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Structure on peirs

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.

RE: Structure on peirs

For a house with that aspect ratio, I would just assume the floor diaphragm has sufficient stiffness to distribute the loads evenly-ish to all the piers.

RE: Structure on peirs

My concern would be less about what load to design the piers for vertically, but rather how to ensure the lateral stability of the house on stilts. Are you assuming the pier cantilevers up from the footing and designing the footings for said moment?

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

Why is it a problem if the footings are interconnected?

Dik

RE: Structure on peirs

Houses on elevated piers, or driven piling are common in south-eastern US coastal areas. IMHO, footing should be (and almost always are) independent to prevent localized erosion during hurricane storm surge from compromising the entire foundation. For the same reason, your proposal #1 is the way to go. Of course no guarantees the house will survive erosion during flooding, but at least there is a chance.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Structure on peirs

(OP)
The thought process is that the footings will support the moment generated by the pier. Connecting the piers together greatly reduces the demand on the footing. The town is rejecting this idea for what I believe is the reasoning listed by SilderRuleEra even thought everything would be located below the scour depth (the rejection is coming from entities outside of the building department).


RE: Structure on peirs

SRE... thanks... would have thought that by interconnecting them, there would be a redistribution in the event one failed...

Dik

RE: Structure on peirs

Perhaps you could construct a distribution truss at the top of the piers using corrosion protected steel angles etc. My concern with something like this is that it's probably difficult to tell what the intended lateral load path was, or even if there was an intended lateral load path.

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

dik - I'm with you on interconnection. With scour you want redundancy, not isolation.

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RE: Structure on peirs

(OP)
JAE, redundancy is the reason why we went with the original interconnected design. However, as they say, you can't fight city hall.... and in this instance that is exactly who we are fighting.

RE: Structure on peirs

Quote:

...project that consists of raising a house and putting it on pedestals.

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).



www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Structure on peirs

(OP)
SlideRule, nice picture. When I inquired about what the deal was with the work space they told me that they were going to move the house to a different area on the same property and then move it back. Essentially move the house to the side and then bring it back. But I get the point.

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