uplift on steel beams

huangyhg

uplift on steel beams
when calculating wind uplift from wood trusses that goes to beams, is it proper to just use mwfrs or c&c? thank you in advance.
find a job or post a job opening
you ought to also post this in the truss engineering forum.
my understanding is the first few ones should be checked for c&c
check the other forum where there are guys who do this for a living
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   
i agree, it doesn't make sense to exclude a   
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.