# MWFRS vs. C & C in ASCE 7-02



## McEngr (Nov 22, 2006)

In ASCE 7-02, I'm a bit confused as to how to apply the main wind force resisting system wind load from a component. For instance, I have a rigid frame that is obviously a part of the MWFRS with a cantilevered beam. The cantilevered beam on the side of the frame, naturally, would not receive the same wind load. Does someone have any advice? I can't seem to get a definitive answer from my boss.


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## McEngr (Nov 22, 2006)

What I've done in the past is applied the C &amp; C wind coefficient to the MWFRS. This seems very conservative, but still works. Anyone have input?


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## scottiesei (Nov 24, 2006)

Try designing the component itself to C&amp;C. The overall structure to MWFRS. i.e: A roof truss on a structure. The truss itself shall be designed to C&amp;C loading, but the reactions to the building shall be taken as MWFRS. I actually wrote an exhaustive article on this several years ago and had FBC and building officials back me up. Also, there is a good book, I believe entitled "guide to the use of the wind load provisions" also by ASCE.


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## RIP - VTEnviro (Nov 24, 2006)

Damn, there's too many acronyms in this thread.

:hung:


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## scottiesei (Nov 24, 2006)

LOL, WTF. Just because the ASCE devlopes the codes for the WL doesn't mean that you should hang yourself. Us structural guys are stuck with ACI, PCI, NDS, LRFD, ASD, IBC, OMRF, SMRF, LBW, etc. If you know the meanings, you should be qualified to pass the SEI. But good point, there R a shit load of acronyms. WFCM, PCA, FE, G40, G60, ASTM, SYP, SPF, DFL, LVL, PSL, TJI, OJ, UNO, TYP, OC, MIN, MAX, PS, FBC, FRC, HTS, LSTA, HTT, BRG, ENG, WTCA, TPI, BCSI, AASHTO, AISC, MSJC, WWF, KSI, PSI.....

:drunk:


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## McEngr (Nov 27, 2006)

> Try designing the component itself to C&amp;C. The overall structure to MWFRS. i.e: A roof truss on a structure. The truss itself shall be designed to C&amp;C loading, but the reactions to the building shall be taken as MWFRS. I actually wrote an exhaustive article on this several years ago and had FBC and building officials back me up. Also, there is a good book, I believe entitled "guide to the use of the wind load provisions" also by ASCE.


Scottiesei,

I understand that an overhang should be used as the roof coefficient from the MWFRS. However, when the overhang is below the eave, it would seem that the wind coefficient, especially for designing for service loads in the MWFRS, would not fall under the figure for low-rise buildings. As usual, I design the beam for the overhang as a C&amp;C. But when it comes to reactions and the overall building envelope for MWFRS, I get hung up on which coefficient to use.

Any more comments?

McEngr


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## scottiesei (Nov 27, 2006)

We are not blessed with GCps for overhangs using the low rise calcs. I thought that I read somewhere thats way the end zone is "2a" ILO "a" as with the CC. The reaction to the building should be reported using the MWFRS.


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## petermcc (Nov 27, 2006)

Agree with Scottiesei.

The "Guide to the wind load provisions" previously mentioned has an example where a roof purlin is checked for bending loads due to the C&amp;C pressures, and axial force from the MWFRS.

my '.02'


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## McEngr (Nov 27, 2006)

> We are not blessed with GCps for overhangs using the low rise calcs. I thought that I read somewhere thats way the end zone is "2a" ILO "a" as with the CC. The reaction to the building should be reported using the MWFRS.


Thanks Scottiesei and petermcc,

One thing that I'm not convinced of is the internal pressure. Would I use a 0.69, for instance on a 1:12, and not include any internal pressure? It would be more conservative to use 0.69+0.18(enclosed), but it doesn't make sense if it's outside the building envelope.

BTW, I don't have the copy of the design guide. I had it at my previous employer because they had a decent library. My current employer doesn't seem to care about such things... don't get me started (see the "I think I'll go shoot myself" thread in the civil pe forum for more info...).

Thanks.

McEngr


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## McEngr (Nov 27, 2006)

Actually, I guess I would use the edge wind and not the internal wind. Therefore it would be something like 1.07 instead of 0.69. Correct?


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## scottiesei (Nov 27, 2006)

I would use 1.07 (don't forget that it's negative, suction!). Agreed the internal pressure doesn't make much sense to use outside the structure but keep in mind that using the GCpf you are finding a mutiplier for the actual pressure. The difference in applying the internal pressure coefficent to the small overhang will have a negligible effect on your design. For ease, I would keep it in.


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## saibi (May 14, 2011)

A very well explained article on various methods of truss analysis is here Truss Analysis


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