Design footing WITH moment

[LearnerN]

In my undergrad textbook for designing footings, it appears to only deal with column footings that have no moment. So then how would a person design a footing that has vertical and horizontal loads or moments, like a footing with a pipe supported on the pier...which would have wind and seismic horizontal loads? (In terms of all the design, especially pier reinforcing)

 

[SteelPE]

The same way as if the vertical load was applied eccentric to the footing.... With an eccentricity e =moment/axial load.

 

[hokie66]

You would first get the geometry correct by applying the loads, and by trial and error, adjusting the size of the footing and pier to achieve satisfactory stability factors and soil pressures. With that done, it is just a matter of standard concrete design.

 

[LearnerN]

So do the vertical load and the applied moment on the pier...make the pier treated and designed like an eccentrically loaded column, you mean? And then the moment at the base of the pier would be added to the moment in the slab of the footing under the pier.

 

[hokie66]

It is all statics. You can have vertical loads, horizontal loads, applied moments. Then you have the resisting forces, which include vertical soil pressure, and forces resisting sliding, which can be passive soil pressure and/or friction.

 

[LearnerN]

hokie66, I understand the statics aspect, eccentrically loaded footing on soil pressure calcs, etc. What I'm trying to figure out is how to the concrete design of the pier of the footing. Do I treat it as an eccentrically loaded column to size the reinforcing I need? And then I'll add the moment at the base to the moment in the footing from soil pressure to ensure proper reinforcing sizing of the footing.

 

[steellion]

Take all of the footing equations you got from school and throw them away. The only equations you need is P/A + M/S < q(max) and P/A - M/S > 0. All of the other equations are derived from these, and it just frankly confused me when trying to apply all of the modified equations to a different situation. The 2 simple equations always work.

Yes, you have to design the pier and the footing for the moment at the base. Your pier will likely have to be at least twice the dimensions of the base plate to get ACI Appendix D to work for your anchor rods.

Be generous with the footing size to keep the eccentric load within the kern (e < L/6) if you can. This will keep the entire footing with at least some bearing on soil. "If you keep the load in the kern, the footing will not overturn"

 

[KootK]

Do I treat it as an eccentrically loaded column to size the reinforcing I need? And then I'll add the moment at the base to the moment in the footing from soil pressure to ensure proper reinforcing sizing of the footing.

Yeah, that's about it. Often these kinds of piers are governed by minimum reinforcing limits. It's common to see 0.5% vertical steel and ties to suit column code rules. Of course, if you loads require more, then you'll have more.

At the top of the pier, you'll have an anchorage connection to design as steellion has mentioned. At the bottom of the pier, you may require two features not common to systems carrying purely axial loads:

1) You may need top steel in your footing and;
2) You may need to turn your starter dowel hooks inwards which is cumbersome.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.