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RE: Beam crack at support

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It sounds to me as if the cracking maybe a combination of shear and tension (flexural and/or other).  Is there any source of tension in the system like post-tensioning or restrained shrinkage forces?

The flexural compression stress at the bottom of the beam may have prevented the crack from propagating to that level.
 
Make sure that you know the cause before you repair it, otherwise it may re-crack.
 
Is the design adequate? Was all the reinforcing properly located? Any misplaced or missing stirrups? Is the in-place concrete strengths proper?
 
Jim K.
 
 
 
 -----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)utahisp.com]
Sent: Friday, February 07, 2003 9:06 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Beam crack at support

It sounds like a tension crack (although diagonal tends to be explained as shear, and it may be shear).  If the beam is continuous or cast into the column, the top of the beam will be in tension and the bottom in compression.  This would also explain why the beam is not cracked at the bottom.  You should be able to do a frame analysis and determine the moment at the column face.  Once you know that, you can estimate the tension stress in the concrete.  Its hard to tell when concrete will crack in tension, but if your stress is above 5 square-roots of f`c or so, it is likely a tension crack.  You will need to make a judgment call.
 
As for the fix, call Master Builders or one of the other chemical companies and talk with their technicians.  They can be incredibly helpful.
 
Jake Watson, P.E.
Salt Lake City, UT
-----Original Message-----
From: Andrew Mifsud A&CE [mailto:andrewm(--nospam--at)maltanet.net]
Sent: Friday, February 07, 2003 1:41 AM
To: seaint(--nospam--at)seaint.org
Subject: Beam crack at support

Hi,
 
I would appreciate feedback on the following problem. I was called to inspect a reinforced concrete beam which exhibited a crack at the support. The structure is composed of r/c columns, beams and in situ slabs.
Within the slabs there are occasional non-structural cracks running parallel to the main reinforcement.
 
The reported crack is hairline - less than 1mm in width, and it runs diagonally from the face of the column at mid-depth of the beam, to the underside of the slab. Therefore the crack is present in the upper half of the beam cross-section. The beam is 21" by 16" in cross-section and has a span of 20 feet. The floor loading above the beam should be well below its capacity.
 
Could this be a structural shear crack even though it does not start from the bottom of the beam cross-section? What kind of repair do you suggest?
 
Thanks
Andrew Mifsud