Dynamic behavior of buildings subjected to vertical ground motions

The dynamic behavior of buildings in the horizontal components of motion has been extensively studied, allowing for horizontal demands to be routinely considered in the design process. In contrast, although there has been growing awareness of the effects of vertical ground motions on buildings, little understanding has been gained regarding how structures respond in the vertical direction. The two main pieces of information needed in dynamic analysis to estimate the vertical structural response of buildings subjected to earthquakes are the vertical ground motion component and the building's dynamic parameters in the vertical direction. Previous studies of the vertical component of motion have mainly focused their efforts on understanding the characteristics of the vertical component of ground motions with comparatively less research devoted to the vertical dynamic characteristics of buildings. The studies of ground motion characteristics have led to the addition of a vertical design response spectrum in ASCE 7, which accounts for the site-dependent maximum considered earthquake as well as soil type conditions. Even though the inclusion of the design response spectrum furthers the advancement in structural codes, giving structural engineers a tool to incorporate the effects of the vertical ground motion in structural design, building design standards are lacking in guidelines and requirements for how to accurately account for the effects of the vertical ground motions from the dynamic point of view. For example, except for the long-stablished static approach of using 0.2SDSD as the primary method to include the vertical ground motion loads in the design process, ASCE 7 provides little additional guidance or criteria for considering vertical ground motion effects in design. This dissertation characterizes the dynamic behavior of buildings in the vertical component using parametric relationships and examples of how buildings respond to vertical ground motions. We studied the influence of modal dynamic parameters of buildings in the vertical component obtained from eigenvalue analysis—modal periods, mode shapes and modal participation factors—on the structural response, and we contrasted the vertical modal dynamic parameters characteristics to those of the lateral components. We also validated the vertical dynamic parameters' characteristics obtained from theoretical models, using a structural system identification procedure to calibrate the set of dynamic parameters that best reproduce the recorded response of instrumented buildings subjected to earthquakes. Additionally, we present the patterns of contribution of modes in the vertical component to the structural responses most affected by vertical ground motions, and we propose modeling practices to accurately estimate structural responses in the vertical component. Furthermore, we propose two modal combination rules in response spectrum analysis that accurately predict the maximum response of buildings in the vertical component and that exceed the performance of existing modal combination rules widely used for the response to horizontal components of ground motions. Finally, probabilistic risk studies of reinforced concrete and steel columns designed for gravity loads demonstrate that risks of exceeding strength limit states to vertical ground accelerations are generally relatively low, due to the inherent overstrength introduced through load factors that are applied to dead and live loads in design, along with other behavioral effects

Advisor: Deierlein Gregory G. (Gregory Gerard)
Miranda Eduardo (Miranda Mijares)
Baker Jack W.