Torsion Dilema - Cutting hole in floor for stair 1

[cieg22]

Novice in need of HELP!!

We are cutting a hole in a floor of an existing building for a new stair. I'm calculating that this will cause a great deal of torsion in the beams that run along the sides of the opening--mainly because I'm assuming we are removing their torsional restraint. I'm hoping this assumption is an overconservative mistake, and that the beams will be okay. More explanation below:

The floor slab that we're cutting into is 6" reinforced concrete. It spans 7' to the secondary beams, which are W12X30's. The W12X30's span 17'. We plan to cut out an entire section of floor (the opening will be just less than 7'wide by 17' long). The remaining W12X30's will still support the sections of floor that remain along the outside of the opening.
I'm calculating that if these beams are considered to be unrestrained, that the load from the remaining floor will cause torsion in the beam. I've assume that since the beams would no longer be connected to the removed section of floor, that there would be no torsional restraint (this is where I think I may be making an overconservative mistake). With this assuption plugged into my model, I'm calculating that there would be severe rotation and torsional failure--even when I'm just looking at the slab dead loads.

Have I made the correct assumptions, or should I retry my model, assuming that the remaining portion of the slab (the one delivering the torsion) could also act as a torsional restraint.

THANKS EVERYONE FOR YOUR TIME AND HELP!!!!

 

[Galambos]

i would first question why a novice is making major modifications to a building.
secondly, a fixed connection would be required between the slab and WFs in order to induce torsion into these beams. would you consider a slab resting on a flange a fixed connection?

 

[cieg22]

Good points, Galambos. I've been in the business for 8 years--I just don't have much experience with torsion (hence, the "novice")

There are no records available, but based on observations, it looks like the slab is intergral with the beams--the beams are encased (although the condition of the floor and the encasements leave something to be desired).

Thanks for responding so quickly. Please let me know if you have any thoughts on this.

 

[cieg22]

One more note to clarify my last posting--the encasements were so bad in some areas that the beams were exposed enough to be measured. The model only accounts for these encasements as dead-load--and neglects any structural contributions they may make.

 

[Lutfi]

As I read your post, you will not have torsion. If the slab is simply spanning from beam to beam, you should not have torsion. I suspect this is the case.

You will have torsion in the floor beams only if you have them loaded a distance from the shear center (vertical centroid of he beam. Other factors cold induce torsion. However, I am not privy to you situation fully.

The presence of the floor slab will not prevent nor produce torsion. The load/reaction/load path has to be transmitted away from the beam centroid to induce torsion.




Regards,
Lutfi

 

[cieg22]

Thanks, Lufti.
What if my slab is integral with beams (and effectively a fixed connection)? Based on what I gather from observations, that apears to be the case.

 

[JAE]

Are your beams fixed at their ends? How are you calculating the applied torsion to the WF? Are you analyzing the slab as having a fixed end?

 

[Lutfi]

cieg22,

How did you determine that the slab is integrally fixed to the beam flange?

I would verify the following:

1. is the slab composite construction?
2. Is slab supported on some sort of floor deck system? If so, how the deck is is attached to the beam.

I would verify the slab to beam fixity before you can render final judgment.




Regards,
Lutfi

 

[cieg22]

Thanks, guys!
There is no deck pan--the slab is just reinforced concrete, about 6" thick. It's also not in the best shape.
I've been thinking about this a lot since my last post. I may have jumped the gun in assuming it was integral--it seems like the only way to handle this is to require the contractor to field verify the conditions once he get in there.

Please let me know if you have any further thoughts on this.

THANKS AGAIN!!!

 

[cieg22]

One more thought on the whole integral/not integral issue:
The top of the top flange of the secondary framing sits roughly 3" below the top of the top flange of primary framing. The bottom of the slab appears to sit on the top of the primary framing. Therefore, I calculate that the encasement of the beams in question (the secondary framing) includes a 3" haunch.

Could this be an indicator that the floor slab may not integral with encasements of the secondary framing?

 

[apsix]

I would have no problems in assuming torsion can be ignored in this case, subject to my comments below.
Whether the slab is intergral or not, it is likely that the support beam will twist only as far as dictated by the rotation of the slab at the beam (due to the slab defection), and therefore induced torsion will be negligible. Think what would happen if somehow the beam twisted more than the rotation of the slab; the load would then be applied at the flange tip remote from the slab span and therefore self-correct the twist until the slab bears on the full width of the flange.
Torsion will be an issue if the slab is cut on the span side of the beam shear centre.

What would concern me is the assumption that the slab will be adequate. As I understand it you are changing what are now interior spans into endspans. These new endspans will have increased positive moment midspan, how do you know that they are adequate?

 

[cieg22]

Thanks, Apsix! You're right, the increase in moment is also a concern, especially given the condition of the slab.
I'm planning to add extra support for the new end sections.

 

[dougantholz]

I would agree- you don't have torsion. The beam that is encased in concrete shouldn't have any more fixed a connection to the slab then shear studs connecting to the top. If this were a new building with a new slab edge would you check the beam for torsion with only a few inches of overhang?

The encasement can also help you. Look in the 4th or 5th edition of AISC and you will see steel beams encased in concrete can be treated as composite becuase of the strain compatability.

By the way I have done a similar modification to a number of buildings and never seeen a beam twist because slab was removed from one side.

Also a note - even though you may be missing a large amount of the encasement there is likely enough there to create a large torsional cross section to resist what ever small torsion may be present.

 

[J1D]

I thought pretty much the same. Stiffness and forces attracted are corresponding in the indeterminate structures.

1.If the beam has a negligible torsional stiffness (such the W12x30’s beams without encasement) or there is no torsional constraint between beam and slab, the torsion is negligible (as Lutfi and JAE pointed out).

2.If the beam is encased and torsionally connected to the slab, as a concrete edge beam with c.i.p. slab, they’ll work together to resist and transfer the forces, including torsion. The torsion along the beam should not be uniform, it varies depending on the restraint condition. This can be confirmed with an analysis model.