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Hold-down Assembly
The hold-downs in shearwalls connect the wall end studs on an upper level to either the corresponding stud on a lower level or anchored to the foundation. Continuous tie rod systems extending over multiple building levels are not included in this version of Shearwalls.
Vertical Connection
Hold-downs include either an anchor bolt or threaded rod which connects upper and lower brackets or straps, or a continuous strap extending from upper to lower level.
Horizontal Fasteners
The connection from bracket or strap to the upper and lower studs is made via bolts or nails, however this distinction is not implemented in the program as an overall capacity and displacement is specified for each hold-down, and the strengths and slippages of the components of the hold-down not needed.
Single or double bracket
Hold-downs are designated as being either single-bracket or double bracket, indicating that the hold-down has a bracket or strap on one floor or both. By default, hold-downs on the ground level are single-bracket, and upper-level hold-downs are double-bracket. The data in the hold-down database are published for one bracket only and are doubled when the hold-down is designated as double bracket in the Shearwalls program.
Shrinkage Compensating Device
You can designate that the hold-down includes a mechanical device to adjust for the shrinkage of the perpendicular-to-grain wood between the extreme hold-down fasteners, so that such shrinkage is not included in the calculations for shearwall deflection.
Displacement and Capacity Sources
There are three possible sources of vertical hold-down displacement that affects shearwall deflection:
Similarly, the capacity of the hold-down takes into account the possible failure in tension of the bracket or strap, the anchor bolt in tension, and the connection of the horizontal fasteners to the wood studs.
The published data are assumed to include all of these sources, except that displacement values include the elongation of the anchor bolt to a maximum length. Elongation of the portion of the anchor bolt greater than that length is analysed separately.
Method of Determining Displacement
Hold-downs are designated according to the method we use for determining the vertical displacement under loading, as follows.
Displacement at Actual Force
With this method, ratio of the capacity of the hold-down to the maximum capacity is multiplied displacement to give the displacement used for deflection analysis and storey drift. This assumption of linear may not be correct, due to the non-linear effects of fastener slippage. This would yield non-conservative results for storey drift determination. However, the choice also affects load distribution to and within shearlines using stiffness analysis, for which the effect may be conservative or non-conservative.
Displacement at Maximum Capacity
With this method, the published displacement at maximum capacity is used regardless of the shearwall force. This ensures conservative storey drift calculations. This choice also affects load distribution to and within shearlines using to stiffness analysis, for which the effect may be conservative or non-conservative.