It's totally based upon the tributary area that the beam supports. If the beam takes more that 700 sf of area, (per ASCE 7) then you can use the MWFRS values. See ASCE 7-02, section 6.5.12.1.3.
If the area is < 700 sf, then you'd use the C&C values, but if the beam is also part of the MWFRS (say a collector beam) then you ALSO have to check it under its MWFRS loads as well.
JAE is correct. I believe the point of the 700 ft^2 trib area is that the MWFRS pressure is the LOWEST it can see. Basically, if you check it for C&C (assuming a small enough trib area that it doesn't qualify for MWFRS loads), there is no need to check it for MWFRS because that will be lower.
Well...there is a need to check the MWFRS if it is part of the lateral system.
What I mean is, you may have an eave beam on the edge of a roof that sees uplift under C&C wind. But also, it may serve as a collector beam or a part of an X-brace that is under that eave beam where the overall building lateral load is proceeding down through your beam and into the X. In that case you really have two wind load situations to verify as they are taking wind in different manners.
It is a little confusing based on the definition ASCE provides for components. Intuitively it seems that based on the tributary area it could be classified as a component, but by the definition of C and C:
"Components and Cladding: Components receive
wind loads directly or from cladding and transfer
the load to the main wind force-resisting system....Examples of components include fasteners, purlins, girts, studs, roof decking, and roof trusses..."
So if the roof truss is a component, then the supporting roof beam would always be designed as MWFRS since it is receiving wind from a component(the trusses). Do you agree?
I got into a big discussion of this over at the ICC bulletin board and a lot was said about the definition of a member being designed by C&C wind if it takes wind directly and a member being designed by MWFRS wind if it takes wind via other members that are directly taking wind.
I know the ASCE 7 commentary seems to imply this but based upon the theory of the wind pressure concepts, what is important to remember here is that the higher C&C pressures are due simply...simply...to the fact that you get wind pressure peaks over small areas and as the tributary area increases, the variations of wind pressure average out and the overall pressure goes down.
I think the concept of saying "a member will be designed by either C&C or MWFRS wind based upon whether it receives wind directly" is not accurate.
The use of C&C or MWFRS depends on function of the member (is it truly a part of the MWFRS) and based upon tributary area.
Member with At < 700 sf and not part of MWFRS: Use C&C
Member with At < 700 sf and part of MWFRS: Check both C&C and MWFRS wind loadings
Member with At > 700 sf and not part of MWFRS: Use MWFRS
Member with At > 700 sf and part of MWFRS: Use MWFRS
I'd say yes if the At < 700. Think about a small one story bay where an interior tube column supports a light roof. The uplift on that part of the roof might be quite higher than the dead load under the .6D + W combination so the footing does then try to hold the column down under C&C.
Can anyone elaborate where the 700 sf value comes from? I find this an interesting topic because if you believe that C&C or MWFRS is only based on tributary areas, than footings, shearwalls, and diaphragms would need to be designed for C&C loading if they had small enough tribs. My point is I don't agree with this since the C&C graph has trib areas ranging from 1 sq foot to 1000 sq feet. If the intended cutoff was 700 feet why doesnt the graph stop at that point? What wind loads would you use to design a wall stud connection to the foundation that is supporting roof trusses? Do you use C&C wall pressures based on the trib of the stud (shear connection) and C&C uplift based on the trib of the truss (tension connection)? Do you use C&C based on the trib of the entire assembly so one pressure for shear and tension calculation?
With that said, we tend to use the surface approach. If something receives its wind load from one surface, than C&C is appropriate, if it's from more than one surface a MWFRS approach is used.
So in my opinion, the answer to the question is C&C needs to be checked for the beam since it receives wind only from one surface, the roof truss (I consider the truss top one surface, even if it's dual pitched). The trib area should not determine what type of loading to use, only how much loading if it's determined to be C&C. And MWFRS would need to be checked if it was also part of the lateral resisting system.
As a final comment I should state all my wind opinions are based on ASCE 7, so they may not apply to other codes I have not read or studied
It says it is permitted to be treated as MWFRS when > 700, it doesn't have to be. It may be that some of those that wrote the ASCE provisions agree with using a surface based definition, what we are talking about is more engineering intuition.
