Understanding stem wall or no cripple wall in seismic retrofit designs is by far the most important tool you can have in evaluating contractors’ bids and determining exactly what your home needs. A bad design means an expensive and ineffective retrofit. A good design means an affordable effective retrofit. If you install too much hardware, or not enough, or the wrong type of hardware, or in the wrong places, or in the wrong way, you can end up with an expensive retrofit that will not withstand a strong earthquake.
It is Simpler Than You Can Imagine
Seismic retrofit designs for a house without cripple walls works like this:
- Find out the anticipated ground acceleration for your location. It can vary from 0.10 to 3.5 depending on the fault and where the house is located. The USGS has maps that show the anticipated ground accelerations for different locations.
- Multiply this by the weight of the house as determined by these exhaustive engineering calculations.
- The result is the pounds of force your retrofit must resist.
All hardware is designed to resist a certain amount of force measured in pounds. Install enough hardware to resist this much force, and our retrofit is complete.
For example, if the USGS maps tells us the anticipated ground acceleration will be 0.186. If a house weighs 80,000 lbs. and we multiply that by 0.186gs we get 14,480 pounds. We therefore need to install enough hardware to resist a minimum of 14,480 pounds of force. We look through the hardware catalog and see a particular foundation anchor (bolting hardware) that can resist 1,480 pounds of force. This means we need ten of these foundation anchors for this house. We also find shear transfer tie hardware that can resist 740 pounds of force, so we install 20 of those. That’s all there is to it!
In the example below, we need to resist 16,000 pounds of force. In order to retrofit the house, we need foundation anchors that can resist 8,000 pounds of force on each side. We also need shear transfer ties that can resist 8,000 pounds of force. Add that hardware and our retrofit is done.
The No Cripple Wall Retrofit VIDEO shows how to install this hardware and what it looks like.
This is how seismic retrofit designs work.