Torsion or not?

[StrP88]

I appreciate if you can tell me if the beam A (upper beam) causes torsion in beam B (lower beam)

Thank you

 

[StrP88]

 

[handoflion]

Critical to your question is what is happening past your extent lines.

 

[StrP88]

The upper beam (beam A)sits on a similar beam to beam B (4feet apart)
The lower beam (beam B) spans 15 feet between columns.

 

[handoflion]

If detailed correctly, the moment from the point load at the cantilever can by taken out by a couple in the two beams below beam A, i.e. no torsion required in beam B.

 

[Teguci]

Yes. There will be some torsion placed on beam B when Beam A is loaded. Further, the other beam B (not shown in your sketch but 4 ft to the left) will have even more torsion placed on it since its support location isn't near an inflection point.

However, just ignore this torsion and run your load path assuming no torsion. We can redistribute this load because the beams and connections can deform a little and allow stiffer load paths to take more of their share of the load. This would only present a problem if you were dealing with a heavier load and thousands of load cycles (fatigue). Never really thought of steel "cracking" until I saw it with my own eyes.

 

[StrP88]

handoflion (Structural)
You mentioned, " if detailed correctly the no Torsion"
May I ask whats a correct detail for no torsion?
A sketch may help - Thank you very much

 

[BAretired]

If the load produces a slope in Beam A where it connects to Beam B, Beam B will rotate slightly to match the slope of Beam A. If Beam B is torsionally fixed to its supporting columns, then Beam B will experience compatibility torsion which is normally neglected in design.

If Beam B is free to rotate about its axis at the supporting columns, there will be no torsion as it will simply assume the slope of Beam A over its entire length. Such a connection would be unusual.


BA

 

[JoshPlumSE]

It's going to be based on relative rigidities. If the torsional stiffness of beam B is significant, then it will have significant torsion. However, most of the time, this type of beam is very flexible in torsion. If so, then the flexural stiffness of Beam A's back span will be much larger. So much so that you can assume the entire cantilever moment goes into the backspan and no torsion is produced in B.

 

[canwesteng]

As long as the system is ductile enough, you can distribute the load in any reasonable manner that satisifies equilibrium. So generally no one would try and take torsion in beam B

 

[gwynn]

If you are concerned about torsion, and that is a purely bearing connection as it appears to be, throw a rocker bar above the web of beam B and call it a day.