Should I Retrofit my Old Foundation?
Foundation cracks are a common concern. Historic Building preservation structural engineer Nels Roselung wrote an excellent article on foundation cracks that will answer most of your questions on this subject. In summary it says, Foundation Cracks are caused by settling of the ground, not any defect in the concrete itself. When a house has foundation cracks, doors often stick in different seasons. Cracks in the plaster or sheet rock walls seem to get larger and smaller and, once repaired, simply return a few months later. Expansive soils that expand and contract, depending on the amount of moisture in the ground, cause this to happen and also cause foundation cracks. Expansive soils are found throughout the country. You can read more here about expansive soils
If your house does not have any serious tiling of the floor then the foundation is holding up the house just fine.
Foundation Cracks are Caused by Expansion and Settlement
Hold both ends of a pencil and snap it in two. This is equivalent to expansion of the soil. Snap it in a downward direction and that is settlement. The point at which it begins to crack is analogous to a foundation crack. Foundation cracks are a normal part of settling and unless the crack is rapidly expanding there is nothing to worry about.
Concrete is very weak when subjected to expansion and settlement.
This is irrelevant from a retrofit point of view because earthquake forces put the concrete in compression, not expansion or settlement. Foundations put in compression do very well. Imagine pushing a pencil on both ends towards the center. No matter how hard you push, the pencil will not break.
If you see a half inch crack, it means the foundation heaved upward or sunk downward because of the expansive soil. Expansive soil is the rule rather than the exception in the Bay Area such that cracks in foundations are quite common.. Cracks like this are not a cause for concern:
If the foundation is holding the house up, it is dong exactly what it is supposed to do and need not be “fixed”. If the crack is irrelevant in terms of retrofitting the house, then the foundation will serve in that function as well. In short, if a foundation with a crack will and can perform as exactly intended by the building code, why repair something that is not broken.
Some foundations in the San Francisco Bay Area have minimal amounts of mortar, used contaminated water from the bay, are made of brick, or have other very serious structural issues that need to be addressed before a retrofit is performed. In these cases the foundation must either be replaced or supplemented with segments of good foundation.
Old Foundations and Retrofit Foundation Bolts
Fortunately, foundations with serious structural problems are very rare and it can be presumed that your foundation is just fine if the house is attached to it with a retrofit. Previous earthquakes and laboratory tests have proven this over and over. All retrofit building codes don’t even address working on the foundation because it is assumed they will not be a failure pont.
Tests and Codes
Tests done on existing foundations by the Structural Engineer’s Association of Southern California supported their conclusion: “The difference in the strength of the concrete did not appreciably affect the performance of the foundation anchorage systems (Bolts) which work with new foundations just as effectively as with weaker concrete. The predominant failure mechanism was the wood sill plate in both the good and weak concrete foundations.”