Designing a Cripple Wall Retrofit

By Howard Cook, Founder and General Manager of Bay Area Retrofit

Seismic Retrofit Engineering for a Cripple Wall Retrofit

(It is much simpler than you think)

Seismic retrofit engineering for houses is much simpler than you might imagine.   It uses a simple formula, called the base shear formula, that tells a contractor how many foundation bolts, linear feet of plywood, and shear transfer ties a retrofit will need to resist the force geologists tell us to expect.  If you don’t use enough components the earthquake will fail.  If you use too many, you are wasting your money.  This information comes from engineering calculations that are the engineering basis of the regionally accepted guideline called Standard Plan A.  The author was a member of the committee that wrote this guideline which is why he has a copy of the calculations.

This formula  is much closer to arithmetic than it is to “engineering.”  It  represents a merging of geology, physics, and empirical evidence to tell us how much earthquake force is going to attack the base of a building where it is connected to the foundation.     Watch the video and find out how the base shear formula is used to design a residential retrofit.  This information prevents overdoing it and spending more money than you need to, but makes sure you get the protection you need.  The video will only make sense if you know what  cripple wall retrofit components do and look like.

Applying the Base Shear Formula to an actual House 

All cripple wall retrofits use bolts, plywood, and shear transfer ties.  On this page you will find the engineering basis of their use in seismic retrofits. With this engineering background you will be able to determine the amount, strength, and type of the retrofit components you will need.

The easiest way to learn this three-step design process is by seeing it applied to an actual house.  The design below involves a house that has a cripple wall.  It is a two-story house of 3,000 square feet, with a 1,500-foot foundation footprint, weighing 90 pounds per square foot.  It has a composition shingle roof, stucco siding, and plaster interior walls and ceiling.  You can also substitute a close estimate of the weight of your house.

DEFINITIONS

SqFt=Square Foot

psf = Pounds per Square Foot

plf = Pounds per Linear Foot

Bolts: Attach the bottom of the cripple wall to the foundation.

Shear Transfer Ties: Connect the floor to the top of the cripple wall.

Plywood:  Braces the cripple wall and prevents it from collapsing.

 

CALCULATIONS

3,000 SqFt two-story home with a foundation footprint of 1500 SqFt x 90 psf x 0.2Gs (G-force and weight come from engineering calculations found in Standard Plan A) = 27,000#.  This equals the pounds of earthquake force the house must resist

Divide 27,000# by 2 because half the force will strike one side and half the force will strike the other.Cripple wall retrofit designed with bolts and plywood

House must resist 13,350# of force on each side.

 

BOLTS NEEDED

13,500# = Force each side must resist.

Bolts with Mudsill Plates resist 1350#.

13,500#/1,350# = 10 bolts.

Number of bolts is sufficient to meet anticipated earthquake force.

 

PLYWOOD NEEDED

13,500#  = Force each side must resist.

With nails spaced 2″ apart, plywood can resist 870# plf (pounds per linear foot).

13,500#/870# = 15.52 linear feet of plywood needed each side, rounded up to 16.

Linear footage of plywood is sufficient to meet anticipated earthquake forces and more.

Note- Nails that are installed further apart will increase the linear footage of plywood needed, though the number of bolts and shear transfer ties will remain the same.

 

SHEAR TRANSFER TIES NEEDED

13,500# = Force each side must resist.

Shear Transfer Ties can resist 600#

13,500#/600# = 22.5 Shear Transfer Ties each side, rounded up to 23.

Number of Shear Transfer Ties are sufficient to meet anticipated earthquake force.

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