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.
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.
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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.
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.
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.