huangyhg

unbraced length of a beam
as most of you probably know, the strength of a beam in bending depends on the unbraced length of the compression flange. my question is, what defines braced? if i have a channel spanning 20 feet with another channel alongside of it 2 feet away, and they are connected to each other with a plate welded continuously along the top flange of each, are these beams continuously braced? i have another pair of these channels 6 feet away and they could be connected to the first pair on, say, 5' centers. if the first pair aren't considered braced, would this give them an unbraced length of 5' or is it still too wobbly to be called braced?
normally, the bracing comes from either a wood or concrete diaphragm, or wood or steel purlins framed 90 degrees to the beam you are bracing.
to rely on elements that are parallel to the same beam is not a good idea, unless they are really stiff elements. that's a judgment call.
mike mccann
mccann engineering
or do a custom section and call it not braced.
your pair of channels with a plate welded continuously to both is not braced, but the assembly forms a beam with a top (compression) flange which is 2 feet wide, so is much stronger than just the two channels.
you could check this as a beam lying on its side, 2 feet deep. the only load the beam lying on its side needs to resist is the buckling force in the two channels (say, the summation of 2% of the force in each channel's top flange). the stress induced by the buckling force would be added to the stress induced by the direct vertical loads.
daveatkins
i would say-tie the bottom flanges of the channels every so often and call it a box shape and forget about the lateral torsional buckling. given, that the plate that you have welded on the top is sufficiently rigid.
dave has a good answer - if in doubt, do the numbers. there are explicit rules in the code regarding what loads the braces need to take.
i would not provide a continuous weld though, a staggered weld would be less likely to warp the channels.
if you are treating it as a built up section then you need to check the shear flow in the welds using v = vq/it.
i'd say quit trying to home-brew the definition (just ribbing, guys! ) of braced and read aisc 13th ed. app. 6 and the yura/helwig bracing seminar notes. the notes, especially, talk about stuff like the proposed situation.
it seems most people are saying this situation is unbraced. please see the following thread,
sorry, try this
"bracing 2 unbraced members together does not help as they could then both buckle in the same direction."
this is a common misperception. bracing two unbraced

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