Crack Control
Crack Control
(OP)
If I have a beam that is going to crack, i.e. the stresses are more than sqrt 7.5F'c, does the ACI equation just control the crack for stress in the rebar, or does it also limit the visability of the crack? I konw its going to crack, but how big of a crack will it be. I thought it used to give you a crack width






RE: Crack Control
Calculating the crack widths with any level of accuracy is nearly impossible (I'd venture to say you'd be lucky if your guess is within an order of magnitude of the actual width of the crack)
RE: Crack Control
RE: Crack Control
RE: Crack Control
Nominal code rules for crack control normally try to limit the width of a crack to .3 - .4mm (about .01"). So you can see it but it is not a gaping crack.
This all assumes that you have allowed for all applied loadings in design (eg restraint to shortening etc).
RE: Crack Control
Normaly for crack width 0.2mm you will find that you use more steel than required for strenght
RE: Crack Control
The ACI ASD provisions limit the tensile stress in the steel based on how large of a crack you want to allow.
A good computer program to estimate crack widths is Response 2000 (search for it on Google).
RE: Crack Control
I am used to desgining uncracked concrete panels, and desgined a few stairs, and just wondered how much cracking would show.
RE: Crack Control
Tom
RE: Crack Control
Before the '99 code, ACI prescribed calculation and limitation of "z" in kips/inch in an effort to limit objectionable cracking in beams and slabs. ACI 350-01 had the same method, with more strict requirements for "z".
RE: Crack Control
The new provisions for spacing to control cracking eliminates this problem since as long as you meet the spacing requirements you "meet the code" for crack control.
RE: Crack Control
The Pre-1999 use of this method was not to predict crack widths, but as a means of considering the distribution of the flexural reinforcement to minimize objectionable cracking, just as the current method is. In fact "z" was specifically defined as "quantity limiting distribution of flexural reinforecment"
With respect to "meeting the code", I do not feel there was a problem "meeting the code" using this method, as the ACI Code (Both 318 and 350) set specific upper limits for the resulting "z". Providing adequately distributed reinforcing to meet the code requirements was never a problem for me. The current method and the old method both rely on the accuracy to which fs can be predicted. The objective for the analyses are the same, the method has just been updated.
I will say, I found few engineers who actually took the time to check the z value. It may also be interesting to note that Enercalc 5.8 still calculates the z factor. Haven't got V6 yet.