Effect of shear at column base on foundation design

[agallag]

Hi,
I just started a new job and my new collegues contradict some previous teaching I had at the old place. So I'm fairly confused...
One such thing is the effect of shear at a column base on the foundation design. I had been previously told that if you take your reactions from the structural model for a pinned steel column at the location of a bracing you get an axial load(comp. or tens.) combined with a shear. Now, if your column base is supported on a concrete pier above the top of foundation (say 3' above) you take your shear and multiply it by the distance to the foundation and then design your foundation for the combined axial load and moment. In these cases you'd generally design a rectangular footing then. But some people disagree with this and say there is no moment so that they end up designing for an axial transfer only and square footings...what is the right way? Thanks...sorry if this is a basic one...

 

[frv]

Statics.

If I understand correctly, you have a pinned column supported by a 3' pier fixed at the base to a footing.

There will be a moment on the 3' pier. I'd like for your new colleagues to explain how there would NOT be a moment.

In fact, you will also have P-Delta moments in the pier, although I imagine that if you went through the calcs they'd be negligible.

 

[BAretired]

agallag,

No need to be sorry...basic questions are the best. If the column load is P, the horizontal shear is H and the moment at the baseplate is zero, then if the baseplate is 3' higher than the footing, from statics, the footing must be designed for a load of P and a moment of 3H unless the horizontal force is handled in another way, such as a horizontal tie.

The footing can be rectangular, square or offset, whatever you choose.



BA

 

[lexim]

agallag,

BAretired has made the explanation very simple and I designed this situation of reactions for a few electrical street light poles with stub columns at about 1.5m high from foundation base.

Thanks BA.

Lexim

 

[BAretired]

You are welcome, lexim.

BA

 

[Lion06]

I actually design the footing for a moment of H*(pier ht + ftg thickness).

 

[BAretired]

No argument from me, SEIT.

BA

 

[agallag]

Thanks...that was what I thought...

 

[kslee1000]

agallag:

Check out if your colleague considers the pedestal is tied at the floor level, thus minimize/eliminate the shear and OT at the footing base (in this case, the pedestal is one leg of a portal frame).