IBC 2012 and ASCE 7-10 Wind Design

[MacEng23]

The jurisdiction that I do most of my work in is preparing to make the switch to IBC 2012 as the governing code. I am in a coastal hurricane prone region with a Vasd = 120 mph. Using the IBC 2012 charts for Risk Category II the Vult = 130 mph. I understand the differences in the codes and modified load combinations but using the new code both my MWFR and C&C design pressures are about 30% less with IBC 2012 than with IBC 2009. Has anyone else run into lower design wind pressures? I thought that even though the nomenclature within the code has changes, design values would be similar.

 

[RobertHale]

Yes, I ran into the same thing on a project in the Florida Keys. V(05) = 150 MPH and V(10) = 180 MPH and my wind pressures were about 10% less. I think it is a reflection in the hurricane areas that the importance factor in 05 only approximated the change in MRI. I compared the maps and their appears to be a slight variation in the contours between Risk categories, which I took, along with some of the commentary, that the old importance factor in fact varied based on location, with the hurricane prone regions being more conservative.

 

[JedClampett]

Same here. I did a design under 2007 FBC and posted the wind loads. The cladding manufacturer checked the siding to ASCE 7-10 and got significantly lower wind pressures. And I thought they would be more, since it was Category IV design.

 

[MacEng23]

Robert - I agree with the fact that the values in the 7-05 may be conservative. I could talk myself into a 10% difference but 30% seems pretty drastic. If I am doing a house on the ocean, I feel much better about the preivous MWFR of 25 psf instead of the new value of 17.5 psf (which is just above the minimum value of 16 psf).

 

[azcats]

I usually see a much bigger spread in wind speed between the '05 and '10 maps than 10 mph. Any chance you make a mistake reading one of the maps?

Perhaps double check here:

http://www.atcouncil.org/windspeed/

 

[MacEng23]

azcats - I am in the coastal area of Delaware, the maps for this area are pretty clear. I have also double checked with wind speed by zip code websites and get the same results: Vult = 123 mph and Vasd - 112 mph (I never interpolate between lines).

I just checked for the areas west and north of here and they have the same design wind pressure regardless of IBC. Vasd = 90 mph has almost the same design pressure as Vult = 115 mph.

Is it possible that the IBC 2009 was 30% conservative for this area? This seems awfully high.

 

[wannabeSE]

See ASCE 7-10's commentary for Hurricane Wind Speeds on page 510. A newer hurricane simulation model is used to estimate the wind speeds. The commentary states: "The new hurricane hazard model yields hurricane wind speeds that are lower than those given in ASCE 7-05 . . ."

 

[RFreund]

Yes basically the new wind speed maps consider new 'hurricane research' and are able to incorporate the importance factor based on geographical location. Thus the design loads are more 'accurate' for strength level calculations and, as others have found, seems to result in reduced loads in various regions. 30% does seem like a lot but I could see it. Make sure you are looking at the correct map with the correct building category as there are different maps for each and also note that the maps are based on exposure C. Take a look at the ASCE7-10 commentary C26.5.1. I think I had a blog post about this at some point, not sure how detailed it was...


EIT

http://www.HowToEngineer.com

 

[JedClampett]

i would advise using the following website for determining wind speeds. It's similar to the seismic spectra development, but possibly even better. It gives wind speed for a specific location for various design categories. No interpolation required. It even translates addresses to longitude and latitude.

http://www.atcouncil.org/windspeed/

 

[MacEng23]

Jed - that is the site that I have always used ever since it was recommended to us by Simpson Strong-Tie at a seminar 2 years ago. Here are the results that I have been referencing:

http://www.atcouncil.org/windspeed/...ce_705=112&asce_793=85&zoom=5&maptype=roadmap

After reading these responses and the ASCE commentary, I guess that what I am getting is accurate. It just seems to a huge drop to me. We have had a lot of close calls with hurricanes but no direct hits in my lifetime.

Thank you to everyone for your help

 

[southard2]

MacEng23,

I had a project recently in Northeast Florida (St. John's County or maybe Clay County) and found a similar drop in wind pressures once making everything apples to apples. They have advertised this as either resulting in the same pressures or 10-15 % less pressures. In fact the wind contour changes were exactly the same as yours. A category II building going from ASCE 7-05 wind speed equal to 120 mph up to only an ASCE 7-10 wind speed of 130 mph. A seminar I went to said the contours are based on a lot more hurricane data than the old maps and that is the reason. At any rate my reaction was just like yours. I was very nervous at first.

So on an apples to apples basis if you are getting lower wind pressures it has more to do with the recalibration of the maps. And even though I've been working with the new ASCE 7-10 for about a year now I still find the new load factors to be annoying. I don't mind all the new wind maps for each building type such a big deal. But the load factors are a pain.

John Southard, M.S., P.E.

http://www.pdhlibrary.com/