Testing In-Situ Historic Masonry

[Matt PE]

Hello,

I have a project where the client would like to anchor a fairly large sign to an existing, historic brick building. Presumed two wythes (to be confirmed.) The preliminary concept is to have a steel plate on the exterior to which the sign is mounted, and then through-bolt through the brick to a similar plate on the interior. This will tend to cause rotation of the assembly in the vertical plane (from dead load) and in the horizontal plane (from wind), and the brick must be checked for a crushing failure under the plate. I would also check the brick for something akin to a "punching shear failure" (more on this below.)

I need to know the compressive strength of the historic brick. I would prefer to take a rational approach as opposed to estimating a value from literature, if possible. It is likely not possible to remove a prism from the historic structure.

I have contacted 3 testing agencies in my area, and they do not perform in-situ testing per ASTM C1196.

My idea: Cast concrete of known strength (break cylinders) and then use this to calibrate a Windsor Probe. Take several probes of the brick and the mortar. Develop a weighted average based on relative volume of brick and mortar.

Question 1: Thoughts on this method? It seems it would underestimate the contribution of the mortar? Are there other methods with proven track records?

Question 2: re: the "punching shear failure". I would not expect a regular shaped failure, based on the plate, through both brick and mortar. I would expect the wall to break back to the next joint, and fail through the mortar. So I am dependent on the shear strength of the mortar. If I estimate the compressive strength of mortar using the Windsor probe, can I use the ACI relation 2 sqrt f'c (divided by appropriate FS) to estimate an allowable shear strength of the mortar?

Thank you in advance.

 

[masonrygeek]

ASTM C1196 is a pretty sophisticated test and only firms like WJE, Atkinson Noland and WDP & Associates have that capability. Doing impact echo testing is a possibility, but getting the right calibration is critical. Hamid and Schuller's book has a lot of good info on NDE of masonry: https://masonrysociety.org/product/assessment-and-retrofit-of-masonry-structures/. You might also want to look into SoundQA as they have been doing some acoustic testing of masonry that is relatively simple (https://soundqa.com/).

 

[TRAK.Structural]

I like the idea of getting actual data in order to substantiate material values, but it seems like that could add up to real cost to the project. Have you done any preliminary checks with some low values to see if the limit state checks will be an issue for your particular situation?

For your "punching" check and any other calculations, I think you should be using TMS/ACI 530 (if you are in the U.S.). f'm values are used in masonry design as opposed to f'c values for concrete.