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RE: Beam crack at support

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Jake,

I am not so sure about that.  From the desciption given, it does not sound
like a flexural-tension crack.  If it was, then the crack should originate
at the top of the beam near the slab up against the column and then
propigate down and possibily away from the column.  From the description
provide it sounds like the opposite...crack starts against the column at
beam mid-height and then propigates upward and away from the column.
Thus, it does not seem that it would be a flexural-tension crack.

It could be a diagonal shear crack.  Shear cracks will typically be
diagonal and will tend to be most prevalent near the supports.  But, I am
not too sure on that.  The location and extend of the crack seems a little
odd.  Even a diagonal shear crack many times starts as a flexural-tension
crack (i.e. mainly running vertical as a flexural-tension crack and then
turning more diagonal as it "turns" into more of a diagonal shear crack).
The fact that the crack starts at mid-height of the beam and runs from
there up and away from the column makes this a weird one to me.

Now all the above is based upon an assumption of just looking at gravity
loads.  Obiviously, if some significant lateral loads have been imposed it
the "correct" direction, then the orientation and location of the crack
could make more sense for a flexural-tension crack, still the crack
seeming to start at mid-height of the beam against the column is what
throws me off.  If it was a flexural-tension crack it should be still
"starting" from the bottom or top of the beam, depending on the direction
of applied lateral force.

As well, if it was a flexural-tension crack, then the crack should not
stop at the bottom of the slab but proceed through the slab depth.  Now it
may does so and Andrew just did not mention that or cannot determine that.

HTH,

Scott
Ypsilanti, MI


On Fri, 7 Feb 2003, Jake Watson wrote:

> 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
>


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