CFSEng - I really disagree with you. Wind varies extremely up close but averages out over large areas. M
My example above - designing a footing to resist C&C wind uplift on a roof. The footing is separated from the application of the direct wind by what...5 or 6 elements? (anchor bolts, base plate, column, beam, joist, deck, etc.)
But if the bay is only 10' x 10' then the potential for a large variation in wind pressure at that column location via the roof uplift is much greater than if the bay was 50' x 50'.
Using MWFRS for that footing would be foolish.
My take is that the basic wind that you use for EVERYTHING is C&C wind. If you have large trib areas - or if it is truly part of the main wind force resisting system then use MWFRS wind.
This isn't all that complicated to understand. If you've ever had a wind tunnel test done on one of your structures you would see how there are peak variations of wind that occur all over the place.
I agree that ASCE 7 isn't all that consistent. In fact - I think the wind provisions are a complete mess.
I think what's important about distinguishing between MW and CC is that MW is providing stability for the globl structure, CC is for a local element of the structure. Getting hung up on the "receives wind from more than one surface" pseudo-definition is that ASCE does not use this as a definition, but rather only as a hint toward intent. LFRS do receive wind from more than one surface generally, but that does not suggest that every element that receives wind from more than one surface should be construed as MW.
I agree with JAE and have always followed using MW for things like braced frames, shear walls, diaphragms and so one are MW because these elements of the structure provide stability for the global structure. All others, including trusses and footings, are CC unless the tributary area is greater than 700 ft2.
Well as professionals we have the right to disagree. My take is you do not use CC wind for everything. I agree this is not too complicated, CC is for localized pressures, MWFRS is for envelope. In JAE's example, I agree the footing would be checked for CC uplift since it receives wind from only one surface (the roof). That's not to say it couldn't also be checked for MWFRS if it was part of the lateral system used to resist overturning, base shear, etc
My point is I would check this for CC not because of its trib area, but because of how it receives the load.
CFSEng - well put. I agree we disagree on this and this is nothing new - I know a lot of engineers who go in circles trying to meet the intent of ASCE 7 wind - I'm one of them.
Reading the definition of components in the ASCE7 Commentary, I think the key word is "directly." Unfortunately, "directly" is not defined. Examples are given. The beam is not one of the examples.
In my mind, "directly" means recieving the wind through a rigid path, that is, through members that work in tension, not bending. A bending member, such as a wood truss, has the ability to relieve local peak pressures before dumping the force to the beam. The shock element is gone, the way I see it. I don't believe ASCE intended this member to be designed for C&C pressures.
Just a comment about CC vs MWFRS (I apologize for hijacking this thread but I feel this is important to understand)
Take a look Figure C6-6 in the commentary of ASCE 7-05. Note where the peak pressures occur in the picture. You can see why they require high edge zone pressures when looking at a deck attachment at the windward edge or on the leeward side of the ridge. Now you can see on the opposite side (leeward) near the wall the uplift suction is very low. If you were to analyze this frame using just the CC tables, your pressure distribution would be much higher than the picture shows since you would use edge zone pressures in areas that do not recieve edge zone pressures simultaneously. This is why you have to consider how something gets its wind load, not just its trib area. This is also why the truss people argue the truss reactions should be designed for MWFRS and not CC pressures.
Trib areas on the CC table only account for load averaging, they do not account for wind directionality
Girts are mentioned in the Commentary as a "component" taking wind load from the siding which is considered "cladding". Is a wind column with eff. area < 700 sf also considered a "component"?
If the wind column gets its wind load from the siding, I would say yes. If it gets its wind load from the girts, I would say no. By "Components recieve loads directly from cladding", I think the code means "in a straight line." Once the loads have to turn a corner and put a member (the girt) in bending, that is no longer direct.
The wind column receiving load from a girt is not really part of the MWFRS. It seems more like a component but does not receive load directly from the cladding.
Are you saying it is part of the MWFRS or to just design it for the loads from MWFRS?
