This article could never have been written without the guidance and support of my mentor and friend historic building preservation structural engineer Nels Roselund with 50 years experience. Not only is he a fine human being, but also the best seismic retrofit engineer I have ever known.
Why This Article on Brick Foundations
If you have a brick foundation a contractor or engineer may have told you to replace it. Until I researched this article I looked at brick foundations suspiciously and told people replacement is probably their best choice.
Now I know better. Now I usually tell most people not to worry. I may prove to you why you don’t need to worry either. This article might save you a ton of money.
This article was a process of discovery. I knew very little about brick foundations when I started this article. All I knew is that there were a lot of opinions which divided themselves along two lines. In one line were those who said “always replace a brick foundation”. In the other line were those who said “consult a structural engineer.” Where the engineers would land was anybody’s guess but for the most part they were in line one.
I set out to discover the facts and see where they led me.
A foundation contractor recently told me a brick foundation replacement costs on average $65,000 in the greater San Francisco Bay Area and that the average price in San Francisco proper is $120,000. I looked at a brick foundation last week in San Francisco and a contractor gave her a bid of $250,000 so I am not really sure how much it costs.
Limitations in Research on Brick Foundations
Building departments and other investigators have done very little research on the earthquake resistance of brick foundations. This is surprising because many Bay Area contractors built homes with fired brick foundation between 1870 and 1930. Homeowners replaced many of them over the years (probably unnecessarily), but foundation replacement is so expensive many are still there.
If you Google “retrofit, earthquakes, brick foundations” or similar keywords, almost nothing comes up. After multiple searches the only thing I found is this very limited report. It further promotes the misconception that brick foundations perform badly in earthquakes because, well, because they are brick foundations. No science or evidence to back up this claim. Just conjecture.
The only other reference I could find on the earthquake resistance of brick foundations can be found on page 211 of the book, EARTHQUAKE STRENGTHENING: For Vulnerable Homes consists of one sentence. Structural engineer Thor Matteson concludes, “I am not aware of brick foundation failures that lead to a collapse of wood structures.”
Mr. Matteson makes it clear that this is an opinion, rather than a verifiable fact. It does not answer the only question that matters: Has there been, or can there be, significant damage directly attributable to brick foundations?
It All Comes Down to Testing
I was hoping to find a “rule of thumb” with which one could address retrofitting brick foundations. I discovered there are an almost infinite number of variables in brick foundation quality. All these variables made developing a rule of thumb impossible.
In the final analysis, I concluded the only way to know if a brick foundation can withstand lateral earthquake forces is with testing. Just Google “materials testing labs” and tell them you want your brick foundation tested.
If you happen to be a random oddball who is curious about the history of mortar, especially in America, brick quality, testing procedures, what these tests mean, the mechanics of retrofitting homes with brick foundations, etc. you might find the rest of this article interesting.
Here is an excerpt from a San Francisco Chronicle article on brick foundations which exemplifies a common misconception.
“There are basically two things glaringly inadequate about brick foundations in general. Unlike rebar-reinforced concrete foundations that are monolithic, brick foundations are pieced together one brick at a time with no reinforcement. If the compressive load of the house shifts, the mortar joints in the brick will fail.
What is the Author Trying to Say?
A “compressive load” means pressure on top of the foundation caused by the building’s weight. “If the compressive load of the house shifts, the mortar joints in the brick will fail.” Is a fancy way of saying “if the heavy house slides on top of the foundation, the mortar will fail.” Is there any evidence of this? I certainly don’t know of any.
It is always the weakest shear plane that fails first. The base of the house, called the mudsill, sits directly on the brick. The mudsill is mortared to the brick. The contractors uses mortar to level the mudsill on the brick. Mortar will not stick to the wood mudsill because wood contracts and expands with moisture, breaking any wood to mortar bond. Therefore, for the typical brick foundation, rather than the mortar joints failing, the mudsill to brick connection will be the weakest shear plane and will be the first failure point
I am not sure why the author even mentions rebar-reinforced foundations. Most existing concrete foundations are not rebar-reinforced (rebar consists of long steel rods, usually 1/2 inch thick that are inside the concrete foundation). Foundations without rebar perform quite well in earthquakes. Tests for the earthquake resistance of bolts are all done in concrete that does not have rebar. In addition, tests done by the Structural Engineer’s Association conclusively proved that the presence of rebar is irrelevant.
