Shear Walls: A Structure First Defense Against Earthquakes

Shear walls are required on all structures that are built in California, both for lateral loads and for loads due to cantilever action. They are built to reduce their damage to the structure and its contents drastically. They can as well be load bearing or non-load bearing, giving the designers the ability to position them wherever they like. If the shear wall is properly designed it will fulfill its purpose of transferring horizontal forces to the next load-bearing element on its path below like the footings, slab, foundation walls, floors or other shear walls. Normally smaller structures will only have them in the exterior walls, however for larger structures they might have to be included as well in the interior wall design.

When using wood structural panels for shear wall construction, its sheathing will have the allowable shear strengths recommended in the Uniform Building Code. Both Oriented Strand Board (OSB) or Plywood can be used for this purpose, however it has been found that structural 1 grade plywood with a minimum of four-ply panel construction. Some designers will recommend Plywood to be used on certain areas as it is more resistant to moisture. The minimum thickness for shear wall sheathing is of 15/32”. Important design elements like opening in the shear panels affect the overall strength of the wall, these edges should be adequately reinforced to transfer all shear-stresses. It was found during the aftermath of the Northridge Earthquake than shear walls constructed with plywood work better than those using other sheathing materials like Portland cement or gypsum wall boards.

In order to be efficient shear walls, have to be aligned vertically, if they do not then additional strengthening will be required on other parts of the building.  The design of this walls will very commonly be designed in a height to with ratio of ½ : 1 for low and long walls, or a ratio of 2:1 in tall and short walls. As well if the structure is on a raised foundation then the subarea will have to be reinforced so that it is capable of distributing the shear-stresses. Normally this can be achieved with a cripple wall that will create a continuous load path.

Shear walls can either be built on the jobsite, or premanufactured by companies like Simpson Strong-Tie or HardyFrame. However whenever using any of their products it is very important that you follow their recommendations. This will include do not expanding any predrilled holes and hand tightening bolts with a wrench and never with an impact drill.

If you are building a new structure in San Diego or another area of Southern California, give our team a call to (619) 882 – 9309 or Contact Us Online so that our expert team can assist you in the proper framing and installation of your shear walls.

References:

• Shear Walls at https://abag.ca.gov/bayarea/eqmaps/fixit/manual/PT07-Ch-3A.PDF
• Chapter 23 of the California Building Code