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RE: FW: RE: technical References and Advice of CMU Wall Cracking

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Hi Jerold,
If there is no out of plane deformation it sounds like contraction to me although  I couldn't be sure, especially not having seen it.  A horizontal string line can show lateral deflection and a plumb line, of course shows vertical movement.  Can you check the outside at any point to confirm the same crack size on both sides?  Do you know  if it is solid grouted and what the reinforcing (especially the horizontal) is?
I would expect that a 35 foot wall would have an expansion crack or two.  It may be that they ocurred under the posts because they divide the wall into equal segments and also because the vertical load and anchor bolts create a small stress concentration at that point which is just enough to provide a starting point to initiate the crack.
These are all just ideas for you to consider in your evaluation.  It is always a judgement call and I certainly have not had a perfect score on the ones I have done.
Good Luck.
Richard 
 
 
-----Original Message-----
From: jerold taylor [mailto:jerold_taylor(--nospam--at)yahoo.com]
Sent: Friday, December 21, 2007 7:45 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: FW: RE: technical References and Advice of CMU Wall Cracking


Richard
 
Thanks for the reply, and the thoughtful questions.
 
The CMU is directly visible for inspection on the inside of the Crawlspace.  And cracks are systematic every 12 feet under the 6x6 posts.  On the exterior, there is stucco, and it periodically also shows cracking on the stucco face.
 
The cracks appear to be about the same width at the top as the bottom, and are mostly just a bit over hairline, but noticeable cracks nonetheless, that proceed as I mentioned earlier, pretty much straight down through the block, then a mortar joint, then a block, etc.
 
The two sides of the Residence are only about 35-40 feet long, so I discounted temperature shrinkage and expansion.  Our climate here is fairly moderate also, with some freezing in the Winter, but not a lot.
 
The geometry of the walls is square (ordinary wrap around porches).
 
Thanks
 
Jerold
 
 
 
Original Message:
-----------------
From: Richard L. Hess RLHess(--nospam--at)HessEng.com
Date: Fri, 21 Dec 2007 13:31:52 -0800
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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