Second Misconception: The Foundation Must Be Replaced If The Mortar is Bad.
Mortar exposed to weather deteriorates. However, most of the mortar is not exposed to weathering and repointing the brick is probably all it needs. Bricks can remain serviceable for centuries.
It is true that mortar exposed to weather can suffer severe deterioration. However, if the mortar is not exposed to weather the bricks can remain serviceable for centuries.
Ground movement caused by normal settling can cause cracking. Inn some cases very large cracks ,but this should have minimal impact on ability of an otherwise healthy brick foundation to be part of a seismic retrofit. Poor drainage causes cracking in most cases.
What is Mortar?
From the day the first person tried to build a structure out of soil or stone, man has been using mortar of some kind to hold these building materials together. Mortar uses several materials depending on what could be found in the area. The first sun dried clay bricks with clay mortar date 8,300 B.C. E. The Egyptians mortared the pyramids with plaster of Paris. The Great Wall of China used mortar made of rice flower and lime. The Sumerians produced the first kiln dried bricks around 3,000 B.C.E. and used bitumen, a form of asphalt. The first description for bitumen mortar can be found in the story of Tower of Babel in the old testament. The Romans created mortar using lime and volcanic ash as shown in this video.
Northern Europeans mortar originally mixed lime, sand, and water to create mortar. This all changed in 1824 when Joseph Aspid in Leeds England fired powdered limestone and clay together to make Portland cement. This additive made the mortar much harder and significantly strengthened its ability to firmly bind bricks together. The United States started producing Portland Cement 1871 and due to its superior bonding it quickly migrated all over the United States, including the San Francisco Bay Area. Even so, mortar mixing was not uniform and there is still a lot of variation in Bay Are Homes.
Let’s Get Technical
Portland Cement and Mortar Strength
A scientist at the Portland Cement Association had this to say:
“Portland cement was beginning to be added to mortar around the time (early part of the century) of that construction. So it’s hard to know if the mortar in question does or does not contain portland cement. It might. Samples could be taken and tested for an idea of components and for strength. There is a good discussion of historic mortars in a National Park Service brief:
A simple scratch test would give an idea of how strong the mortar is. By that I mean take some hard piece of metal like a nail or screwdriver and see how hard it would be to scratch the surface of the mortar. That would be a very rudimentary way of assessing mortar strength.
Others on this inquiry can speak about the relative effect of mortar strength on wall strength. My inclination is to agree that if the mortar and wall are in decent shape, they likely don’t have to be replaced (for mortar, that means it doesn’t need to be ground out and repointed).
If there is concern about the wall’s structural resistance to earthquakes, there are surface-bonded wraps or strips that can improve out-of-plane resistance: Quakewrap is one example of that type of product.”
What Do We Know About Brick Foundations?
1) Mortar binds the bricks together. The stronger the bond, the more resistant to earthquakes the brick foundation will be.
2) Brick quality has an impact on mortar’s ability to bind bricks together.
3) Mortar deteriorates when exposed to weather. The action of wetting and drying degrades mortar over time. Therefore, mortar exposed to the weather has more deterioration than the mortar below grade.
4) Mortar, like stucco, is brittle. Once it loses its adhesion to brick, it has very little shear resistance.
5) Failure of the mortar bond holding the bricks together is the primary cause of damage.
6) Mortar quality varies significantly from foundation to foundation. Only testing can determine mortar bond shear strength
Why Do We Care About Mortar Condition?
As mentioned previously, mortar holds the bricks together. Weak mortar could lose all cohesion could cause the bricks to slide on top of each and the house might lose vertical support with catastrophic consequences. I have seen more than a few houses brick foundations where the mortar is similar to sand. This is especially true in San Francisco, where contractors sometimes mixed Portland Cement with salt water.
