Graduate Thesis Or Dissertation
 

The effect of hold-down misplacement on the strength and stiffness of wood shear walls

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  • The objective of this study was to determine the effect of misplaced hold-downs on the static and cyclic behavior of wood shear walls. Three shear wall configurations were considered in this study. The first configuration (control) had hold-downs at the ends of the wall (wall type 1), the second configuration had one of the hold-downs misplaced to the first interior stud (wall type 2), and the third configuration had a misplaced hold-down with additional nailing applied to the stud attached to the misplaced hold-down (wall type 3). All shear wall specimens were 2.44 x 2.44 m (8 x 8 ft.) frames sheathed with oriented strand board (OSB). The frames were constructed with 38 x 89 mm (2 x 4 in. nominal) No. 2 kiln dried Douglas-fir studs spaced at 406 mm (16 in.) o.c., connected with 16d (3.33 mm dia. x 82.6 mm long) pneumatic gun driven nails. The 1.22 m x 2.44 m x 11.5 mm sheets of APA rated sheathing Exposure 1 OSB were attached to the frame with 8d (2.87 mm dia. x 60 3 mm long) pneumatic gun driven nails. SIMPSON Strong- Tie® PHD-2A® style hold-downs were used. Two wall specimens for each configuration were tested under both static and cyclic loading conditions, for a total of four tests per configuration. The recently developed CUREE protocol was used for all cyclic tests. Results from this study show that misplaced hold-downs cause reductions in strength and absorbed energy. Wall type 2 specimens achieved strength values 42 percent lower under static loading, and 35 percent lower under cyclic loading when compared to wall type 1. These strength reductions are much higher than the 17 percent anticipated strength loss due to the reduction in effective wall width. Application of a denser nail spacing to the stud attached to the misplaced holddown (wall type 3) helped achieve higher strength. Wall type 3 specimens reached an average strength that was 21 percent lower than wall type 1 specimens under static loading, and 19 percent lower under cyclic loading. This value is closer to the strength loss anticipated due to reduction in the effective wall width. The effect of additional nailing has little effect on the initial stiffness of walls with misplaced hold-downs. Denser nail spacing significantly increased the amount of energy absorbed by the shear walls with misplaced hold-downs. Wall type 3 specimens tested under both cyclic and static protocols absorbed nearly 36 percent more energy than wall type 2 specimens. Failure mode for the control walls was in the form of nail withdrawal from the center stud. Hold-down misplacement to the first interior stud (wall type 2) shifted the nail failure away from the center stud. Wall type 2 specimens had nails withdrawal primarily from the stud attached to the misplaced hold-down. Application of denser nailing to the stud attached to the misplaced hold-down shifted the nail failure back to the center stud (wall type 3). In both wall types 2 and 3, the frame underwent significant deformation due to vertical deflection of the unrestrained portion of the sill plate. The static and cyclic test results show that undetected misplaced holddowns have detrimental effects on the structural performance of wood shear walls. Application of denser nail spacing can help to regain strength.
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