Proper Lateral Bracing
Proper Lateral Bracing
(OP)
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
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






RE: Proper Lateral Bracing
The most often place I see single struts is for plan bracing of girders. This is generally for transfering lateral loads to supports before the deck is in place.
I would not rely on friction to brace a beam either. Shear studs are cheap, so I don't see why you wouldn't include enough to provide bracing.
RE: Proper Lateral Bracing
RE: Proper Lateral Bracing
RE: Proper Lateral Bracing
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
RE: Proper Lateral Bracing
For more info, try to get a copy of the Yura/Helwig AISC bracing seminar notes.
RE: Proper Lateral Bracing
DaveAtkins
RE: Proper Lateral Bracing
RE: Proper Lateral Bracing
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.
RE: Proper Lateral Bracing
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.
RE: Proper Lateral Bracing
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.
RE: Proper Lateral Bracing
RE: Proper Lateral Bracing
Mike McCann
McCann Engineering
RE: Proper Lateral Bracing
With a span that length the construction drawings deserve a look if they are still available (unlikely but worth a shot). You may find that at least some of the girders have brackets and ties welded to the top fange which provide adequate connection to the slab for bracing.
lkjh345
Fair point on stud cost. All of the instances I've looked at where shear studs could be used we've had 1500 studs minimum.
RE: Proper Lateral Bracing
RE: Proper Lateral Bracing
With precast concrete floors it also is not unusual for the floor to be treated at the restraint due to friction. I would assume this could also apply to insitu concrete floors.
Alternatively you could have vertical fin plate welded to the top flange of the beam at intervals during fabrication.
RE: Proper Lateral Bracing
RE: Proper Lateral Bracing
RE: Proper Lateral Bracing
Just a thought.
Best of luck,
Daniel Toon
RE: Proper Lateral Bracing
However if the slab underside is below the top of the flange it is probably OK as is.