Combining Pad Footings in the Field
Combining Pad Footings in the Field
(OP)
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?
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.






RE: Combining Pad Footings in the Field
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
RE: Combining Pad Footings in the Field
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.
RE: Combining Pad Footings in the Field
RE: Combining Pad Footings in the Field
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
RE: Combining Pad Footings in the Field
RE: Combining Pad Footings in the Field
Similar but not identical loads.
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.
I want to not worry about it. I agree that the arrangement is safe so long as corrosion doesn't take hold.
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.
RE: Combining Pad Footings in the Field
RE: Combining Pad Footings in the Field
RE: Combining Pad Footings in the Field
RE: Combining Pad Footings in the Field
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.