This webpage only applies if your proposal was written by Howard Cook rather than Jeff Bailey. If Jeff wrote your proposal, watch this video.

Your proposal contains a sketch with a description of the material we will use and where we will put it.  This is probably the most important part of your proposal, and if you understand it, everything else will make sense.

I have put relevant information in colored boxes so you can find the information more easily on this page.  I suggest you print the sketch in your proposal and read it along with this webpage.  This will help you understand what the sketch means.  Your drawing may not have the information shown in the red box but the same methodology was used in your design.


How a Retrofit is Designed using a Simple Engineering Formula


Engineering Calculations from Standard Plan A

The information in the red box is the most important because it is the engineering background behind the regional guideline Standard Plan A.  These exhaustive engineering calculations are specific to the Bay Area and are a gold mine for any retrofit contractor or engineer. Let’s see how these calculations apply to this particular design.

0.186 Gs. is the ground acceleration we should expect according to geologists.

Standard Plan A engineering calculations tell us how much a house weighs, in this  case 95,296#.

When we multiply the weight of the house by the ground acceleration the 17,762# result equals the amount of force that will strike its base at the foundation level .  This is called the Base Shear.

Given 17,762# of force will strike the base of the house we must install enough Foundation Anchors and Shear Trasfer Ties to resist 17,692# of force caused by the anticipated 0,186 Gs ground acceleration

Each type of Foundation Anchor bolting hardware and each type of Shear Transfer Tie is designed to resist a certain amount of force and we need enough hardware to resist it.  In this case the design provides enough Foundation Anchors and Shear Transfer Ties to resist 19,724 pounds of earthquake from each direction, which is slightly more than we actually need.

Earthquake Strength of Shear Transfer Ties to Attach Floor MudsillThe blue box tells how many pounds of earthquake force each of the different type shear transfer ties can resist.  We need to use enough of a certain type of Shear Transfer Tie, or a combination of several types, to resist 16,119# of force or more.

Foundation Anchors use to Bolt House to FoundationThe green box tells us how many pounds of earthquake force  the different types of Foundation Anchors can resist.   We need to use enough of a certain type, or a combination of several types of Foundation Anchors, to resist 16,119 pounds of force or more.  It is always best to be able to resist a little extra as a margin of safety.

Chart Showing how Many Bolts and Shear Transfer Ties Needed to Retrofit HouseThe purple box tells us how the different components all work together.  Let’s assume that on Wall line C we want to resist 4,300# of earthquake force.  In order to do this we need to bolt the mudsill with 3 pieces of FA hardware along this foundation along with various quantities and types of Shear Transfer Ties.  We just need to make sure the FA hardware and Shear Transfer Tie hardware can resist 4,300# of force.    Exactly which hardware to use is determined on site.

Hardware can be Used in Combination to be Enough for the Retrofit