Wednesday, September 28, 2011

Re: Allowable flexural crack width

What are you using to base you statement of "obviously the steel inside has yielded"?  Keep in mind that reinforced concrete design is essentially based upon the assumption that the concrete in the tensile portion of a beam _WILL_ crack...and that crack _WILL_ propagate up to the neutral axis.  In terms of concrete design methodology (in 318), the STEEL takes the tension...the concrete below the neutral axis is basically just there to "tie" the steel in the tension steel to the concrete in the compression area...and to help try to protect the steel from corrosion.  So, JUST because you see cracks, that does not mean that the steel has yielded.  From a structural engineers point of view, cracks in a flexural elements does not mean that the beam has "failed" least not alone they don't.  Now, from the perspective of freakin' the crap out of "lay people", you generally don't want really noticeable cracking...but the point is that cracking _alone_ is not an indication that the steel has yielded.

Now, I have to admit that I may not be completely "seeing" what you are describing.  And with that in mind, it kind of appears that you might have a crack that goes full height (i.e. potentially into the "compression zone" that is "above" the neutral axis)...but I could not effectively picture the structural system that you were describing to see if that really "jives".

As to permitted crack width, that is generally a function of the exposure of the concrete element.  Generally speaking, if you have something that is more at risk of elements that might corrode the rebar, then you will want to keep the cracks smaller.  ACI 318 _USED_ to have empirical equations for crack widths and provisions for limiting those crack width.  ACI 318 went away from that.  This cause the "water tanks" folks to get upset and let to their own concrete code (aka ACI 350).  For a long time, they kept those crack width provisions in 350 and still might do so (it has been a LONG time since was a staff engineer for ACI and thus a long time since I actually looked at ACI 350).  In 318, crack widths are "implicitly" handled by section 10.6.4 (correct section at least in 318-08).  In theory, if you meet the current ACI code provisions (including 10.6.4), then your cracking should be "generally acceptable" (quoting the commentary for section 10.6.4) for typical use.  Section 10.6.5 does offer, however, that in more corrosive environments or for "water tight" structures "special investigations and precautions are required", which could entail using ACI 350.

Now, an 1/8" crack in a flexural member does sound concerning...and that could indicate that maybe the steel has yielded, but it is not definite.  But, since you found that the existing element "does not work", that might explain it...and "does not work" implies that there might be insufficient steel...which if true, would likely meant that it definitely yielded.  Since it "does not work", it is kind of moot as you mentioned.

End result, the answer to your question is that if designed properly to ACI 318, the cracks should be "generally acceptable".  If you really want to try to do some crack width calcs, you could try looking in ACI 350...they may still have those provisions in there.  If not, you could dig out an old ACI 318 and use those provisions as an approximate "gut check"...the provisions should be around the same area as 10.6 or so (I am too lazy to go grab my 1995 318).  But, the idea behind ACI 318 thinking is that they are trying to get you to get the appropriate about of reinforcement and have it distributed appropriately, and if you do that, then cracks should not be a concern.



On Sep 28, 2011, at 11:50 AM, Jeff Hedman wrote:

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.