Ordinary vs. Special Moment Frame Selection for a Two-Story Office
A two-story steel office building in a region with Ss = 1.8g and S1 = 0.7g is being designed with a special steel moment frame (SMF) perimeter system. The fabricator proposes substituting an ordinary moment frame (OMF) to reduce cost, arguing the building is only two stories and the frame is not highly stressed. The building is assigned to Seismic Design Category D. The frame has two bays per side, and the beam-column connections are prequalified welded unreinforced flange-bolted web (WUF-B) details. The architect wants larger windows, which would require deeper beams and a stiffer frame.
1. Evaluate the OMF substitution request against the seismic design category and system limitations.
Read the worked answer
The OMF substitution is not permitted in SDC D for this configuration. ASCE 7 limits OMF systems to SDC B and C (with height limits); in SDC D and above, steel moment frames must be SMF or IMF with the appropriate detailing. The high spectral values (Ss = 1.8g, S1 = 0.7g) confirm a high-hazard location.
Even if the code permitted it, the OMF would have much lower ductility and would not reliably form the beam-hinge mechanism assumed in the analysis. The prequalified WUF-B detail is an SMF connection; using it in an OMF would not change the system classification.
The correct response is to retain the SMF and address cost through connection optimization (e.g., reduced beam section or bolted flange plate connections) rather than downgrading the system.
2. The architect's larger windows require deeper beams. Explain the effect on the SMF design and the checks required.
Read the worked answer
Deeper beams increase the frame's flexural strength and stiffness, which changes the SMF design in several ways:
- Strong-column/weak-beam ratio: Deeper beams increase beam moment capacity, so column sizes may need to increase to maintain the SCWB ratio greater than 1.0. This is a code requirement for SMF systems.
- Panel zone demand: Larger beam moments increase panel zone shear; doubler plates or thicker column webs may be required.
- Connection design: The prequalified connection must be re-evaluated for the larger beam depth; the WUF-B prequalification has limits on beam depth and span-to-depth ratio.
- Drift and period: A stiffer frame shortens the period, potentially increasing seismic demand; the analysis must be rerun.
- Foundation: Larger overturning moments increase uplift and footing demands.
The design team should rerun the analysis, confirm the connection remains prequalified, and verify SCWB and panel zone capacity before accepting the deeper beams.