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The Three Components of Retrofit Design The three areas of retrofit reinforcement are:
The illustrations below show where each of these areas is located. If these illustrations are not clear to you, see the drawings at Retrofit Terminology.
If any one of these three areas fails in an earthquake, the building could suffer serious structural damage and maybe even total collapse. A retrofit must therefore resist base shear forces in each of these three damage areas. This means a properly designed retrofit for the house in our engineering example (see Basic Retrofit Engineering ) must resist a minimum of 13,000 pounds of force where it is sits on the foundation, AND a minimum of 13,0000 pounds against the cripple walls, AND a minimum of 13,000 pounds of force where the floor sits on top of the cripple wall or mudsill. It is an engineering principle that earthquake resisting elements (bolts, shear walls, etc.) can resist only those forces This information allows us to calculate how much hardware and plywood is needed to protect these three weak areas. All bolts, nails, plywood, shear transfer ties, etc. are rated in terms of the amount of shear force they can resist. For example, a 1/2-inch bolt with a plate washer can resist 1000 pounds of force. Each linear foot of high capacity plywood bracing can resist approximately 500 pounds of force. Good shear transfer ties can also resist around 500 pounds of force. The house in our example will be attacked by 13,000 pounds of earthquake force in each direction. We know that a 1/2-inch bolt provides 1,000 pounds of earthquake resistance. To determine the number of 1/2-inch bolts we will need, we divide 13,000 by 1,000. The answer is 13. This means we need a total of 13 bolts to protect the house in the north-south direction and 13 bolts to protect it in the east-west direction. For additional safety, we will round this number up to 14 and install 7 bolts along each Next we need to address the bracing of the cripple walls. A medium capacity plywood shear panel can resist 500 pounds of shear force per each linear foot. If we divide 13,000 by 500 we get 26. This means we need 26 linear feet of plywood in the east-west direction and 26 linear feet of plywood in the north-south direction, or 13 feet of plywood on each wall. The same method is used to calculate the number of shear transfer ties needed to attach the floor framing to top of the cripple wall. Good shear transfer ties can resist 500 pounds of shear force. 13,000 divided by 500 equals 26. This means we need 26 shear transfer ties for each direction or 13 shear transfer ties along each foundation wall. While this simple example can give you a basic idea of the theory of retrofit design, it is vastly different from what is encountered in real life. So many factors must be considered for a properly designed retrofit that full understanding of earthquake forces, intense training in engineering principles and knowledge of the most up-to-date hardware are absolute musts. A retrofit designed by an untrained contractor can actually do more damage to a house than if the house was not retrofitted at all. A retrofit designed by an engineer who is not up-to-date in the latest hardware and retrofit technology can waste your money while providing you little or no protection. Just because someone has a degree doesn´t mean he knows what he is doing. It cannot be emphasised enough...pick your contractor or engineer with care! The next article in this Retrofit Design series is Bolting: attachment of the mudsill to the foundation. |
that are generated parallel to the foundation. In the illustration at the right, this means earthquake
forces generated in the east-west direction will be resisted by the bolts, plywood, and other hardware that are installed along foundation walls A and C. Forces generated in the north-south direction will be resisted
by bolts and other hardware installed in foundations B and D.
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