Graduate Thesis Or Dissertation
 

Examination of lateral stiffness and strength of pitched residential roof diaphragms with implications for seismic design.

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  • There are about 80 million single-family dwellings (SFD) in the United States, predominantly of wood-frame construction. Of these, 68% are owner occupied. A home is typically the largest single investment of a family, and is often not covered by earthquake insurance, even where it is available. Of all SFD in America, 50% were built before 1974, and 76% built before 1990. Most wood frame SFD (WFSFD) were built to prescriptive code provisions before seismic requirements were introduced. This dissertation examines a broad field of seismic research applicable to WFSFD seismic analysis and the design, and to the retrofit of existing WFSFD. It summarizes the "state-of-the-art" of seismic experimentation and seismic evaluation, and provides observations and recommendations for future research. The study showed that performance based design (PBD) needs to develop a consensus on performance objectives for WDSFD with respect to damage and repair costs, including new design techniques which balance life-safety with the effects of damage to building finish materials. There is also a strong need to develop a better understanding of the effects of WDSFD components, attachment, the lateral force resisting system and how loads are distributed to these elements within the structure. To address concerns with limited pitched light-frame roof diaphragm experiments, ten full size (3.7 x 4.9 m) plywood roof diaphragms with metal plate connected (MPC) common and hip wood trusses or joists, typical of WFSFD construction were tested according to ASTM E455. Specimens included three gable roof slopes of 33, 67 and 100%, a hip roof of 33% slope, and a flat roof. Gable and hip roofs experiments examined the effect of eave sheathing and gypsum ceilings on the bottom chord. Results showed eave plywood had negligible effect on diaphragm apparent stiffness; gable roofs had apparent stiffnesses that were about 50% that of the flat roofs; and gypsum provided more than 1/3 of the total roof apparent stiffness for gable roofs at slopes of less than 33%. There was no effect of pitch on ultimate roof strength in any configuration and all exhibited approximately the same ultimate shear strength. Failure modes of roofs included nail withdrawal, nail tear-through, metal plate tear-out on trusses and chord tensile failure. This dissertation also examines code provisions applicable to WFSFD seismic design, compares rigid and flexible diaphragm analyses for different geometries of “L” shaped WFSFD applying stiffness reductions due to differing roof geometry and pitch. These analyses are applied to historic earthquake damage reports and compared with a practical rigid, semi-rigid or flexible diaphragm plate FEM analysis method. This study determined that most WFSFD should be designed using an envelope method due to a mix of diaphragm types and the effects of roof pitch and geometry on the stiffness. Cases may occur where determination of semi-rigid or flexible diaphragm behavior is difficult because the code prescribed analysis is contradictory or fallacious. This suggests that use of semi-rigid finite element model (FEM) or a manual envelope method is prudent. The use of RP, FP or SP FEM methods can be simple and practical methods for analyzing WFSFD with a reasonable level of detail and effort.
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