The “Screwdriver Test”
Before conducting the mortar bond strength test recommended by the International Existing Building Code (IEBC), examine the overall condition of the mortar. Do this by prodding the mortar with a screwdriver or awl. Mortar condition falls into 1 of 4 categories:
- The mortar is practically disintegrating. In these cases the mortar resembles something like sand.
- It is fairly easy to insert the screwdriver or awl into the mortar joints but eventually it hits hard mortar.
- The mortar chips away with difficulty.
- It is very hard to penetrate the mortar with a screwdriver.
What Do We Do Now?
In case 1 the foundation should not be considered as part of a retrofit strategy.
However, you may still be able to use the footing. Check the quality of the mortar in the footing, its width, and its depth below grade. Mortar in good condition can contribute to an effective retrofit in two ways: Where the foundation touches the ground at its base and side, resistance to sliding is provided by friction when the bottom of the brick footing tries to slide on the soil it sits. Sliding movement is resisted when it the foundation rams into the soil at each end.
If the brick is in poor condition, but the footing is in good condition, I recommend building a new concrete footing next to the existing brick footing in the crawl space. Be sure and key it into the brick footing on each end. The keys allow the designer to take advantage of the soil bearing resistance and the end bearing resistance of the brick foundation.
Create a key like this: remove a segment of brick from the footing at each end of the new concrete foundation. This will create two cavities, one on each end of the new foundation.
Pour the new concrete into the footing forms. Fill these cavities with concrete as part of the new footing. To effectively connect the new footing to the existing footing, the brick foundation will need to be the form for the concrete on the inside of the footing, with wood form on the outside.
How to Check The Mortar Shear Strength Of a Brick Foundation
in cases 2, 3 and 4 of the screwdriver test, where the mortar may be in serviceable condition, it should be tested as described in the 2015 IEBC Appendix A, chapter 1 Seismic Retrofit Guidelines for Existing Buildings. the Seattle edition of this appendix can be downloaded here or you can view the IEBC edition online. The two documents are identical.
- First of all, remove a single brick as shown by the red X.
- Next, remove the mortar from the head joint in the adjacent brick.
- Third, place an hydraulic jack with a pressure gauge inside the cavity left by the removed brick.
- Fourth, exert pressure on the head brick to the left of the jack.
- Lastly, as soon as any mortar falls from the horizontal grout lines above or below the brick, read the gauge to see how much pressure measured in pounds was needed to move the brick.
- Divide the pounds of pressure by the surface area of both sides of the brick measured in square inches. This is the shear strength of the mortar bond between the test brick and the bricks below and above it measured in pounds per square inch (psi).
What Exactly Does the Building Code Say About Brick Foundation Mortar Tests?
This is how it is described in Appendix A of the International Existing Building Code
“[BS] A126.96.36.199 Mortar tests
The quality of mortar in all masonry walls shall be determined by performing in-place shear tests in accordance with the following:
- The bed joints of the outer wythe of the masonry shall be tested in shear by laterally displacing a single brick relative to the adjacent bricks in the same wythe. The head joint (where two bricks meet at a corner) opposite the loaded end of the test brick shall be carefully excavated and cleared. The brick adjacent to the loaded end of the test brick shall be carefully removed by sawing or drilling and excavating to provide space for a hydraulic ram and steel loading blocks. Steel blocks, the size of the end of the brick, shall be used on each end of the ram to distribute the load to the brick. The blocks shall not contact the mortar joints. The load shall be applied horizontally, in the plane of the wythe. The load recorded at first movement of the test brick as indicated by spalling of the face of the mortar bed joints is Vtest in Equation A1-3.
Do this test in several locations, concentrating on those area where the mortar appears weakest.
We Did The Test, Now What?
I know this information made be tedious and If you want to forget about the following word problem and go straight to the answer, which is the important thing, skip down to the example in bold.
Let’s assume our test brick has a mortar test value Vto= 640psi. This is the shear resistance of the mortar on the top and bottom of the brick.
