Steel Beam Design With Precast Conc. Floor

[strguy11]

We are designing and office building with precast concrete floor on concrete beams. I have 2 questions.

1. In designing the steel beams, how are most people attaching the concrete to the beams? Either with embed plates or shear studs and grout? (We will be using a 2" topping for leveling purposes). This appears to be the only way to say the flange is braced.

2. Is the steel beam braced during construction? It appears that with either of the methods above, the steel beam would need to be braced by the erector, or the beam would need to be designed for dead load, and construction live load for an unbraced length equal to the span. How is this normally done, or what do i need to specify?

 

[structural01]

I've used embedded plates and welded to the beam.

The steel beam is not braced during construction. So, you must design the beam for full unbraced length of the beam with all dead loads and construction load. If you design the beam for this condition, than there's no need to specify anything more other than general notes.

If you design the beam for specific bracing requirements, then you should specify that the concrete slab shall be shored prior to attaching to beam.

 

[strguy11]

Thanks for the information. I assume this is typically "non-composite" with the steel beam. Also, do you typically put rebar (say 2'o/c) into the hollow core and then grout?

 

[structural01]

Yes. It is non-composite.

I don't specify any rebar for the pre-cast. But, it is up to the pre-cast manuf. to design the embedded plates for the beam to be welded to.

 

[geoffdale]

We actually do connect 10M (#4) rebars at 2'-0" or so spacing (z-bars) to the u/s of the top flange into the cores of the pre-cast, as well as anti-rotation bars. The steel beams are designed as top flange braced after construction, unbraced (unless temporary bracing is installed, normally the case in Canada) during construction.

 

[astructurale]

It sounds like you are using hollowcore slabs. You may want to visit

http://www.coreslab.com/atlanta/DetailsATL.pdf

They have a detail similar to the one geoffdale mentioned.
If your product is extruded you'll want to use details similar to the ones they use. However, if your product is a wet cast system then you can use welded plates. It's really dependent on the precaster. They will usually have a preference.

As for the beam it should be braced or designed to withstand any temporary conditions during erection.

The precaster you are using should be able to answer these questions for you. If this is prior to selecting a precaster then you can always call one in your area. Most are very happy to assit you or at least point you in the right direction.