Combining Pad Footings in the Field

[KootK]

I've got a series of concentric pad footings that have been designed as separate entities but will almost surely be constructed as one, monolithic footing (see sketch below). The question has arisen as to whether or not these footings should have top steel if constructed this way. The concern is that, under load, the large monolithic footing may develop large cracks at random topside locations between columns, allowing water to reach the reinforcing steel at the bottom of the footing and initiate corrosion.

How serious of a concern is this? Is top steel necessary in this situation?

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.

 

[rlflower]

I am assuming these three columns do not carry the same amount of load. If that is the case, then we should expect differential settlement of the footings corresponding to the loads of these columns if the pads are poured separately. If a monolythic pour (and I agree it is likely this will be the case), then both top and bottom steel should be continuous. Also check for punching shear through the footings.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[bookowski]

Can they just put in a bond break to separate them? Piece of tar paper, cardboard, anything to separate. Your sketch shows the rebar discontinuous but if it's actually continuous then that idea wouldn't work.

The proportions of the footings, depth to width, probably would influence how likely you feel it is that there is actual flexural behavior causing top tension rather than direct strutting through the concrete down to the bearing.

I've had this same question doing design where you lay out the footings and think that it would make more sense to combine them, then you check it and it's more steel by analysis.

 

[XR250]

Certainly easy enough to put top steel in so might as well do it just so you don't have to lose sleep over it.

 

[manstrom]

If it cracks, they won't become separate footings, the crack will be essentially a key/pin that will continue to share load. I wouldn't worry about it.


When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[dcarr82775]

Rigid insulation bulkheads would allow for a single pour while while maintaining three independent footings. If it was cast monolithic I would make the bottom steel continuous. Whether or not I added top steel would depend on size/loads/etc.

 

[KootK]

Thanks for the feedback gentlemen. I suspect that this happens more frequently than we know. If one considers more complicated situations, such as differing footing thicknesses and bearing elevations, it would really become quite cumbersome and cost prohibitive to start getting into properly detailed, continuous top and bottom bars.

I am assuming these three columns do not carry the same amount of load.

Similar but not identical loads.

Can they just put in a bond break to separate them? Piece of tar paper, cardboard, anything to separate. Your sketch shows the rebar discontinuous but if it's actually continuous then that idea wouldn't work.

They could but then that forces the separate footings to be separate pours. The rebar could be discontinuous or continuous. Although I suppose that you'd lose some economy due to laps/handling with continuous.

If it cracks, they won't become separate footings, the crack will be essentially a key/pin that will continue to share load. I wouldn't worry about it.

I want to not worry about it. I agree that the arrangement is safe so long as corrosion doesn't take hold.

Rigid insulation bulkheads would allow for a single pour while while maintaining three independent footings. If it was cast monolithic I would make the bottom steel continuous. Whether or not I added top steel would depend on size/loads/etc.

Clever. I'll pitch the rigid insulation idea. Part of the reason I like discontinuous bottom steel is that I'm hoping that shrinkage will cause cracks to form right between the bottom bars, where I want it. Wishful thinking no doubt.

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.

 

[dcarr82775]

I agree the crack really should just happen where there is no steel, but concrete is a fickle mistress that way. Even if it didn't I doubt corrosion would be a problem.

 

[Signious]

you could put 2" v-notch crack control cuts between the footings after a single pour too (if you stick with discontinuous steel)

 

[aggman]

I would agree with the bond break and would probably insist on that if they don't want top steel. It's pretty simple to do, even if they place plywood at the theoretical footing ends. Down the road when we are all dead someone will want to increase the footing size for some unknown reason or add a structure right up against it and when they scratch off the dirt and uncover a footer that has a big crack in it they will have a few heart attacks before everything gets straight and the building/ structure is deemed "safe"...

 

[TLHS]

As a side note, my worry when this happens is that I'm going to accidentally restrain my base for things I didn't originally plan for.

You've now got a member that can hold moment, tension and compression spanning between the two members. If anything, it makes your superstructure stronger, but it may increase the loads you're going to see in your footing. Your base can't spread or rotate as easily.

I'm pretty much always willing to assume that a single post on a single footing can be treated as a pin, since unless I go significantly out of my way to design it to resist moment the movements in the soil will be more than enough to release that restraint. That's not necessarily true if you have a stiff common footing shared by multiple supports. If you've got a stiff braced frame, it's probably still mostly pinned. If it's a moment frame, depending on the stiffness of the structure and your connection detail, you may now have a reasonable amount of moment going into your footing.