Design of Footing with very large moment 1

[StewardMM]

Good day!

I believe this one is just simple, what makes it complex is the large moment to be considered in the design.
I have an anchored steel pole with base reactions M=220kN-m, P=28kN, V=14kN,. (M=moment, P=Axial, V=Shear).

I have already design the pedestal to support the pole, my concern is the footing.

May i ask for idea how can i design the footing with these much moment in it. it might be critical for overturning.

what i found in textbooks are just those with minimal to no moment considered in the footing design.,
Thanks

 

[ishvaaag]

Make a model of the footing as a slab with the pedestal reactions in place; use compression only springs, for a range of modulus of subgrade reaction. This way you can ascertain the stresses in the footing, modeled as a slab. If a narrow footing is suitable, even using a beam to model the footing might be practical. But by using a slab or a solid FEM model for the footing you eliminate most practical doubts about what is happening in your footing.

 

[hokie66]

There are a few choices:

1) Large footing with enough mass to resist the overturning. The simplest and usually the most economical. Do your first one by trial and error. You will soon get a feel for it.
2) Bored pile which resists the overturning by passive soil pressure. Often used for lighting towers.
3) Pile group with pile cap designed to resist overturning by push-pull on the piles. Relatively common for portal frame buildings, especially those with cranes.
4) Footing with ground anchors, in which the anchors provide tension force, soil pressure on footing bottom provides compressive force.

 

[blihpandgeorge]

i think hokie66 is right and they are listed in order of preference to my mind. a few other comments i may throw in:

- all depends on your ground conditions and ability to excavate / pile / get good bearing pressures
- if your steel pole is sensitive to deflections at the top, ie a camera on top of a mast, then a piled solution may be better
- for a simple steel post (say lighting post) where there is lots of room then a pad is likely to be simple & cheap (if not massive!) - a first guess is something like 2.5 x 2.5 x 1.2 m !

 

[Guest262653]

There is another option. You can use block foundations and take into account the passive resistance of the soil for moment equilibrium. Check Frolich's method or something similar.

 

[paddingtongreen]

I agree with hokie's order for a one-off but if there are multiples, I would go with the drilled in pier. We found that the most cost effective for some high voltage pole lines.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[WARose]

In addition to what others have suggested, I’d like to add that I’d be wary of supporting a large moment that is sustained with a spread footing. It can lead to uneven settlement that creates problems. You say it is a pole so I would assume this moment is caused by [temp] lateral load.....but I just wanted to bring that up.

 

[TLHS]

Yeah, I'd prefer the single bored pile, personally, if you can make it work. You can add a block to the top to activate extra lateral capacity if you need it. My experience, though, is with larger loads than you're looking at, so for this scale spread footings might make sense?

I've often had trouble making spread footings work in a reasonable size (plus the fact that I work in areas with huge frost depths tend to make them uneconomical). I've had problems in the field installing the caps for multiple pile installations. The types of contractors that do these little jobs aren't necessarily the most experienced and you end up having a few critical reinforcing details where things can get a little congested. Even with fairly detailed drawings I've found issues on site inspections. Depending on your depth you can also end up having to do a very annoying strut and tie model with several different cases including load reversals to check everything properly.

ishvaaag, I don't know if you need to go so far as the whole beam on a spring foundation system you're describing. In this kind of situation you're likely to have a very thick slab to so your internal deflection is going to be fairly negligable. You can likely just assume that the slab deflects as a block and assume a reaction that varies linearly with the distance from the neutral axis. If you're concerned at the end, you can check the deflection at the tip of the foundation under the applied soil reaction and make sure it's insignificant in comparison to the expected soil deflections. The more complicated analysis obviously wouldn't hurt, though, if you can do it quickly.

 

[fcu45]

That 220 kN.m is not large moment, and you can easily design an isolated pad foundations (Footing) that will resist the overturning (OT) mode of failure which will control your footing dimensions, you may need to make it around 0.8-1.0 meter deep and the length and width will depend on the FS of OT which usually ranges between 1.5-2

Good luck

 

[fcu45]

"Reinforced concrete analysis and design" By S.S. Ray
Is having pad footing design section that would be very useful and informative reference for your case here, it lists the analysis and design into simple steps.

 

[rowingengineer]

I would go with a pier footing - brom's method is the method I would use.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[rowingengineer]

in this thread there is a good paper that if you wish to design using piers is worth a read.

http://www.eng-tips.com/viewthread.cfm?qid=272903

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."