Simple Footing design
Simple Footing design
(OP)
I'm currently working on a project. I am extremely new into this, having recently graduated and I'm trying to estimate the amount of rebar that would be needed in a simple footing design. It's 24feet by 24feet, and it's 6feet deep. We're trying to cost this item, but I've not been given very good instructions about the rebar requirement. Any input would be very valuable at this point, or an idea of where to look for future problems! Thank you so much.





RE: Simple Footing design
RE: Simple Footing design
RE: Simple Footing design
Then get your factored downward load and divide it into 576 square feet. This is your distributed load. Analyze the footing as a cantilever from the column to the edge of footing. Calculate the moment. Design the footing for this moment according to ACI 318, Chapter 10. If any of this sounds new, get some help. We've all been these. No one started out with all the answers.
By the way, a 24 ft. square footing is pretty big. What are you supporting, anyway?
RE: Simple Footing design
RE: Simple Footing design
Best Wishes
www.SlideRuleEra.net
RE: Simple Footing design
I wasn't trying to be flippant - I really do think that a 24 x 24 foot footing is NOT a simple footing design. In fact, its quite a large footing with multiple issues to consider. First, you have almost a mass concrete situation where thermal problems could result. Second, such a large footing implies a very large downward load and/or a very large uplift. In this case, "simple footing" design isn't applicable. Uplift forces would create a requirement for top layers of reinforcing.
For such a non-typical footing, running to the PE isn't a thing to avoid, its an opportunity to learn. And your PE supervisor should be expected to mentor you in this process, not wait for you to just solve it on your own.
I guess I was indirectly suggesting that this was unique enough to warrant better direction from the PE, not that you were wrong to seek out help here.
RE: Simple Footing design
While I applaud your efforts to learn more and be prepared when you go back to discuss it with your PE, be careful with trying to assimilate broad forms of information and utilize such without any experience. You are fortunate that you've been given some good "rule of thumb" information here that can be useful, but I think the most important thing you might have learned here is that you can go back to your PE and emphatically state that you have some general idea of the rebar, but the potential design complexity requires that you tie it down closer with some actual computations.
If you are providing an "Engineer's opinion of cost" then ballpark is OK but you still don't want to be very far off (just makes you look like you don't know what you're doing). If you are providing an estimate, then you need to be very close but must work from more information than you have provided. There's a difference between those two levels of effort and a difference in the liability that goes along with them.
Go back and kick your PE and get him to walk you through this. If he's unable to do so, then perhaps your next step would be to get your resume in order, because you won't learn much from him/her.
RE: Simple Footing design
RE: Simple Footing design
Short of doing a preliminary design, you can't get but so accurate. The best you can do is try to make an educated guess based on experience while realizing that the "real" answer may be a bit different. Based on the size of your footing, I'd guess that your column will be "fixed" to the footing. Therefore, footing stresses will be high. You'll also likely have piles under the footing. If so... don't forget to do an estimate for those also.
For a quick ballpark approximation, you might try this:
Assume #11 bars every 6" in both directions for the bottom mat of steel. Assume the same for the top mat. Assume #6 stirrup ties ( vertical bar with hooks at each end ) placed at 12" intervals in both directions over the entire plan view of the footing. Depending on your "confidence" in this guess, you might then consider upping your rebar quantity by an additional 10%.
Dan
www.dtware.com
RE: Simple Footing design
RE: Simple Footing design
Dan
www.dtware.com