How Many Retrofit Foundation Bolts Do I Need?
Many contractors and even engineers do not understand that earthquake forces always concentrate in the stiffest part of the house first. This also means only the bolts attached to that part of the house will actually do anything. This is the reason why the placement of the retrofit foundation bolts is so important. When trying to figure out how a house will fare in an earthquake look for the stiffest element. That is probably the strongest part of the house. Studies on the performance of stucco confirm this.
Let’s take an example of a house with stucco siding that has been retrofitted. First, all the force will go into the stucco because it is stiffer than plywood and into the bolts attached to the stucco. After it cracks and can no longer resist lateral forces the force goes into the plywood and then into the bolts attached to the plywood. That is why we say only the bolts attached to the plywood will do anything once the stucco fails. For houses with wood siding, bolts attached to the plywood are the only ones that every do anything.
That is why the most advanced guideline in the country, FEMA P-1100 Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings makes this clear. Other guidelines such as Standard Plan A are based on the formulaic, use the put-a-bolt-every-six-feet approach. This can leave some cripple walls lacking the bolts they need and other cripple walls with too many bolts. In other words, on the one hand there are not enough bolts and the retrofit might fail, and on the other hand, there are too many bolts and money will be wasted.
This is from Standard Plan A and all the other retrofit guidelines contain something similar.