huangyhg

grating to transfer shear (lateral load)
i have a simple frame of w10 beams. a mechanical unit will rest on it. i am placing w-19-4 grating on the frame. there is very little lateral load coming from unit. what should i specify the contractor to do with grating and w10 joint so that i can assume that grating is acting as shear diaphragm?

two things to consider:
why do you want to use the grating as a shear diaphragm if there is very little lateral load?
does the grating manufacturer recommend their product for transferring in plane shear loads?
good luck.
i once contacted a grating manufaturer to see if the grating could be used to cut down the unbraced length of the beams. they said no way. i wouldn't count on the grating for any type of lateral stability.
i have always used angles placed diagonally under the grating to take lateral loads. usually just loose angles field welded to the underside of the top flange of the beams.
i'm not sure of your layout, but you can design the support beams to carry biaxial bending.
i agree with bbruins that grating doesn't provide much diaphragm shear values.
i would never use the grating in this fashion. there is no data that can be used to back up this utilization.
i normally add horizontal diagonals for bracing.
regards,
lutfi
agree with mwpc and lufti. use horizontal braces as required to stabilize the frame.
one further point. if you do use the grating in this fashion - how is anyone else going to know? you have a duty to inform future engineers who may need to modify your work.
horizontal bracing is a good idea as long as there are no conflicts with ducts that come out the bottom of the unit. sometimes they come out the side of the unit, but more often they come out the bottom. if there would be a conflict, i would then design the beams to handle biaxial bending like whyun says and omit horizontal bracing. this gives you heavier beams, but it's also easier to erect. i use knee braces on the posts, but you have to keep the work point at the bottom of the brace up far enough so that the roof penetration can be flashed with a boot. also, if your framing will be hot dipped galvanized bolted connections are prefered because welding through zinc is not recommended (although doable).
i assume this is a rooftop platform that you are doing. if my assumption is correct, another consideration is the height of the platform above the roof. you should leave enough space between the bottom of the platform and the roof surface so that when it's time to re-roof, workers can reasonably work under the platform to put the new roof down. depending on the size of the platform, roughly 3 feet is good. this could be less for a small platform.

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