Structure on peirs
Structure on peirs
(OP)
I am tasked with a project that consists of raising a house and putting it on pedestals. The reasoning behind the raising has to do with getting the house out of a flood plain (so I have been told). The house is 40’ x 17’ and we have placed columns under the house in an 8’ square grid. To cut down on the footing size we interconnected the footings with grade beams on the same grid. After submission of the drawings the client called us to let us know the plans were rejected by the town because the footings were connected together. So now we are tasked with designing the system using isolated footings. Required code (IRC 2009/ASCE7-05)
My question is in regard to the design of this system. Currently I don’t have any information on where the shear walls would be inside of the house (if there are any). Distributing the lateral loads to the piers (pedestals) is proving to be a challenge. I have two options:
1) Distribute the lateral loads evenly to the piers (all of the piers receive the same load) and design the system accordingly.
2) Distribute the lateral load only to the end/side walls of the house and design only these piers to resist the lateral loads.
I know Item 2 will work; however, this is causing some very large issues with the foundation design as the footings need to be isolated. I like idea #1 however this is going to need to rely on the wood diaphragms above the pier to distribute the loading evenly.
I realize others may have ideas for a better foundation system; however, a spread footing type system is what I have been told to use. So I do not need recommendations of some other system that might be better.
My question is in regard to the design of this system. Currently I don’t have any information on where the shear walls would be inside of the house (if there are any). Distributing the lateral loads to the piers (pedestals) is proving to be a challenge. I have two options:
1) Distribute the lateral loads evenly to the piers (all of the piers receive the same load) and design the system accordingly.
2) Distribute the lateral load only to the end/side walls of the house and design only these piers to resist the lateral loads.
I know Item 2 will work; however, this is causing some very large issues with the foundation design as the footings need to be isolated. I like idea #1 however this is going to need to rely on the wood diaphragms above the pier to distribute the loading evenly.
I realize others may have ideas for a better foundation system; however, a spread footing type system is what I have been told to use. So I do not need recommendations of some other system that might be better.






RE: Structure on peirs
RE: Structure on peirs
If that's the case, I could see the floor diaphragm acting stiff enough to go with option 1, provided there are no major discontinuities in the floor plate. I don't see how one line (or two lines) of piers could overturn without forcing the entire array of piers to resist the lateral movement.
For the additional vertical load from shearwalls, I would likely keep those to the outside walls only.
RE: Structure on peirs
Dik
RE: Structure on peirs
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RE: Structure on peirs
RE: Structure on peirs
Dik
RE: Structure on peirs
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: Structure on peirs
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RE: Structure on peirs
RE: Structure on peirs
The work area will be under the (temporarily supported) house. For both cost and safety reasons, probably best to minimize foundation excavation, construction, backfill, etc. go with the simplest design (independent piers).
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RE: Structure on peirs