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Shear wall flanges and eschewed walls

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In the past year, I have had communications with users of the shear wall analysis that was added as an appendix to the open-source wood shear wall spreadsheet authored by Dennis Wish. The following is a summary of those conversations. I am very interested how others are designing shear walls for concrete and wood. Please respond.

David Merrick, SE
Sacramento, CA
(I am no longer a member of SECB)

The following is only for discussion only and may contain errors. Please respond if you disagree.

The lateral force distribution for one or two story building depends on assumptions that are different from tall buildings. When a tall building uses shear walls that have cross-walls acting as flanges, then the tall building analysis might be more similar to that of the short building. A short building sheer wall analysis is simplified by making the assumption of rigid soils and mostly shear strain controls the rigidity.

Cross walls used as flanges can have higher overturning reactions at intersections for a seismic analysis not in the principle axis with or without eschewed walls. Both UBC(1633.1) and IBC(1620.2.2) allow the SRSS combination of loads or add 30% of the other direction’s analysis.

In a short buildings, using cross-walls as flanges, assumes that the soils are soft. If the cross-walls, being used as flanges, are very long then their full length may not be effective as a flange. The softer the soil, the longer portion of the cross will can be used as a flange. An infinitely rigid soil or foundation, results in very little of the cross wall being used as a flange. Estimates of the effective cross-wall on a rigid foundation will be less than the height of that cross wall. Wood shear walls are too flexible to use cross-walls and flanges in bearing. Wood shear walls not anchored may have some uplift independent of the foundation and possibly can use a cross wall to resist uplift.

Using cross walls as flanges allows the footing under the shear wall to be less for resisting the overturning bearing stresses on the soils. Such a foundation design will create a softer foundation, and lead to an analysis that may more resemble a tall building shear distribution.

For a flexible foundation, a shear center must be considered, in lieu of the simple center of rigidity, for a wall set that uses cross walls as flanges when the wall, does not intersect cross-walls at their centers. An example would be a set of walls forming the “C” shaped section, when viewed in plan. Over a soft foundation, the wall’s overturning motion pulls up and pushes down on the end cross walls. That rotates the cross-walls and forces them to pull laterally on the building in opposite directions. The shear center is to be used as a center of rigidity for that set of walls.

For a flexible foundation, the smaller wall rigidity will more depend on the overturning stiffness of the foundation as well as the flexural stiffness of the wall.

Using cross walls as flanges for shear walls has its advantages. However, the simple one or two story building, lateral analysis uses assumptions that seem to limit the cross wall to act as a flange.

The one or two story shear wall system also has assumptions about the rigidity of diaphragms and the rigidity of the foundations. For concrete walls, it is usually assumed that the diaphragms and foundations are rigid. For wood shear walls, diaphragms are usually considered flexible, a rigid diaphragm sometimes needs to be considered, and the foundations are assumed rigid.

Eschewed shear walls are easily misunderstood. An eschewed wall is oriented diagonally in plan. To test a valid analysis, remove all wall rigidities in one direction and add a force in the other direction. The eschewed wall reaction, minus the diaphragm torsion affects must equal zero. An eschewed wall can only resist an “x” or “y” force when the wall is braced in the perpendicular direction. The eschewed wall stiffness diminishes as the perpendicular brace (cross-walls) rigidity is reduced. To design the walls, use a combination of “x” and “y” forces whose resultant aligns with the principle axis or combine the “x” and “y” results using the SRSS (square root of the sum of the squares) method. Alternatively, the UBC(1633.1) and IBC(1620.2.2) requires 30% of the perpendicular analysis to be added to the direction being considered.

The following code rules are confusing. For columns supporting intersecting shear walls, the UBC(1633.1) requires the principle axis analysis if 20% of the reaction of the other direction’s analysis is added. This seems to be in addition to the omega factor and SRSS. The IBC(1616.4.1) “the design seismic forces are permitted to be applied separately in each of the two orthogonal directions and orthogonal effects are permitted to be neglected.” This is not as compelling as IBC(1620.2.2)

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