Storm Surge Loading On Structure 2

[ali07]

Hi Guys,
Our company has got a project consists of two storey concrete structure which have to resist storm surge loading. We haven't design for such type of loading. Can somebody guide regarding design, how we can calculate such type of loading in addition to normal seismic loading.
Structure will have to be near ocean that's why anticipated storm have to be resisted by structure.

Appreciate for design guide.

 

[structuresguy]

FEMA has a design guide which covers coastal construction called "FEMA 55 Coastal Construction Manual". It is a 3 part manual. You can order it from the FEMA website.

It covers coastal construction in great detail, showing typical design details and requirements for structures subject to wave and storm surge loading.

As for calculating the actual loading, I recommend that either your geotech or civil engineer provide the storm surge design parameters, or you (or the owner) may need to hire a coastal consultant. Typically on projects we do here in Florida which are coastal, there is a coastal consultant hired who provides a report identifying all the coastal design parameters. Again, here in Florida, the state has determined general storm surge elevations for the entire state. But site specific determinations can reduce your required loading, so it pays to hire someone familiar with your local requirements.

The key parameters that you will need (might vary based on local geography) are:

1. Wave crest height (max height of waves on top of surge height)
2. Scour elevation (describes how much the soils we be scoured away due to waves washing over the soils)
3. Wave load pressure curves (pressure versus height which varies from zero at wave crest to max at mean wave height)
4. Existing grade elevation
5. Final desired finished floor elevation

Florida Building Code requirements state that the lowest elevation of the horizontal structure of habitable structures must be above the wave crest height. With a few exceptions, walls built below the wave crest height must be break away, so as to not transmit wave loads to the structure. Also, all structures in coastal regions must be supported on piles.

One thing you want to avoid is locating any horizontal structure, such as pile caps or grade beams, within the wave loading area. This typically is from the low point of the scour elevation up to the wave crest height. Otherwise, you will find extremely heavy wave loads on the structure. I have seen loads as high as 3000 psf on pile caps. Of course, this varies with the vertical elevation of the structure. Typically, the wave load on a 16" pile would be in the range of 4000-8000 pounds, located at the mean wave height. So you can see the loads get to be very high very quick.

Due to the break away wall requirements for habitable structures, we typically set the elevation of the lowest horizontal structure to be just above the wave crest height. Then, depending on the required thickness of the structure (say perimeter beam supporting 1st floor), we set the final finished floor elevation. This frequently results in the 1st floor being elevated several feet above the existing grade elevation. You then either need to fill the site up to the structure, or provide ramps/stairs up. It can be somewhat challenging for the architect to accomodate this requirement, so be prepared to defend the elevations.

For low rise structures (1-3 stories), we typically design the foundation with 16" diameter auger cast piles, with the required pile depth starting at the scour elevation (which could be several feet below existing grade). Then we run the piles up to the perimeter beam, which is above the wave crest elevation. We design the piles as cantilever columns, fixed at the top in the perimeter beam (and usually a structural slab supported by the perimeter beam). You will need to work with your geotech engineer to determine the appropriate soil resistance and moment distribution in the piles. We typically then design for 1/2" max of lateral deflection due to the wave loads on the piles.

Hope this gives you some insight into design of coastal structures. I'll be happy to try to answer any other questions you may have.

 

[ali07]

Appreciate for your detailed guidance. We'll look into coastal construction manual for further help as you referred.