Understanding your Seismic Retrofit Plan
The webpage only applies if your proposal was written by Howard Cook rather than Jeff Bailey. If Jeff wrote your proposal watch this video.
Your proposal contains a drawing with a description of the material we will use and where we will put it. This is by far the most important part of your proposal and if you understand it everything else will make sense.
I have put the information in colored boxes so you can find the information more easily on the page. I suggest you print the drawing portion of your proposal and read it along with this description of its contents.
The information in the red box is the most important because it is the engineering/mathematical and geological background upon which your entire retrofit is built. It tells us how many pounds of earthquake force your house must resist and where the retrofit components must go. In this example the house must resist 16,119 pounds (#) of earthquake force and the retrofit has been designed to resist 17,400 pounds of earthquake force.
The blue box tells how many pounds of earthquake force each of the different type shear transfer ties can resist. We need to use enough of a certain type, or a combination of several types, to resist 16,119# of force
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 16,119 pounds of force
The black box tells us how much earthquake resistance one linear foot of plywood has. We must install enough linear footage of plywood to resist the 16,119
The Simplicity of a Retrofit Design
Figuring out how much hardware or plywood we need to install is very simple: Divide the pounds of force you must resist by the force a certain type of hardware can resist and you get the information you need. If we want to know how many 4LTPs to use in our example we simply divide 16,119# pounds by 670# (the pounds of force a 4LTP can resist) for a total of 24 4LTPs needed for this retrofit. We do the same for the other components
Bolting can consist of one type of bolting hardware or a combination thereof depending on existing site conditions. Many types are made for this very reason. The same is true with Shear Transfer Ties; you just don’t which one you will be using or which combination thereof until you will need until you have studied the house for awhile at the time of retrofit installation. Plywood is plywood, you just need to nail it to resist as much earthquake force as possible.
The orange box tells us how much overturning force the Hold Downs must resist.
This means at wall line F in the sketch this will be the location of the different retrofit components described in the column for wall line F.
More precisely along wall line F we will need two FA10s + Four 90L or Five 4LTPs or Twelve 23A We say “or” “or” “or” because no two houses are built alike and we just don’t know which Foundation Anchor or shear transfer tie or combination thereof we will be using until we get under the house. We carry each type with us in our vans.
The same things applies here. “Along wall A” in the first column we need 4′ Plywood, and 3 bolts (in this case 1/2 inch bolts), 4-90Ls or 6-HT10Rs or 6-LTPs or 7-23As or 8 Lags plus 2 Hold Downs where on the drawing you see the letter A in the addition. . Again, say “or” “or” “or” because no two houses are built alive and we just don’t which shear transfer tie or combination thereof we will be using to resist the 16,119# of earthquake force the base shear formula tells us we need to resist.