Penndot Bar7 - Lateral Torsional Buckling

[bpstruct]

I am checking through a noncomposite steel beam analysis for a bridge beam (inventory/operating rating). It seems that the software output produced by the other engineer does not check lateral torsional buckling. I can't make sense of why the software is not checking it. The diaframs are spaced at about 15 feet which is beyond even lr for this beam. I would think for the compression flange to be considered braced, the friction between the deck and top flange alone would not suffice. That is the only rationale I can come up with behind why it is doing this. Has anyone else used this program?

 

[BAretired]

Perhaps the program assumes the top flange is fastened to the deck with connectors and is not relying on friction.

BA

 

[homostructural]

What are the boundary conditions of the bridge?
Is it simply supported?

All about Eurocodes:

http://www.DesignToEurocodes.com

 

[Qshake]

How is the bridge deck formed at the top flange of the girder? Does it have a 3" wide haunch on each side of the flange? Or does the slab sit directly on top of the girder flange? If it is the latter, there are several papers written on composite action with this additional concrete even though there are no shear studs for a conventional composite beam analysis. This additional concrete may be considered to provide the necessary bracing. I've seen some designers use and others not.

Regards,
Qshake

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[Qshake]

Another thought that's not mentioned as part of the description is whether or not there are metal stay-in-place forms used in this application. That also may be used by some to provide lateral resistance to the flange.

Regards,
Qshake

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[paddingtongreen]

1. Please explain "15 feet between diaphragms", is that 15 feet between lateral beams?
2. Are you saying that the metal deck was not welded to the beam? ("the friction between the deck and top flange alone would not suffice").

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Qshake]

Paddingtongreen - It is not uncommon for bridge decks to be formed using the old conventional way using plywood forms. Other options are 3" precast concrete panels that sit atop the flanges of the girder and metal SIP forms that are tied to the top flanges of the girders.

It is also not uncommon to find non-composite construction depending on the span length or that the bridge is old enough no composite connection was used. In cases where no shear connectors are used and the concrete deck doesn't extend past the top of the top flange a reliable connection to provide lateral restraint can't be had.

Yes, the spacing of the 15' is between cross-frames or diaphragms (or both as used in bridge terminology.). Diaphragms generally refer to transverse members that use both shear and bending to resist forces between bridge girders and cross-frames describe transverse members that resist forces primarily in axial action, truss members.

Regards,
Qshake

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[paddingtongreen]

Qshake, Whenever I used conventional plywood forms, I set them below the top of the beam so there was a positive lateral lock, I don't know of anyone doing this differently. If the bridge in question relies on friction, the concrete must have been placed level or above and haunched down to the top of the beam. I can't imagine why anyone would do that, it seems like going out of ones way to cause a problem. I was taught to set the concrete below the top of the beam when I started in the early fifties so it isn't new, it has been going on since the comparatively dark ages.

I assumed it was metal deck from the OP but that would have been puddle welded to the beam and would provide lateral restraint, even if not composite.

I have to admit though, I never ventured into the dark arts of bridge design, so I might well be missing something, especially nomenclature.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Qshake]

I've worked in many states each with differing construction techniques and it seems that all are staying away from embedding the top flange (composite or not) nowadays. I suspect this is due to the additional forming costs. Also it seems to be just going away due to the popularity of precast concrete deck panels. Though not all DOTs are allowing the panels due to reflective cracking. At any rate, what was once standard construction, embedded flanges, is no longer the case now.

All that said, the OP didn't elaborate on the deck construction nor the forming method so at this point it's all conjecture. Not even sure what era this bridge was constructed.

Regards,
Qshake

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[bpstruct]

Sorry for such a late response, and I appreciate all of the feedback. The question I had was pretty specific to a particular software "Penndot Bar7". The software output doesn't show anything about the deck forming input. Based on the details and age of the bridge, I have assumed that it was in fact plywood formed. The deck does not extend below the top flange. There is a haunch above the top flange...bigger on the center two steel beams, smaller on the outside (to account for crown).

Thanks again.

 

[Qshake]

In that case, if the bracing provided isn't spaced close enough then I'd have a hard time with the compression flange being braced. And would report it as such.

Regards,
Qshake

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