existing moment connection

[ukengineer58]

hi. i have an existing structure originally designed with moment connections to several beams although not high loads. under the new condition the moment Capacity is exceeded. so my question is could assume that the connection will not transfer the full moment and recheck the beam as simply supported. it passes. The truth is somewhere between i guess but to my mind the connection will transfer as much moment as it can then the rest back to the beam. what are your thoughts.

 

[MrHershey]

Concrete? Steel?

In any case, though the member may be fine I would be wary of serviceability concerns at the connection. If concrete you'll get a plastic hinge forming with excessive cracking. If steel you'll start fracturing at the welds which could start adversely affecting strength of both beam and column/girder.

How are deflections for the simply-supported case? Is strengthening the connection an option?

I'd be really wary about allowing a plastic hinge to form at the connection under normal loading.

 

[ukengineer58]

sorry steel. defections ok. my conjecture is that no matter how you design a connection the degree of fixity will never mean that you will transfer the design moment exactly. i.e. Simple connection will transfer some moment its just not Quantifiable.
so if it can't take the moment the forces will be taken by the beam. same way that in a simple connection some moment will transfer unless the connection or column yields locally and it goes back to the beam.

 

[hokie66]

Engineering judgment. Based on what you have related, I judge it to be acceptable, but the call is yours.

 

[JAE]

I think this is a case where you have to satisfy "deformation compatibility" in the connection.

In other words - when you analyze the beam for a simple span condition, there is a rotation that occurs at the beam ends where your moment connections occur.

That would be a maximum rotation that could possibly occur in the beam.
You would then impose that rotation on the connection and back-derive the forces/stresses in the steel based upon that deformation. The question is whether the connection can deform to that extent - either elastically or inelastically - without failing.

The concern here is that you may have enough flexural capacity in the beam at midspan assuming a simple span, but you may not have enough deformation capacity in the connection - it may start failing (unzipping is a term we use) and you would possibly lose your shear capacity of the connection - fall-down-go-boom.