Download PDF version (296.7k) Log In or Register to view the full article as a PDF document.

Retrofit Evaluation

When I inspect a house for a seismic upgrade, I start with the foundation. The foundations I see range from poorly mortared sandstone and brick to unreinforced block or poured concrete. In some cases, there is no foundation at all — just a series of wood posts resting on masonry pads set on the ground. (In such cases, it’s just a matter of time before the whole house is sitting on the ground.) When there is a foundation, I check the strength of the concrete, block, brick, or stone by drilling a test hole or chipping at the surface with a steel bar. I have to find out if the masonry is hard enough to hold a bolt or other hardware. Working up, I next explore the condition and placement of existing hold-down bolts attaching the mudsill to the foundation. I also look for rot and termite damage to wood members. In many cases, there are no sill anchors at all; in other cases, the mudsill is too rotten for the anchors to do any good. Many homes in our area have cripple walls — short stud-framed walls that sit on the stemwall foundation and support the floor framing above. Strong cripple walls are critical for earthquake resistance. Not only must they be anchored to the foundation and secured to the framing above, but they must also receive shear panel sheathing to handle the severe lateral forces that a quake causes. At the top, the cripple wall has to be secured with hardware clips to the floor framing above. I inspect the cripple walls for rot, and make sure they are framed properly to receive shearwall panels. Probably the most common condition I run into, however, is an unsecured mudsill sitting on a foundation with the floor joists resting directly on it. Such low-clearance situations make retrofit work difficult and typically require special hardware.

The Basic Retrofit

Once I have defined the problem, the next step is to choose the proper metal connectors to tie the foundation to the mudsill or cripple wall plate. Every retrofit job is different. Let’s look at a basic job first — a firm masonry foundation with a sound cripple wall, having insufficient anchors but plenty of room to work. For areas that can be easily reached by an impact drill, we typically drill 5/8-inch holes through the plates and about 41/2 inches into the foundation. We then insert lengths of 1/2-inch threaded steel rod, epoxied into place (see Figure 1). This is the least expensive solution.

497sc6.gif (14107 bytes)
Figure 1. Where there is good access, the least expensive retrofit involves threaded rod anchors epoxied in place, plywood shear paneling, and framing anchors between the top of the cripple wall and the floor framing.

Advantages of epoxy.

Because most of the foundations we see are made of old concrete, we prefer the epoxy anchoring system to expansion bolts, which might crack the stem wall as they expand. We use Simpson’s Epoxy-Tie Adhesive Anchor system (Simpson Strong-Tie, 4637 Chabot Dr., Suite 200, Pleasanton, CA 94588; 800/999-5099). The epoxy costs $22 for fifteen 5/8-inch-diameter holes. The dispenser costs over $100, but can be rented for around $5 a day. Before epoxying either threaded-rod or the Simpson anchors, it is important to clean out the predrilled holes. We scrub the hole with a nylon brush and then blow out the residue with compressed air. The hole must be perfectly clean: Any dust will reduce the epoxy’s bond. It’s also important to monitor the mixing of the two-part epoxy. A consistent gray color indicates a proper mix, which means the epoxy will cure to full strength. After filling the hole half full with epoxy, we insert the anchor and slowly turn it until it contacts the bottom of the hole. We wait a full day until the epoxy cures before disturbing the anchor.

Cripple Walls

Once the epoxied anchors are in place and the sill is bolted down, we turn our attention to the cripple walls. Here, we follow the guidelines provided by the City of Santa Barbara, which recommend that 4-foot-wide 1/2-inch structural sheathing be installed at corners and every 25 feet along the house’s length. Shear panels are also installed at the sides of any access doors or vent openings. The city also recommends that the length of each shear panel should be at least twice its height. The required fastening schedule is 8d nails 6 inches on-center on the edges and 12 inches on-center in the field. This work goes quickly with a pneumatic nailer. The next step is to drill 3-inch holes into the plywood at the top and bottom of each stud bay to allow for ventilation. To keep rodents out, we cover the holes with 1/4-inch hardware cloth. Finally, at the top of the cripple wall, we usually install Simpson H1s or H5s to every other joist to secure the floor framing to the cripple wall plate.

Limited-Access Retrofit

Limited access — when an unbolted sill sits right on top of the stem wall in a tight crawlspace, for instance — usually dictates the use of more expensive connectors. Occasionally, we get lucky and find that the floor is framed with 2x12 joists. This gives us enough room to use our Hilti TE15 right-angle hammer drill in the restricted area between floor and plate. Then we can drop in an epoxied threaded-rod anchor or a Simpson RFB retrofit bolt. More often, however, the joists are smaller, so we have to use a Simpson FA6 or FA8, an

L-shaped 12-gauge steel connector that rests on top of the plate and laps over the side of the stem wall (Figure 2). Figure 2. The Simpson FA series anchors are useful in tight quarters for anchoring the mudsill to the foundation wall. The side plate bolts to the foundation wall with epoxied threaded rod; the top can be nailed with a palm nailer. This configuration allows us to drill into the concrete horizontally, instead of having to position the drill upright. The top of the FA anchor can be nailed to the plate with a pneumatic palm nailer.