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| The violent shaking of an earthquake can cause a building to fall off of its foundation. Seismic retrofitting strengthens the connection between the wood framing members of a building and its concrete foundation. In houses built before 1940 there may be no existing bolts, or the existing bolts may be either weakened or too far apart to be strong enough for earthquake resistance. Studies show that a properly retrofitted home is highly effective and greatly reduces the amount of damage a house receives from an earthquake. In many cases, seismic retrofitting will result in a substantial reduction in the premium cost for earthquake insurance. |
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Foundation Bolting. Foundation bolting ties the wood framing members of a building to its concrete foundation. The mudsill is bolted to the stem wall by usually using 5/8" diameter anchor bolts embedded a minimum of 7". At times where there is no access to bolt the mudsill vertically, alternative methods can be used such as the placement of Universal Foundation Plates. The type, size and shape of the bolting washer can also make a difference in the bolting strength. Current retrofitting standards call for thicker and larger square washers as compared to the small, thin, round washers found in houses built prior to the mid 1990s. This allows the sill to be connected more securely to the foundation by reducing the chance of the washer pushed through the mudsill in an upward motion quake.
In situations where the concrete has been weakened, possibly due to exposure to water, an epoxy method would be applied to alleviate any potential stress caused by the use of expansion bolts. Retrofitting allows a house to withstand much greater earthquake forces with much less structural damage. |
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Cripple Walls. Most houses have a short wood-framed wall in the crawl space. This cripple wall may be anywhere from a few inches to several feet in height, extending from the top of the concrete foundation stem wall to the bottom of the floor joists. Cripple wall bracing is the application of structural plywood to the cripple wall. The ½” structural 5-ply APA rated plywood sheathing is custom cut with ventilation holes, covered with galvanized wire mesh screens and securely nailed to the cripple wall framing according to structural nailing schedules. For the bracing of cripple walls which are greater than 4 feet in height, the services of a structural engineer are required by City of Los Angeles Department of Building and Safety.
Cripple wall collapse is a main source of earthquake related failure. The installation of framing anchors and bracing of the cripple wall with plywood to keep it from collapsing during seismic movement is a very important part of earthquake retrofitting. Sinai Construction, Inc. has the expertise and engineering services available to design a retrofit at an affordable price. Our tenured crews have been performing earthquake retrofits on a daily basis for many years; we pay close attention to detail and we are watchful for any potential problems which should be brought to your attention. A typical medium house bolting and bracing can be completed in 3 days or less and at a very reasonable cost. |
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