component and cladding pressures question 1

[hippo11]

Looking at 6.5.12.4 of the ASCE7-02, it tells you how to calculate your C & C pressures...

Can anyone tell me where I can find a detailed description in the code of when C & C pressures are used, where they apply, and not MWFRS pressures? For instance, if I have a big C & C uplift on the roof, do I design gravity foundations for that C & C uplift? How do I know?

Please point me to a section in the code where I can read about this, and don't just tell me "you design cladding for C & C pressures...."

Thanks!!!

 

[JAE]

Ahem,

Here's what I have described on past posts on this subject:

[blue]1. Wind pressures vary across areas of surface with peaks and valleys and variabilities.
2. The highest areas of pressures tend to occur around corners, edges, ridges, etc. and the lowest areas of pressures occur near large field-areas.
3. For both edges, ridges, etc. and large field-areas, there is still peaks and valleys and variabilities.
4. The ASCE 7 (and wind codes in general) have developed two separate wind conditions for use in building design - MWFRS and C&C.
5. When designing an element (doesn't matter if it is a deck, girt, truss, column or footing) the amount of wind that the element will feel is significantly affected by the tributary area of building skin that the element supports on the load path to the ground.
6. C&C wind pressures are higher than MWFRS as they tend to deal with elements that resist wind from smaller areas. The ASCE 7 in fact limits C&C wind pressures to 700 s.f. Anything higher than that and you are permitted to use MWFRS wind.
7. MWFRS wind is lower because, for most cases, the main wind force resisting system is taking load from very large (i.e. > 700 sf) tributary areas.[/blue]

So in summary:

[red]A) You use C&C wind for ALL elements of the building unless they have trib. areas >700 s.f. or if they are part of the MWFRS for the wind direction it is used.

B) You use MWFRS wind for the main wind force resisting system (braces, collectors, moment frames, etc.) AND you use MWFRS wind for elements and components when the tributary area is >700 sf.

C) Some members must be checked for both MWFRS and C&C. An example would be a column that is part of an X-brace and which also supports wall girts. You would first check the column for the MWFRS for the wind direction associated with the brace. Then you would check it for C&C wind perpendicular to the wall and X-brace for the girt reactions. Both checks including the appropriate vertical roof winds, sidewall winds, etc. [/red]

ASCE 7, in the commentary, tries to explain this but sometimes uses descriptions that appear to relate to whether the element receives wind directly or indirectly. This, in my view, is not very accurate.

The wind loads are based on the statistical variability of the wind where large surface areas would average out all the peaks and valleys and thus result in a lower overall wind pressure for a large-area element.

The interior column, resisting uplift from wind, does not "know" whether the wind is directly applied or not. It only reacts to the statistical average of the peaks and valleys of the wind pressure depending on the area it supports.

Hope this helps.

 

[civilperson]

hippo.
ASCE 7-05 is the current standard for wind loads and has Method 1, (Simplified Procedure), for buildings meeting the criteria.

 

[Lion06]

I don't have ASCE in front of me, but it is based on the tributary area of the member in question. The larger the area that the member sees, the closer the wind pressures get to teh MWFRS pressures. I believe the cutoff is 700 square feet. If you have a trib area of 700 square feet or more then you can use MWFRS pressures. Otherwise you have to use C & C, but the C & C pressures are still dependent upon the trib area. Look in ASCE & in the wind section.

 

[hippo11]

Thank you guys, that was very helpful....where in the code can I read about the 700 sf rule?

Thanks

 

[JAE]

If you look in ASCE 7 - under the various wind coefficient tables, there is usually a footnote for C&C wind that refers to the 700 sf limit.

 

[OldPaperMaker]

In ASCE 7-05, on page 26, Section 6.5.12.1.3, titled 'Tributary Areas Greater than 700 ft sq" you will find the authority to use MWFRS.