Thursday, September 29, 2011

Cracking concrete beam

I can't answer your question in its entirety, partially because I do not really understand
what's happening and what you want to achieve.
However, keep in mind that strain at failure, epsilon u, is not something commonly tabulated.
(Strain at max stress, epsilon m, is much better known.)
Epsilon u can be reasonably estimated for most concrete grades.
I have done that in one of my recent papers.
If this bit of info could be of help, I can email you the text.
Gregory from Oz

Subject: Allowable flexural crack width

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I have a concrete wall of a subgrade parking structure that is on expansive
soils.  The foundation wall also acts as the grade beam spanning between
piers.  The voids have been compromised and we are in the process of
restoring the proper void depth. One side of the garage is like a walkout
basement with large ventilation openings in the wall. At this point the
grade beam goes from a 10' tall grade beam to a 42" tall grade beam.
Instead of placing piers at each side of the opening, one pier was placed in
the center of the opening.  I had previously observed vertical cracking at
the bottom corners of the openings.  Now that the concrete slab has been
removed to restore the void spaces, the cracks are clearly flexural cracks
that start at hairline widths at the top and taper to up to 1/8" at the
bottom.  Looking at the bottom of the grade beam and the exterior face of
the grade beam, the crack extends completely through the grade beam and it
looks to be the same on each side of the wall.  So I checked the grade beam
right at the point where it tapers down to 42" tall and it doesn't work
according to the details on the existing plans.  So we are going to install
additional piers on each side of the openings.  Now I finally get to my
question, say that the grade beam did work calculation wise.  Obviously the
steel inside has yielded, there is no concrete crushing damage at the top.
How much crack width is acceptable?  In the event the calculations work,
would it be reasonable to calculate the strain in the steel and determine
the elongation from that.  Does anyone know of any documents, text books,
etc. that provide guidance on this?  I have done a lot of searching on the
internet and can't seem to find anything useful.