Composite beam/girder

[hdn32]

Hi everyone,

Currently, I'm working on the addition to an existing mezzanine. The existing floor framing system consists of FSD composite concrete deck, composite filler beams and girders.

I am planning to utilize the same framing system for the new addition by adding one new girder parallel to the existing perimeter girder and filler beams span between the mentioned girders. I am aware that I may have to reinforce the existing girder to accomodate the new loads.

And following are my questions
1. What would it be the effective width of the existing girder in proposed condition (by adding a concrete deck to its side)? or in other words, should I utilized the minimum of 1/8 beam span and 1/2 beam spacing because of the addition of concrete deck for capacity calculation?

2. If yes, should the new concrete deck be tied into the existing? Dowel or ..?(FS Deck is parallel with girder)

3. I also have to fill large opening in the existing mezzanine with concrete deck, where new FSD will run perpendicular to existing perimeter filler beams. Again, can I tie the new concrete deck to the existing to obtain a wider effective width instead of adding a new beam parallel with the existing to catch the egde of new deck?

Thank you in advance for any recommendation.
Regards,

hdn32.

 

[msquared48]

My initial reaction was that the existing beams would not be able function as composite tee beams unless the new concrete had continuity over the existing steel beam. I did not see how this could be achieved with what amounts to an existing cold pour construction joint at the edge of the beam.

Drilled dowels do not work for me to achieve any continuity as the slab could be cracked and spalled to install them. I have seen dowels sawcut into the top of the slab and epoxied, placed perpendicular to the construction joint of the two slabs (not cutting the slab rebar though). With enough of them, you might be able to justify the dowels as shear studs to engage the action of the slab addition. If you do not want to do this, I would use only the flange to one side.

Mike McCann
McCann Engineering

 

[lkjh345]

hdn32:

FWIW, my personally opinion is I would go the ultra-conservative route and not count the on the new concrete acting compositly with the existing beams and deck.

I suppose there would be a way to tie the new and old slabs together to justify adding the new deck into the transformed section but being rather conservative, I would not.

You may want to look at re-analyizing the existing beams using LRFD. If the existing mezzanine is fairly old, it may have been designed using ASD. Re-analyizing the existing beams using LRFD may squeeze a bit more out of them.

Also, keep in mind, if the orginal framing was designed using unshored construction, it is often the unshored condition with wet concrete that drives the beam design. After the beam goes composite with its deck, it may be considerably stronger than it needs to be.

TTFWIW.

 

[jike]

I would suspect that if you real careful in your detailing, you might be able to take advantage of it, but only for load applied after everything is cured out. Unless you have a very high LL, this may not be advantageous. The other problem is you may not have enough studs to transfer the load. Additionally, you must transfer shear across the interface between slabs (vertically for gravity loads and horizontally for the composite action).

I would more than likely just go with what you have and not count on additional composite action from the new slab. In fact, I probably would add a new beam only a few feet away from the existing to support the new slab.

 

[hdn32]

Gentlemen,

Thank you very much for sharing your kind opinions.
Regards,

Hdn32