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Pouring a New Foundation

Unfortunately, sometimes the foundation isn’t stable enough to hold bolts, so we have to pour a new concrete stem wall. A recent retrofit job illustrates the point. When I first visited this residence, I was pleased to find two rare conditions for a retrofit — a very comfortable clearance and a lighted crawlspace. The owner wanted recommendations for strengthening the subfloor framing and a labor-and-materials bid for bolting his cripple walls to the existing concrete foundation. This particular house sat on a sloped lot, but instead of stepping the foundation, the builder simply sloped the stem wall to follow the grade — making it much more vulnerable to slipping in a quake. The cripple walls were inadequately bolted and unbraced. Even worse, the foundation concrete had been mixed with unwashed aggregate and beach sand and was in very poor condition. It was obvious the existing concrete would not hold an anchor bolt, let alone hold up under seismic stress. One alternative was to raise the house on large steel I-beams, remove the existing foundation, then form and pour a new one — an expensive proposition. But once we determined that the existing floor was level, we decided instead to pour a new stepped foundation tight to the inside of the existing one (Figure 3).

Figure 3. In cases where the old foundation is too weak to hold anchors, it’s necessary to pour a new wall. In the case shown here, a new stepped stem wall was poured tight to the existing one, with new cripple walls built on top. Simpson A35F clips secure the cripple wall to solid blocking between the floor joists above (see photo). We first excavated for a new footing, slightly "undermining" the existing footing. Then we drilled and placed #4 rebar dowels in the old stem wall on 4-foot centers. In the new footing trench, we installed continuous #4 rebar around the perimeter, and attached it to the dowels with short vertical pieces of rebar. After pouring the new foundation, we plated and framed new cripple walls around the perimeter. These were bolted to the new foundation with standard 5/8-inch J-bolts. To connect the floor to the new cripple wall, we blocked between the existing floor joists directly over the cripple wall and used Simpson A35F metal brackets to connect the blocking to the double top plates. We covered the cripple wall with 1/2-inch structural-1 plywood edge-nailed 6 inches on-center, with 12-inch centers in the field.

Posts and Piers

Many of the houses we work on have a center girder supported by a series of posts and piers. The posts are typically resting on the piers with no mechanical anchor and are attached to the girders above with nails only. In that case, we install new 2-foot-square by 1-foot-deep pier footings between existing posts. We also replace any posts located under girder splices. To form the new pier footings, we lay a grid of four #4 rebar pieces beneath a precast block that comes with embedded straps for attaching to the post (Figure 4).

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Figure 4. New post-and-pier supports are built in place using precast pier blocks set in a 2-foot-square concrete footing. Note how the original post, at right in the photo, has no mechanical anchorage. After pouring the new piers, we install new posts, strapped at the bottom and connected at the top with a plywood gusset. Finally, we use Simpson H-series clips to tie the floor joists to the girders.