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RE: CMU fire damage

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In your investigation, try to obtain the fire report from the Fire Department.  Sometimes there will be information that will assist your decisions.

Neil Moore, SE, SECB
shingle springs, ca


At 06:34 AM 10/4/2006, Andrew Kester, PE wrote:
I have looked at quite a few CMU walls and slabs damaged by fire, and their surviveablity is high...
 
I would be interested in some more info about the fire damaged building:
-Size of building : length of walls, height of walls, stories
-Any original CDs available so you can see where the reinforcement was supposed to be? Clean outs visible?
-Bearing condition of walls - steel floor framing, flat plate concrete, wood trusses; how are the walls restrained? Loading in walls?
-Cracks in the mortar or face shells of the masonry, especially around doors and windows and any changes in direction or where it may be restrained
-Spalling of the CMU or slab
-What caused the fire, how long did the fire burn, what did the remains look like, severity, temperature of fire estimate?
 
Without any spalling, it is highly unlikely the reinforcement reached a temperature that would lower its yield stress.
 
From Ratay, Forensic SE Handbook, the strength of concrete is first affected @ 575 degrees. Compared to steel, concrete is relatively slow to increase in temp when heated..
 
Heat from the fire will cause the CMU to expand just like regular thermal expansion, and possibly crack at restrained points along the wall or openings, or other weak spots. Cracks are usually easy to spot with they are blackened with fire because you can see the white of the CMU shell beyond inside the crack. But it may be necessary to have the wall pressure washed to get a really good look. I personally base the strength of the wall on the lack of cracks in the CMU. If you don't have any cracks or spalling, it is my opinion that the CMU did not reach the temp required to cause damage and lessen the strength. IF there are isolated cracks I would address those using epoxy injection or doweling in additional reinforcement and solid grouting, on a case by case basis. It may be necessary to actually remove a section of wall and replace it, depending on the severity.
 
I do however reserve the right to be wrong!
 
HTH
Andrew Kester, PE
Lake Mary, FL