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Evaluation of direct stucco-woodframe connectors for enhanced seismic performance Mastschuch, Tatiana

Abstract

Observations of earthquake damage indicate that stucco shear walls often fail at the connection (staples/nails) between stucco lath and wood frame members. To improve the connection of stucco to wood frame, special devices called Shear-locks have been developed. Shear-locks provide a direct connection between stucco and wood frame members that is stiff, strong, and has significant ductility. The objective of the current research project was to experimentally evaluate the specially developed shear transfer devices. The project involved a detailed examination of a number of variables and their influence on the performance of the connectors including the material properties and geometry of the connectors, the spacing and edge distance of the connectors, the stucco compressive strength, as well as the loading protocol used to conduct the tests. Because of the large number of tests that were needed, small specimens with a single piece of wood representing either the bottom, top or end stud connected to the stucco segment with two Shear-lock connectors were used. The element tests were conducted using a specially developed testing apparatus, which simulated the correct boundary conditions of the elements. The experimental study was divided into four phases and 68 specimens in total were tested. Phase I served as a pilot study to evaluate the specially designed testing apparatus. Phase II investigated the portion of the Shear-lock that is embedded in the wood (ductile portion). The tests examined variables that influence the displacement capacity of the Shear-lock. Phase III investigated the stucco (strong) portion in order to determine the anchorage strength of the Shear-lock in stucco. Finally, Phase IV involved full connection tests with no artificial strengthening of ductile/brittle portion of the connectors. For comparisons, tests were also conducted on element specimens of regular stucco construction, and plywood specimens. The results from the different elements are compared in terms of simple parameters characterizing the complete load - deformation response such as peak load, effective stiffness, displacement capacity, and displacement ductility. The results from the element tests demonstrate that Shear-locks with optimum characteristics are capable of resisting a shear force of up to about 2 kN (450 lbs) per connector while allowing displacements of up to 30 mm (1.2 in.) between stucco and Spruce-Pine-Fir (SPF) wood frame members with typical moisture content. The optimum characteristics of the Shear-locks include annealed fasteners and a sleeve with a smooth inner transition. Tests using over-strength fasteners indicate that the minimum pull-out resistance of Shear-locks from stucco with typical in-situ compressive strength is about 4 kN (900 lbs) per connector. Thus for normal strength fasteners (which yield at 2 kN) attached to typical SPF wood frame members there is a factor of safety of 2 against pull-out. The anchorage strength of Shear-locks is not significantly reduced when the connectors are spaced as close as 76 mm (3 in.) apart or located within 45 mm (1³⁄₄ in.) from the edge of the stucco. The final series of tests were done using very dry Douglas Fir (DF) wood frame members. These tests indicate that the species and moisture content of the wood has a very significant effect on the performance of the connectors. The stiffer wood resulted in a larger yield force (greater strength), but significantly reduced ductility as the final failure involved connector pull-out from stucco. The dry DF wood frame members also resulted in reduced performance of the nailed plywood elements. Additional element tests are required to more completely examine the influence of species and moisture content of wood frame members on both Shear-lock connectors and nailed plywood connections. As a future phase of the research program is expected to involve full-scale shear wall panel tests, a complete proposal is provided on how these tests should be conducted, including the type of specimens and their construction, testing apparatus, instrumentation, loading protocol, and acceptance protocol.

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