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RE: technical References and Advice of CMU Wall Cracking

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The house is approx. 35 ft. x 40 ft.  Posts are spaced at approx. 10 ft ? 12 ft. apart max.
 
There are no control joints.  The posts and cracks are under each post at the aforementioned 10 ft. to 12 ft. apart.
 
The cracks are pretty much straight vertical and therefore go through the blocks themselves.
 
Jerold
 

From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu]
Sent: Friday, December 21, 2007 6:45 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: technical References and Advice of CMU Wall Cracking
 
My initial thought is along the same line as Richard's second thought...maybe shrinkage cracks.  How long at the two sides of the house with these posts?  And how far apart are the posts/cracks?  Are there any control joints in the CMU?  Are the cracks in the CMU units themselves or the mortar joints?
 
Regards,
 
Scott
Adrian, MI
-----Original Message-----
From: Richard L. Hess [mailto:RLHess(--nospam--at)HessEng.com]
Sent: Friday, December 21, 2007 4:32 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: technical References and Advice of CMU Wall Cracking
Jerold,
Are the cracks on both sides of the wall and of equal width? That is, could they be caused by out of plane horizontal forces or a P-delta condition?  And, are there expansion joints or geometry to preclude contraction cracking?
These are just two things that I look for with vertical cracks.
Richard Hess SE
 
-----Original Message-----
From: jerold taylor [mailto:jerold_taylor(--nospam--at)yahoo.com]
Sent: Friday, December 21, 2007 12:11 PM
To: seaint(--nospam--at)seaint.org
Subject: technical References and Advice of CMU Wall Cracking
In the recent past, I created a report for a Homeowner who has cracks in the perimeter masonry (CMU) walls of his home.  The Builder has brought into question my interpretation of what may have caused the cracking, so I wanted to ask for some additional judgments and ideas, and possibly some forensic references that I could use to back up my assessment.
 
Specifically, the home is a very nice residence with tall perimeter (8 ft. +/-) walls and interior CMU bearing piers.  The interior bearing CMU piers ?look? adequate (40?x40?) and the perimeter walls do not resist any lateral grade loading from the perimeter (not retaining or basement walls), just a tall crawlspace.
 
Around two sides of the perimeter, typically two levels of decking and porches span to 6x6 posts and  then bear directly on the perimeter CMU wall.  Predominantly under each of these posts, a crack has developed. The cracks are predominantly vertical.
 
My interpretation on what may have happened is that it does not appear to be a settlement issue.  I had often thought that a differential settlement would expose itself with a masonry wall via cracking that is more ?rainbow? shaped and mostly along the mortar joints.  Whereas a straight vertical crack implies either overloading of the CMU wall or the effect of a sudden, impact load (or combination of the two).    In other words, it looks like a brittle, sudden, overload crack.
 
This interpretation makes sense to me, but I am having a hard time finding references in my text books or on line to back up my interpretation that the vertical crack is presumably due to an impact load or over loading from above.  
 
By the way, the 6x6 posts are anchored, but the Simpson tie used is eccentric and it allowable uplift resistance is less than the calculated net uplift per my calculations, so it seems a very strong wind could have lifted up the post and jolted it back down.  We are in 130 mph, Exp. C location.
 
Please feel free to let me know if my interpretation is off base, in your judgment.
 
Thank you
 
Jerold Taylor PE
 
 
 

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