Beam Connection - Fixed or Pinned 1

[MER3]

We are debating this in the office. I am designing frame that will be constructed as shown on the attachment. My question relates to how to treat the connection. My coworker is saying to treat the connection between the perpendicular beams as a simple connection (ie no moment transferred to the bottom I-beam as a torsional moment), but my gut is telling me that this isn't conservative. I don't expect the top beam to develop a full fixed end moment at each end, but I expect there to be some moment capacity that then develops a torsion in the bottom beam. My reasoning for this is that if this connection detail were used for a cantilevered beam, anyone would assume full moment transfer to the bottom beam and design for torsion.

I realize that no connections are ever perfectly simple or fixed and that certain assumptions have to be made regarding moment capacity. Therefore, what is considered prudent in this situation? I could take a WAG and assume a percentage of FEM is resisted by the end connections.

 

[KootK]

I vote for not bothering to account for the incidental moment that will arise in the connection. In most scenarios, there will be quite a bit of flexibility in the system to allow for redistribution. I agree that this is somewhat unconservative with respect to connection moment and beam torsion. I'm okay with that. Practical engineering is about identifying and dealing with that which is important. This is not important in my opinion. The cantilever situation really is quite different in that there is nowhere to redistribute the moments to in that case.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DETstru]

Second to KootK.
Pinned and don't worry about any torsion in the supporting beam.

 

[rb1957]

3 ... how stiff is the flange in resisting FEM ?

another day in paradise, or is paradise one day closer ?

 

[atrizzy]

Am I missing something? If the upper beam was cantilevered, why would this be treated much differently?
The cantilevered beam is infinitely stiffer to take moment in its strong axis direction than the top flange of the lower beam is to resisting the torsion. Seems like any imparted torsion would be quite small indeed.

 

[MER3]

Atrizzy - To answer your question, if the upper beam were cantilevered, the strength of the assembly would no doubt be limited by the torsional capacity of the top flange, not the flexural strength of the top beam.

Everyone else, thanks for the help. You all confirmed my hunch that this is normally disregarded. It's good to sometimes revisit fundamental questions like this to validate assumptions.

 

[dik]

The stiffness of the beam is flexure is so much greater than the supporting beam in torsion... good reason to neglect it... treat it as pinned as everyone else notes...

Dik

 

[BAretired]

If the top beam had been cantilevered, it would be a lousy detail.

BA

 

[SAIL3]

in any event, it is self-limiting...the support bm will only roatate as much as the supported bm deflects...