twin beams combined torsion

[tchconsulting]

I have a disagreement with a local authority checking engineer concerning torsional restraint for interconnected beams and would welcome other views.

The side/side two beams 8 meter length(305/165 UB & 305/127 UB)carry the two skins of a domestic cavity wall and are intended to be interconnected at 1.5metre centers by means of bolts/spacers at the center of the web and with 100mm wide steel plates (welded or bolted) to the full width of top flanges at the same positions. My design assumes effective length for the beam of 1.5 meters and my feeling is that the beams will restraint each other against torsional bucking. The checking engineer's view is that the twin beams could buckle "as one". Is this possible, is this realistic, or it just an oversafe consideration?

 

[jayrod12]

A sketch would go a long way in helping us understand the configuration.

 

[tchconsulting]

approx detail attached

 

[EngineeringEric]

Both beams can buckle together. Both beams can "sway" to out/in together, there is nothing bracing them.

think about the case when both are loaded with large loads, when beam-1 wants to buckle-out then Beam-2 will "brace" Beam-1 by taking lateral load, if Beam-2 doesn't have sufficient capacity to resist the lateral load and since it is also loaded... this lateral load will induce buckling in beam 2 as well. now Beam 1 and Beam 2 want to sway out and there goes the system.

Basically you do not have anything bracing the system except the composite weak axis section properties... which the bar isn't doing anything substantial.

Or that is how I would look at it...

 

[XR250]

Use a continuous plate on top to connect them

 

[jayrod12]

Agree with the EEs. I could see them buckling as a single unit. Even if only one of the beams was loaded.

 

[tchconsulting]

Your views are most appreciated. I can understand the views of the EEs and have in the past used the option of full length plate to the top flanges as means of considering full restraint, however, my thinking was that if i use an effective length equating to the spacing of the restraint positions, I could then still assume some degree of resistance to torsion, rather than treating the beams as two separate entities. Please feel free to tell me why I am going in the wrong direction

 

[EngineeringEric]

You can get some resistance... just very little relative to anticipated loads. Think about what is bracing Beam 1 from buckling out, What is bracing Beam 2, What is bracing both? How much lateral load can each beam take at each brace once both are fully under vertical loads?

 

[KootK]

I agree with your assessment completely TCH. Presumably, lateral torsional buckling is the issue here, not straight up weak axis buckling. A composite section LTB buckling mode might be possible but the capacity on that would be astronomically high.

I think that you should lose the top plate altogether. What you need here is vertical shear transfer between beams. The tubes will get that done and the top plates will make little difference.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

I just realized what you're doing with the bolt in the spacer. That is questionable.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

Ok TCH, I still think that you'll be okay here. Check out the attached sketch for what I see as the LTB resisting mechanism. If you're going to do battle with a code checker over this, you may want to assess the stiffness of the bracing mechanism before suiting up. I'm not sure how this is done in Europe but there are definitely provisions in AISC.

If you do this again, I'd recommend omitting the tube spacers and adding a few extra bolts as I've shown in the lower portion of the sketch.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

I like your suggestion Kootk, however how would you tighten the interior bolts you are proposing. I'm assuming if his plates are at 1.5m centres that this "beam" has a long span and so they won't be accessible from the beam ends.

 

[KootK]

I was hoping that there would be enough space between beams to get a hand inside and do it from the top. If not, maybe one could tack weld nuts to the underside of the flanges. At that point, it might make sense to just weld the plate down.

If space allowed the installation of bolts, I'd really like to have stiffener plates in each beam that overlap in the space between in a bolted connection. Wishlist.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

A poster in another thread supplied this paper (attached) which is quite relevant.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.