Development and assessment of wood light-frame unibody structures for enhanced seismic performance

Past earthquakes have shown wood light-frame structures in the United States provide adequate collapse safety, but they can get severely damaged in the process. This is in part the result of building codes allowing the structure to undergo large displacements to achieve ductile behavior, by using response modification factors (R) as high as 6.5. In the US alone, more than 90 percent of single-family and multi-family residences have been constructed out of wood over the last decade, with 50 percent of the population susceptible to damaging earthquakes. Therefore, it is crucial to develop new cost effective design methodologies that can significantly reduce the risk of damage and resulting displacement of residents. The objective of this research is to demonstrate that enhancing the strength and particularly the stiffness of the light-frame residential buildings by engaging the architectural components leads to dramatically better performance. This new methodology is known as ``Unibody'' construction method. By using cost effective off-the-shelf connectors such as construction adhesives, stronger lag screws and hold-downs, the lateral stiffness of a unibody interior walls can be up to 2.6 times larger than that of conventional interior wall with hold-downs. Similarly, the lateral stiffness of a unibody exterior wall can be 3.7 times larger than that of a conventional exterior wall with hold-downs. On the other hand, the lateral strength can be 1.5 to 3 times larger than the conventional walls. Large scale quasi-static tests of four full-scale unibody room specimens were conducted and demonstrated improved performance resulting in high strength and stiffness, corroborating previous work from other researchers. In addition, these tests assessed the collapse performance by loading the structures to approximately 10 percent drift with residual strengths that were on average approximately 30 percent of the maximum strength. To further validate the proof of concept, full-scale shake table tests on a full-scale two-story unibody house were conducted and confirmed the same improved behavior seen in quasi-static tests. The tests also revealed that with careful attention to tie down detailing and contribution of transverse walls, the unibody structure can drift as much as 25 percent drift while maintaining a residual strength approximately 20 percent of the maximum strength. All tests showed ease of constructibility by professional contractors. Numerical finite element models were successfully calibrated and validated to capture the response of cyclic and dynamic tests of the unibody specimens. Modeling recommendation are provided using nonlinear hysteretic models. Using the calibrated models, general equations and adjustment factors were developed to account for wall length, wall end-returns, and perforation factors (i.e., door and window openings). Current building codes and standards do not allow the use of adhesives or architectural components as part of the lateral resisting system. To help demonstrate the benefits and overcome restrictions to utilizing these materials, design procedures were developed and comparative analyses with conventional construction were performed. Archetype building models were desgined using the conventional and unibody design procedures. In addition, nonlinear numerical models were created in OpenSees to compare the performance of conventional and unibody construction and evaluate the proposed design procedure. An R of 1.5 was used for the unibody design and an R of 6.5 for the conventional design. The archetype building models were evaluated using nonlinear dynamic analyses. The maximum mean story drift from the design level earthquake (10 percent in 50 years intensity) was compared between the unibody and the conventional structure. The unibody archetype building models experienced drifts that were on average less than 0.15 percent, while the conventional design sustained drifts that were on average 0.5 percent. The latter is larger than the median drift value of 0.2 percent for initiation of damage in gypsum walls. The results showed the unibody archetypes were essentially damage free at the design level earthquake based on the design procedure. In addition, the performance of a hybrid unibody structure was evaluated. The hybrid unibody is conventional wood shear with sheathing attached to wood framing with construction adhesive in addition to mechanical fasteners. This enhancement is done to the shear walls only. The hybrid unibody structure, on average, saw drift levels of about 0.30 percent when subjected to the to the design level ground montions, with is about 1.8 times less than the drifts seen for the conventional structure. Finally, the additional cost to implement the unibody construction was also quantified. On average, the additional cost to implement a unibody structure is about 2 percent of the total construction cost of a conventional structure. This value is 2.5 times smaller than minimum deductible of 5 percent required for residential earthquake insurance in California. These initial estimates were developed for single structures and can be further reduced if mass scale production is considered. In addition, a loss assessment study to compare the conventional and unibody construction was also performed. It was found that the loss ratio (which is the repair cost over the replacement cost of the building) for a unibody structure was on average about 7 times smaller than that of a conventional structure. The hybrid unibody structure had on average, a loss ratio that was 1.8 times smaller than that of the conventional structure. The design recommendations and the cost studies were done to provide practicing engineers with the tools to successfully implement the unibody method in new construction.

Advisor: Deierlein Gregory G. (Gregory Gerard)
Fell Benjamin
Miranda Eduardo (Miranda Mijares)