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FEMA Open Foundation Requirements

FEMA Open Foundation Requirements

FEMA Open Foundation Requirements

Hello, everyone. I'm working on a flood risk mitigation project in a coastal V zone. We are raising an existing house above BFE, and it has to be on an open foundation. I've scoured the FEMA literature, but I can't find a hard definition of open. I know any lattice or slats between piers/columns/piles must be at least 40% open. Does that mean I only need 40% of the total area of the foundation "wall" open (a 3' wide "pier" at 5' o/c would meet that)? Or is it an engineering judgement call based on a consideration of the flow of flood waters/waves through and around the piers and resulting loads applied to the foundation?


RE: FEMA Open Foundation Requirements

I believe you can also satisfy the requirement with knock-out panels too. Until any flood, this should provide better security below the structure.

Mike McCann, PE, SE (WA)

RE: FEMA Open Foundation Requirements

Thanks, guys.

msquared - knock outs aren't an option for this house, but to follow that train of thought: what would be the allowable ratio of fixed pier area to knockout panels? If properly designed, the resulting ratio would be the same that I'm looking for - total open area/total fixed pier area.

I'm thinking more and more this is a judgement call rather than a prescriptive rule. Quantifiable performance levels for open foundations aren't even specified in FEMA documents or NFIP regulations - just that the flood waters have to be able to flow through it without "significant obstruction."

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