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The fourth term in the deflection equation relates to the displacement of the shearwall anchorage devices and the movement of the wood material at the hold-down location. The following sections give the various components which are added to give vertical hold-down displacement da.
Displacement
Refers to the elongation in tension of the hold-down brackets or straps plus anchor bolt elongation, plus the slippage of fasteners attaching the brackets or strap to the wall studs.
Database value
The hold-down database contains the strength-level displacement that occurs at the maximum capacity.
Displacement/Elongation at Maximum Capacity
If this method ( see ) is selected for a particular hold-down, the program uses the database maximum value regardless of the force.
Displacement/Elongation at Actual Force
If this method is chosen, then the program divides factored hold-down force by the capacity, then multiplies this ratio by the strength-level displacement.
Additional bolt length
In some cases, separate elongation of the anchor bolt is added to the database deflection. This happens when the published displacement or elongation is for an anchor bolt which is shorter than the one input in Structure input view for the level the hold-down is on. The elongation for the additional length is calculated. Note that in this case, for double bracket hold-downs, the published length is doubled before being compared to the actual length in the program.
The elongation of the length L of bolt that is to be analyzed is PL/AE, where A is the bolt cross-sectional area, E is the steel modulus = 29000000 psi and P is the strength level hold-down force at that location.
Shrinkage
Refers to the wood shrinkage that occurs between fabrication and service of the perpendicular-to-grain wood members spanned by the hold-down.
It is calculated when the hold-down does not include a shrinkage compensating device.
Calculation
The vertical shrinkage displacement is 0.002 x (% fabrication moisture content – % in-service moisture content) x shrinkage length for that building level from the Structure input view.
Moisture content input
The fabrication and in-service moisture content are input in the Design Settings. Previously you could input only whether it was greater or less than 19%, for use in nail withdrawal design. Now the actual moisture content is input.
In-service Greater then Fabrication
If for some reason in service moisture content is greater than fabrication, shrinkage is set to zero.
Crush
The wood crush as input in the Hold-down settings is applied to all hold-down locations in the program. Typically ranges from 0.2 – 0.4”
Additional Components
The additional components in the “Other – miscuts/gaps” input of the Hold-down settings are applied to all hold-down locations in the program.
Anchorages
Wood panels
Vertical displacement of anchorages as opposed to hold-downs is determined for walls with wood panels via the equation in 9.7.1.1 for this situation. If the wood construction of one side of the wall differs with the other in any way, both sides are calculated and the smaller deflection of the two is taken. This is equivalent to ignoring the contribution to stiffness of the weaker side.
Gypsum wallboard
For walls sheathed entirely with gypsum, the displacement over-ride from the Hold-down settings is used, as there is no guidance for this in the O86. If a wall has wood on one side and gypsum on the other, the gypsum is ignored.
Unblocked walls
For unblocked walls, a nail spacing of 150mm is used in place of the actual nail spacing to comply with CSA O86 9.7.1.2.