Footing Design

[ToadJones]

I'm designing a footing that is more of a grade beam that carries moving loads.
I have pos & neg moments, but the locations and incidences of the moments vary...
For example, varying the position of the loads may produce a small negative moment and a large positive moment simultaneously in the beam.
If I place the loads near the end of the beam, it produces a large negative moment and zero positive moment.

1). Is it proper to design top and bottom reinforcing separately for the worst case positive and negative moments?

2). Should I be designing this as a doubly reinforced beam?


Sorry if this question seems academic, but most of my footing design experience is with spread footings and this "footing" is really a beam.

 

[ferrarialberto]

1) yes, you can;
2) a symmetric reinforcement? I would not.

Regards.

ing. FERRARI Alberto -

http://www.ferrarialberto.it

 

[ekaj2000]

Analyze the grade beam for all anticipated loading conditions, and develop an enveloped shear and moment diagram. Make sure your reinforcement is adequate to carry that enveloped shear and moment. Don't forget to check deflections!

As for designing it as a doubly reinforced beam, I wouldn’t. You could analyze it as doubly reinforced, but is the increased time of analysis worth the small cost savings you may get by analyzing it this way? In my experience, it usually isn’t.


Jake

http://www.pelicensemanager.com

 

[ToadJones]

thanks guys
I'll just design top reinforcing for worst case negative moment and bottom for for worst case positive. The loads are not large enough to warrant a more detailed analysis, I don't think.

Since there are moving loads, I think I'll need stirrups the whole length.

 

[msquared48]

No reason not to for the stirrups Toad - you need them to hold the cage intact anyway - just use a max spacing of "d", and 2 root fprimec for the allowable shear. Do it all the time.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

Mike-
"...2 root fprimec...."

Not following you here

 

[msquared48]

Plain, unreinforced concrete is (f'c)^.5 for the max shear - no stirrups.

With nominal stirrups as you describe, using a max distance of "d" for the stirrup spacing, it is twice that, or 2(f'c)^.5

Sorry if I confused you...



Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

This seems a bit over kill to me...

My beam is 24" wide and 18" deep over all.

d= 14.5"

Vu= 31 kip

phi * 2 fc'^0.5 bd = 33.0 kip

Per 11.5.5.1 since Vu (31 kip) is more than 1/2 phi Vc (16.5kip), therefore I need min shear reinforcing.

The min spacing is d/2 or 24" ....this means I need stirrups @ 7.25" anywhere that Vu > 1/2 phi Vc?

 

[msquared48]

yes.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[DST148]

@ToadJones- Yes. You need stirrups at 7.25" anywhere that Vu > ......
In most cases the critical section for shear is at a distance d from the face of the support. But it seems in your case it may not make that big a difference so as to avoid stirrups altogether.

 

[ToadJones]

"...so as to avoid stirrups altogether...."?

You suggesting I dont need them?

 

[msquared48]

Toad:

If your beam section is large enough, technically you don't need them. However, I would never do that in my high seismic area - cheap insurance,

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

yea Mike, my thoughts were the same as yours....having them will make building the cage a lot easier anyway. In fact, its got to be the easiest way to go.

Company that I am doing the design work is building everything themselves and I suspect I'll be hired to QC and do some construction management and supervision. So, it will make this part of the job a lot easier for me too!

 

[DST148]

@ToadJones - I was looking only from standard design provisions point of view, seismic provisions are different animal....
My last post ... "so as to avoid stirrups altogether" . What I wanted to convey was that you don't need stirrups from strength design point of view. You could provide stirrups at any spacing just to keep the top and the bottom reinforcing together.

 

[ToadJones]

Thanks DST

 

[BAretired]

Toad,

It seems like you are dealing with a beam on an elastic foundation. Theory gets a bit messy, but it isn't too bad. Probably you can simplify the problem without too much trouble.

BA

 

[ToadJones]

BA-
You're right.
It is a BOEF and that's how I analyzed it (after several iterations of matching up Roark & Youngs formulas to a STAAD model).
My shears were only high when the loads moved very near the end of the beam. In reality, the loads can't get but about 5ft from the end so I sharpened my pencil a bit and found the shears to be reduced quite a bit.

 

[nac521]

Footings and slabs are not required to meet the minimum shear reinf. from ACI 318 11.4.6(a) Is a BOEF a footing or grade beam? I would still provide them in this case at a larger spacing than 7.25" to hold the cage together.

 

[ToadJones]

I consider it a beam.

 

[ToadJones]

You can't really determine the moments and shears in this beam the way you would a typical footing so I think it has to be considered a beam/