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Short Wall Segments - RIgid Body Assumption
Short shear wall segments are typically modeled as rigid bodies undergoing rotation due to overturning, so that vertical dead and wind uplift loads applied at the top of the wall are counteracted at the bottom by a reduction or increase in the tensile hold-down reaction at one end and the compressive end stud reaction at the other end. The distribution of partial loads, triangular loads, point loads, etc, to each end is done via beam statics.
Longer Wall Segments - Partially Rigid
Many designers believe that the rigid-body assumption breaks down for longer segments, so a setting in Shearwalls allows you to define the distance from the end of the wall segment over which dead and wind uplift loads are concentrated at the segment ends and, in the case of dead loads, counteract overturning or contribute to compressive stress.
In such a case, the loads within the defined distance from the end are distributed to the end chords as if it was a rigid body, but the load in between is transferred directly to the level below without affecting overturning calculations.
Dead Load Completely Counteracts Overturning
If a dead load is large enough to completely counteract overturning, there is no rotation and the rigid-body assumption breaks down. In this case we assume any load in addition to that needed to counteract overturning is passed to the level below as a line load and does not contribute to compressive stress at the end chords. This is true regardless of whether we are limiting the distance over which the dead loads act via the setting.
Examples with Uniform Line Loads
The case of not completely counteracting overturning and completely counteracting overturning are illustrated in the subtopics. For simplicity, we will consider just dead loads without wind uplift, neglect forces from loads over adjacent openings, and assume uniform line loads at the top of the wall. Similar results occur for triangular and point loads, but with more complicated algebra.
A more general conceptual model that includes wind uplift loads, loads over adjacent openings that concentrate as point loads at the end studs, hold-down or compressive forces that are transferred from upper levels, and line loads transferred from upper levels, is given in Effects of Wind Uplift, Upper Levels, and Adjacent Openings.