ASCE7 Minimum Wind Load definition 1

[mafiucivil]

I don't understand the definition of the minimum load on the ASCE7-05.

The minimum wind load (10psf or 480 N/m2 )needs to be applied horizontally to the entire vertical projection of the building.

I know that the preassure is proportionally to the V^2, but i want to know how did the ASCE7 determine this 480 N/m2. The minimum wind velocity in USA is 85mph (38m/s) and if I follow the equation for the preassure:

q=0.613*Kd*Kzt*Kz*I*V^2
q=0.613*0.85*1*0.705*1*38^2=530 N/m2

Kd=0.85
Kzt=1
Kz=0.705 for exposure B and heights < 9.1m
V=38 m/s

This 530 N/m2 does not match with the minimum load of 480N/m2

Please help me, I will apreciate that

 

[JedClampett]

For one, Exposure B is not that usual.
Also, this is the basic wind pressure. You need to use the figures in ASCE 7 to apply it. Some of the figures increase the pressure (like in corners) and other reduce the pressure.
There's also internal pressures, depending if the building is enclosed or partially enclosed.
You've also cherry picked a very low wind pressure. 85 mph is not the usual wind pressure for the vast majority of the USA, 90 mph (or more) is.
Finally, your case is the reason they have a minimum wind pressure. The code wants to make sure even under unusual wind combinations, no one ever uses less than 10 psf.

 

[charliealphabravo]

You should not be surprised that a number as round as '10', and applied to the projected area of a structure regardless of size or shape or enclosure, would have no rational meaning. It was likely an armchair committee consensus. I don't see anything in the commentary about it.

 

[mafiucivil]

Thanks for the answers!
I really wanted to know if this "10 psf" are some kind of conventionally number tipically or if it has a scientific meaning.

I'm studing to implement this standard in Chile ( with other velocities, the minimim is 30m/s) and thats why i'm asking for. If i have a low velocity obiously the minimum preassure needs to be a lower value than 10psf.

Other standards (equal to asce7) used different numbers for the minimum load, but there's no more literature to obtain this values ..

 

[frv]

You are confusing several issues. The minimum velocity is usually statistically derived, the minimum pressure is not. One really has nothing to do with the other. And, as Jed mentioned, don't confuse the pressure coefficient qh with the actual design pressure. The pressure coefficient is just one component in deriving the design pressure. Design pressures can be much higher than qh or somewhat lower.

The reason for inclusion of the 10 psf minimum lateral load is simply to ensure that there is no odd combination of factors that would lead to unexpectedly low design pressures. It ensures a minimum lateral capacity for the structure.

 

[charliealphabravo]

The standard can give you zero, near zero, or negative pressures under various conditions of roof pitch, enclosure, exposure, etc.

Any of these occurrences may result in negligible net pressures for C&C, MWFRS, or global stability. I think the 10 psf is intended to assure some minimal integrity and global stability of the building. Again it cannot possibly have any rational basis other than educated engineering consensus.

 

[mafiucivil]

Thanks for your answer frv! I knew that i was wrong in something. But i do not understang how did the ASCE7 established that minimum load (scientifically, in a equation o if it is a convention like charliealphabravo says)

I'm trying to understand how can a define a minimum load for my country (Chile). We have 67 mph as wind speed in our maps ...

other countries like Colombia, Venezuela changed the minimum load:

Colombia has 38 mph and 8.4psf
Venezuela has 43mph and 6.3psf (smaller than Colombia)
USA has 85 mph and 10psf

It seems that the pressure arent proportionally to the V^2.

 

[charliealphabravo]

Maybe try reverse engineering the number. Consider the common minimum levels of conventional construction in your country for a sign or wall or roof connector or whatever that you want to continue to allow. Then calculate the corresponding wind pressure.

Alternatively, you could select a trial pressure and check it against examples of conventional construction that you do not wish to prohibit.

 

[frv]

Be careful about mixing things up. You can't necessarily separate wind speed from design. The wind speed may be 38 mph in Colombia, but the design requirement (or safety factors) much larger than in the US, so that the reliability index is somewhat similar. I am not familiar with the codes in South America, but the wind speeds are extremely low if the design is to be carried out how we do it in the US; I strongly suspect that the safety comes heavily on the resistance side.

And yes- the pressure is proportionate to the square of the wind speed (everything else being equal), but the wind speed has little to do with the minimum pressure. As others have noted, it's just a "feel good" number to ensure a minimum amount of lateral capacity.

 

[a2mfk]

I think Charlie hit the nail on the head. Its a minimum load to be used in design (its a prescriptive requirement) because there are cases where you would have a pressure close to zero or negative that may not produce the worst case total lateral force for stability analysis. So that some lateral force is considered in the stability of certain structures, the powers that be came up with a minimum prescriptive amount of 10psf. This is how I understand it. This has also been in the Florida Building Code for as long as I have known.

Example:
Take a monoslope open roof structure, freestanding, like a picnic pavilion or carport. You would apply the ASCE wind pressures perpendicular to the roof surface for your basic design. But when considering MWFRS lateral forces for overall stability of the structure, you take the horizontal component of this force. In the case of the wind direction going "up slope", this horizontal component would actually be in the opposite direction of the wind (which to me is a bit counter intuitive).

However, the magnitude of this force may very well equate to less than 10psf applied horizontally to the vertical projection of the roof depending on the wind speed and roof slope. Therefore, you use the minimum 10psf.

Another very similar case would be the same structure, with a traditional two sloped gable type roof. If you take the pressures perpendicular to each slope of the roof, if the slopes are the same, the horizontal components would cancel each other out. Therefore, you have the 10 psf minimum.

I think I have this right....

 

[deBono]

mafiucivil
Have you made any progress in implimenting ASCE7-05 in Chile yet?
If not what is the current standard for windloads on signs?

 

[msquared48]

The wind blows in a different direction in Chile, so the forces are different.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ajh1]

Note with ASCE7-10 the wind loads are now ultimate loads, rather than service level loads. The new minimum wind pressure value is 16 psf up to the eave line of the building and 8 psf on the vertical projection of the roof. Given the change in load factors in the load combinations the16 psf gives basically the same value as the old 10 psf.