Breakaway (frangible) Walls 1

[slickdeals]

All:
I have a project close to the ocean (VE 15 Flood zone) which require the design of breakaway walls.

The walls are about 11' tall (ground floor), 8" reinforced masonry picking up pressures due to a 146 mph wind.

I need to detail a connection to the slab on top and footing below that can withstand wind loads, but needs to breakaway during floods. I think the loads due to a surge are approximately 3 times higher than wind loads.

Any ideas/previous experience on how to detail this fuse?

 

[ash060]

The only thing I can think of is the FEMA 55 Coastal Construction Manual. Have not done it in a while, but I think there are details and design methods in it.

 

[msquared48]

Quite frankly, I'd be more worried about the damage to the structure from the churning flotsam than the water pressure.

Breakaway panels won't deal with that, but several well placed concrete bollards will.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[concretemasonry]

Just after and later in Katrina country, after looking at the damage and typical failures, I can only offer observations and not too much in specifics.

The most severe conditions for storm surge were on the Mississippi coast there there was a 28' or so surge and and and equally damaging outward surge because of the debris. This was a situation were the surges were generally perpendicular (within 30 degrees) to the coast line and normal tracks for most storms.

Anything with either steel, timber or concrete piles under an elevated home was gone and there where few blow-out walls that were in evidence after the surges. The reinforced concrete homes with a good foundation survived well and were habitable or cleaned up within a few days if you could get to them. The other structures that survived were the elevated homes with a rigid concrete frame on deep spread footings or reinforced CMU columns with designe "blow-out walls".

The temporary/sacrificial blow-out walls (perpendicular to the coastline) were generally 6" CMUs (virtually unreinforced) and just were used for general protection and enclosure of lower bathrooms/showers, utility rooms, garages, dens, TV rooms and workshops. This system was used on many during the early rebuilding.

Your estimated factor comparing a storm surge in comparison to wind may be realistic for small, slow surges. The major frequent storm surges and flood levels are not adequate for some typical surges to determine a structural analysis. If it is a gradual river flooding the current and associated forces can be estimated using approximations. Very little real information can be drawn for New Orleans because of the quality of construction, the rise was gradual and may not have been classified as a "surge", but just a rise. In New Orleans, I had two homes (12" water in the home) that I visited for loss verification and in both cases the owners said they actually manned the pumps to equalize the water levels between the problematic political parishes and they returned to see the homes flooded and qualify for aid and assistance from the U.S. government.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[nhstruct]

Typical what I do is simply design it so that the fasteners that hold the wall in place will failure just below the force required by the water pressure, Typically I will use just screws into wood, and powder actuated fasteners into concrete. place just enough to resist wind.

 

[a2mfk]

Slick- does your construction require CMU? I know metal stud and wood walls are pretty easy to design for this condition, and you think that'd be safer. If you must use CMU, I am thinking a light gage steel angle attached at the top with tap-cons, you should be able to design it to resist the wind but fail from the water pressure loading. I think with the uncertainties involved in fastener design, I may opt to make the light gage angle the failure mode, which may involve some notching or something to make it fail in block shear or tear out.

I'd also get my hands on that FEMA document that Ash mentioned.

Do you also have to design for scour? I had a project in SW FL on a bay once and the local jurisdiction required something like a 10ft scour. This can be tricky when you are designing your piles...

 

[ron9876]

Like ash060 said. The DEP used to have standard details for this type of wall.

The walls like this that I have designed were several years ago. They also retained soil. We didn't make a physical connection. We let the soil hold it in place and when the soil eroded the wall would fall out.

Be sure to check with local building department to be check if they have any requirements.