Evaluation of system effects and structural load paths in a wood-framed structure

approved: Rakesh Gupta The objective of this project was to develop an analytical model of a lightframed wood structure using a prevalent structural analysis computer program in order to evaluate system effects and define load paths within the structure, especially under extreme wind events. Simplified modeling techniques and material definitions were developed and used throughout the analysis. A three dimensional 30-ft by 40-ft building was modeled using SAP2000. The building had a gable roof system comprised of Fink trusses. Wall and roof sheathing was modeled using SAP’s built-in thick shell element. Conventional light-frame construction practices were assumed, and the model was linear with all joints considered to be either pinned or rigid. Also, the effect of edge nail spacing of the wall sheathing was incorporated by way of a novel correlation procedure which eliminates the need to represent each nail individually. Instead, a single sheathing element represented each wall and property modifiers were assigned to that wall element based on the nailing schedule. The NDS 3-term shear wall equation was used to derive the correlation procedure and the correlated model was compared to full-scale testing results with good agreement. The computer model was validated against both two and three dimensional experimental studies (in-plane and out-of-plane). Once validated it was subjected to uniform loads to gain insight into its uplift behavior. Uniform uplift pressure was applied to the roof, and vertical foundation reactions were evaluated. In this phase of the investigation, the building geometry was altered in several different ways to explore the effect of these variations. Next, the model was subjected to several uplift loading scenarios corresponding to worst-case simulated hurricane events. With these inputs, the same uplift reaction profiles were generated. Finally, for comparison the model was loaded using the “Component and Cladding” pressures determined at a comparable wind speed, as given by ASCE 7-05 (lateral and uplift). The ASCE 7-05 uplift pressures were found to adequately encompass the range of uplift reactions that can be expected from a severe wind event such as a hurricane. Also, the analytical model developed in this study inherently takes into account system effects. Consequently, it was observed that ASCE 7-05 “Component and Cladding” pressures satisfactorily captured the building’s uplift response at the foundation level without the use of “Main Wind Force-Resisting System” loads. Additionally, it was noted that the manner in which the walls of the structure distribute roof-level loads to the foundation depends on the edge nailing of the wall sheathing. Finally, the effects of variations in the building geometry were explored and notable results include the presence of a door in one of the walls. It was revealed that the addition of a door to any wall results in a loss of loadcarrying capacity for the entire wall. Moreover, the wall opposite the one with the door can also be significantly affected depending on the orientation of the trusses. In general, it was determined that complex, three-dimensional building responses can be adequately characterized using the practical and effective modeling procedures developed in this study. The same modeling process can be readily applied in industry for similar light-framed wood structures. ©Copyright by Kenneth G. Martin January 29, 2010 All Rights Reserved EVALUATION OF SYSTEM EFFECTS AND STRUCTURAL LOAD PATHS IN A WOOD-FRAMED STRUCTURE by Kenneth G. Martin A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of