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CMU Cracks Structurally a Concern? 1

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TonyES

TonyES

Structural
Oct 2, 2007
37
The quick question: when do cracks in a cmu wall become a structural concern? Most posts I viewed deal with the cause of the cracks. Once that cause has been determined and fixed than when is the building structurally in jeopardy? I'm not thinking about minor shrinkage cracks but larger cracks. How can I tell if the cracks can be just patched over and when they need some structural strengthening? I've seen this Sikadur 33 but I'm unsure how it will provide structural strength - any experience?

Story: two story cmu building that has raised in one corner of the building due to chemicals that are freezing and expanding in the soils. The building has elevated up to 5" in one corner thus twisting the entire building and leaving cracks everywhere. The solution is to install helical piers to support the foundation and thus cmu building and than remove 6 inches of soil below the footings to allow the soils to continue to expand. (they don't think they can get rid of the bad chemicals in the soil).

Thanks
 
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I saw something similar in a freezer building. The system to keep the soil from freezing broke, and the saturated soil beneath the footings froze and created very large differential settlements (uplifts in this case).

If you have large X-cracks your wall shear may be been used up. Research some of your FEMA publications (FEMA 310, FEMA 351) for evaluations of existing structures. I believe those have provisions for allowable crack widths.

If you can fit the end of a dime in the crack, it's getting too large. Possibly you can jusify your masonry wall for shear by relying on the reinforcing only. Is there high seismic demand?
 
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Who came up with the solution? I would be interested in any pictures if available.
 
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Read Day's Forensic Geotechnical and Foundation Engineering, 2nd Ed. You know how when look in an index for "crack" and you might see 6 or 7 lines? His book has an entire index page just for cracks. Also, I've always subscribed to the notion that we should stick to crack classifications and severity scales, and frame concerns through that language. Day's classification system (architectural, functional, and structural cracks) and severity scale (from "very slight" to "very severe") has served me well over the years. Specifically, Tables 4.1 and 4.2, and Figure 4.4. One of his other books has a similar table describing the extent of repairs at each step of the severity scale.
 
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If the significance/insignificance of the crack to the stability of the structure is not intuitive, as is sometimes the case, then I think you might do an analysis where you tabulate the loads and attempt to quantify the remaining resistance in the masonry for the actual shear/compression/tension loads. Obviously the tensile and shear resistance might suffer more from large cracks/displacement than compressive strength for example.

If this type of analysis is not practical or reliable then some conservative assumptions are needed which basically amounts to neglecting some or all contribution to resistance in suspect construction and specifying a repair/retrofit or partial/total replacement to make up the difference.
 
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Seems to me that if they are going to go to that extent with 5" of expansion already having occurred, they should remove a lot more than 6" of soil. What is the rate of expansion? I think a foot would be more warranted on a cost basis.

What makes them think that the pile would not also be subject to frictional uplift from the expanding soil? I assume the Geotech considered this...

Mike McCann
MMC Engineering

 
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Mike: the geotech did consider uplift and I agree that 6" may not suffice, we will recommend more, in fact we have to have something there else the soil around will just cave in so 12" of some compressible material should work.

haynewp: I just joined this project and I think it was the geotech, I still can't get past the fact they don't care about solving the actual problem. Drives me nuts to not have a solution to a problem. Instead just a fix.

Thanks for the input: I purchased Day's book and we'll be strengthening the short walls since their capacity is more critical.

Tony
 
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