Compressive Strength of Existing 1913 Brick 1

[BSVBD]

The building i am working on requires remodeling such that i will have the base plates of new steel columns bearing on an existing 8" brick wall. Not yet at Demo stage, design-wise, we are assuming a 3-wythe thickness / layout / configuration of the existing wall.

I WILL have demo'd, as necessary, to install grouted concrete masonry bond beams on top of the existing clay brick, if necessary.

I have a 6" thick "Masonry Technical Manual", filled with "Technical Notes" for Brick and Concrete Masonry. The dates on most of the notes range from the 1960's through 1980's.

The seemingly most applicable table i could find says "Minimum Compressive Strength" of Clay Masonry Units, depending on the Grade (SW, MW or NW) is, (the lower of the three), 1,250 psi. A foot note states, "Many units have compressive strengths far greater than these required minimum."

Conservatively, 0.2f'm = 0.2 x 1,250 = 250 psi. I can deal with that!

However... is this reasonable? Other Tech Notes suggest much higher compressive strengths. I certainly want to be conservative, but, not unreasonable.

Any other thoughts to determine a safe, allowable compressive strength for existing brick manufactured around 1013?

Thank you!

 

[SlideRuleEra]

Good well-burned brick has an average ultimate compressive strength of about one hundred thirty tons per square foot...

Perhaps more importantly:

,but when it is laid in lime mortar in the usual way, the average strength of the combination is not more than half of this. (70 tons)

Brick And Clay Record - March 13, 1917

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[robyengIT]

in a field manual for engineering (in Italy)dated 1934 I found the same : 70 tons per square foot

Similar data on a manual dated 1890

 

[Ron]

One other point to be aware of is that early brick firing caused a hard, brittle outer shell and and relatively soft interior. This is particularly problematic where water has been going through the brick and the interior of the brick becomes friable.

I would suggest removing a few bricks in various locations and have them tested by compressive testing and petrography.

 

[EngineeringEric]

If you are remodeling old brick you will probably go back with softer mortar (not type S). Maybe type N... therefore your compressive capacity is governed by new mortar/brick matrix weakest element... soft mortar.

 

[bookowski]

You say 8" existing wall and then 3 wythe existing wall - an 8" wall is going to be 2 wythe.

I find a large variation in brick quality in anything of this age, and mortar condition can vary from ok to non-existent/sand. Depends on what you have but 250 allowable is higher than I go, I like 150 or less even if in good condition - less if questionable.

Not sure of what you're doing but also note that 8" (2 wythe) has very little out of plane/buckling capacity, I would assume zero tensile strength so any eccentric loading and/or out of plane loads can kill it easily.

 

[dhengr]

BSVBD:
Some testing labs have the equipment and ability to test brick prisms in place, for bond, shear and compressive strength, and the like. It would seem that you might want to look into this, as testing a brick alone is not particularly meaningful. You also want much more info. on the existing mortar type so you can match it during any repair or repointing, etc. Using current mortar mixes, along side or mixed in with existing mortars, often turns out disastrously, as they just perform much differently with the existing brick. You should do some minimal demo, in locations were you will do further work, so as to be the least destructive, even before you start any design. This so you really do know the makeup and condition of the wall sections before you move too far down the pike. This is doubly true where you will be putting large concentrated loads on these old walls. I agree with Bookowski, that caution should be your byword, there might be wide variations in wall condition and quality from area to area.

 

[BSVBD]

SlideRuleEra & robyengIT - based on the 70 tons per square and allowing for 1/3 voids, i am figuring 175 psi.

Ron, thank you for the advice, i will consider. There will be plenty of demo'd brick to test if we deem necessary.

bookowski - you are right regarding the discrepancy. We are field verifying the thickness.

Thank you all!

 

[BSVBD]

dhengr - Thank you for the further advice and caution.

 

[concretemasonry]

If you are interested in using the modern technical information to use for current wall standards, definitely take some samples and construct prisms (masonry units and mortar to determine the combined strength of the masonry (masonry units and mortar) in the wall.

Masonry is not designed on a myopic basis using the strengths of the individual materials, but how they work together. This is spelled out well in the ACI document C530. Since it is internationally recognized standard for masonry design and analysis based on over 50 years of research of walls and prisms. As an example,strength of materials and have seen hollow block prisms testing at 4800 psi that were made with 8500 psi hollow concrete block and mortar testing at 2200 psi. Prisms made with the same block and 1500 psi mortar tested at 4600 psi. The mortar strength is of little importance of masonry analysis as most people recognize.

ACI 530 is the consensus standard for all masonry masonry materials and it written by the following: ACI, TMS(The Masonry Society), ASCE, PCA, the BIA(Brick Institute of America), NCMA(National Concrete Masonry Association) and is used extensively.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.