chapters on wind loading 2

[Iasonasx]

This may be a very difficult question but as we have these methods to follow, each of which may give us different values in terms of wind pressures, I wonder which is the method most colleagues use. I am in FL this moment and I am not sure which method will be the best to go with. I may be leaning toward chapter 30 and design for the most conservative value, but I would like to hear comments from other colleagues.
Thank you

 

[phamENG]

I'm assuming you mean ASCE 7? Each chapter has a specific purpose, but you can break it down generally into 2 groups: Main Wind Force Resistance System and Components and Cladding. MWFRS loading (directional and envelope) is used to determine the loads on the MWFRS...shear walls, portal frames, braced frames, cantilevered columns, etc. It's what the building has to resist to prevent it from racking over. Components and cladding is for individual pieces and connections. Using these loads the MWFRS would be excessively conservative. Directional is applicable to lots of different building shapes and sizes. Envelope is restricted to a limited scope of common buildings. Each have subsets of procedures tailored to certain scenarios.

Wind isn't constant, and it isn't uniform. In a design level wind event, an individual screw somewhere may experience a momentary spike in pressure of 35 or 40 psf, but the average pressure across the building will never reach that level - it'll be more like 26 or 27psf. Read the commentary - all of it - and probably some of the references, too. If you're designing buildings in the great state of Florida, you need a deep understanding of how wind behaves, how it generates loads on a buildings, and how the building responds.

Of course there's also the Wind Tunnel procedure. That's for those buildings that don't fit the mold for the rest of ASCE 7 wind provisions.

 

[manstrom]

Teach me to be an engineer, I don't care if it takes all day.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Iasonasx]

Well, it is between the directional method and the envelope method, and then getting to the MWFRS and Components and cladding, so I was wondering if the directional method was more popular or the directional. I assume that the wind tunnel procedure in chapter 31 is just out there for special cases and perhaps for areas like NY downtown manhattan w/ exposure A.

 

[phamENG]

Depends on what I'm doing and where I'm doing it. I usually use directional - I feel more like an engineer when I do math rather than look up a value in a table. But if I'm in a hurry and the building is a rectangle with a gable roof, I grab the info from the envelope method. And yes - wind tunnel is for the unique edge cases that a) are sufficiently irregular that the assumptions in ASCE 7 break down or b) they have unique geometric features that can cause vortex shedding, cross wind response (where the aerodynamic effects make the building deflect perpendicular to the wind direction - neat stuff!), and other things that ASCE lists out.

 

[Ron]

@Iasonasx....I'm in Florida and have done lots of wind analysis (600+ designs). I use the directional approach and the envelope approach, depending on which is more critical to the structure. Various structures and geometries behave differently and it our responsibility to design for the critical conditions.

Wind tunnel analysis is not just for NY/Manhattan, Exposure A. It is used in Florida, particularly for special cases and high rise buildings.

 

[Iasonasx]

Ron, thanks for the reply. That's a good lead you give me. So how do you determine which one of the two methods is more critical to the structure?