Retrofit Shear Walls & Sheet Metal Blocking
In existing retrofit shear walls it is quite common to find no blocking at horizontal seams or discover that one must put the plywood components of the shear wall together in pieces. For the former condition, the most common way this is addressed is by removing thpe plywood, installing the blocking, and rebuilding the shear wall. This is a very labor intensive and expensive process. Technical Note N370C confirms: “Allowable shear values for blocked structural panel diaphragms are significantly higher than those for unblocked diaphragms. Where blocked diaphragm values are required, panel edges are typically blocked with 2x lumber framing and fastened in accordance with recognized schedules in order to achiever desired shear values. There are instances, however, where installation of lumber blocking may not be convenient. One alternative is to substitute sheet metal strips for lumber blocking at panel joints. The technique has been evaluated by APA-The engineered Wood Association and is discussed in the Technical Note”
This is also quite useful when stitching plywood together because large pieces of plywood will not fit through the crawl space opening.
Here is an example of a shear wall that does not have blocking on the horizontal seam. If it had been blocked one would see horizontal rows of nails on each plywood edge where the two pieces of plywood meet. This blocking earthquake forces to travel from the moving floor, down the upper piece of plywood and into the block. The lower piece of plywood is nailed to the same block so the two pieces of plywood now function like one solid piece of plywood.
Unblocked horizontal seam
Using the information in APA Technical Note N370C we are able to repair shear wall defects like this with sheet metal blocking as shown below at a fraction of the cost of rebuilding the shear wall. According to the APA 5/16 long 7/16 crown staples are supposed to be used which are no longer available. A suitable substitute are the readily available 3/8 inch long, 3/8 crown staples made by Senco.
Use of this technique is often quite useful in new seismic retrofit work as well, such as when the retrofit shear walls are larger than the access opening under the house or when shear walls must be built while standing on ladders where the installation of blocking is difficult.