We do not subtract PD + L as shown in the formula. For one thing it is not possible to subtract vertical loads from lateral loads. And even if it were, the amount of weight on a single brick would be so insignificant, as well as impossible to measure or calculate, that it should be ignored. In any event, the press (we assume they mean pressure), whatever it is, will increase, rather than decrease the shear resistance of the mortar joint. If a brick is 8″ by 4″ the surface area on each side is 32 square inches.
Only one shear plane of mortar fails due to sliding in an earthquake. To determine the shear resistance of a single shear plane in our test brick, divide 640 psi/ 2 = 320 psi. This is the Vto (tested mortar shear value) of one face of the 4″ by 8″ (32 square inch) test brick. 320to/32 square inches = 10 psi.
How to Use This Information
For example, let’s figure out the shear resistance of a 10 feet long brick foundation. 10′ x 12″ (conversion of feet to inches of feet to inches) / 8 inches length for each brick), = 18 (bricks) x 320 psi tested shear resistance = 5,760 shear resistance over the 10 foot span.
This example is for educational purposes only. As shown in this report, mortar is usually several magnitudes greater than 10 psi so the probability of the bricks becoming separated from each other is very small.
You Also Need To Test The Brick
The brick must also be tested to see if the bolts will crush the brick. Especially the top brick. The best way to do this is with a crush test. Do it just like you did the mortar test. First thing y0u do is Epoxy a 5/8″ piece of allthread through at last three courses of brick with Hilti HY 270 epoxy. Lastly, push on the side of the bolt with the ram and see if the brick holds up.
Vertical allthread has not been tested with epoxy or acrylic adhesives in brick walls. However, the epoxy manufacturer Hilti tested their Hy 270 epoxy horizontally and it performed well. Hilti HY 250 is the brand we use.
If Both Tests Prove Positive Or You Decide to Trust the Brick as Is.
- Determine the base shear on a particular wall line.
- Calculate the linear footage of plywood and bolts that will be required.
- Look at the mortar shear test results. Decide if the existing mortar is strong enough to keep all the bricks together.
- Test the brick. If it passes the test, retrofit the house as you would a house with a concrete foundation
- In the event the brick fails the test, put in more bolts 2-3 feet apart along the entire length of the foundation. The bolts distribute the earthquake force along the entire foundation. This pevents the concentrated in any particular area.
- Do the same thing if you didn’t test the brick. This makes sure the earthquake force goes into the entire length of the foundation. If each bolt resists a small part of the earthquake force, the bad foundation might be good enough. The image below shows bolts going into the foundation about every 2 feet.
- Make the plywood as long as possible to resist overturning. Do a pull test on the brick to see if hold owns are an option.
- Finally, connect all mudsill breaks together with steel straps.
Brick Foundations and the California Historic Building Code
This code applies to a “qualified historical building or structure deemed of importance to the history, architecture, or culture of an area.”
According to table 8-805.1 in this cose, the mortar bond may be allowed, without testing a maximum ultimate strength of 9 pounds per square inch.
I personally think this is quite odd. Allowing a seismic retrofit of an historic treasure using an untested brick foundation does not make sense.
Lets assume tests prove the mortar shear strength is high and the brick passes the crush test. This means the foundation is suitable for an effective retrofit. If you find out your foundation is fine, don’t forget to say a prayer of thanks to Mr. Aspid of Leeds England for saving you thousands of dollars.
As you can see this article led me to the Great Wall of China, Ancient Greece and the Pyramids. I even paid a visit to the Tower of Babel and met a young mason and backyard inventor. His name was Joseph Aspdin of Liverpool who invented Portland Cement. This invention changed the history of the modern world.
It made me think of how arbitrary the universe is. Life on this planet would be completely different if the molten orb that was once the earth had not by chance created an abundance of limestone in the cooling process. Mortar built kingdoms, the Roman Colosseum, and the lack of mortar caused other kingdoms to perish.
The study of a humble brick led me all over the world and back into ancient and modern history. I discovered brick and mortar interconnects all cultures together.