Wednesday, September 28, 2011

Allowable flexural crack width

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.