Simple Footing design 4

[newgrad5]

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.

 

[JAE]

Foundation / Footing design is a basic structural engineering task that should either be done by a person with the appropriate education/experience or by someone who is designing under a PE who has the education/experience. You shouldn't be designing such a footing as this if you are in neither catagory. Find your PE supervisor and learn from them. This is basic design.

 

[newgrad5]

Well thanks for the input JAE, but that I am already aware of. I'm not working on this by myself, I am merely preparing the bid to have it reviewed by my PE. I figured that before I went running in and asking a standard question, I would post here for some input so that I could do justice to this project. I am merely nervous in my new role, and I don't want to be making incorrect assumptions.

 

[JedClampett]

To estimate the thickness, do a punching shear calculation. Just get your column load, factor it according to ACI 318, Chapter 9 and run this number through the calculations in ACI 318, Chapter 11.12. Solve for "d." Add a few inches and round it to an even depth. Don't try to go less than about 15 inches thick.
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?

 

[UcfSE]

Are you looking for cost or for engineering? You say estimate rebar in the footing, not calculate, trying to "cost" this item (typo?) and preparing a bid for your PE. That doesn't sound like engineering to me. Will you please clarify?

 

[SlideRuleEra]

newgrad5 - Sounds like a good assignment for a new engineer. One way to set a lower bound on the quantity (lb. of rebar per cu. yd. of concrete) is to consider the ACI minimum reinforcement requirement. Since the crossectional area of rebar must equal or exceed 0.5% (in each direction) you can compute the rebar weight per cubic yard (steel at 490 lb/cu. ft.). I get a minimum of 132 lb. rebar / cu. yd. From my days as a contractor, it believe that 150 to 200 lb/cu yd. is more typical (but it is still just an estimate).

Best Wishes

http://www.SlideRuleEra.net

 

[JAE]

newgrad5,
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.

 

[Ron]

newgrad5...JAE is right on target. Your supervising PE has not taken an opportunity to mentor you on this. This is not a "let's throw him in the water and see if he can swim" simple design as JAE pointed out.

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.

 

[jjeng2]

I agree with JAE on all the issues he raises but FYI the CRSI manual has a lot of tables that simplify footing design. In your case I would consider using them for a rough estimate only but NOT for the final design because a footing of that size probably has other stuff going on.

 

[DanTex]

The amount of rebar will depend on the column size and thus the loads imparted into the footing. This is not an uncommon size for a bridge footing. So... at this point, it all depends on how accurate your estimate has to be.

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

http://www.dtware.com

 

[dawn836]

well if the footing depth is more than (36 inch) shall i use side bars (skin Reinforcement) all around the footing to avoid shrinkage of concrete?

 

[DanTex]

Side bars may certainly be warranted ( depends on the code you are using and some engineering judgement too ). Part of my suggested "Additional 10%" of rebar was to take into account the potential for such extra bars. Remember, at this point... you're just trying to capture approximate cost. Short of doing a full design, you're going to be guessing no matter what you do. You can be conservative and try to guess slightly high everywhere. However, if you want to bid a job and you are conservative everywhere, you can kiss the job goodbye. Of course, underestimate everything and win the bid, and you can kiss your future paychecks goodbye. In the end.. it all comes down to understanding how accurate you NEED to be and what the stakes are.

Dan

http://www.dtware.com