This article could never have been written without the insights and guidance provided by research scientist and engineer Ed Keith. He has 44 years of shear wall testing experience with the American Plywood Association, the largest shear wall testing laboratory in the world.
Soft Story Retrofits using Rotation will Save you Money.
This is an advanced retrofit procedure. It is advised that you not only read the material, but spend some time studying the images and the linked-to webpages. This webpage describes in simple terms a fairly complex retrofit principle. It should not be attempted without the aid of a retrofit professional.
Soft Story Conditions
A garage soft story condition is caused when a floor is supported by narrow walls or posts at a garage door opening. If you imagine the floor above a two-car garage, weighing approximately 15,000 pounds, rocking back and forth on top of these post or narrow walls, it is easy to see how the it can collapse. This can be addressed by using a moment column or moment frame, or through the use of shear walls as described below.
Using Shear Walls to address Garage Soft Story Conditions
If you install a shear wall on the back of the garage, and if the floor above the garage is more or less either a square or a rectangle and the depth of the garage (from garage door opening to the back garage wall) does not exceed the width of the garage, you can protect the building by putting shear walls on the back and two sides of the garage, without doing anything to the garage door opening itself. Using shear walls to protect a building like this is known as using the principle of rotation.
- This image represents a floor with a garage below it where wall line “D” is the garage door opening. The view is as if you were looking down at the garage from above.
- The red lines represent earthquake forces, which will attack the garage from all sides.
- The dark line on the perimeter represents the foundation, which is missing on wall line “D”.
- The red arrow in the upper left is pushing down against a side wall of the garage. This movement is resisted by Shear Wall A.
- Another arrow in the upper right is pushing down against the opposite side wall of the garage. This movement is resisted by Shear Wall B.
- A third arrow in the lower left is pushing against the back wall of the garage on wall line “C”. This movement is resisted by Shear Wall C.
- The fourth arrow, on the left, is pushing on the garage door opening on wall line “D”. There is no shear wall to resist this force. What happens?
- The entire floor tries to twist or rotate, as shown by the rotating circular black arrow, pushing down on the right and up on the left as it rotates . This rotating action transfers all of the force to the back wall and side walls of the garage, where it is resisted by shear walls A- C. This retrofit procedure is using the principle of rotation.
The remarkable thing about using rotation is that this resistance can be achieved by only using 3 shear walls instead of 4.
With forces attacking from the side, 1/2 the force goes to the floor above the garage door opening on side “D”. As mentioned earlier, this force causes the floor to twist or rotate and transfer the force into side “C” at the back of the garage. The other 1/2 of the force attacks the floor at the back of the garage directly, which is likewise resisted by side “C”. The shear wall on side “C” must resist earthquake forces from both sources. In other words, shear wall “C” must resist all the force attacking the garage from the side. For this reason the shear wall at the back of the garage must be twice as strong as the shear walls on the sides of the garage.
In the photograph below it looks like the earthquake pushed against the front of the garage and caused the walls on either side to almost collapse. What really happened is that the garage roof rotated and twisted, pushing the tops of the narrow walls to the left as shown by the illustration on the right.