Footing Design
Footing Design
(OP)
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.
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.






RE: Footing Design
2) a symmetric reinforcement? I would not.
Regards.
ing. FERRARI Alberto - www.ferrarialberto.it
RE: Footing Design
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
RE: Footing Design
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.
RE: Footing Design
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Footing Design
"...2 root fprimec...."
Not following you here
RE: Footing Design
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
RE: Footing Design
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?
RE: Footing Design
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Footing Design
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.
RE: Footing Design
You suggesting I dont need them?
RE: Footing Design
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
RE: Footing Design
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!
RE: Footing Design
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.
RE: Footing Design
RE: Footing Design
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
RE: Footing Design
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.
RE: Footing Design
RE: Footing Design
RE: Footing Design