Flexible Moment Connections (PR, Type 2, etc.) 1

[enginerding]

We are helping a fabricator with some connection design where the EOR simply provided drawings that specified "Steel fabricator shall provide full moment connection between all beams to columns with the design of full capacity of the section".

We were directed to provide field welded connections, so we designed connections with bolted shear tabs with CJP welds from beam flanges to column flanges or stiffeners in column webs, depending on the orientation of the connection.

This design was rejected by the EOR, and the EOR stated that they want Type 2 connections and pointed to page 4-100 of the 9th Ed. AISC Manual. (This requirement was not stated in the drawings or specs.)

I was under the impression that FMCs were used so that the connection could be designed for something less than the full moment capacity of the beam. Am I missing something? Doesn't it kind of miss the point when we have to provide the capacity of the full section?

According to AISC, FMC connections result in heavier beams, so my design moment, the "full capacity of the section," is even higher than it would be if the whole structure were designed with FR connections. Do I understand this correctly?

 

[ishvaaag]

Well, a fully fixed connection will have typically the maximum moments at the connections and won't need any relative rotation at the joint, nor it seems there will be any such understanding in the calcs. So a proper understanding of the matter needs to know how the design has been made. Maybe the EOR is privy to the info of the beams having been designed for partial fixity at the connections, what may be causing them wanting the change of your design.

Just as an example, the lateral forces assumed taken very rigidly by some walls, the beams may have been designed for economy for qL^2/16 both positive and negative moments (instead of the controlling qL^2/12 negative plus qL^2/24 positive moment) but this relaxation from the elastic behaviour can't happen without some relative rotation at the joint; a case like that may require both the full moment capacity of the beam at the connection and some degree of relative rotation, as in your case. Maybe talking with the EOR or designers can explain the why of your particular situation.

 

[Teguci]

FMCs are used to allow some rotation at the connections but to ultimately resist lateral wind loads. The thinking is to "ignore" some of moment at the connection caused by the dead load and just provide frame action for lateral loads.

The EOR will need to revise his drawing (ie you are no longer providing FMCs as called for on the current drawing) and the contractor should revise his bid. You should get paid to redo the connection designs but thats up to you to negotiate.

 

[connectegr]

If the EOR's intent is to provide a FMC, the connection design forces should be given. If flange plates or angles are provided for the full moment capacity of the beam, the expected ductility may not be provided due to the required connection material thickness. The connection detail alone does not provide all of the flexibility, the connection material must be proportionate to the load. To provide the expected performance the connection should be designed for the real forces.

If the FMC was not clearly shown or stated in the contract documents, there is a strong argument for and extra. FMC connections are not a typical choice and would not be the default choice if insufficient information was given.

http://www.FerrellEngineering.com

 

[Lion06]

If they say full moment connection and don't exclude full pen welds, then There is nothing preventing you from providing full pen welds.

Providing type II connections for a fully fixed beam makes no sense on any level. The benefit of Type II connections is that the moment demand is significantly reduced by designing for wind moments only. To design them for the full section capacity is unrealistic and impractical. Also, you can only two bolts in tension from angle to column. There are no prying equations for two rows of bolts on a single angle.

Additionally, the 9th edition manual isn't even a valid standard any more - it's not referenced in any building code - unless maybe they're using a very outdated code on the project, but it's unlikely.

 

[enginerding]

Thanks all.

We were able to get design moments for these connections in an RFI, so this helps a lot. Additionally, we are being paid for the revision.

The jurisdiction requires the 2003 IBC which references the 9th AISC Manual with Supplement 1.

I have had to use 1984 BOCA within the last couple of years because that was the code referenced in a local city - and not a small one either.

I have very recently had to use another current "homemade" building code that referenced the 1956 ACI 318. That uses working stress design!

So be careful before saying things like it's not referenced in any building code...

 

[Teguci]

As long as its not the Code of Hammurabi you should be good!