Seismic analysis and design of rigid wall -- Flexible roof diaphragm buildings

PhD Student: Maria Koliou

Publication Year: 2014

Advisor: Andre Filiatrault

Abstract: In this dissertation, the seismic response and design of Rigid Wall-Flexible Roof Diaphragm (RWFD) buildings under extreme ground shaking was investigated. RWFD buildings are a type of low-rise industrial construction widely used in North America that incorporates rigid in-plane concrete or masonry walls and flexible in-plane wood, steel or ―hybrid‖ roof diaphragms. This type of structure has exhibited poor seismic response during past earthquake events, where the global system response was dominated by the response of the roof diaphragm. This is mainly attributed to large diaphragm displacements, which can significantly exceed the displacements of in-plane walls.

A two-dimensional (2D) simplified numerical framework of RWFD buildings was developed to investigate the seismic performance of this type of structure and validate current and proposed design approaches. The modeling approach is detailed enough to capture the nonlinear response of RWFD buildings, and simplified enough to efficiently conduct a large number of nonlinear time-history dynamic analyses. The 2D numerical framework is based on a three step substructuring approach including: (1) hysteretic response database for diaphragm connectors, (2) 2D inelastic diaphragm model incorporating hysteretic connector response and (3) 2D building model incorporating hysteretic diaphragm model response. This numerical framework was validated using experimental and analytical data available in the literature.

A case study investigating the out-of-plane wall anchorage forces for a typical RWFD building archetype was performed in this dissertation. The main objectives of this case study were to: (i) evaluate the out-of-plane wall anchorage force distribution along the roof span, (ii) evaluate the influence of wall-to-roof diaphragm connection stiffness on the magnitude of the out-of-plane wall anchorage forces, and (iii) evaluate the correlation between the in-plane shear forces in the roof diaphragm and the out-of-plane wall anchorage forces.

The seismic collapse capacity of RWFD buildings designed to current code provisions in the United States was evaluated by applying the FEMA P695 methodology for a large set of representative archetype buildings. This study showed that certain ―weak‖ aspects of the code provisions referring to the seismic response of RWFD buildings and the underestimation of their fundamental period of vibration. Two fundamental period formulas (a rigorous mechanics based period formula and a semi-empirical period formula) were recommended to address the underestimation of the fundamental period.

The concept of distributed yielding in the flexible roof diaphragm by weakening certain intermediate diaphragm zones was explored as a cost effective means to improve the seismic collapse capacity of RWFD buildings and mitigate their seismic vulnerability. The efficiency of the proposed concept was numerically investigated in this dissertation for two case study RWFD buildings incorporating a steel and a hybrid flexible roof diaphragm, respectively.

This dissertation concluded by proposing a new rational seismic design approach for RWFD buildings based on the concept of distributed roof diaphragm yielding. This approach relies on forcing yielding of the roof diaphragm as the predominant inelastic response under extreme ground shaking instead of the vertical elements of the seismic force-resisting system (SFRS). In a force-based design framework, a response modification factor (R) for the roof diaphragm is recommended to be used along with the R-factor for the vertical elements of the SFRS as currently used in seismic design codes. Moreover, the proposed design approach includes stronger end diaphragm regions, to spread yielding deeper into the roof diaphragm, by introducing an overstrength factor increasing the design shear forces for the diaphragm boundaries. The proposed design approach was validated through the FEMA P695 methodology for a large set of building archetypes.