How to Read your Cripple Wall Retrofit Plan

This page only applies if your proposal was written by Howard Cook and does not apply to proposals written by Jeff Bailey.  If Jeff wrote your proposal watch this video.  

Your proposal contains a retrofit design sketch with a description of the material we will use and where we plan to put it.  This sketch is probably the most important part of your proposal and is the basis of your cripple wall retrofit design.  If you understand it everything else will make sense.

Relevant information is in colored boxes so you can find the information more easily on this page.  I suggest you print the sketch in your proposal and compare it to this webpage as you read.  This will help you interpret the sketch.

Cripple Wall Retrofit Plan with Bolts, Plywood, and Shear Transfer Ties

The Engineering Formula

The information in the red box is the most important because it is the engineering background behind the cripple wall retrofit 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 work.

Engineering Formula Used in All Home Retrofits

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

Application of Standard Plan A calculations determine house weights, in this  case 95,296#.

When we multiply the weight of the house by the ground acceleration (0.186 x 95,206#) the 17,762# result equals the amount of force that will strike its base where the wood floor and the concrete foundation meet .  Hence this force is called Base Shear. 

17,762# of force try and dislodge the wooden base of the house from the concrete foundation so we must install enough bolts, shear transfer ties, and plywood to resist 17,692# of force.

Strength of Bolts

Bolting Hardware seen in Retrofit Plan

The green box tells us how many pounds of earthquake resistance  different type of bolting hardware have such as standard bolts and the different kinds of Foundation Anchors.   We need to use enough of a certain type, or a combination of several types, to resist 17,692 pounds (#) of force

Strength of Shear Transfer Ties

Floor Connection Hardware Seen in Retrofit Plan

The blue box tells how many pounds of earthquake force each of the different types of shear transfer ties resist.  We need to use enough of a certain type, or a combination of several types, to resist 17,692# of force

Strength of Plywood

Strength and Quantity of Plywood in Retrofit Plan

The purple box tells us how much earthquake resistance one linear foot of plywood can resist and how much force various lengths of plywood can resist.  The resistance of the plywood is  determined by nailing.  In this case the plywood has been nailed to resist 870# per linear foot. We must install enough linear footage of plywood to resist the 17,692# of force.

Plywood, Bolts, and Shear Transfer Ties all Working Together

Total Number of Components Needed in Retrofit Design

The black box tells us what combination of the different types of hardware and linear footage of plywood we need along each foundation  to meet or exceed 17,682#  For example, if Along Foundation B we have 6 feet of plywood also need 4 bolts or a combination of bolting hardware to resist 17,682#.  The same logic follows the shear transfer tie hardware.

Showing how Hardware All Works Together to Resist 17,762# of Force

In this retrofit we installed enough components to resist 19,724#, exceeding the code requirement by 1962#

If it isn’t clear yet, this video on retrofit engineering might help.

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