Load redistribution on a line of footings supporting continuous beams 1

[TonyMPE]

An existing building has 3 levels of continuous beams. Every other column originally terminated at the first floor, but those short columns were extended to the roof to eliminate beam overstress. Foundations for the system of columns and beams have generally benefited from the column extensions, but the footings supporting some of the extended columns have become overloaded. Also, the longstanding footing overload at the original construction 3-story columns has reduced, but significant overload still exists and is worse than the overload of the footings at the extended columns. The code-required extent of footing reinforcement must be determined.

One view is that no investigation or reinforcement is necessary because the evening out of the footing loads is a benefit to the system. Considering that individual footings have been brought into noncompliance, this does not square with the code-mandated check of structures affected by structural alterations. On the other hand, reinforcing only those more lightly loaded footings could increase differential settlement, damaging new partitions, and may therefore be considered to be bad engineering. In this view, all of the overstressed footings would require reinforcement, half of which have seen a load decrease. There is a quandry whether all, some or none of the overstressed footings are reinforced.

I've been thinking that I should require reinforcement of footings supporting extended columns that are newly non-compliant, to advise the owner to reinforce the footings supporting original construction 3-story columns and to omit their reinforcement from the design only if instructed by the owner to do so in writing. This view is based on the idea that the building code is imperfect and that I would be overstepping my role if I force the owner to abide by a more restrictive requirement than is written into the code. Also I don't believe that the idea of groups of footings acting as a system is well-enough recognized to favor an all or nothing approach. This approach theoretically leaves future owners with some risk of partition damage and foundation rehabilitation expense, but such risk does not arise from any code violations with the current work. Any thoughts?

 

[csd72]

In your situation, the existing soil under the footings will have consolidated and may be able to take a larger bearing pressure than originally designed for.

I would suggest you get a geotechnical engineer to do some investigation, this may save the client a lot of money.

 

[TonyMPE]

The borings show that the footings are on sand, so, unfortunately, the bearing capacity cannot benefit from any time dependent effects. Thanks for your input.

 

[kslee1000]

The load on the extended column now is approx. 3 times higher than the original design. if it is founded on spread footing, you have 2 problems at hand - inadequate footing bearing area & inadequate footing base pad strength. You should check the original footing design drawing, sometimes the designer keeps the same footing size for simplicity, and/or expecting changes you have mentioned/future additions.

 

[TonyMPE]

There are no drawings, but I had some test pits dug and checked those 2 problems. The footings don't seem to vary with the number of stories supported and the overstress is as described in the original post. Thanks, kslee100, for your input.

 

[kslee1000]

You welcome.
At this time, you may want to consult with a geotechnical consultant, looks like "under pinning" could help here.

 

[PanamaStrEng]

My $0.02:

First a general comment: I believe that even tough the short columns where extended, most of the dead load has already been applied to the original foundations and the only beneficial effect is in better distribution of superimposed dead and live loads. This will of course depend on how the retrofit of these short columns was handled (any shoring, jacking, etc to redistribute dead load). I assume that has been considered, because the “evening out” of load is relative and the benefit may be small.

Now to your question:

My first line of thought is that you are already aware of the situation and need to take action. If footings have recently become, or have always been overloaded is not the issue. It is your responsibility to let the owner know about it and suggest corrective actions if you are involved in further work on this structure and will be stamping structural drawings. I would move to reinforce all foundations that are known to be overloaded.

I would, however, try and obtain the most advantageous bearing capacity I can get from my geotech consultant if the problem is related to that. Area there any signs of settlement or bearing failure in the original foundations, supposedly grossly overloaded? Maybe you could get away with a smaller number of reinforced elements just by changing the geotech recommendation.

 

[TonyMPE]

Thanks, PanamaStrEng, There are no signs of settlement, etc., but borings data & bearing capacity equations indicate overload. Also, it doesn't seem kosher to leave that overload in a building that is undergoing extensive renovations. The owner has been kept in the loop and is against the foundation reinforcement. I either need to require some or all of it or leave it undone.

 

[apsix]

If footings have been 'overloaded' for many years without ill effect and there will be no load increase, I would argue to leave them alone. Despite what the theory says, in practice they are adequate.
If you are increasing load the footings have to be shown to be adequate. This could mean increasing the size or some enlightened geotech investigation and analysis.
The client should be warned of the possibility of differential movement and its consequences.