Load redistribution on a line of footings supporting continuous beams
Load redistribution on a line of footings supporting continuous beams
(OP)
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?
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?






RE: Load redistribution on a line of footings supporting continuous beams
I would suggest you get a geotechnical engineer to do some investigation, this may save the client a lot of money.
RE: Load redistribution on a line of footings supporting continuous beams
RE: Load redistribution on a line of footings supporting continuous beams
RE: Load redistribution on a line of footings supporting continuous beams
RE: Load redistribution on a line of footings supporting continuous beams
At this time, you may want to consult with a geotechnical consultant, looks like "under pinning" could help here.
RE: Load redistribution on a line of footings supporting continuous beams
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.
RE: Load redistribution on a line of footings supporting continuous beams
RE: Load redistribution on a line of footings supporting continuous beams
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.