From the previous page we learned that shear walls based on different materials do not function together. Let's look at an example.
In the San Francisco Bay Area construction market, the
price of property forces owners to buy below their expectations then look
to remodeling to rectify the situation. This means work for contractors,
but it can also mean problems if you don't correctly identify the lateral
force-resisting system. The diagram below highlights these
problems…
| Remodeling Project #1 | |
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| In this remodel, the addition relies on a resheathed rear wall for lateral support. |
In this plan, a 1950's style home is expanded by adding a strip of building to the rear. The original construction was cement plaster over tar paper on the exterior and wood lath and plaster on the interior. The new portion is cement plaster over plywood on the exterior with gypboard on the interior. The original double sliding door and kitchen window will be removed to create pass-throughs between the buildings. The roof will be disassembled and a new ridge extended to make one roof.
Problem: The old exterior wall finish needs to be removed so the studs can be finished with gypboard.
Solution One: Since the original cement plaster functioned both as a finish and a shear-resisting material, new plywood should be installed prior to the gypboard to recoup the lost lateral load resistance. New hold-downs and posts should be added as required at the ends of the sheathed wall sections. Threaded rods can be epoxied into the existing foundation or a dead man (a block of concrete poured under the existing footing) can be installed to resist the hold-down tension.
| Remodeling Project #2 | |
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| In this version, a new wall is built along side the existing wall. |
Solution Two: Leave the existing lateral force resisting system alone and build a new wall and new footing adjacent to the existing footing. The doorways and pass-throughs would be thicker visually, but not unreasonably so. Savings could result from not digging into the older structure. A cleaner job overall.
Here's another example - a variation of the above plan…
| Remodeling Project #3 | |
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| Looks easy, but you could get into trouble. |
In addition to other work, the owner would like to open up one side of the existing structure with a solarium leaving the solid wall in the new portion to recoup the loss of lateral strength.
Solution: The original stucco finish and new plywood wall will not work together to share lateral loads. Strip the entire side and sheath with plywood then stucco to match.
