Foundation Cracks

By Howard Cook, Founder and General Manager of Bay Area Retrofit

Should I Retrofit my Old Foundation?

 

 

Foundation cracks and settling are a common concern. Historic building preservation structural engineer Nels Roselung wrote an excellent article on settling and foundation cracks that will answer most of your questions on this subject.  In summary it states foundation cracks are caused by settling of the ground, and not any defect in the concrete itself.  When a house has foundation cracks and settling, which are the rule and not the exception, floors become uneven, cracks appear in stucco and plaster walls, doors often stick in different seasons, existing 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.  Expansive soils are found throughout the country, especially in the Bay Area.  In this article you can read more here about expansive soils  

Foundation cracks or no foundation cracks wouldn't make a difference here.

No Retrofit will Work here.

If your house does not have significant tilting of the floor where you can see actual and obvious movement of the foundation from its original location, then the foundation is holding up the house just fine.  If the retrofit is connected to sound segments of foundation, even if it has settled significantly and  there are a few cracks in it, then attaching the house to this foundation and protecting the house from earthquakes will not be a problem.

How Expansive Soils cause Cracks and Settling

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.   This is how foundation cracks are created.

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.” 

 

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