Reinforcement of composite Beam

[Italo01]

Hello,

I was hired to design a reinforcement for two parallel and equal composite concrete-steel beams. The beams were not designed for the load that they are subject and the deflections are above code limits. The problem that i'm facing is that i checked the quantity of stud bolts and the beam has partial shear connection. I thought of jack up the beam, eliminating the deflection, and weld a plate below to increase the strength and stiffness of the beam, but at the moment i do this, the value of η decreases and the value of the lower limit of η, given by eurocode increases, so i cannot increase the strength and stiffness without affecting the ductility. Does anyone have a suggestion to increase the strength and stiffness of the beam and comply with this code requirement?

PS: Although i'm using the brazilian code, i posted the Eurocode criteria because they are similar and i suppose a lot of people here are familiar with the eurocode.

 

[GeorgeTheCivilEngineer]

I think I'd be tempted to ignore the composite action now and just use it for lateral restraint.

If it's been over deflecting, what assurances do you have over the condition of the studs or the concrete interactions?

 

[Italo01]

Thanks George. I will check the beam without composite action, although i think will be complicated because the inertia of the composite section is much, much bigger than the steel section. If i could add a Tee section to increase the height maybe would be possible but the clearance is not good.

 

[Italo01]

Just so i could understand, does anyone know why the inferior limit of η increases if the bottom flange area is greater than the top flange?

What difference does it make to the ductility if the same stress must be transmitted from the concrete to the steel?

 

[GeorgeTheCivilEngineer]

Just a guess, but considering the shear flow, a large bottom flanges moves the neutral axis downwards, putting more shear through the studs. I'm not sure how much of these equation are analytical or empirical. There is a Commentary on Eurocode 4, but I don't have a copy.

Another strengthening option would be to insert new beams between the existing. Just to reiterate, excess deflection in services would be a red flag that composite action is already compromised.

 

[Italo01]

When you say that composite action may be already compromised, you think that i cannot guarantee te compression force of the model on the concrete?

Thanks again.

 

[Italo01]

With respect with the reinforcement. I was thinking of reinforcing both flanges because in that way, η inferior limit would not increase.

 

[JoshPlumSE]

Some ideas:
a) I've seen a case study (can't remember where) in which they shored the beam. Then ripped out some concrete in locations so they could add in another few studs. That was kind of shocking to me.

b) In a case where they REALLY needed to increase stiffness, I saw them add in a queens post truss to the bottom of the beam. By this I mean they added in a steel post at mid-span, then added in some poste tensioned cables to this post to take out the undesirable deflection. This can be tricky, of course, when you've got height restrictions on the beams.