Before going any further it is important that you understand what a shear transfer tie is. Please watch this video to find out. Start at minute 15:30
When there is sheet rock on the ceiling, there are two ways to connect the floor to the shear wall. One way is to remove a strip of the sheet rock 12-18 inches wide next to the top of the wall and use conventional shear transfer ties. The other way is with lag bolts.
Here you are looking at someone driving a lag bolt up through the top of the cripple wall and into the floor framing. This is for illustration purposes so that you can see how the lag bolt is driven through the top plates into the floor framing which is normally hidden by sheet rock.
A perfect example of a wall where lag bolts will work as shear transfer ties.
The drawing below shows how a lag bolt can do the same thing as a shear transfer tie without removing the plaster. Here you are looking at it from the end.
If for some reason there is no floor framing above the cripple wall for the lag bolts to bite into, it will be necessary to remove the a strip of sheet rock from the as shown here and use conventional shear transfer ties.
How many lag bolts?
Lag bolt strength is defined by the inches of penetration into what is called the “main member.” The main member in this case is the floor joist that the lag bolts are driven into.
A lag bolt with 1 ½ inches penetration can resist 300 pounds of earthquake force.
A lag bolt from 1 1/2″-2 1/4 inch penetration can resist 450 pounds of earthquake force.
A lag bolt with over 2 1/4 inch penetration can resist 550 pounds of earthquake force.
Here is an example:
Let’s assume, based on this page on plywood science, you have a 10 foot long shear wall and you have nailed the plywood with 8d common nails 3 inches apart on the edges. This shear wall can therefore resist 5,500 pounds of force.
With an 8 inch lag bolt we will need to go through the 2″ thick lower and 2″ thick upper top plates, which means the lag bolt will penetrate 4 inches into the joist. A lag bolt with over 2 1/4″ penetration gives us 550 pounds of resistance. Divide 5500/550 and we discover we need 10 lag bolts for the shear transfer tie component of our shear wall.
Nailing the bottom and top plates together. The interface where the upper and lower top plate touch is called an “unsecured shear plane” and there is potential slippage here and must be addressed. The way this is done is be nailing the two top plates together with 12d common nails that can resist 188# of force each. This connection must equal the shear transfer ties connection. In our example we put in 10 shear transfer ties that can resist 5500# of force. We divide that by 5,500#/188# and we discover we need 30 nails.
Lag bolts are a far superior way because when they are used it is not necessary to remove the sheet rock from the ceiling, replace the sheet rock, texture it, and paint it to match the sheet rock next to it. Even when this is done by a highly skilled (and expensive) professional, imperfections are always visible and you can never make the repair completely match.
Most contractors will leave this up to you, which can add considerably to the total job cost.
When hiring a contractor ask them how they intend to attach the shear wall to the floor once they take the sheet rock off the wall. If they tell you lag bolts, you have a professional with experience. If they tell you “I need to tear out part of the ceiling to do it,” you will be facing a future expense with patching the ceiling.