Crack widths for RC Section under Flexure and Axial Loads

[nglclrk]

I'm trying to find a method (compliant with American codes and standards) for determining crack-widths in RC sections under combined flexure and axial loads only (without success). I'm used to working with BS8110 and BS8007 which gives specific crack-width limits and also an equation to calculate the crack width if the strain at the face of the concrete is known.

I think ACI-318 limits the crack-width to 0.4mm (10-6 commentary) and gives guidance on the spacing of rebar in order to limit the cracking. I've even looked at ACI-350 but again I've come up empty.

Can anyone point me in the right direction or give me some references that will enable me to calculate crack widths under flexure and axial loads?

Thanks

 

[frv]

This topic has been discussed previously (or maybe it was only flexural loads)...

Are you looking for some numbers that will make you feel good, or for some level of precision?

If it's the latter, just forget about it.

I had a concrete professor tell me about a competition (I honestly forgot who sponsored it- maybe ACI) where a certain beam, with known dimensions and reinforcing, was loaded and the competitors were asked to predict the deflection..

The actual deflection was somewhere around an order of magnitude different than anyone predicted- mind you, the participants included academics, researchers and practicing engineers with years of experience.

Crack widths are an extension of this.

 

[Lion06]

See ACI 318-05 10.6

 

[nglclrk]

Thanks guys.

I'm not really looking for any precision - I think that anyone who tries to use the theory of elasticity to accurately predict the behaviour of a very non-linear structure such as an RC section is deluding themselves. I'm just trying to satisfy the ACI code requirements and I couldn't find any code-sanctioned method of determining crack widths.

You were spot-on StructEIT. The equation in 10-6 was the one I needed but it was cleverly disguised; or maybe I was just being dumb. After I submitted my post, I found that the equation was based on work done by Frosch and I was able to get the complete equation that includes w (crack width).

Thanks again

 

[yakpol]

nglclrk,

AASHTO LRFD is using the same method as ACI 318 for crack control, but provides very usefull comments connecting rebar spacing requirements with crack width calculations(see attached file). Finding steel stresses under combined flexure and compression service loads you will need to solve two simultineous equations to determine neutral axis location and maximum concrete stress (in triangular diagram).

Hope it helps!

 

[nglclrk]

Yakpol,

Thanks for your post (and for the attached file), it helped a lot. I'm used to designing to british standards and suddenly having to switch to american codes is taking me out of my comfort zone. Thanks again.

 

[mannyp2k4]

Hi,

ACI 350-06 has more stringent crack criteria than the ACI 318-05, so you should make sure that you are using the correct code. Although section 10.6.5 equation is the same on both codes, 10.6.4.1 and 2 of the ACI 350-06 gives you a lower allowable maximum stress than that of the ACI 318-05 which leads to more steel. If you have severe enviromental exposure its even worst.