can use steel beams instead of rebar for my reinforcement ?

[catxla]

I would like to know if I can use steel beams instead of rebar for my reinforcement in my concrete footing.

I have a 15'x15'x19" footing and since I have compressive and uplift forces I need to doubly reinforce my footing (top and bottom). However, based on my calculations I need a lot of rebar. I want to know if I replace the rebar with steel beams (that are connected by angles welded to them or running thru them) if I can accomodate for the area of steel required.

Any references, suggestions, or comments are welcome. Thank you.

 

[broekie]

Is your footing depth limited? What about a deeper footing so less rebar is needed? 19" seems quite shallow for a 15' x 15' footing.

 

[SlideRuleEra]

As broekie has noted, you have a very thin footing. If this is necessary because of space limitations, you could consider an old style grillage footing. Steel members are the load carrying members with the concrete just encasing them. Most reference books from the 1940's and before cover this subject. One reference would be "Foundations, Abutments & Footing" by Hool & Kinne.

 

[LPPE]

19" sounds really off for a 15'x15' footing. Have you checked punching shear?
To answer your question, yes, as Slideruleera stated, it is acceptable.

 

[J1D]

Not quite clear how your footing is loaded and supported. Is this a slab-on-grade or structural slab? Is the load transferred to supports 15ft apart?

If it is a structural slab (supported by piles, for example), 19" is not bad. The slab with normorl reinforcement rebars should take quite heavy loads. If it is a footing slab on soil with pedestals, the 19" may not be enough for the upward pressure. But in any case, conventional reinforcing should be considered first.

 

[CAOranger]

Whether you use a bunch of rebar or a solid steel beam, the problem will not go away -- you need to privide adequate development length for bars to be effective.

Try with increased depth, which is a economical solution.

 

[3dboy]

Like the one dude said up above, you better check punching shear. Also another big problem you have going w/ wide flanges is getting the proper development length, although If it was me, I would calculate the required force to develop the steel and try getting that by welding headed studs to the web of the WF member. Try going thicker on the slab first though, as that will probably solve most of your problems

 

[catxla]

Thank you all for your recommendations and suggestions.

My footing depth is not really limited other then I don't want to go too high. My footing is above ground on top of limestone. I have a pedestal that is 3'x3'x18" (my base plate is 2.5'x2.5'.) Which makes it already 3' above the ground including the 19" from the footing (15'x15'x19"). I ran different scenarios and I also get a deisgn of 15'x15'x17" for the footing and 3'x3'x12" for the pedestal.

My allowable soil bearing is 2500psf. My column loading is a compressive load of 280K, an uplift load of 167K and horizontal load of -55K (N-S)and 89k (E-W). I ran my numbers thru Enercalc 5.8 and StruCalc 6.0 both gave me very similar results and required areas for reinforcement.
I know since I have uplift forces I need to be reinforced top and bottom. The cost of rebar exceeds the cost of the concrete currently.

Am I going about the footing design incorrectly by using the same program for designing a footing above ground?

 

[jike]

You mentioned an uplift load of 167 kips. Your footing and pier together only weigh 55.4 kips (for 150 pcf concrete) which is not enough to resist the uplift. Is there other dead weight that you haven't mentioned? If uplift is caused by wind, you may need a 1.5 factor of safety, which gives 1.5 x 167 - 55.4 = 195.1 kips of additional dead weight. If uplift is caused by gravity loads, you may need a factor of safety of 2 which requires 278.6 kips dead weight.

The required factors of safety that you actually need may be different, of course, because it depends upon the governing code that you are working under.

 

[catxla]

I accounted for a 100K of dead load in the calcs. My structure weights about 50kips and I added 50 kips for the weight of the concrete.

 

[jike]

I don't know if I understood you correctly, but if you have only a 100 kips of dead weight and 167 kips of uplift, it seems to me that you need substantially more dead weight to hold the structure down and provide an adequate factor of safety.

 

[catxla]

Yes, you are correct jike. I defintely need more weight.

Let me run this by you to make sure I understood correctly:
I would need a total of 200k of concrete with a F.S. of 1.5.
[1.5 X 167K = (50K + Xk); where X = concrete weight]

So, based on that, if I were in the ground (vs being ontop)I would in addition add the weight of the soil acting on the fotting to counteract the uplift to make my concrete requirement smaller?
[Where 1.5 X 167k = (50k + Xk + Yk); where X = concrete weight, Y = weight of soil acting on footing]

 

[boo1]

The 2500# bearing of the limestone seams low..

 

[SimpleStructures]

Adding a steel beam to solve this problem is not a recommended solution unless you're supporting the USX building in Pittsburgh.

First, recheck your shear calculations - you footing thickness is inadequate.

Second, solve your uplift problem by increasing your footing thickness and/or mobilizing adjacent foundation dead loads. As a last resort, you can use rock or soil anchors to resist the net uplift.

Third, now that your uplift problem is solved, you only need bottom reinforcement in the footing. If you've adequately sized the footing thickness for shear, the bottom steel should not be significant.

Good luck!

 

[catxla]

2500# is for the soil below the limestone.
The limestone is a packed about 9" on top of the soil.
The plan is for the footing to be on top of the limestone to avoid digging into the limestone.

 

[Focht3]

Do you mean crushed limestone base? If so, the 2,500 psf may be quite reasonable - particularly if it's on top of a natural clay.

You can offset the uplift with drilled and underreamed piers. This is a lot easier than the approach you seem to be taking -



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?"