Graduate Thesis Or Dissertation
 

Lateral force resisting pathways in log structures

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  • In recent years, log structures have been marketed as an alternative to conventional light-frame wood structures. Log structures are constructed using round timbers (or manufactured timbers) that are stacked vertically and have interlocking corners. Thru-rods and lag screws are used to provide inter-log connections. This project was conducted to evaluate the lateral force resisting pathways that are developed by anchor bolts, thrurods and friction. An experimental study was used to investigate inter-component friction, force-displacement behavior and yield mode of sill log-foundation details. A family of finite-element models was developed to assess the force-displacement response of log shear walls without the corner connection for a set of construction variables that included foundation connectivity, aspect ratio, wall perforations, and thru-rod hole diameter. The experimental study used test specimens that represented common construction details for sill log-foundation anchorage. One detail had the sill log on the floor diaphragm and the other detail had the sill log in direct contact with the sill plate. A sinusoidal cyclic testing protocol was used to assess friction between the sill log and plywood surfaces. It was shown that a reasonable value for the coefficient of friction is 0.4. The sill log-foundation details were tested statically and then with a fully reversedcyclic quasi-static test protocol. The force-displacement curves showed an initial stiffness, slip, and post-slip stiffness and capacity. The open shape of the hysteresis diagrams suggests that energy dissipation occurs primarily through friction rather than bolt yielding and material damage. Connection details were shown to have capacities at least 4.8 times greater than that needed for an upper bound on design base shear as calculated following the Uniform Building Code. A finite-element model using ANSYS 6.0 was developed for a representative unit log shear wall and a sill log-foundation assembly. The wall model was eight logs high (2438 x 2438 mm) and included foundation anchorage, friction, and thru-rods. Linear springs were used to model loglog normal contact, while nonlinear springs were used to model log-log friction and thru-rod behavior in oversized holes. The wood materials were given linear elastic, planar isotropic properties. The force-displacement behaviors of the wall model and the sill log-foundation model were verified with test data. These models were then used in a parametric study to evaluate the lateral force resistance response of a log shear wall given a range of slip force and design alternatives. It was shown that log-log friction affects the initial wall stiffness and slip force. A 50 percent loss in thru-rod tension decreases the log-log slip force by the same amount. Oversized thru-rod holes can be the source of increased lateral displacement; an increase in hole size of 13 percent can increase the wall displacement at the plate log by 31 percent. Window and door openings are accompanied by additional thru-rods, and the additional thru-rods improve wall forcedisplacement response relative to walls with minimum thru-rod hardware and no openings. This research did not address the three-dimensional system behavior that is expected to develop in a box-like structure with integral corner connections. Further research is needed to assess the role of integral corner connections in three-dimensional response to lateral loading.
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