UC San Diego

Stairs serve as an essential nonstructural system within buildings, providing a safe means of egress as well as much needed access for emergency responders following an earthquake. Unfortunately, the state of knowledge regarding the seismic behavior of these critical systems is limited, despite the fact that past earthquakes repeatedly reveal that stair systems are highly vulnerable to damage even in low- or moderate-intensity earthquakes. As a result, disruptions to building functionality, delayed rescue operations, and life safety hazards ensue.

To this end, this dissertation presents a first-of-its-kind experimental investigation of the seismic behavior of a prefabricated steel stair system incorporated in a full-scale five-story building shake table test program. These system-level tests allowed investigation of the seismic behavior of these systems under realistic installation and dynamic loading conditions. In this work, the modal characteristics of the stair system identified using white noise base excitation test data as well as the physical observations of the stair system during earthquake tests of increasing intensities are presented. Analysis of the earthquake response of the stair system particularly focuses on associating their behavior with the seismic demands of the test building and the associated implications on seismic design. These shake table tests highlight the seismic vulnerability of modern designed stair systems and in particular the importance of improving the deformability of flight-to-building connections.

The second part of this work involves a comprehensive computational study to capture at first the salient seismic response characteristics of prefabricated steel stairs in isolated configurations and subsequently the key response characteristics of building-stair systems. Detailed three-dimensional finite element models of a pair of prefabricated steel stairs are developed and their efficacy evaluated through extensive comparison with experimental data. The validated modeling approach is then used in a parametric assessment of the seismic behavior of stair systems using a broader range of design variables commonly found in practice. Subsequently, a system-level numerical study is conducted to investigate the interaction between buildings and stair systems and the associated impacts on building response characteristics as well as estimated stair loss using a probabilistic seismic analysis framework. Simulation results indicate that although the presence of stairs does not significantly modify the seismic response of buildings, enhancing the stair deformability can effectively mitigate the severity of seismic damage and loss of these critical nonstructural systems.