Shear Walls

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  • Topic: Drywall, Earthquake engineering, Concrete
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  • Published : April 5, 2013
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Shear Walls 17

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SHEAR WALLS
by Timothy P. McCormick, P.E.

Seismic Retrofit Training

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Shear Walls

Seismic Retrofit Training

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SHEAR WALLS
This section provides an introduction to shear walls and how they resist earthquake and wind forces. This section also shows how to properly construct the shear walls and the parts that make them up. With this knowledge, contractors can build proper shear walls and inspectors can recognize the errors untrained contractors make.

GETTING THE BIG PICTURE
WHAT IS A SHEAR WALL?
Shear walls are vertical elements of the horizontal force resisting system. They are typically wood frame stud walls covered with a structural sheathing material like plywood. When the sheathing is properly fastened to the stud wall framing, the shear wall can resist forces directed along the length of the wall. When shear walls are designed and constructed properly, they will have the strength and stiffness to resist the horizontal forces. (Fig. 3.1)

Fig.3.1 - Typical Shear Wall

WHERE SHOULD SHEAR WALLS BE LOCATED?
Shear walls should be located on each level of the structure including the crawl space. To form an effective box structure, equal length shear walls should be placed symmetrically on all four exterior walls of the building. Shear walls should be added to the building interior when the exterior walls cannot provide sufficient strength and stiffness or when the allowable span-width ratio for the floor or roof diaphragm is exceeded. For subfloors with conventional diagonal sheathing, the span-width ratio is 3:1. This means that a 25-foot wide building with this subfloor will not require interior shear walls until its length exceeds 75 feet unless the strength or stiffness of the exterior shear walls are inadequate. (Fig. 3.2) Seismic Retrofit Training

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Shear Walls

Fig.3.2 - Location of Shear Walls Shear walls are most efficient when they align vertically and are supported on foundation walls or footings. When shear walls do not align, other parts of the building will need additional strengthening. Consider the common case of an interior wall supported by a subfloor over a crawl space and there is no continuous footing beneath the wall. For this wall to be used as shear wall, the subfloor and its connections will have to be strengthened near the wall. For new construction, thicker plywood or extra nailing and connections can be added. For retrofit work, existing floor construction is not easily changed. That’s the reason why most retrofit work uses walls with continuous footings underneath them as shear walls. (Fig. 3-3)

Break in Load Path

Exterior Shear Wall

Interior Shear Wall Interior Partition

Cripple Wall

Floor must support Shear Wall

Continuous Load Path

Fig. 3-3 Horizontal Alignment of Cripple Walls
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Fig. 3.4 -Vertical Offset of Shear Walls Another type of alignment problem occurs when the ends of shear walls do not align from story to story. This condition creates the need for extra framing members and connections in the walls for holdown devices. Holdown devices must transfer the uplift from the shear wall to framing members that can resist it. When full height studs are not available, special connections must be added. These connections must assemble enough of the structure’s framing to resist the uplift. (Fig. 3-4, 3.5)

WHAT TYPES OF FORCES DO SHEAR WALLS RESIST?
Shear walls resist two types of forces: shear forces and uplift forces. Connections to the Fig.3.5 - Shear Wall Over Floor Beam structure above transfer horizontal forces to the shear wall. This transfer creates shear forces throughout the height of the wall between the top and bottom shear wall connections. The strength of the lumber, sheathing and fasteners must resist these shear forces or the wall will tear or “shear” apart (Fig. 3-6).

Fig. 3.6 – Shear Force on 3-Ply Plywood

Fig. 3.7 - Uplift...
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