Wind Loading on Column

[mahagna]

If I have a braced bay column with girts attached to the exterior flange of the column, is it redundant to consider the wind loading caused by the girts and the bracing at the same time when looking at the strength of the column?

In my analysis, the column bending caused by the girt points loads was calculated using components and cladding forces, while the bracing forces were calculated using MWFRS. If both sources of wind load should be considered at the same time, is it appropriate to mix the different loading methods or should only one be used?

Thanks

 

[phamENG]

It depends. You need to look at the wind direction while determining these. When wind is parallel to your bracing, you'll have maximum in plane (MWFRS) loads and minimum C&C loads (wind suction on the side of the building). In this case, you should look at the MWRFS loads present in the bracing and C&C in the columns. You should not, however, take the girt reactions and apply them to the column. You should recalculate the C&C loading based on the tributary area of the column. Wind is not a truly uniform load. The values we used are based on probabilistic models that predict worst case effects at various scale levels. When you're looking at a stud that's 8' long and and has a trib width of 1.33ft, for instance, the odds that it will experience a strong gust over its entire length is pretty good, whereas the odds that the beam supporting the top of a row of studs will see that same gust over its 25' length is much lower (a few studs may see the gust, but several will see a lower, nearly steady state wind). That's why the C&C values decrease as trib area increases.

You'll need to consider each wind direction and associated load cases in turn (0deg, 90deg, 0deg w/ ecc, 90deg w/ ecc, etc.) and look at the in and out of plane effects on your column.

Before you go any farther, I suggest you stop and read the commentary chapters in ASCE 7 for wind (all of them). This will give you a better understanding of the loads you're applying and will help you with how to apply them.

 

[KootK]

It's been a good long time since I've read all of the ASCE7 wind commentaries so please give phamENG's comments greater weighting than mine.

If I have a braced bay column with girts attached to the exterior flange of the column, is it redundant to consider the wind loading caused by the girts and the bracing at the same time when looking at the strength of the column?

I don't feel that it is redundant. Almost by definition, you'll have wall pressure acting on those girts concurrently with the MWFRS loads. That said, in my experience, designers ignore this on a pretty regular basis. A common example is a concrete shear wall at the exterior of a building. By way of engineering judgment, of course, that's bound to be a less critical situation with respect to the concurrent loads.

If both sources of wind load should be considered at the same time, is it appropriate to mix the different loading methods or should only one be used?

In the interest of keeping your efforts simple and preserving your sanity, I would be inclined to go with MWFRS for the in plane loading plus C&C for the out of plane loading; and just live with the conservatism that implies.

One could make a pretty good argument that, if combined actions governed the member design, then the load case considering both in plane and out of plane loading can't logically occur without a very large wind collection area in play. And that would steer one towards a MWFRS load level. Obviously, this becomes greatly complicated by the fact that the in plane and out of plane wind loads will likely tax the columns in very different ways.

 

[Hokie93]

I am not aware that ASCE 7-10 (or any other edition of ASCE 7) requires the concurrent application of wind loads in orthogonal directions when the element in question is a MWFRS element in one direction and a C&C element in the other. ASCE 7-10 does require the concurrent application of wind loads for the design of the MWFRS when the Directional Procedure of Chapter 27 is utilized. This accounts for quartering winds or winds acting at some orientation other than along the orthogonal axes. A very similar issue is addressed as one of the "frequently asked questions" in the ASCE "Guide to the Wind Load Provisions of ASCE 7-10" publication.

 

[rowingengineer]

For those of us whom are not states based what does mwfrs stand for.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[JAE]

Main Wind Force Resisting System
C&C stands for Components and Cladding.

 

[rowingengineer]

Thanks

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."