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The method for distributing loads via the torsional effects of rigid diaphragms, described in Rigid Diaphragm Analysis, requires the relative rigidity of the shear lines in each direction to determine how much load goes into each line. Distribution of shear forces to shear resisting elements within a line also depends on the relative rigidity of the element, regardless of whether rigid diaphragm or flexible diaphragms are used.
This setting allows you to indicate to the program how it is to determine the relative rigidity of shear walls and shear wall segments. You have two options:
Shear wall capacity (wood panels and fiberboard only)
With this method, the program uses the shear wall capacities, initially determined from the Flexible Design Method, to approximate the relative rigidities. This method is allowed by SDPWS 4.3.3.4.1 only for wood structural panels and fiberboard, and is subject to a penalty for narrow segments with aspect ratios between 2:1 and 3.5:1.
Using this method, the rigidities appear as force units ( lbs or kips) in the Torsional Analysis Details output, that is, the rigidity expressed as force / length multiplied by the shear wall length.
If this method is chosen along with the All shear walls have identical materials and construction setting, then force per unit foot is constant along the entire line unless there are perforated wall factors Co or height-to-width factors for short segments along the line that cause variations in shear wall capacity .
If this method is chosen, the program ignores the contribution to shear resistance and to stiffness from gypsum-based materials and lumber sheathing to comply with SDPWS 4.3.3.4.1. Accordingly, in the Design Settings, the Ignore contribution of... Walls entirely sheathed with gypsum-based materials or lumber sheathing setting is checked and disabled when this setting is checked.
Deflection of wall segments, perforated walls, or force-transfer walls
This method uses more rigorous rigidity analysis that takes into account framing bending, sheathing shear stiffness, and nail and hold-down slippage to determine deflections of shear resisting elements.
Using this method, the program use the sum of shear-resisting element stiffnesses on the line, or S Fi/Di, to calculate rigidity, where Fi is the force on the element and Di is the deflection calculated for that element. When deflections are equalized on the line, this is just F/D, where F is the shear line force and D is the common deflection. For segmented walls, the shear-resisting elements are full-height wall segments between openings, for perforated or force-transfer walls, the deflection of the entire wall is used in the calculation.
Using this method, the rigidities appear as force / distance units (kips / in) in the Torsional Analysis Details output, that is, the force required for a unit deflection.
According to SDPWS 4.3.3.4.1 a condition for using this approach is that forces are distributed such that deflections are the same for all shear resisting elements on the line. The program attempts through an iterative procedure to equalise deflections on the shear line by redistributing the shear force v to the segments until the deflections calculated with SDPWS Eqn. C4.3.2-1 are the same for all segments. Refer to Force Distribution to Equalize Deflections for more information.
Note that this method can increase design processing time because these iterative procedures, and can result in many low-stiffness segments or sides of walls taking zero load, especially on lightly loaded shear lines.