FEMA Open Foundation Requirements

[phamENG]

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?

Thanks.

 

[TerryScan]

According to FEMA documents, the 40% refers to the opening ratio of the lattice or louver material.

https://www.fema.gov/media-library-data/20130726-1511-20490-8359/fema_tb_9.pdf

https://www.fema.gov/media-library-data/20130726-1537-20490-1176/fema499_8_1.pdf

https://www.fema.gov/media-library-data/20130726-1511-20490-9526/fema_tb_5.pdf

 

[msquared48]

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)

 

[phamENG]

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."