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Shear flow in web to flange welds in a composite girder

Shear flow in web to flange welds in a composite girder

Shear flow in web to flange welds in a composite girder

(OP)
Hi all,

I am currently designing an asymmetrical plated girder acting compositely with a RC slab and trying to calculate the shear flow on the interface of the web and the flange.

While I think this is straightforward for a non composite girder, using the well known f= VQ/I applied on the larger of two flanges, my question lies how this is changed when analyzing a composite girder.

Assuming a plastic distribution of the longitudinal shear force on the shear studs meaning there is a constant longitudinal shear along the entire beam acting from the RC "flange", I am guessing the shear flow due to this shear on the top flange to web interface will be zero?

In that case, assuming we have elastic stresses at ULS, would the shear flow on the top flange be calculated through f=VQ/I by determining:
Q= (steel flange area only) x (lever arm to composite ENA)
I= second moment of area taking into account the full composite section (steel girder plus the "equivalent steel" concrete flange.

In which case, the shear flow on the bottom flange would command due to normally being larger size in a composite girder and also due to distance to ENA being larger because of the concrete flange driving its location upwards on the steel section.

I hope all these makes sense and please provide your feedback.

Thanks.

RE: Shear flow in web to flange welds in a composite girder

Quote (OP)

I am guessing the shear flow due to this shear on the top flange to web interface will be zero?

I don't think so. If your plastic neutral axis falls below the t0p flange to web interface, than all of the shear load delivered to the flange by the studs will make it's way to that interface. And, even with the plastic neutral axis higher than the flange to web interface, you'd only get rid of all the stud shear if the tension in the top flange were able to balance all of the compression above the neutral axis. And that would make for a pretty strange composite beam.

For the situation that you've described, I think that I'd just assume that the tension flange yields and then develop that force along the interface welds with a distribution matching the shear diagram. That is, of course, if I even bothered to vary the welding at all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear flow in web to flange welds in a composite girder

It also looks like you are mixing elastic stress distribution with ultimate strength design. I would take the ultimate compression in the slab plus assuming the steel flange yields and then design the flange to web interface for that total load.

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