Help with Min. Steel Requirements for Foundations

[CTW]

The ACI requirements in chapter 10 for flexural members, structual slabs, and footings is fairly straight forward. But what about footings that are "thick" due to the extra mass required to resist overturning? I'm thinking about a recent project where the foundation was a large mat supporting a silo. The mat was only 2 feet thick (min. steel controlled) so meeting the requirements of section 10 (which refers back to temp/shrinkage steel in chapter 7) for structural slabs and footings was satisfactory. So I began thinking, what if the mat was 3, 4, or 5 feet thick? The requirements for structural slabs and footings in ch. 10 becomes excessive in my opinion when min. steel controls. I'm curious how others are interpreting these requirements.

 

[JedClampett]

We only consider temperature and shrinkage steel on a 24 inch maximum section. If you're looking for a place where that is referenced, see ACI 350R-89, section 2.5. It may also be in other places, but I have that handy.

 

[Focht3]

I guess it depends on what you are building, and what properties are important to you. I've seen quite a few mat foundations for mid-rise and high-rise buildings (40 stories+) with LOTS of N^o 18 bars. Overall stiffness controlled the design, not minimum code requirements.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"