ASCE 7-10 Table C26.5-2 Saffir/Simpson VS. ASCE Ultimate Loads

[structuralengr89]

Does anyone know if the Gust Wind Speeds indicated in table C26.5-2 are ASCE 7-10 Ultimate Design Wind Speeds (Vult) or ASEC 7-10 Nominal Design Wind Speeds (Vasd)?

I always thought that this was an error in the printing (I have 2nd edition ASCE7-10), but have not seen anything in the errata on this.

I have to say, that the people changing the way we handle wind speeds every few years (and I guess this is a great way to sell publications), create great confusion amongst the public. An individual paying for a commercial building or house, just wants to know....Will this thing stand a Category III, IV, etc. hurricane?


So a building which was designed to handle a Category II under ASEC7-05...now in the same location has to handle a Category IV, yet the building pressures have gone down????

This was something someone else posted in 2017 on the same issue:




HSBNOLA (Structural)
(OP)
22 Mar 17 14:36
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

 

[dianium500]

I found your post searching for the same thing. I live in Florida, and it seems that the engineers that write the ASCE 7 code are disconnected with what's reported in the news. In Florida, the news reports go by hurricane category and sustained wind speed. My clients see that 150 mph wind speed and automatically assume they are safe for up to a CAT 4 storm. This is simply not the case. When doing research for comparison of the hurricane category to ASCE 7-10 wind speed, I found the same discrepancy you have posted. The pressures from ASCE 7-05 are higher than those of ASCE 7-10. So why is the hurricane category now higher on ASCE 7-10? It is in my opinion that this is an error in the code and should be addressed immediately. We should also be required add hurricane categories on drawings in hurricane prone regions. If the wind speed falls in the middle, select the lower category to protect the public.

There is an ASCE convention in October that I plan on attending. I would love to tell them the things I hear from my clients. I also find this is an issue with flood zones. The news reports a storm surge of 6-10 ft, and I am asking myself 6-10 ft from what? Mean sea level? High water line, low water line? NAVD, NGVD? I met with a FEMA engineer last night at an information hearing for the FEMA flood maps and asked him that same questions. He said, he had no clue and that drives him crazy too. I just want to know how much water should I be expecting in my house. If you have found out any information on this, I would love to know if it's been corrected.