Breakaway walls 1

[SteelPE]

We were recently contracted to design a foundation for a metal building. It seemed simple enough at the time until our client informed us that we had to certify that the building was in compliance with ASCE 24 “Flood Resistant Design and Construction”. I have never had to deal with these requirements before and I find them a little confusing.

The building is located in a V16 zone with a BFE (Base Flood Elevation) = 17’. The elevation of the SOG is at 7.5’ meaning our structure will be 9’-6” below water when the design flood occurs. This particular structure will house a facility that manufacturers/repairs boats. The second floor of this facility is located well above the flood elevation (23’ off the slab on grade). Basic Wind speed = 110mph.

I am a little confused with the requirements for breakaway walls. I know I need them because the building is located in a V zone. However, wall needs to fail at a maximum of 20psf unless certified by an engineer or architect under the following conditions:

1) Breakaway wall collapse shall result from a water load less than that which would occur during the base flood.

2) The elevated portion of the building and supporting foundation system shall not be subjected to collapse, displacement, or other structural damage due to the effects of wind and water loads acting simultaneously on all building components (structural and non structural).

Because the wind loads are greater than 20 psf I have to certify the wall will fail under the above conditions. Can I design the stud wall to fail under 1.5x the wind load (which will be less than the hydrostatic load) and have the Metal Building Manufacturer make the required adjustments to the design? How do I go about certifying that something is going to fail when most codes deal with safety factors? Am I worried to much about this?

 

[tolchijb]

I have never designed anything that far below the design flood, but can't you just provide flood vents to allow the water pressure to equalize inside and outside? I cannot remember the exact requirement, but I believe it is 1 square inch of flood vent for 1 square foot of footprint or something along those lines.

 

[SteelPE]

tolchijb

Thank you for your response. It appears you have a little more experience than I do in dealing with ASCE 24.

I believe the section of the code that you are talking about does not apply in this instance. My building is in a V16 zone, which is a zone that is subjected to high velocity wave action. The code defines this area as a Costal High Hazard Area (CHHA).

The 1 square inch per 1 sqaure foot of floor areas is for flood hazard areas that are not identified as costal high hazard areas.

I agree that this building is in an area with a ton of flooding.... but this building is going to be put next to 10 others that already exist.... Unfortuntaly they have been there for a while (I say at least 20 years) and my client is having a hard time understanting why all these additional requirements are required for his building.

 

[InDepth]

FEMA has several publications on the subject. Try these links:

http://www.fema.gov/plan/prevent/floodplain/publications.shtm

I believe the coastal construction is pretty good:

http://www.fema.gov/hazard/flood/pubs/lib55.shtm

There was a CD or online course at one point for it.

 

[SteelPE]

Thank you In Depth. There is some more information I found using your links.

I still interested in how other would design a wall to fail at a specified load? I need to design the breakaway wall so it can satisfy the wind loads (at 110 mph) together with the breakaway requirements of the NFIP codes.

Would you design a wall as you normally would and not worry about the safety factors applied to the building materials?

 

[rd78]

Try the following-

http://www.fema.gov/library/viewRecord.do?id=1722

Design and Construction Guidance for Breakaway Walls Below Elevated Coastal Buildings

That provides some prescriptive details you can look at.

 

[abusementpark]

We had to design a building in a V-zone with breakaway walls recently. We used light gage stud walls.

From what I recall, you basically design the walls for the wind and put the stress ratios right on the money. The flood loads are usually gonna be so much higher that the wall will fail under the flood loads based on the design for wind.

The biggest thing I remember is to make sure have a reliable failure surface.

 

[SteelPE]

Abusementpark

At least now I kind of know I am heading in the correct direction.

An exterior metal stud wall was what I was leaning towards. However, we usually write a performance based spec for the metal stud walls. I don’t really want to design the studs myself as I am not really familiar with the process. I am now considering a minimum load and maximum load for the stud wall… say min = 25psf / max = 35psf.

I also agree that the force on the wall from the flood is going to greatly exceed the wind loads for this project (600psf at the base of the wall). So the odds of the wall failing under the flood loads are pretty good.

 

[InDepth]

Steel PE. Steel Studs are really easy to design, especially when you have the SSMA catalog (see below).

Like abusement park indicated, a realiable failure mechanism is needed. You should probably at least provide a schematic detail with the performance spec so you can ensure the behavior you want.

SSMA

http://www.ssma.com/documents/ssmatechcatalog.pdf

http://www.icc-es.org/reports/pdf_files/UBC/4943p.pdf

http://www.ssma.com/technical_library.aspx