Proper Lateral Bracing

[Buzzbromp]

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

 

[gwynn]

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 members with a moment connection, this is fine. The strut will have bend for torsional buckling to occur.

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.

 

[strguy11]

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.

 

[hokie66]

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.

 

[msquared48]

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

 

[271828]

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.

 

[DaveAtkins]

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

 

[CTW]

I've seen metal deck considered as lateral bracing when the metal deck was puddle welded to the beam at a specific spacing.

 

[lkjh345]

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.

 

[MarcbSE]

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.

 

[Buzzbromp]

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.

 

[MarcbSE]

plates welded to the top flange and then bolted to the concrete deck would be a good solution in your case. It provides an easy to install positive brace for the beam.

 

[msquared48]

You mentioned a "pier". If so, and it is on piling, then I would not be so concerned with thermal issues as any movement so induced is probably accommodated by the lateral movement of the piling. But this is probably the devil in the details.

Mike McCann
McCann Engineering

 

[gwynn]

I agree with msquared48 on the thermal expansion issue. Piles or pipe columns often take up the thermal expansion with little resistance.

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.

 

[hokie66]

Thermal movement is probably the cause of the spalling you have observed beside the beam, but I still don't think you have a problem with the beam being braced. Is the beam visibly deflected horizontally?

 

[Vinny7]

I design using BS5950 (UK) In this it states lateral restraints have to be of adequately stiffness and strength. It states this can be achieved if the restraint is capable of resisting a lateral force of not less than 1% of the max. factored force on the compression flange.

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.

 

[Buzzbromp]

The beam has not visibly deflected horizontally. Its just that if I assume the beam is braced adequately, my bending stress is approximately 80% of my allowable, while if I'm not braced adequately, my bending stress is approximately 150% of my allowable. I would say that either my concrete deck is bracing my beam or the deck is never nearly loaded to the design. I'm just wondering as this seems a judgement call, would you typically be conservative and go ahead and install bracing, or since nothing has moved, leave it as be and check on it during a following inspection (years later).

 

[miecz]

I would say the answer lies in the amount of spalling. If the unsupported length within the spalls is way less than L[sub]c[/sub], then I believe you are braced. If you are approaching L[sub]c[/sub], then I would check Section 1b of Appendix 6(13th Ed), with L[sub]b[/sub] is somewhat longer than the spall length.

 

[DTGT2002]

In the interest of providing the suspenders to the concrete lip belt already in place, would drilling holes in the beam and installing some concrete anchors e.g. Hilti Kwik Bolts provide a reasonable solution, with an acceptable strength in shear depending on the spacing without being an overwhelmingly painful installation (vs welding overhead to add plates - fabricating stiffers w/ moment connections etc).

Just a thought.

Best of luck,

Daniel Toon

 

[apsix]

You could also use something doing the same job as a rail clip. A plate bolted to the slab, acting as a keeper, but not welded to the beam.

However if the slab underside is below the top of the flange it is probably OK as is.