Question about Chapter 6 (wind) in ASCE

[faromic80]

I've been reading up on chapter 6 (Wind loads) of ASCE 7 and have some questions. What is the difference between "fastest mile per hour wind" and "3 second gust wind"? Which one is larger? which one do you design for? I'm having trouble gauging wind speeds and what they mean in real life. Using section 2 I calculated that an 85 mph wind is equal to a 31 psf. Is 31 psf a lot? I'm supposed to check for a load of 75 psf as well since it's an industrial structure. The siding has to fail at 75 since the superstructure is only designed to handle 75 psf. 75 psf is high, but how high. I can't "visualize" it since it is intangible. Is the wind due to a tornado?

 

[civilperson]

75 psf comes from approximately 175 mph wind speed.

 

[swearingen]

I'm pretty sure it was between the 7-88 and 7-95 codes that the wind speed was changed from fastest mile to 3-second gust. You shouldn't be dealing with fastest mile at all.

The fastest mile speed was the average speed of a particle traveling with the wind over the distance of one mile.

The 3-second gust speed is the highest average speed over a 3-second duration.

From these definitions you can see that the fastest mile speeds will be lower than the 3-second gust speeds. Back in the '88 code, most of the U.S. was covered in a 70 to 80mph zone whereas the latest code coats most of the country in a 90mph zone - a reflection of this definition change.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[chichuck]

swearingen is correct in his explanation of fastest mile speed. He is also correct in that ASCE 7-95 was the first standard to use the 3-second gust speed to define the wind used for design.

To expand on what he said: the 3-second gust speed is higher than the fastest-mile speed. They are simply two different ways of defining the same wind. When ASCE 7-95 was developed, they changed many of the coefficients used to calculate the wind pressure, and the pressures calculated from the 3-second gust speed was not much different tnah the pressures calculated from the earlier fastest-mile speed.

In the US, you should be designing according to the current codes, and therefore designing using pressures calculated from 3-second gust speeds.

Regarding the 31 psf wind pressures; these seem large to me. I am used to seeing pressures on the wall of a building on the order of 20-25 psf. These pressures are common for relatively low structures, and represent the maximum pressures that occur on the walls. These maximum pressures occur at the top of the windward wall. Leeward wall suctions and sidewall suctions on most buildings are somewhat smaller and for most buildings will not vary from top to bottom of the walls. I don't know where the 75psf load you cite is coming from, for an industrial structure, the wind pressures would not be any different than for a non-inductrial structure, but if you have another (non-wind)load case that is 75psf then use it. Even if that 75psf is from wind, then I would not be designing the siding (and girts?) to fail at 75 psf. I would design both the siding and the structure to not fail at the 75 psf load. (That said, the code calls for the siding and girts to be designed for wind pressures calculated based on components and cladding coefficients, while the main structure is designed for wind pressures calculaed based on main wind force resisting systems coeffieients. The cladding pressures are higher than the MWFRS pressures. This is defined and explained, aboeit not to well, in the code and commentary.)

Lastly, a 75psf wind pressure, both for cladding and for MWFRS, is very high, as civilperson has pointed out. It is probably not a tornado wind because usually tornado winds are not part of design criteria. See the wind map in the commentary to ASCE 7-05 for an idea of tornadio wind speeds that have been defined in the US, The 175 mph quoted by civilperson is in the ballpark for some areas of the country.