Flood Resistant Design

[RFreund]

Has anyone been required to certify that the construction of a building is in accordance with ASCE 24? I am wondering if there is anything noteworthy or what sort of undertaking this is. We were asked to evaluate a residential addition which is in a flood plane. There is a crawl space which follows the detail that allows water in/out and there are deck piers. I'm assuming we will need to check piers and house foundations for the flow of water. Are there prescriptive requirements for residential projects?

I'm just trying to get an idea if it is worth it for us to do it and what sort of fee we will need to propose.

Thanks!

EIT

http://www.HowToEngineer.com

 

[JedClampett]

We've dabbled in this. Just a few seashore structures required by a client. It's a short document, but luckily not terribly clear.
Anytime you need to train yourself and might get corrected by the building official, don't be shy with you fee. See Link

 

[WoodDesignCat]

I'm working on a project like this now. A new nonresidential building in a river flood plain. This is my first project doing this as well. See ASCE7-10 chapter 5 for flood loads. See Chapter 5 commentary in the back of ASCE for more in depth explanation of debris impact loads. I made a post recently asking about breaking wave loads and how I interpreted it.

http://www.eng-tips.com/viewthread.cfm?qid=418834

. ASCE 24 mainly talks about limitations on what you are and aren't allowed to do in different types of flood planes, we are leaving that portion of the project up to the architect for the most part.

I'm not entirely sure when you do and don't need to look at breaking wave loads, but from ASCE 24 section 4.1.1 the definition of "Coastal A Zone" seems to be any area with a stillwater depth above ground surface of 1.9 ft or more.

 

[WoodDesignCat]

Update: ASCE 7-10 defines "COASTAL A-ZON"E: An area within a special fl ood hazard
area, landward of a V-Zone or landward of an open coast without
mapped V-Zones. To be classified as a Coastal A-Zone, the
principal source of flooding must be astronomical tides, storm
surges, seiches, or tsunamis, not riverine flooding, and the potential
for breaking wave heights greater than or equal to 1.5 ft (0.46
m) must exist during the base flood.

So for "riverine flooding" you don't need to design for wave loads, but you do need to design for hydrodynamic, debris loads, and hydrostatic load of 1'.

 

[RFreund]

Thanks WoodCat!

EIT

http://www.HowToEngineer.com

 

[RFreund]

@WoodDesignCat -

Any suggestions for Debris Loads?

Sorry, as you said, "see commentary". Good info there, got it. Thanks.


EIT

http://www.HowToEngineer.com

 

[SteelPE]

One thing you need to do s to check the flood insurance rate maps (FIRM maps) from FEMA. These are the maps I used to see if my projects were in A zones or V zones. When you get into V zones things can get tricky so hopefully you are out of there.

 

[hokie66]

If you are in a river flood plane, it is not enough to design the structure. You also need to prove that the structure does not worsen the flooding. I don't know what agency in the US controls that, maybe the Corps.

 

[WoodDesignCat]

hokie66
I found the section that defines to what extent you cannot worsen flooding.
ASCE24 section 2.2 Development in Floodways
Structures and fill shall not be constructed or placed in floodway areas unless it is demonstrated that those structures and fill will not, during the design flood (1) increase flood level, and (2) reduce the conveyance of the floodway.
If the design flood elevation has been determined and a floodway has not been designated, structures and fill shall not be constructed or placed unless it has been demonstrated that the cumulative effect of proposed structures and fill, combined with all other existing and anticipated development will not increase the base flood elevation more than 1 ft.

Doesn't that basically mean you can't build new structures in designated floodways, because it would increase the flood level? Or everything needs to be on stilts? We are replacing an existing structure, so we should be able to work something out with the building department.

 

[RobertHale]

You might want to check out FEMA P-55 Coastal Construction Manual. It has a lot of good info and might be worth your perusal.

Robert Hale, PE

 

[cvg]

if the structure is built in a flood plain, than the flood plain manager will need to approve it. you may need to prove that you are not blocking flow. if this is in a FEMA jurisdictional floodplain, it may trigger a revision to the flood insurance rate maps. That would require review and approval by FEMA. you do not want to go there. If you are working in the floodway, you may also need a permit from the corps of engineers. you don't want to do that either. most flood plain management is done by either the City or the County.