huangyhg

proper lateral bracing
i have 2 questions regarding the lateral bracing of beams.

first, when can a concrete deck sitting on a beam be considered to adequately brace the beam laterally? i know you can put shear studs in the beam, which would obviously brace the top flange from lateral movement. if there are no shear studs, and the form work for the concrete pour was placed just below the top flange, can that layer of outside concrete really be considered to brace the beam? also, after years of service, there will most likely be spalling in some areas along this beam, which would make me think i would only want to consider a deck as a lateral support if the beam has shear studs, or the pour occured at some depth of the web. i don't think i would want to really on friction as a lateral brace either.
second, can you brace the beam with a strut to the adjacent beam? i would think that you shouldn't brace to a beam that also needs bracing, and then consider both beams adequately braced unless the bracing was not perpendicular to their webs. yet, i've seen this kind of bracing a lot under bridges.
i've looked in the asd 9th edition, and skimmed through the aisc 13th edition, and have yet to find any information regarding this. does anyone have a good resource?
thanks a lot for any input
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in terms of a single strut bracing two sections which need bracing, it depends on the connection and what they are bracing against as to whether this works. if you are only bracing against lateral torsional buckling and have the single brace connected to both
does the concrete deck have a metal deck form as well? if so, when the deck is perpendicular to the beam, we consider it braced. when parallel, i consider it unbraced, unless other beams are framing into the top flange.
if the concrete slab is placed on formwork level with the bottom of the top flange, i think the top flange is well braced laterally.
i agree with gywnn. i cannot envision concrete on top of steel without shear studs. composite action saves so much steel it is unreal.
with the shear stids, lateral bracing of the top flange really becomes a non-issue. however, you still have to consider the bottom flange where negative moments occur over columns at any frames or cantilevers. intermediate purlins or diagonal struts back to the slab work well here when needed.
mike mccann
mccann engineering
take another look in your 13th edition manual->appendix 6.
for more info, try to get a copy of the yura/helwig aisc bracing seminar notes.
i consider the situation you describe as a continuously braced beam. i don't think it matters if the deck is perpendicular or parallel. according to salmon and johnson, it doesn't take much to brace a beam flange.
daveatkins
i've seen metal deck considered as lateral bracing when the metal deck was puddle welded to the beam at a specific spacing.
fwiw and slightly off topic:
imho: shear studs are cheap only above a certain threshold. it is my understanding that the stud welding equipment must be calibrated and tested every time it is moved to a new job site. this takes some time and expense. if you only have a few studs on a job, it may be cheaper to go with non-composite steel.
steel fabricators and erectors in my area (nebraska) tell me somewhere around 500 to 750 studs for a job is the break even point. if you have a project with a small mezzanine, for example, with say 10 beams, it is unlikely that using composite action will save enough steel to pay for the calibration and testing of the stud welding machine.
jmho.
i agree with daveatkins: if there is a metal form deck attached to the beams then i would consider the beams fully braced.
also fwiw and also slightly off topic:
it is important to consider the use of a building before deciding to go with a composite system. i've seen several cases in industrial facilities where the original floor design used a composite system and due to changes in the processing they needed to cut new openings through the floor. this is very difficult to do with a composite floor.
thanks for the inputs.
this beam is an existing beam and did not have any shear studs installed. there was no metal deck form, and the concrete around the top flange has spalled along its length in various magnitudes. the fixed strut connection is a good point, that makes sense that it would be considered braced because for the beams to move it would have to bend the bracing.
one more quesion on shear studs. when does thermal growth/contraction become an issue on the studs? this beam is perpendicular to the length of the pier, and the deck on this side is very long with no expansion joints (~400 ft). if the expansion is not an issue, i would think another option would be to weld a plate to the top flange and bolting it to the underside of the deck.

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