Seismic Behavior of Steel BRBF Buildings Including Consideration of Diaphragm Inelasticity
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Date
2020-03, revision 2021-02
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Abstract
Compared to vertical elements of a building’s seismic force resisting systems, our understanding of the horizontal elements, i.e. the diaphragms, is grossly lacking. Recent research showed that diaphragm design forces that have been in the building codes for decades are not sufficiently large to protect the diaphragm from inelastic actions. That research led to the development of the alternative diaphragm design provisions in ASCE 7-16 which use larger diaphragm force demands, but also allows reduction by a diaphragm response modification factor, Rs, that accounts for diaphragm ductility.
In this study, the effect of different diaphragm designs on the behavior of steel buildings is investigated using three-dimensional computational building models that consider nonlinear behavior in both the vertical and horizontal elements of the seismic force resisting system. Three different diaphragm design scenarios are investigated: 1) a conventional design using typical diaphragm design procedures from Section 12.10.1 of ASCE 7-16, 2) an alternative design based on Section 12.10.3 of ASCE 7-16 with Rs = 1.0, and 3) an alternative design with Rs = 2 for composite deck diaphragm and Rs = 2.5 for bare deck diaphragm. A series of 1, 4, 8, and 12-story archetype buildings with 100 ft x 300 ft plan area and perimeter lateral force resisting system consisting of buckling restrained braced frames (BRBF) were designed to the current U.S. building code. The computational models are three-dimensional assemblies of frame elements and truss elements that are capable of capturing yielding of the buckling restrained braces, plastic hinging of the beams and columns, nonlinear behavior of the diaphragm and geometric nonlinearity (i.e., second order effects). The nonlinear behavior of the diaphragm is captured using truss elements with calibrated hysteretic behavior to match past test data from cantilever diaphragm tests. Using these nonlinear computational models of the archetype buildings, modal analyses were conducted to study their modal properties, nonlinear pushover analyses to investigate their static behavior, and nonlinear response history analyses to evaluate their seismic performance including probability of collapse.
Results of the eigenvalue analyses showed that the consideration of diaphragm flexibility led to an increase in first mode period between 13% and 48%. A comparison of results from pushover analyses and response history analyses indicated that even though the pushover analyses (based on a first mode load pattern) identified the BRBF as being weaker than the diaphragms and therefore dominating the inelastic pushover behavior, response history analyses demonstrated that the diaphragms can experience substantial inelasticity during a dynamic response. The response history results also suggest that there would be a significant difference in seismic behavior of buildings modeled as two-dimensional (2D) planar frames as compared to the three-dimensional (3D) structures modeled herein. For instance, the median of the peak story drift was approximately 1.5 to 2 times larger than the median of the peak story drift in each of the two orthogonal directions. Furthermore, the observed final collapse mode involves an interaction between large BRBF story drifts combined with diaphragm deformations that are additive and exacerbate second order effects leading to collapse.
The percentage of 44 sets of ground motions that are predicted to cause collapse across all buildings and diaphragm designs is 3.5%, 16.4%, and 32.6% for the design earthquake (DE), maximum considered earthquake (MCE), and an earthquake scale level from FEMA P695 associated with an adjusted collapse margin ratio where 50% collapse is allowable (ACMR10%), respectively. A comparison with results of similar studies in the literature using 2D frames shows that the current 3D models experience more collapses, likely due to consideration of 3D behavior with deformable diaphragms and bidirectional ground motions which results in larger story drifts and larger second-order effects. Although the number of collapses at the DE and MCE hazard levels is larger than desirable, it is expected that these collapses are primarily associated with 3D effects other than diaphragm design. This is further supported by observing that the change in median story drifts was negligible when the Rs value was changed from 2.0 for composite deck diaphragm and Rs = 2.5 for bare deck diaphragm to 1.0.
Computed adjusted collapse margin ratios for all buildings satisfied the FEMA P695 criteria for acceptance. Therefore, it is concluded that the alternative diaphragm design procedure with proposed Rs values listed above (Rs = 2 for composite deck diaphragm and Rs = 2.5 for bare deck diaphragm) are reasonable for use in design of these types of structures. Further research is required to understand the behavior of 3D models that consider diaphragm deformations as compared to the more widely used 2D frame analyses, and to define more refined performance objectives (e.g. collapse ratios).
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Technical Report
Keywords
BRB, P695, 3D Building Model, Seismic Performance, Diaphragm