the steel frame, is generally considered a rigid diaphragm in structural analysis. AISC 360, and by extension the IBC, defines a rigid diaphragm as one where the lateral deformation is so small compared to the story drift that it can be neglected.
Why the other options are wrong
- A. Flexible diaphragms (e.g., wood structural panel sheathing without a concrete topping) distribute forces to vertical elements based on tributary area, not stiffness.
- B. Semi-rigid diaphragms exhibit some flexibility but also distribute forces based on stiffness; this is a more complex analysis and not the typical assumption for concrete slabs.
- C. Non-structural diaphragms do not contribute to lateral force resistance and are not a recognized diaphragm type for structural design.
Rigid Diaphragm
A rigid diaphragm is a floor or roof system that is assumed to have infinite in-plane stiffness, distributing lateral forces to vertical resisting elements (shear walls or frames) based on their relative stiffness.
- Typically includes cast-in-place concrete slabs.
- Assumed to deform negligibly in its own plane.
- Distributes lateral forces based on element stiffness.
- Simplifies lateral load analysis for seismic and wind.
Memory trick: Diaphragm's dance, lateral chance, stiffness's glance.