What is an Earthquake Retrofit? How to Protect Your Home from Quakes
Ever since the Ridgecrest Earthquakes struck in July 2019, several friends and customers of mine have asked for my construction advice - they all...
3 min read
We all know that California is ‘Earthquake Country’, but did you know that nearly every neighborhood in Los Angeles is near an earthquake fault?
In fact according to the Southern California Earthquake Center (Plesch, 2007) there are over 140 active faults in Southern California. Nervous? Don’t be, you can protect your home with a Seismic Retrofit. If your home has a raised foundation and was built before the 1970s, a Brace + Bolt Seismic Retrofit might be right for you. Our short article will help explain what a Seismic Retrofit is, how long the installation will take, and why retrofitting your foundation is a smart idea.
What Does Brace+Bolt Mean?
An earthquake bolt down is a seismic retrofit that helps reinforce the connection between your home’s foundation and the home’s frame. This type of seismic retrofit is an economical way of preventing your house from ‘sliding off’ its foundation. The ‘Brace & Bolt’ namesake comes from the bracing of the frame/subfloor and the bolting to the foundation that will be required as part of the seismic retrofit.
URFP Plates Installed at a Recent Foundation Retrofit Project
Let’s examine the wood-frame of the typical home in Los Angeles, and remember that the goal of a Brace & Bolt Retrofit is to reinforce the connection between the framing and the foundation. Every wood-framed house has four (4) main framing systems, including the below.
- Roof System – Roof framing, roof rafters, and ceiling joists
- Wall System – Wall studs of 2x6 or 2x4 dimension lumber.
- Floor System – Floor Boards of 2x8 or 2x12 dimension lumber, held up by 4x8 girders. The top of the floor joists usually consist of either plywood or diagonal 1x6 boards.
- Foundation System- Concrete stem walls with footings, with piers holding up the floor system.
During an earthquake your home can experience lateral shear forces that will cause it to sway in a side-to-side motion. The connection points in the home’s frame will take the brunt of these forces, something that they were never designed to do, they are not intended to resist lateral force. If the lateral shear loads are big enough, the floor system could separate from the foundation as a result, causing the house to ‘slip off’ the foundation. This type of earthquake damage is extremely expensive to repair and yet relatively affordable to prevent. In the picture below you can see how this home’s foundation was damaged after the recent Ridgecrest Earthquakes. The Retrofit Pros team visited both Ridgecrest, CA and Trona, CA to observe damaged structures and to learn how to prevent earthquake damage in homes. Our team observed several homes with foundation damage, it was the most common type of structural damage we encountered in Trona, CA.
Common Foundation Retrofit Techniques
Retrofit contractors have traditionally relied on metal hardware components from Simpson Strong-Tie and Mitek USP to retrofit homes with raised-foundations. These metal connectors make retrofitting a wood frame structure much easier. These foundation plates ensure a solid connection between the concrete foundation and the wood-frame of the home. Below are some examples of the different foundation plate hardware used when bolting down a foundation.
These steel connectors are truly technical marvels, however, it is important to note residential homes are of wood-frame construction and are only as strong as the current condition of the lumber. This means that if your home’s frame has termite damage or rotting wood, your foundation is in need of repair, you don’t just want to rely on the steel plate connections. Connecting steel brackets and plates to damaged lumber will diminish the effectiveness of a brace & bolt foundation retrofit. A true construction expert will give careful consideration to the condition of the existing wood members in the floor system.
Foundation Retrofit Components
As a general contractor specializing in seismic retrofits for residential homes, we use four main components to create a better connection between the foundation and the frame. A Brace & Bolt Foundation Retrofit uses four main components. Our team prefers the Simpson Strong-Tie line of products and so we’ll show you just some of the products we use when we retrofit foundations.
Retrofit Hardware Used by Retrofit Pros:
- 1 - Simpson Foundation Plates (URFP/FRFP)
- 2 - Simpson A35 Framing Angles
- 3 - Heavy Angle Transfer Angle (HSLQ)
- 4 - TITEN HD Concrete Anchor
Wood-Frame Strengthening Techniques
During our larger soft-story retrofit projects our team found a need to create a framing technique to strengthen the existing wood frame, we dubbed it the ‘Rodriguez Wood-Frame Transfer Technique. No other company uses this type of framing technique to bolt down a house. We use this framing technique to strengthen a buildings wood frame and to create a predicable load path transfer of seismic forces. This technique is so strong and dependable, it allows the transfer 27,000 lbf pounds of force along the load path. Because it is a proprietary construction method, we can only share how this framing technique works during our in-person meeting.
Want to Learn More?
References:
Plesch, A.; et al. (December 2007), "Community Fault Model (CFM) for Southern California" (PDF), Bulletin of the Seismological Society of America, 97 (6): 1793–1802,
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