Standard Home Construction
The photo on the below is a diagram of how typical older, wood framed homes are constructed in our area.
What is a seismic retrofit?
A seismic retrofit is the technical process of transferring the lateral forces generated from an earthquake in a building structure down to the foundation. This process is typically accomplished by modifying a buildings structural system to reduce or eliminate known seismic deficiencies.
We need to address the three main weaknesses in how a structure (home) could fail in the event of an earthquake: racking, sliding and overturning.
In order to maintain a continuous load path from the house to the foundation, anchor bolts, shear panels, shear transfer ties, and sometimes hold-downs, may be required to accomplish an effective retrofit.
How to Retrofit a Home
If you have a home that was constructed prior to 1976, in most cases, your home is resting on its concrete foundation by nothing but gravity. Prior to that date, there were no building codes that required newer homes to be attached to the foundation. Evergreen Seismic address those key major weaknesses by attaching the first-floor framing platform, down to the foundation by using the ABC’s of retrofitting; Anchor, Brace, and Connect. By completing this work, it significantly improves the chances of a total loss of your home being displaced from the foundation and most importantly, life safety. Below is a detailed depiction of the ABC’s.
In rare situations, homes may have existing anchor bolts. These usually are not up to current building codes, due to their size and spacing, and may even be rusted through because they are not galvanized.Call us today for best seismic retrofitting services.
Most Seattle area homes have short wood-framed walls in the basements and/or crawlspaces. These walls are called “cripple” or “pony” walls. They span the distance between the top of the concrete foundation to the first-floor framing platform of the home, and can vary in height.
This type of construction is the most susceptible to failure in an earthquake event, due the fact that the entire weight of the house sits directly on top of these short, unsecured walls that have little shear value. These unreinforced walls fail by racking, or folding over, due to excessive lateral forces during an earthquake; if these walls fail, the potential of a home falling off of its foundation is extremely high.
We address this vulnerability by installing shear transfer ties, or framing clips, which resists lateral forces, similar to anchor bolts. Sometimes, if there is a full-height concrete foundation, this area will be from the rim joist to the mudsill.
Hold-downs are a piece of hardware that is installed at the end of shear wall to resist uplift or overturning forces imposed on the wall do to “in-plane” lateral load applied at the top of the wall. They are drilled, cleaned and epoxied several inches into the concrete foundation with threaded rod, and bolted into the existing framing.