Seismic Retrofitting Houses without Cripple Walls
Seismic Retrofits of Houses without Cripple Walls Transfer Earthquake Forces into the Ground
Basic to understanding what and how much seismic retrofitting your house needs, you should understand the earthquake engineering that applies to the seismic strengthening of your home and to bolting your house to the foundation.
The crawl space photograph below is to help you visualize what it looks like under the floor you walk on and how the floor joists, shear transfer ties, and foundation anchors that suffice as foundation bolt substitutes, all work together to protect a house from earthquake damage . When a house does not have a cripple wall the floor is to close to the foundation so that the floor can be connected to the foundation directly with steel. This direct floor-to-foundation steel connection makes these retrofits extremely effective.
Foundation anchors do the same thing as foundation bolts. There are four types of foundation anchors and three types of Shear Transfer Ties. Any type of shear transfer tie that is rated to resist the earthquake force the building code tells us to expect will be effective. The specific type we use is determined by what we find under your house.
A Sound Load Path in Every Effective Seismic Improvement
The colored arrows below correspond to the sliding motion of the floor caused by an earthquake. The load path transfers the earthquake force from the floor, through the shear transfer tie, into the foundation anchor, into the foundation, and finally into the ground where the earthquake force dissipates. This process is known as the load path. The colored letters below correspond to the way earthquake forces are transferred into the ground.
Different Types of Foundation Anchors
Shear Transfer Ties
The houses below had a bolt every 4 feet but the end joists were not attached to the mudsill with nails or Shear Transfer Ties.
Below is an image of what it looks like under a house without a cripple wall. The red arrows represent earthquake forces trying to push the floor off the mudsill. If this happens, the house will slide off the foundation. The joists that sit on top of the mudsill are called end joists on one side of the house and rim joists on the other side
The way this sliding is avoided is by attaching the end joists and rim joists to the mudsill with Shear Transfer Ties, sometimes known as framing anchors. A large variety of shear transfer ties are available and a contractor must know which one to use when confronted with every imaginable framing configuration. Given many houses were built without a building code, the varieties of floor framing are legion.
Below are the types of Shear Transfer Ties available.
L70, L90, LS70, LS90, A23, A35,
LTP4, H10R, H10, H2.5, lag bolts
Each Shear Transfer Tie, also called a framing anchor as expressed in the drawing above, can resist a certain amount of force. You want this hardware, or a combination thereof, to resist the force that will be attacking your house.
Below is a photograph of a single L90 Shear Transfer Tie. It is nailed at the back to the floor framing and at the bottom to the mudsill. Shear Transfer Ties transfer earthquake force into the mudsill
Types of Shear Transfer Ties
There are many types of Shear Transfer Tie hardware with different strengths. Some are suitable for retrofits and some are not.
f you would like an evaluation of your building or home, please fill out this form. I will take a look on Google maps and email you back information relevant to your building or house and provide approximate costs. If you are doing your own retrofit or have any questions about your house I am happy to help you free of charge.