Standard Home Construction

The photo on the below is a diagram of how typical older, wood framed homes are constructed in our area.

On the right of the diagram, you will see a full-height concrete foundation. In these conditions the house is resting directly on top of the concrete foundation, eliminating the vulnerability of racking due to the fact that there is no existing pony walls. These homes have less weaknesses than the pony wall conditions, but are still very much capable of sliding off their foundations because they lack the proper hardware needed to resist lateral movement in the event of an earthquake.

The photo on the right is a diagram of how typical older, wood framed homes are constructed in our area.

The left side of the diagram illustrates how most Seattle area homes are built, with 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.

Project 3d Daring

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.

Project drawing

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 most situations, homes rest directly on a piece of lumber, called a mudsill, that is sandwiched between the concrete foundation and the first-floor platform (joists). If the mudsill is not attached to the foundation, the entire house could slide off of its foundation.

This problem is addressed by adding anchor bolts through the mudsill, embedded several inches into the foundation. In addition, it is very important that each anchor bolt has a bearing plate to prevent the mudsill from splitting due to the lateral force. It is always guaranteed that homes that will need a seismic upgrade will require anchor bolts.

anchor

In situations where the home has a full-height foundation, and access is limited vertically, alternative methods will be needed, such as side anchor plates. These types of anchors require five lag screws that are installed into the sides of the existing mudsill and two anchor bolts that are bolted directly into the side of the foundation. The purpose of these anchors is to resist lateral side to side movement.

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.

As previously mentioned in the first diagram above:

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.

brace

We address this vulnerability by adding shear panels, which are attached securely with very tight nail spacing, to the cripple walls to add rigidity.

Note that not shown in the diagram, in conditions where work may occur in unheated areas such as a crawlspace and some basements, round ventilation holes will be required in the shear panels. This allows air circulation throughout each stud bay to prevent future rotting. Without ventilation holes, moisture buildup can occur inside the pony walls and create rot.

The third area in which a home could fail during an earthquake is the point of contact between the first-floor platform (floor joists) and the top of the cripple wall. The only positive connection in these areas are diagonal nails, called “toe-nails), that span every few feet. As is the case when a home doesn’t have anchor bolts, this type of connection can cause a house to slide off of its foundation.

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.

connect

Additional hardware, called hold-downs, may be required in areas where overturning forces are likely. Areas where there is a limited length of wall to reinforce, or areas where there are very tall cripple walls to its length, the use of hold-downs will probably be required.

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.

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