Basic Retrofit Engineering

As in all engineering fields, retrofit engineering is based upon mathematical formulas. Retrofitting uses a formula called the Base Shear Formula to calculate the amount of shear force (earthquake force) that will hit a house at its base (the
foundation.) If the connection between a house and its base is weak, the building can be jolted off the foundation and damaged or possibly destroyed in an earthquake. The above photo shows where an entire house slid several feet off its foundation.  Strengthening the connection between a house and its foundation is the sole purpose of seismic retrofit.

Construction materials used in retrofits are scientifically tested by independent laboratories and rated in their abilities to resist specific amounts of shear force (earthquake force.) 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 can resist 840 pounds of force; a plywood shear panel can resist 200 to 475 pounds of force per linear foot, depending on how it is built; and most shear transfer ties can resist 150 to 580 pounds of shear force, depending on which of the many types of shear transfer ties is used.

To design a retrofit in the East Bay Area, the Base Shear Formula is:

• V = 0.185 (W)
   
• V represents the shear force that will be generated at the base of a building.
   
• 0.185 represents anticipated force of ground acceleration from a major earthquake in Berkeley.  This number varies from region to region and is based upon proximity to known earthquake faults.
   
• W represents the weight of the building.  Single story homes weigh approximately 50 pounds per square foot.  Two story homes weigh 80 pounds per square foot of the first floor.

So, the Base Shear Formula tells us the anticipated earthquake force equals 0.185 times the weight of the house being retrofitted.

Example: We have a one-story house with a first story that is 35 feet by 40 feet.  The first story is 1400 square feet (35 x 40 = 1400). If we multiply this times 50 pounds, we determine that the building weighs 70,000 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.

For our example we would use the Base Shear Formula as follows:

V = 0.185 x weight of house
V = 0.185 x (area in square feet x weight per square foot)
V = 0.185 x 1400 sq. ft. x 50 pounds per sq. ft
V = 0.185 x 70,000 pounds
V = 12,950 pounds

Therefore the amount of shear force that is anticipated to strike this Berkeley home at its base (foundation area) during a major earthquake is 12,950 pounds.  For our purposes we will round 12,950 pounds to 13,000 pounds.

Using this information we can determine how much hardware and plywood we will need to protect this house in an earthquake. See the next section entitled The Three Components of Retrofit Design for an explanation of how we design a retrofit that will resist 13,000 pounds of force.

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