This article was written by contractor Howard Cook and Private home inspector David Heilig.
Very little work has been done on the performance of brick foundations. If you Google nothing comes up that is based on observed damage or even testing The only investigation work that has been done, which is extremely limited, was by structural engineer Thor Madison in his book EARTHQUAKE STRENGTHENING: For Vulnerable Homes. Another source is this very limited report without any reference to the author or sources so the information is suspect.
He states “I am not aware of brick foundation failures that lead to a collapse the wood structure.” I too am not aware of failures of brick foundations, nor have I met anyone who has but I believe this is more a result of their having been very little research done on the performance of brick foundations. Perhaps 1 out of 500 houses we see has a brick foundation: making their relevance fairly insignificant.
So little research has been done that Mr. Matteson’s book contains only four short paragraphs discussing this important topic. We simply do not have any research reports that documented the performance of brick foundations. I imagine this is because there are so few brick foundations or professionals who have any interest in this topic. There is no way short of very expensive customized laboratory testing to figure this out. The only thing we have is the “screwdriver test:”
- The mortar is severely compromised and practically disintegrating. In these cases the mortar resembles something like sand. A screwdriver will go through the mortar joints from one edge to the other.
- A screwdriver can be inserted into the mortar joints with some ease.
- A screwdriver can chisel away at the mortar with some difficulty.
- It is very hard to penetrate the mortar with a screwdriver.
In the first case there is no reason to believe the brick foundation won’t turn into a pile of loose bricks. The question is whether or not the building will lose vertical support once this happens after a retrofit has been performed. In other words, will the sheer walls still hold the building up off the ground.
There are many variables, not the least being the quality of the mortar and its adhesion to the brick. When you go through the process of figuring out what conditions will be safe and what conditions will result in failure, you always end up at the same place. You have to know, or assume, what materials are in place and you have to test the materials for strength and degradation. Nobody knows what materials are in place and there is no reasonably accepted method of testing. What you really need to know is “What is the value for the frictional resistance between mortar and brick”?
We know the surface area of the bricks, and we know how much mortar binds the bricks together, however, we do not know how much lateral force produced by an earthquake the mortar bond can resist.
If the mortar passes the “screwdriver test” does that mean it will perform well in an earthquake? Will the bolts in the brick simply cause more bricks to become dislodged? Certainly some bolts are much better than no bolts but is very difficult to know how much one gains relative to the cost. We hope so. But we have no evidence either way.
The foundation below was tested by a construction materials testing laboratory and it was determined that it was suitable for bolting.
Below are some very unique photographs from a brick foundation retrofit done approximately 20 years ago. The photographs are black and white because that is all the engineer had when he took the photographs. Bay Area Retrofit has the structural plans for this kind of retrofit and has found this system very cost-effective in some cases.
Retrofits like this require very complex engineering and very skilled retrofit contractors.