Design of Moment Connections
Design of Moment Connections
(OP)
As part of my small practice, I often design moment connections for steel fabricators, where the EOR has delegated that responsibilty to the steel fabricator. I am often faced with the problem that rather than give the design moments from the structural analysis, the EOR has a blanket statement requiring the full moment capacity of the beam to be developed, even though the connection is only for lateral load resistance.
Many of these situations would result in the moment capacity of the supporting column being greatly exceeded, especially for an exterior column connected on the y-y axis. It is my opinion that this is not an acceptable condition. I would appreciate your input.
Redhead
Many of these situations would result in the moment capacity of the supporting column being greatly exceeded, especially for an exterior column connected on the y-y axis. It is my opinion that this is not an acceptable condition. I would appreciate your input.
Redhead






RE: Design of Moment Connections
Standard practice in my neck of the woods is to provide un-factored reactions at the joints to be designed. A detail showing the required/suggested type of moment connection is usually also included.
RE: Design of Moment Connections
RE: Design of Moment Connections
RE: Design of Moment Connections
I tend to agree that these blanket statements for "developing the full moment capacity of the beam" are improper. Does that mean develop Mp necessarily? Here at my firm I've run into opposition with this when I say we should put reactions on our plans or design the connections ourselves, but the senior engineers here like to do it their way. When I ask about it, "that's just how we do it, we've always done that" and it isn't going to change. I just design the connections myself and that works for me. Typically we use a full penetration weld at the flange connection for moment, single plate for shear if possible and that takes care of it.
Generally I would expect that you would design for either the Mp of the beam or the Mp of the column, which ever is smaller while maintaining the fixity necessary not to alter the frame analysis assumptions too much. Afterall it seems that most beams are governed by deflection, or soemthing other than their moment strength so it doesn't make sense to have to design for the full moment capacity of the beam if you can provide the fixity required. Still, if you are not the EOR you have to follow his rules as best you can even if they are an ignorant way to do things, he is the one in responsible charge for the building.
RE: Design of Moment Connections
1. UcfSE: I am required to sign and seal the design of the moment connections, so I am not off the hook as far as liability is concerned.
2. JedClampett: The beams have been designed and sized as simply supported. The moment connections are purely for wind and seismic (in New Jersey). The seismic requirements are based on R=3.0, i.e. the special AISC requirements are not impacted. The lateral load frame analysis performed by the EOR should have produced actual design moments, which they are either reluctant to release, or can't bother to retrieve.
Redhead
RE: Design of Moment Connections
RE: Design of Moment Connections
Yes, the EOR shares liability. But why should I incur it in the first place with a questionable design?
Redhead
RE: Design of Moment Connections
RE: Design of Moment Connections
The Australian Steel Code AS4100 stipulates a minimum connection moment capacity of 0.5 x member moment capacity, for rigid frames. There are similar rules for other connection types. These rules ensure a reasonable level of robustness.
As for the effect on frame behaviour if less than full capacity is provided at the connections: I analysed a portal frame with and without reduced stiffness at the joints (3 no. total), subject to horizontal and uplift forces. Reduced stiffness was modelled as 43% of member stiffness over a length of 10mm. This resulted in 0.5% greater sway deflection and 0.3% greater peak moment, negligible increases I think you would agree.
John
RE: Design of Moment Connections