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There are three possible sources of vertical hold-down displacement that affect shear wall deflection and three possible means of hold-down failure:
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.
Total Assembly Displacement/Capacity
The current standard practice is to publish displacement and capacity values for to entire hold-down – bracket or strap, anchor bolt, and fasteners into studs. Capacity takes into account tensile capacity of brackets and straps, and wood-connection capacity of horizontal fasteners. Anchor bolt displacement applies to a maximum length; elongation of portion greater than that length is analysed separately.
Displacement/Capacity of Separate Hold-down Components
Previous industry practice was to publish displacements and capacities for the hold-down brackets only, and to rely on the engineer to calculate the capacity and slippage of the fasteners attached to the wood studs, and he tensile elongation and strength of the anchor rod. Shearwalls has retained this option for those users who are using custom-fabricated hold-downs, as follows:
Separate Slippage/Connection Capacity, Combined Elongation/Tensile Capacity
Displacement values refer to the elongation of the brackets or straps plus the anchor bolt, to a maximum length. Elongation past the maximum length is analysed separately. Capacities include only the tensile capacity of the brackets and anchor bolt. Slippage of fasteners is calculated by Shearwalls, and wood connection capacity of the horizontal fasteners is not considered.
Separate Slippage/Connection Capacity, Separate Elongation/Tensile Capacity
Displacement values refer to the elongation of the brackets or straps only. The entire anchor bolt length is analysed separately. Capacities include only the tensile capacity of the brackets. Wood connection design of the horizontal fasteners is not included, and capacity of anchor bolt is not considered.
These choices are set via options for each hold-down in the Hold-down Database Editor.