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The total force on each shear line is distributed to the shear wall segments according to the relative rigidity of the segment.
The determination of this rigidity and other factors affecting shear distribution to are controlled by several Design Settings, as described below.
Rigidity for Shear Force Distribution
CSA O86 11.5.5.3, Figure 11.7 mandates that relative shear wall capacity be used to determine the distribution of forces along a shear line. However, the Commentary to O86 11.5 indicates that this method is not valid for shear lines with variations in materials, construction or hold-down configuration, and that distribution based on relative stiffness of shear wall segments is preferable. Shearwalls therefore provides a choice between using deflection-based distribution and capacity-based distribution:
Use shear wall deflection the calculate rigidity
If this option is selected the program will attempt through an iterative procedure to equalize deflections on the shear line by redistributing the shear force v to the shear-resisting elements until the deflections calculated are the same for all elements. Refer to Force Distribution to Equalize Deflections for more information about the methodology employed to equalize deflections.
Note that this method can increase design processing time due to the iterative procedures, and can result in many low-stiffness segments or sides of walls taking zero load, especially on lightly loaded shear lines. Under certain conditions, the mathematical routine does not converge and deflections are not equalized as expected.
Use shear wall capacity to approximate rigidity
If this option is selected, shear force is distributed according to the factored shear capacity of the wall or segment.
If Use shear wall capacity is checked when using the same materials on the shear line, the force per unit foot will be the same on each segment , unless there are hold-down effect Jhd factors along the line that would cause variations in shear wall capacity along the line.
For dissimilar materials forces vary along the line depending on the wall composition.
The shear wall capacity option is not available if there are Non-wood/Proprietary walls in the structure.
Distribute Forces to Wall Segments based on Rigidity
When checked, the program distributes forces to each wall segment proportionally to the deflection-based rigidity of the segment. It does so for both rigid and flexible diaphragm distribution of loads to the shear lines.
This setting provides the opportunity to use stiffness based-design force distribution for torsional analysis, but capacity-based distribution to distribute forces within the shear line, to comply strictly with the main provisions of the CSA O86 design code.
Refer to the Rigidity for Shear Force Distribution Design Setting for more information about these options.
Identical Materials and Construction
Using this setting, if you choose to allow walls on the shear line to have different material specifications, and have selected some unknown materials, an iterative design is performed with shear wall strengths or stiffnesses from one iteration leading to a redistribution of forces on the next, and possible different walls designed on the final iteration.
If all walls must have the same materials, for capacity-based design, only one force-distribution iteration is needed. For deflection-based design, fewer iterations are required if all walls are specified to have the same materials.