ACI 350-06 - Environmental Factor for Flexure

[Jacst3]

Has anyone used the "Sd" factor when calculating the moment in a concrete tank wall? It seems that the "Sd" factor for flexural steel as defined in section 9.2.6 and 10.6.4 (ACI 350-06) lead to very high factored moment values. I am calculatng factors of 2.25, 1.9 and 1.7 which are used in addition to and based on the load combination factors defined in section 9.2.1.

ACI 350-01 only required the additional "Sd" fator of 1.3 for flexureal steel. Has anyone had any experience with this. Thanks for your insight !

Jason
Pittsburgh

 

[JedClampett]

The load factors are proportionate to the reinforcing spacing and the reinforcement size. It's goal is to reduce cracking and subsequent reinforcing corrosion by encouraging the use of smaller bars and closer spacing. If you're getting high moment load factors, reduce the spacing of the reinforcing and use smaller bars.

 

[Jacst3]

Thank you for the help ! "Sd" is calculated by dividing (Phi)*Fy by (gamma)fs (ACI 350 Sec. 9.2.6). So, fs can be affected by providing (more) smaller bars at closer spacing, but fs need not be taken less than 20,000 psi (two-way action, sever exposure). It seems that when I calculate fs - I get values well below 20,000 psi, so the 20,000 psi seems to govern. With that in mind, I get "Sd" factors of 1.9 for flexural steel. Does that sound accurate to you?

Thanks,

Jason

 

[JedClampett]

I would calculate the fs and not limit it (to 20,000 psi). In the older versions of ACI 350, it was normal to use a factor of 2.21 for flexural steel. Reinforcing is cheap, cracks are expensive.