ASCE 7-10 v Saffir-Simpson

[HSBNOLA]

We are reviewing a building here in New Orleans for wind. The owner would like us to tell them what their building is rated for in terms of Hurricane Category.

From the Windspeed by Location Website you can see:

ASCE 7-10
Risk Category 2 - 144 mph
MRI 50 year - 108 mph
MRI 100 year - 119 mph

ASCE 7-05 - 126 mph

The ASCE 7-10 windspeed produces factored reactions; we would multiply the wind reaction by 0.6 to get the wind component for working stress design when designing our pile foundations, for instance. Alternately, we could use a windspeed of [144^2 * 0.6] ^ 0.5 = 112 mph to get the same reactions. This reaction would be approximately 78% of what the ASCE 7-05 wind speed would produce ( 112/126^2 ), and is what we expected with the ASCE 7-10 update.

Table C26.5-2 in ASCE 7-10 (p 536) defines a 144 mph Gust Wind Speed Over Land as the minimum wind speed for a Category 4 Hurricane. Footnote "c" defines this windspeed as the basic wind speed found in Figure 26.5-1, i.e. the 144 mph.

Table C6-2 in ASCE 7-05 (p 314) defines a 126 mph Gust Wind Speed Over Land as a high Category 2 Hurricane. Again, footnote "c" defines this wind as the basic wind speed from Figure 6-1, i.e. the 126 mph.

So, in summary, when designing per ASCE 7-10 we are designing for a Category 4 Hurricane, and when designing per ASCE 7-05 we are designing for a Category 2 Hurricane. However, the reactions produced by ASCE 7-10 are less and only 78% of the the ASCE 7-05 reactions. This really seems like an error to me - I think the Gust Wind Speed Over Land in Table C26.5-2 (ASCE 7-10) does not correlate to Figure 26.5-1, but should be "de-factored" wind speed from said figure that produces 60% of the pressure, i.e. 112 mph in this case.

Thanks for your thoughts / responses in advance.

hsb

 

[txeng91]

Hmm... it seems that the typicall 0.6 provision starts becoming pretty inaccurate once you get start getting past about 110 mph ASCE 7-05 windspeed which is around the where hurricane zones start. I would go ahead and size everything using service loads with a 7-05 windspeed just to be safe. I will continue researching and let you know if I find anything, but I'm not sure there is anything in the code that covers this.

 

[HSBNOLA]

Thanks for the response. To be clear this is a review of an existing building, not a new design. The owner wants to know what category hurricane their building is good for, i.e. when to evacuate and when to stay.

Presume the building was designed per ASCE 7-05. Per table C6-2 we would say the building was designed for a high Category 2 Hurricane. ASCE 7-10 comes out, our reactions due to wind are less (78% of ASCE 7-05), and now we say the building is good for a Category 4 Hurricane? This is the disconnect that we are seeing.

We are planning on "defactoring" the ASCE 7-10 wind, reducing the 144 to 112mph, which correlates to a mid-Category 2 Hurricane, and is consistent with ASCE 7-05.

hsb

 

[OHIOMatt]

ASCE 7-05 uses wind speed of a 3 second gust. The Saffir-Simpson scale uses the maximum sustained wind speed over a one-minute time span. This is apples to oranges. A Hurricane with a wind speed of 110 mph sustained winds, could easily have a 3 second gust to 140 mph.

One pet peeve that I have always had, is when weather reporters claim that a storm has "winds gusting to hurricane strength". This is simply false and used to grab headlines and clicks. The definition of a hurricane is sustained winds in excess of 74 mph. A simple gust to 74 mph does not compare to a sustained wind.

 

[mike20793]

Check out ICC 500 for hurricane design loads. It uses ultimate wind speed of 200 mph, but that may cover all the way up to category V. I believe there is a section that talks about the derivation of the wind speed in the FEMA 361 companion design guide.