Wind Design Pressures 4

[sme75]

Design pressures at wall corners and roof edges calculated per ASCE 7 are to be applied over a distance a. This is listed as 0.1 x width, 0.4h, minimum of 3 feet. In the 1997 UBC this used to be limited to a maximum of 10 feet. Is ther a similar maximum now in ASCE 7?

 

[twinnell]

There is not a maximum limit. Only those three criteria.

 

[civilperson]

Rather over an area with "2a" as a width.

 

[structuralaggie]

I think the guys that made ASCE 7 looked at the UBC and decided that calulating wind pressures with it was just way too easy and made too much sense. I think there goal was to make it as confusing as possible.

 

[csd72]

ASCE7 is not that complicated, its presentation is just badly set out.

It would be a whole lot easier if you didnt have to flick back and forth to get values from tables.

 

[JedClampett]

I think I can settle this argument by saying that it's both complicated and poorly presented.
I asked at a seminar about six years ago why the code changed wind design so much since it didn't appear to be a problem. The answer was this was more accurate (?) and since everyone had computers, there would soon be a computer program that would take care of it for us.
For a small fee, of course.

 

[structuralaggie]

Amen!

 

[bjb]

ASCE 7 is bad, and the wind provisions are abominable. There was an article in Structural Engineer Magazine recently where they compared wind load calcs using ASCE 7 and UBC 97. The author made the point that the results were practically the same, with the UBC yielding slightly higher results. However, the ASCE 7 calculations were admitted as being far more complicated and taking far longer to perform.

So where's the benefit? I have trouble with any claim that the increased complexity of ASCE 7 is justified by increased accuracy. Definitley a case of false precision.

 

[dik]

In these environs, there is a simple and a complex manner of determining wind pressures... the use of either is discretionary. If ASCE 7 is less conservative, they maybe they should consider a simplified and detailed approach.

Dik

 

[structuralaggie]

Getting all these factors and formulas down to the gnat's eyebrow doesn't really mean you are being more accurate, either. How many perfectly rectangular buildings are they building these days anyway?

The wind loads derived from these codes are not a real representation of what happens in nature, but rather produce expressions that work. I think ASCE 7 has lost site of this.

 

[minorchord2000]

bjb is exactly right. In fact, I will start using UBC 97 and forget about the white coated lab rats and their holier than thou over complicated wind analysis.

Ghosh's articles, (2 articles) written in the Structual Engineer magazines are well written and it is the first grenade launched during this war to simplify the wind provisions.

Excuse my rant, but as structural engineers, who have to deal with this everyday, should scream from the highest mountain that these current provisions are B.S. and we should not have to rely on the computer just to get the forces. It is insane.

 

[Ron]

minorchord2000....we don't have the opportunity to just use any code we want just because it's easier to use. UBC 97 has been superceded in almost all jurisdictions by subsequent versions. Further, many local and state codes do not give you the opportunity to use other than ASCE 7 for computing loads on buildings, not just the wind loads. You cannot mix and match codes either.

Yes, ASCE 7 is a bit more complicated and poorly presented than other code documents. My suggestion is that you boil it down with a throrough going over then produce a spreadsheet with your most common conditions. At least that works for my purposes.

 

[haynewp]

You might think of it this way instead; if structural engineering was made to be really easy, anybody with a PE license could do it and that would bring your job worth down.

 

[jt12]

It is good to have a complex way of designing exactly what wind pressures are, but I agree that there needs to be a simpler conservative method for us to use the 99% of the time we don't need to have wind loads to `gnats eyebrow' accuracy. Structuralaggie makes a good point. Maybe for simple structures haynewp makes a good point, but most of the projects I work on are complex / unique enough where nobody in their right mind is going to work on it if they don't know what they're doing. For complex work, I feel it is mandatory to have conservativeness on all of my loads, so I throw a S.F. on top of ASCE 7 wind loads. The code people don't exactly have this situation in mind when they write codes however, and they should.

 

[haynewp]

Keeping things complicated also helps to keep the architects' noses out of our business too. There is nothing I hate more than when an architect tries to tell me what size something should be. Things being complicated may also help discourage architects from stamping structural work.

 

[haynewp]

Just don't let them know there is not much difference whether you do it the hard way or the easy way.

 

[bjb]

jt12,

In my opinion it would be almost impossible to estimate with calculations exactly what the wind pressures will be in the "real world". What it needed is a simplified method that is truly simple that can be applicable for many "average" structures.

In my state before we adopted the IBC all you had to do was look in a table that listed wind pressure as a function of building height-no calcs involved. I'm not aware of a lot of wind induced failures in my area that have been attributed to a deficiency in design wind pressure. In fact, with V=90 and exposure B my MWFRS pressures are relatively close to the old table values.

I'm not advocating a return to that old system, but simplifications can certainly be made to the IBC/ASCE7 provisions. Calculating wind pressures should not be an academic research project.

 

[minorchord2000]

Right on bjb! Agree 100%.

 

[bkgd]

I thought I would add that in Oregon we use the 2003 IBC. There is an admendment that allows the use of wind provisions of the 1997 UBC, apparently they are going to allow the use of the 1997 UBC wind provisons even when we switch to the 2006 IBC. However there are some fears that corner pressures for high rises are 40 to 50 percent higher when using ASCE methodology. I am not sure if this is true or not as I have not compared them myself, just heard that.

just an fyi for engineers designing in the state

 

[pba]

Nice to see that you Americans are having similar rants to those in the UK when BS6399 wind code was introduced in the late 90's!

We now are all familiar with the code and it is used without too much difficulty as we know the shortcuts...

There is some pretty good software available for free from cold formed steelwork suppliers. I know the US is different but this may well also happen there.

Structuralaggie makes a very good point: "How many perfectly rectangular buildings are they building these days anyway?"

Maybe we should all be lobbying the relevant people to introduce more building types which better reflect those which we actually do design?