Wall‐diaphragm interactions in seismic response of building systems I: Parametric models and elastic response

Abstract

The objective of this paper is to study the elastic seismic response of simplified building models that include considerations of the vertical lateral force resisting system (walls) and the horizontal lateral force resisting system (floor diaphragms) through parametric study. The assumption of rigid diaphragms is often used in the seismic design of buildings; however, research suggests that the diaphragm may influence the fundamental building period and the seismic building response. In this paper, a simplified model is presented for single and multi-story buildings utilizing two degrees of freedom per story for the vertical system, for example, the walls, and one degree of freedom per story for the floor diaphragm. The number of stories, the stiffness of the walls and diaphragm(s), and the mass distribution between the walls and diaphragm are varied to represent a large range of potential building compositions. Eigen analyses of the models indicate that the wall and diaphragm interact when at similar periods resulting in elongation of the overall building period. Response of elastic time-history analyses of the building models are presented in this paper where it is shown that the force demands in both the wall and diaphragm(s) are dependent on the relative stiffness of the walls and diaphragm(s), and the mass-distribution between these two systems. In the companion paper "Wall-Diaphragm Interactions in Seismic Response of Building System II: Inelastic Response and Design" the parametric building study is extending to nonlinear time-history analyses including different levels of assumed inelasticity in both the walls and diaphragm.

Wall‐diaphragm interactions in seismic response of building systems ii: Inelastic response and design

“…One way to ensure reduced ductility demands is for the wall and diaphragm periods to be “long.” This generally results in a longer building period, as discussed in Fischer and Schafer 9 . Figure 7 shows the ductility demands in the v‐ and hLFRS in the models with long periods, that is, $${T}_w \ge 0.5s$$ for the walls and $${T}_d \ge 0.4s$$ for the diaphragm.…”

“…In Fischer and Schafer, 9 it was found that the alternative diaphragm design method can accurately predict the elastic hLFRS forces developed in the models when both walls and diaphragms are stiff, 𝑇 𝑤 < 0.5𝑠 and 𝑇 𝑑 ∕𝑇 𝑤 < 0.5 and in some other instances. However, in the majority of cases, the alternative method is unconservative in its estimate of elastic hLFRS forces.…”

“…This generally results in a longer building period, as discussed in Fischer and Schafer. 9 Figure 7 shows the ductility demands in the v-and hLFRS in the models with long periods, that is, 𝑇 𝑤 ≥ 0.5𝑠 for the walls and 𝑇 𝑑 ≥ 0.4𝑠 for the diaphragm. The minimum, mean and maximum ductility demands are indicated in Figure 7, where it can be observed that the variance in the v-and hLFRS ductility demands are decreased and they are essentially always less than the assumed level of ductility, 𝑅 𝑀𝐶𝐸 𝜇 and 𝑅 𝑀𝐶𝐸 𝑠𝜇 .…”

“…The force levels in both the vertical and horizontal LFRS for a subset of models are compared to the code predictions, followed by an examination of the ductility demands from the time‐history analyses. Special attention will be directed at a few selected models that parallel the SDII archetype building models, as introduced in Fischer and Schafer, 9 with characteristic parameters listed in Table 3. The archetype building models are steel frame buildings with buckling restrained braces (BRB) and concentric braced frames (CBF) as the vLFRS and concrete‐filled steel deck floors and bare steel deck roof diaphragms as the hLFRS.…”

“…However, it is not clear if this assumption is correct in practice. In this paper, the parametric study of the simplified building model presented in Fischer and Schafer 9 is extended to include different levels of ductility in both the vertical and horizontal LFRS. Vertical and horizontal LFRS force demands developed in the model are compared to the current seismic design forces prescribed in ASCE 7–16.…”