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SDPWS 4.3.6.4.1.1 and Equation 4.3-9 require that forces in collectors connecting shear wall segments (drag struts) be divided by the perforated wall capacity adjustment factor Co and narrow segment adjustment given in SDPWS 4.3.5.6 so the shear force used for drag strut calculations is
Sbi is the sum of the shear wall segments, adjusting for narrow segments as per 4.3.3.4 and V is the force on the perforated wall
vmax rather than the actual shear force v is required by SDPWS to account for unbalanced distribution of shear force that can occur within a perforated wall due to the absence of hold-down devices at wall segment ends. For this reason, and to ensure a balance between the accumulated diaphragm force and the design shear force, the drag strut force is calculated by Shearwalls with only one wall segment in a wall being assigned the vmax force. The additional force vmax - v is subtracted from the force distributed to all other segments on the wall with the segment. Then the calculation is made for each wall segment in the line having v = vmax in turn, and the largest drag strut force at each location taken.
Thus the calculation given in drag strut forces is modified as follows:
F(x) = maxs [ ∫ ( vs (x) - V/L ) dx ]
where
= (Vs - vmax bs ) / (Σb – bs ) , for segments other than s
= 0 over openings and gaps between walls.
Drag Struts Calculated Using Sheathing Capacity
A Design Setting allows you to use shear capacity, which we will call vr rather than applied load v to calculate drag strut forces .
The equation for vmax using vr can be written
where vn is the nominal shear capacity, so
This makes sense, because shear resistance does not concentrate on perforated walls as does applied shear. Using shear capacity, drag struts will be calculated for perforated walls as they are for segmented walls.
Elevation View Drawing
In Elevation view, when you select the option in the Show/Hide menu to view forces for drag strut and anchorage design, the vmax force is shown at the bottom of all wall segments, even though for drag strut design it is applied to only one segment at a time. This is because it is also being shown for anchorage design, which requires it on all segments.