Bahamas Building Code - ASCE 7

[wallerdf]

From the Bahamas.gov website the 3rd edition is the most current I can find. In chapter 20 it references the ASCE 7-88. On the website it does say that the code is to provide minimum standards. Any reason I can't use at least the 7-05. The wind speeds have changed from fastest mile in the 88 to 3 second gust now. All the testing on windows, doors, etc. will be done using the 3 second gust pressures, right. It just seems crazy to me to use the 7-88. Has anyone else designed a building in the Bahamas recently?

 

[msquared48]

What did they use for the mega-resort Atlantis? Seriously...

Mike McCann
MMC Engineering

 

[frv]

That's tricky. From an engineering standpoint, not only do I think it's OK, it's probably advantageous (more recent research, wind maps, etc.). From a dealing with an idiot inspector standpoint, who knows. Have you tried calling the building official to ask?

 

[wallerdf]

Contacted the Local Structural Building Inspector/Official and here is a summary.
1. The 3rd edition is the most current Bahamas Building Code.
2. The use of a more current ASCE is acceptable.
3. So for this job (which is in Freeport) ASCE 7-05 with V=150 mph and Exposure D.

 

[frv]

The wind speed seems reasonable, but the exposure category is incorrect. Exposure D applies only to waterfront (or really flat areas) outside hurricane prone regions. By definition, category C applies to water surfaces in hurricane prone regions.

 

[wallerdf]

Agreed but the Bahamas Building Code makes no reference to hurricane prone regions. I does say however that any structure within the coastal building zone (defined as 1500 feet from the mean water line) shall be considered in Exposure Category D.

 

[wallerdf]

I did find this though. The Caribbean Application Document for ASCE 7-05 does include the Bahamas when defining hurricane prone regions. Which would then put me in exposure C per the ASCE 7. Looks like I need to contact the Building Official again and get his opinion.

 

[frv]

Hmm.. seems arbitrary to force you into D. I suspect some "official" with no knowledge of code development or intent thought he was being conservative when he wrote that in the code. Hopefully you won't need to design to D, as you're already going to be dealing with very large lateral loads, without having to add an additional, unnecessary 20 % (or so).

 

[wallerdf]

I'll keep you posted. I have a call in to see his interpretation.

 

[spats]

Don't forget about seismic design requirements. I design buildings in the U.S. Virgin Islands, and the seismic requirements are significant. They use IBC 2009.

 

[wallerdf]

Seismic is not referenced in the Bahamas Building Code but I went ahead and checked it anyway. It's not even close to the forces being applied from 150 mph wind. Thanks for the reminder though.

 

[wallerdf]

This is the Building Officials interpretation.
Exposure D is required for coastal building zones based on the building code. Exposure C is then obviously to be used for those structures not within the coastal building zone. They added this. In addition, because the distance across each island is so narrow it is almost pointless trying to justify not being at the hurricane ocean line. Therefore, the recommendation is to use exposure D.

I guess I could still argue exposure C if I'm not within 1500 feet of the coast but it looks like I would probably lose. I guess my only saving grace is that this project is a really small masonry building with a mean roof height of less than 15 feet. Pressures aren't terrible but definately high.

 

[frv]

The building official is wrong. There is no nice way to say it. Look at the commentary picture of p. 290 (in ASCE 7-05). "Shorelines in Exposure D include inland waterways, the great lakes, and coastal areas of California, Oregon, Washington, and Alaska" The common thread in all of these is that the basic wind speed is fairly low- hence the required "bump" in loads in Exposure D for exposed structures.

 

[wallerdf]

Right. It's like a double bump. I'm already getting high pressures due to 150 mph. Exposure D just increases them even more. This will be the building to run to during the next hurricane for sure.

 

[frv]

Or Nuclear war.