Development of a Unibody System To Improve the Seismic Performance of Lightweight Residential Wood Structures

Abstract eng:
A new design and construction approach is proposed to significantly improve the seismic performance of lightweight residential woodframe structures. We refer to this new approach as “unibody” construction, where the architectural nonstructural walls and finishes, such as gypsum wallboard in the interior walls and stucco in the exterior walls, are better connected to the framing elements and work together with structural walls to significantly increase the lateral stiffness and strength of the structure. Improvement of the connection of wall finishes to framing elements is done through the use of offthe-shelf construction adhesives, stronger fasteners and hold-downs. As a result, the lateral strength and particularly the lateral stiffness of the structure are significantly increased, resulting in significantly smaller lateral deformation demands and better seismic performance. The increments in strength and stiffness together with the corresponding reductions in lateral displacement demands are such that the structure remains elastic or practically elastic even under severe earthquake ground motions. The novel approach is aimed at achieving a nearly damage-free performance at the design level earthquake and limited amount of damage at the maximum considered earthquake, allowing their residents to remain in their residential units with limited disruptions even after severe earthquake ground motions. Over a period of four years, a series of carefully planned experimental tests of increasing level of size and complexity were conducted to develop the unibody approach. The first phase of experiments set out to investigate ways to improve connections between architectural finishes to wood framing elements. These experiments involved several different connector tests and half-scale wall tests, conducted at Stanford University. In the second phase, full-scale walls with different configurations were tested under quasi-static loading at the California State University Sacramento (CSUS). The third phase, investigated the system behavior of unibody structures through the testing of full-scale rooms incorporating floor systems under quasi-static loading at NEES@Berkeley. Using the data and knowledge gained from the first three phases, a full-scale, two-story 3-bedroom wood-frame house was built using the unibody method of construction and tested at the NEES@UCSD outdoor shake table. This final phase, investigated the behavior of a unibody house, subjected to simulated earthquake ground motions at increasing levels of intensity ranging from a service level earthquake to intensity levels 2 and 3 times larger than the maximum considered earthquake intensity. The house remained damage free under a ground motion with an intensity corresponding approximately to that of the maximum considered earthquake and with minimal level of damage even at twice that intensity, demonstrating that the unibody construction approach, with very small increments in cost and small modifications to current practice provides a much better alternative to current methods of design and construction which are only aimed at avoiding collapse.

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