The Base Shear Formula
The base shear formula is: Earthquake Force = 0.2 x weight of house in pounds; where 0.2 is the anticipated ground acceleration measured in Gs
The result = the pounds of force that must will try and separate the house from the foundation. This is why all retrofitting is an attempt to strengthen this connection.
Example: We have a two-story house with a first story that is 25 feet by 40 feet. The first story is thus 1,000 square feet (25 x 40 = 1,000). If we multiply this times 80 pounds, we determine that the building weighs 80,000 pounds. Using this information and the base shear formula we can determine the amount of earthquake force expected to strike this building. We will want to design a retrofit that will resist this amount of force.
This house must have enough bolts or Foundation Anchors to resist a minimum of 16,000 lbs of force AND enough Shear Transfer Ties to resist the same 16,000 lbs. If we need plywood, it too must resist 16,000 lbs of force If each side can resist 8,000 lbs (2 x 8,000 = 16,000) then the house will resist 16,000 lbs of force no matter which direction the earthquake force comes from.