Distribution of Dead And Wind Uplift Loads to End Chords

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. It may affect overturning calculations on the level below if the wall segments are offset, otherwise it is just passed down through the structure.

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 and loads are sent down for that reason as well.. A setting allows you to specify that all loads on the wall are used to determine whether it completely counteracts overturing even when you are limiting the range over which the dead loads act in other circumstances.

In these cases, the combined hold-down force at the tension end is zero, and no hold-down device is required, pictured in Elevation view, or specified in the Hold-down Design table. However, in both Elevation view and the Hold-down Design table, the D (dead), T (overturning), U (wind uplift) and the vertical earthquake force E_v components are shown to allow you to track these down the load path. No vertical element is shown where these zero-sum forces are not at a segment end, and the forces are not repeated further down the structure in that case.

Note that there is a contribution for a segment with tension end force to the the anchorage term of the deflection equation in the Hold-down Displacement table due to crushing at the compression end.

Examples with Uniform Line Loads

The cases 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 or vertical earthquake loads, neglect forces from loads over adjacent openings and from levels above,and assume uniform line loads at the top of the wall. Similar results occur for triangular and point loads, but with more complicated algebra.

Other Load Types and Loads and Forces from Outside the Wall Segment

A more general conceptual model that includes wind uplift loads and the vertical earthquake force E_v, loads and forces 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 Other Load Types, Upper Levels, and Adjacent Openings.