Torsion in beam - Overkill for design?

[liam1369]

Hi All,

As part of a design for a roof perimeter beam, we have a 1.70m high parapet. The beam is 9.0m long and has a steel parapet post in the middle to prop the PFC windbeam at the top of the parapet spanning between main oversailing columns.

Conservatively, we have a moment base at the base of the parapet post, therefore, via calculation it is transferring a torsional moment of 12kNm into the mid span of the beam.

When running a TEDDS calc for moment, shear and torsion at mid-span position it is utilised to 140%!

However, it is not considering the secondary beams which frame in at 2.5m centres, one being very close to the post that comes in. The 130mm deep composite deck that is fixed to along its length and also we have 12 bolt end plates at the ends of the 533x210x109 UB.

I have attached a sketch to confirm section arrangement.

Does anyone have any advice or can pinpoint to some literature or guidance on practical justifications or considerations for perimeter beams like this, otherwise the beam size needs to increase which I think is quite a significant overkill.

Let me know your thoughts.

 

[MotorCity]

If the posts are at the same locations as the secondary beams, the secondary beams will try to resist the torsion or twisting of the perimeter beam. I would probably just check the torsion where any posts are located between the secondary beams. Even then, the slab could offer additional torsional resistance to the beam depending on how it is attached to the beam. I don't think you will find any literature on the design of a perimeter roof beam designed for torsion due to parapet loading with secondary beams framing into it. Simple statics.

 

[WinelandV]

I agree that between the secondary beams and the concrete roof deck, a resisting moment couple should develop.

Question: Is your parapet all the way around your roof? If so, what is the framing (if any) at the roof edge that is oriented 90 degrees to the view in your pdf?

Please note that is a "v" (as in Violin) not a "y".

 

[cliff234]

Here is what I would do,

Shift the post so it aligns with one of the perpendicular floor beams.

Specify a full depth beam-to-girder connection at the post location.

Check local bending in the girder web and add a stiffener on the far side of the girder (opposite the beam) if the web is too thin. Or, locate the stiffener on the same side of the girder as the beam and make the beam-to-girder connection a full-depth single plate connection with slip-critical bolts.) The stiffener (if required) should be welded to the underside of the top flange, but could stop short of the bottom flange (by the “k” dimension).

Don't put torsion in the girder. W-shape beams perform poorly in torsion.

 

[WARose]

Agree with what Cliff234 was saying: try to line up the posts with those girders and let that force go into them as strong-axis moment. And you may not even have to go with a fixed connection.....if the loads are light enough, in many cases I have put a moment on a "pinned" connection like that. (Those connections do have some fixity....not much (I've typically modeled about 10% of a fully fixed connection)....but some.) You've got to check prying and so forth on the angles. And you have to get a idea as to how much rotation will happen as a result of this. (Modeling the fixity as I mentioned.)

 

[liam1369]

Great responses, Cliff, are you able to sketch what you mean. I am trying to visualise but just want to be clear.

 

[liam1369]

Windlandv, fortunately at those locations, there are no secondary beams framing in so the bending moment and shear force etc is small. The wind loading on those parapets is more reduced as well so they are checking out ok.

 

[cliff234]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1659120532/tips/detail_vpofdc.pdf[/url]

Liam1369 - see attached. If the parapet is CFS framing, the most challenging aspect of the detail will be the moment connection of the parapet framing to the top flange to resist the moment. If the parapet post is steel (and angle or small HSS), it will be an easy fillet weld connection to the top flange.

 

[dik]

Can you have an HSS 'stub' going through the concrete with a couple of BAR straps to bolt the C parapet to? or just a couple of straps welded to the beam (have to be careful handling, they want to bend). Can you make the torsion 'go away'... that's the approach I would use. The beam at right angles to the spandrel will take out any torsion?

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

1) I don't like using the concrete deck for this. Why do that when you've got all of the amazing stiffness of the infill beams right there to grab?

2) Detailing a torsion problem out of existence is usually the most economical approach. As such, I would prosecute cliff's approach if at all possible. Even if it meant two posts per span for some reason, that would still probably be cheaper than your other options. If it were me, I skip straight to full depth stiffeners on both sides of the girder web unless there were 2000 of these things and real money to be saved.

3) Per MotorCity's approach, one could quite easily obtain a satisfactory approach designing the beam for torsion like this:

a) use the bi-moment approach from AISC design guide nine to estimate the axial flange stresses resulting from the warping torsion on a 2.5m torsion span.

b) keep flexural flange stresses low enough that that, when the warping stresses are added to them, the result is less than unity.

There is some, inherent waste in the design strategy in that, functionally, it forces you to design your beam elastically rather than plastically as we usually do for flexure alone. Who knows though, as perimeter beam, it may well be governed by deflection rather than strength. And this shouldn't affect vertical deflection at all.

 

[CDLD]

[/Check local bending in the girder web and add a stiffener on the far side of the girder (opposite the beam) if the web is too thin. Or, locate the stiffener on the same side of the girder as the beam and make the beam-to-girder connection a full-depth single plate connection with slip-critical bolts.) The stiffener (if required) should be welded to the underside of the top flange, but could stop short of the bottom flange (by the “k” dimension)]

Cliff, what sort of check would you do for the girder web?
Would the web and stiffener (if required) be checked against the 12 kNm moment?

 

[slickdeals]

Please refer to AISC Design Guide 22 that discusses roll-beams.

 

[Tomfh]

Cliffs answer for sure. Align the post with the secondary beams and design the connection for the full load. The connections, stiffeners etc need to take the load. You can get 12kNm out of it quite easily if detailed accordingly.