Beams on the topside of a concrete deck?

[LarryAndrews314]

This is a picture of a large parking structure being built on the site next to the one I'm on right now. It has me puzzled because it's a concrete deck over a tunnel(about an 18' span) and it has the concrete beams on the topside of what I reckon is an 8"-12"" deck! The two downsides to this I can think of is you lose the continuous bracing along the top of the beam and more importantly ductile failure. My question is do concrete beams do something similar to compression flange buckling and is ductile failure lost in this set up? I'm having real difficulties coming up with any positives besides aesthetic. I could see it being a little easier to place rebar and form if you're ok with a CJ between the beam and deck.

P.S. A review of the photo make's me curious about how much that longitudinal beam is doing. Is it beneficial bracing than it's detrimental pointloading of the span?

 

[JedClampett]

For relatively shallow concrete beams (which these are), it seems that compression flange bracing is not that much of a concern. As a matter of fact, it has only been in the last couple code cycles (I don't have my code books with me) that lateral bracing has even been mentioned for deep members. And it seems if I remember right, it's not quantified even then. It's kind of mentioned as a good idea.
As far as ductility, if you use the code limits for strain, you should be all right. Beams are designed without top flanges (slabs)all the time. This is no different.

 

[hokie66]

Agree with Jed. Upstand concrete beams like these are strong in torsion, so don't need bracing at the top like a wide flange steel beam, which is very weak in torsion. Not sure about the longitudinal beam, would have to know more about the design.

 

[LarryAndrews314]

Cool, I hadn't made the connection between torsional capacity and buckling before. Maybe they're a little ahead of the Washington bridge designers and that longitudinal beam is to prevent the deck coming down if an oversize truck hits it.