Control of torsion in Hollow core supporting edge steel girder 2

[Aakalim103]

Hello all,

We frequently have WQ steel beams supporting hollow core slab floors. For the edge beams where there is significant torsion, we consider that the WQ beam are not capable of taking any torsion and tie the beam to the floor using rebars that are anchored in the hollow core seams on one end and either bolted or welded to the beam web on the other end. This essentially ties the beam to the floor and prevents excessive rotations. I have shown this in the figure attached.

Does anyone have any idea how should i size my rebar for a given amount of torsion?

 

[phamENG]

1) I think it's too conservative to assume no torsional resistance (unless the manufacturer says so - looks like a WQ is proprietary?). A closed box is second only to a round closed section in torsional resistance.

2) I'd approach it like this. Your bar is in tension and you'll have some bearing area. c should equal the allowable bearing stress of the steel on concrete. It may not be so simple as I've drawn it depending on how much grout you have, strength of the grout compared to the hollow core, etc.

 

[KootK]

This is what I'd do. Then a classical concrete flexural analysis per phamENG to get the bar sizing.

I would consider the beam to have no torsional stiffness for this purpose for three reasons:

1) It's conservative.

2) Any real torsional flexibility at the beam supports and it's all going to the slab once it's locked into it.

3) It's difficult to estimate what load winds up where with any accuracy.

 

[Aakalim103]

Thank you phamENG and KootK for your promp and helpful replies!

The WQ beam is indeed proprietary. In addition to the reasons kootk mentioned, I an told that the connection to the columns are not designed for torsion hence we try so minimize the torsion by tying the beam the floor.

 

[KootK]

You're most welcome Aakalim103.

Is some degree of beam torsional end resistance required for the erection load case? Or is the plank shored until the beam is grouted in? Probably the latter.

One thing that I've never been clear on in these situations is to what degree you can rely on the grout plug to not pull out from the plank core when pulled upon.

 

[HTURKAK]

Does anyone have any idea how should i size my rebar for a given amount of torsion?

The edge beams could be asymmetric slim floor beam , rectangular hollow section fabricated beam (RHSFB), even ,down stand beam. Two tie bars T10 mm for each plank are provided in Eurozone.

Lets assume conservatively no torsional resistance for the edge beam.. In this case, shoring the planks and beam will be necessary till the grout is hardened. After removing the shoring, the HC slab will deflect and eventually the friction and adhesion developing between HC slab and steel beam flange will resist to tension forces developing due to torsion. Not the TIE BARS..

The assumption that the side beam and connection to the columns will not resist the torsion is not correct. The side beams and connections shall be designed also for torsion developing due to facade loads and interior RH slabs.

The following SCI publication for this subject will be helpfull

https://www.steelconstruction.info/images/a/a0/SCI_P342.pdf

The following picture shows the edge beam options...