Understanding retrofit design 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 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
Engineering a retrofit for a houses without cripple walls works like this:
- Geologic research tells us our anticipated ground movement acceleration, measured in Gs, will be 0.186 Gs.
- 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 a house weighs 80,000 lbs and we multiply that by 0.186gs (our anticipated ground acceleration) we get 14,480 pounds. We therefore need to install enough hardware to resist 14,480 pounds of force. We look through the hardware catalog and see a 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.
This webpage shows how to install this hardware and what it looks like.