Seismic Analysis and Design of Rigid Wall – Flexible Roof Diaphragm Buildings

Graduate Students: Maria Koliou

Principal Investigator: Andre Filiatrault

Project Completion Date: 09-01-2014

This study investigated the seismic response of Rigid Wall-Flexible Roof Diaphragm (RWFD) buildings under extreme ground shaking was studied. 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. First, a two-dimensional (2D) simplified numerical framework of RWFD buildings was developed to investigate the seismic performance of this type of structure and validate 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 seismic collapse capacity of RWFD buildings designed to current code provisions in the United States was evaluated by applying the FEMA P695 methodology to a set of archetype buildings. The simplified numerical framework was used to conduct this study. RWFD archetype buildings incorporating both wood and steel roof diaphragms of different geometry and fastener configurations were considered. The results of this study identified certain ―weak aspects of the code provisions referring to the seismic response of RWFD buildings as well as the underestimation of their fundamental period of vibration. Two period formulas were recommended to address the underestimation of the fundamental period. These recommendations included a rigorous mechanics based period formula as well as a semi-empirical period formula.

Furthermore, 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. An increase of the collapse capacity of both RWFD case study buildings was obtained when the distributed yielding concept was adopted in their diaphragm design. A better displacement ductility distribution along the diaphragm span was also achieved leading to less damage at the boundaries of the roof. A new rational seismic design approach for RWFD buildings was then developed and proposed in this study, which is based on the concept of distributed roof diaphragm yielding, 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-factor) for the roof diaphragm was recommended to be used along with the R-factor for the vertical elements of the SFRS as currently used in seismic design codes. The proposed design approach was also validated through the FEMA P695 methodology for a large set of building archetypes varying in building size, diaphragm aspect ratio, roof diaphragm construction, seismic hazard, framing and sidelap connector types. Finally, a case study investigating the out-of-plane wall anchorage forces for a typical RWFD building archetype was conducted in this study. 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.

This project was partially supported as part of a project directed by the National Institute of Building Sciences (NIBS) and funded by the Federal Emergency Management Agency (FEMA) under DHS/FEMA Contract HSFEHQ-09-D-0147, Task Order HSFE60-12-J-0002C.