pre-composite stage for steel beam

[h-badawy]

dear experts

i finished a design of composite section (wide flange with concrete steel dick) without shore , the steel dick is corrugated sheet perpendicular to the secondary beams , i designed the secondary composite beams on two stages.
1-pre-composite stage:in this stage i take only the constructions loads plus the own weight of beams and steel dick only without the concrete, but i designed the beams as a single beam with unbraced length equal to full beam length without any flange restrain .
2-composite stage:in this stage i take all loads on the roof and i designed a fully composite beam with unbraced length equal to zero .
the thing is that the reviewer put a comment in my design that all secondary steel section is very conservative because at the pre-composite stage the dick plate will make fully restrained to the top flange of beam so the unbraced length will be equal to zero , and the nominal moment of the beam will be equal to plastic moment. (see attchment below from AISC solved example) .
my engineering judgement is that the plate is very thin (0.8mm ) to make this restrain to the beam and make such a diaphragm even the short time between two stages.

Thank you

 

[jayrod12]

We quite consistently use steel deck to brace beams for bending.

 

[h-badawy]

but in AISC solved example used expression "assumed" in the start so may be he wants to keep this issue to engineering judgment of the designer

Thank you

 

[jayrod12]

That's a fair opinion.

I'm just saying that in the my opinion, and the opinion of many many designers, steel deck is considered adequate compression flange bracing for bending.

 

[BridgeSmith]

Yes, as the designer you have the discretion to ignore the bracing provided by the steel decking. However, for a straight beam, it takes very little to restrain it against lateral torsional buckling (LTB). Even a light gauge steel deck, if it is attached to the beam, and the sections of decking are attached together to form a diaphragm, is usually considered adequate to brace the beam flanges.

Rod Smith, P.E.

 

[JoshPlumSE]

When the deck runs perpendicular to the beam, I usually take the unbraced length to be no more than maximum spacing between the studs. I suspect the deck is sufficient to fully brace the beam for MOST, but not all, cases.

 

[Guest262653]

When the deck runs perpendicular to the beam, I use the simplified provisions of Eurocode 3 and publication P360 - Stability of Steel Beams and Columns (Steel Construction Institute) to assess this issue, comparing the sheeting stiffness with the minimum sheeting stiffness for fully effective lateral restraint established in the code.

P360 can be found legally and for free at the following link: Link
Check pages 93 and 94 for a design example.

From my experience, for typical composite slab spans of 2.0m to 2.5m and for 7 to 9 m composite beam spans, it is usually necessary to consider a double span deck panel to comply with this simplified requirement.

 

[JAE]

Pre-composite stage: Include the wet concrete weight as well.



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[Agent666]

Also be aware that if you have any continuous composite beams that any areas with negative flexure (hogging moments), or cantilevered beams may mean the bottom flange also requires some additional restraint. Alternatively propping could be used, though typically prefer no propping and just design for these intermediate cases.

avscorreia's advice is similar to how I'd prove its effectiveness provided the studs are welded through the deck (this isn't always the case, so make sure you're going to get this on site and contractor understands it is required from a design perspective!).