Composite beam effects and implications to seismic design provisions

Previous studies have investigated the slab effects on the cyclic behavior of composite beams in moment-resisting frames. Of interest in this paper, is the slab contribution to the flexural strength and effective stiffness of the beam. The former influences the strong column/weak beam ratio while the latter affects the fulfillment of serviceability limits. For this purpose, experimental data of composite beams from prior subassembly and frame system tests are assembled in a consistent database format. This database facilitates the evaluation of code-based approaches that are adopted in the seismic design of composite beams in steel moment-resisting frames. In particular, the developed database is used to assess the plastic flexural strength and effective stiffness of the beams according to the current Eurocode, US and Japanese seismic provisions. The database is also used to quantify the plastic rotation capacity of composite beams that is particularly important for nonlinear seismic assessment. In general, all three design codes estimate the sagging plastic moment resistance of the beams reasonably well. On the other hand, the estimated effective stiffness shows greater variability. The proposed composite beam effective stiffness according to the Japanese seismic provisions is found to be the most accurate between the three design provisions. Results also suggest that the plastic deformation capacity of composite beams is higher under sagging bending than hogging bending due to the slab restraint. Although inconclusive, the framing action seems to enhance the plastic deformation capacity of beams intersecting interior joints.