Breakaway wall foundations

[Standard American]

I have a coastal project in an AE zone of moderate wave action. It is a 3 story townhouse building with the 1st floor as a garage. The 1st floor walls will be breakaway CMU with concrete columns supporting the structure. I anticipated supporting the breakaway walls on a thickened slab. However ASCE 24 states the slab has to be frangible or able to withstand the flood loads. A frangible slab does not work for situation so I believe I just need to make sure the edge of the slab is below the scour line. My question is does it really need to be below the scour line if the slab is just going to be destroyed anyway when the breakaway walls torque it and crash down on it? Thanks in advance for the advice.

 

[le99]

I would consider founding the building on piles and utilizing the slab as a rigid cap to tie the piles.

 

[Standard American]

The columns are pile supported. I don't want the columns and breakaway walls to share a foundation so that in the event the walls do breakaway it doesn't damage the main support of the building. Plus the GC would throw me to the fire if I put breakaway walls on piles.

 

[GC_Hopi]

Can you use wood framing for the breakaway? In my experience this is common and there are prescriptive breakaway nailing patterns you can use.

 

[Standard American]

The architect wants it to be CMU because they are dividing the units. Either way I think I'm in the same boat with the thickened slab edge depth. Whether or not it needs to be below the scour line around the perimeter.

 

[GC_Hopi]

That is no fun. You will have to calc the flood loads then since you are doing a performance based design. Check out the FEMA guide, Link You can use grade beams or a slab but it just has to be designed for the appropriate load combinations including flood loading which you have to calc based on your approach.

 

[le99]

So, the walls will have spread footing independent of the piles/columns, then how the slab is connected? I couldn't picture it in my head.

 

[MotorCity]

Any damage to the slab as a result of the cmu walls breaking away I would not see as a slab failure, its simply incidental damage from the wall breaking away. The slab will still be intact, right?

 

[struct_eeyore]

Do you have grade beams? They should form a grid to brace the piles in 2 major directions, and you should be able to bear it on that. Self support the slab of the grade beams to avoid headaches.

You cant turn down the thickened edge below scour - it creates a vertical surface which will cause additional erosion and serve to accentuate scour - in addition you'll be creating non frangible break away concrete - which is what you're trying to avoid in the first place.

Whats the issue with not being able to use a frangible slab for bearing? You can see some examples in FEMA P55 - they show walls bearing on an unreinforced 4" thick, unreinforced slab/footing. There shouldnt be any axial load on a bearing wall other than self weight. I think you'll see that the load flares out more than sufficiently, and the unreinforced slab check out by ACI for bending.