huangyhg

mwfrs or c+c
in a typical one story building with a joist girder/joist/steel deck roof system supported by on interior columns and exterior cmu walls, what kind of wind loading do you consider for design of these items??
cmu walls - mwfrs
columns - mwfrs
girders/joists/steel deck - mwfrs or c+c - the roof diaphram as a whole transfers wind loading back to shear walls, therefore are each of these items designed with mwfrs.
my confusion primarily lies with the roof system since to me they act together to transfer wind loading but as seperate items (not working together as a diphram) they would be more under a c+c item? so waht loading(s) would you consider when designing these items?? also what are the thoughts of connection designs of the deck to the joist and the joist to the joist girder and teh joist girder to the walls? i have heard two schools of thoughts on these connections as well and to me it seems that these connections shoudl definitely be designed to resist the c+c loading.
so waht are your thoughts on this subject??
chris
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you need to design the roof diaphragm and shear walls using the mwfrs loads. for individual elements, like cmu walls, roof girders, roof deck, etc. etc. you need to use the c & c loads. the c & c loads should be higher, since there's more of a chance of a maximum wind load on a smaller area than a whole building.
if it seems like some items are designed in one direction for one load and in the other direction for another load, you're probably doing it right.
i agree with jedclampett, but would also like to add the following: the c&c loads are higher for more of a reason than just the smaller area. c&c loads are also greatly affected by the location on the building - the end zones on the roof and walls see much larger loads.
jed is also right about designing each item for possibly both mwfrs and c&c - the most obvious is the cmu walls. i would design the shear wall loads with mwfrs and the out-of-plane loads as c&c.
first let me say thank you for all your help so far, i really appreciate it. second, let me clarify this:
1. shear walls (cmu walls for my individual case) - mwfrs wind on wall perpendicular to wind and transfer the shear loadings (thru roof diapham) to the walls parallel to the wind (shear walls).
2.cmu walls - (c+c) design individual walls perpendicular to wind load (for bending moment) with the appropriate c+c wind pressure.
*** summary for cmu exterior walls (shear - mwfrs) & (moment - c+c)
3. roof system - joists / joist girders / deck - (c+c) +/- pressures.
****the positive pressure by the wind are rarely controls any design for downward (bending of joists/j girder) in my area of the country.
*** the(c+c) uplift wind forces are provided on the drawings for the joist manufacturers so that they can design their joist and joist girders to resist the uplift pressure.
question: do joist manufacturers design their joists and joist girders to resist the full uplift (c+c) pressures generally or do they reduce it by a fraction of the dl allowed by asce 7?? just curious??
4. roof diapham - mwfs - all connections transfering loading to shear walls (such as the puddle welds between the deck and joists since these cause the roof system to act as a diaphram). am i forgetting any other crucial connections that should be designed under mwfrs wind pressures?
thanks again for your help.
chris k
to answer some of your questiona:
it sounds like you you have the right idea.
some critical design aspects that ae commonly missed are the attachment of the ledger angle to the cmu wall to transfer mwfrs loads. there's also a minimum value in the code.
i'm not sure what reductions in uplift you're referencing, but as far as what the joist people design to, it is kind of up to you. you can tell them no reductions. or better yet, give them a diagram of the downward and uplift forces. it's very common. another trick you can use is to assign a dead load for uplift only, less than the one for the downward cases. when you calculate your dead loads, you're probably conservative with their values. but for uplift, you want to be conservative in assuming that there's nothing there except your roof deck.
for uplift on joists and joist girders, we create a diagram called "net uplift". we calculate this from the equation 0.6d - w to get the "net" uplift.
i hope this helps!
i've put net uplift for joist design on the plans before and calculated it using the dead load i wanted and the load combination 0.6d+w.
it's a little easier in my mind for cmu wall design to think of it as mw for in-plane forces and c&c for out-of-plane forces. joists and trusses are part of the c&c unless they are being used to transfer part of the mw forces, which would call for a separate load case. an example of this would be a joist used for a drag strut. again, that would still be mw for in-plane forces and c&c for out.
keep in mind that if your effective or actual tributary area exceeds 700 ft2, you can use mw regardless. asce 7 6.5.12.1.3.
a metal deck diaphragm is another one where the mwfrs is used for lateral load transfer to vertical wind bracing and c&c loads for uplift and fastener design.
also when using c&c loads make sure to use the right trib area, so that the loads are not larger than they need to be. for masonry walls i usually use l^2/3 which is allowed in asce 7 because masonry walls are typically designed on a per foot basis. this rule would apply to joists as well if the trib area is less than l^2/3 use l^2/3 instead.
good point ash. also, in the same manner, you can use span/3 for the width of wall studs. this will reduce the wind load significantly.
thanks everyone for your insight. i really appreciate it.
you have all cleared up alot of issues for me.
chris

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