Because the controlling component in shear wall design is the nailing, it is no surprise that the correction for field problems generally revolves around the installation of additional fasteners. In some instances we can continue to use nails, but where nails are spaced closely or where the materials have low moisture content due to aging, wood screws are the right choice. Here's our hit list:
|Too Close To Edge Of Panel|
|Subdrill and add wood screws as shown.|
|Too Close To Edge Of Framing Member|
|Supplement with another 2x stud, internailing the
new and existing stud together to transfer the whole
Into Too Narrow A Stud - The 1997
Uniform Building Code requires 3x studs at panel edges where the design shear
exceeds 350 pounds per foot. This is a change from previous codes in that
2x framing was commonly used at 4""o.c. nailing. Word will be slow to get
around and sure as not you'll be looking at a wall with 4" o.c. nailing and
Overdriven Nails - Treat as if the nail was misplaced and follow the recommendations for adding screws.
Box Versus Common Nails - The Uniform Building Code accepts galvanized box nails as equivalent alternates to common nails for shear wall nailing. The galvanization increases the diameter of the thinner box nail, making the plywood think it is bearing on a common nail. If you are using untreated box nails, the shear wall has lost significant strength. This formula will permit you to calculate the reduced allowable load:
[(Allowable lateral load for box nail) / (Allowable lateral load for common nail )] * (Shear nailing value per Code)
Partial Head Nails - As was noted previously, buckling of the plywood is one of the mechanisms by which shear walls tend to fail. The plywood is allowed to buckle because it is insufficiently anchored to the framing lumber. It tends to fail by slipping over the head of the nail. As might be expected, partial head nails lack a full circular nail head. Thus their ability to resist buckling is less than a full head nail.
|Nailing To The Wrong Member|
|The limits of the shear wall are indicate by the arrows.
But when the framer
installs sheathing over the entire face of wall, all indications of the thicker
member attached to the hold-down are obscured. Chalk lines snapped on
the exterior of the plywood would minimize the problem.
Stud Grade - The grade of a stud (No.1, No. 2, Construction, Utility) has some bearing on the strength of a shear wall. Material properties that vary according to this classification alter a stud's allowable strength in compression. Overturning forces can significantly increase the compressive load on studs close to the ends of walls. The solution is to add sister studs to those already present.
Stud Species - The specific gravity (G) of wood varies from species to species. Fastener lateral strength is decreased with decreasing specific gravity. According to the current Code
|Nail Design Values for Single Shear Connections - Combined Table|
|(Both members of identical species)|
If you reference the plywood shear wall tables in the building code, you'll see a message similar to the following:
Allowable shear values for nails in framing members of other species shall be calculated for all other grades by multiplying the allowable shear for nails in Structural I by 0.82 for species with a specific gravity greater than 0.42 but less than 0.49, and 0.65 for species with a specific gravity less than 0.42.
Sheathing Type - As we've seen in the section on sheathing selection, the difference between Structural I and Structural II sheathing can have an effect on the strength of a shear wall. But for all the hooplah made about Structural I panels, the real difference in strength is less than 15 percent. The allowable shear on Structural II panels can be found in the UBC Tables.