Flexible Moment Connections for Seismic Loading

[CJJS]

I know these connection types have historically been used for wind only. But, I was wandering if anyone has used flexible moment connection when designing for seismic loading.

 

[ishvaaag]

Since I don't practice in a seismic zone I have not practiced flexible moment connections against earthquake (at least not by design). However there's technically nothing that prevents to build resisting systems that use some flexibility at the joints and must resist earthquake. However the code could prescribe otherwise and then the answer lies in the ruling code.

 

[JoshPlumSE]

If it is designed to AISC's seismic provisions 341/358 then I don't think there are any pre-approved connections that you can use. Maybe if you're in a Sesimic Design Category that will allow you use R=3 then you can do it.

 

[connectegr]

AISC prequalified connections apply to SMF (special moment frames). But I do not believe flexible moment connections are allowed in any seismic conditions. The AISC seismic moment connections are design for a force greater than the beam capacity. This would make flexible moment connections impossible.

http://www.FerrellEngineering.com

 

[connectegr]

I was mistakened. Today I noticed that AISC 341-10 does have criteria for PR moment connections within the Ordinary Moment Frame information. I have never seen this used.

http://www.FerrellEngineering.com

 

[WillisV]

connectegr - the PR moment connections in AISC 341 are true PR connections that require knowledge of the moment rotation curve - not FMC (Flexible, or in my mind Fake, Moment Connections) which generally mean the Geschwindner / Disque type of Type 2 or Wind-Only type connections.

 

[bookowski]

I have never been able to make sense of these connections. All buildings are designed for some level of seismic force right? So what does it mean when you say that they can't be used for seismic? Is it that they just can't be used in above a certain SDC or with an R>3? If they can be used for low seismic applications how are they modeled - the same way that they are treated for wind?

 

[CJJS]

bookowski... I was referring to low seismic applications with R<=3. I see connections used in mezzanine design that are designed as FMC. The connections I've seen are not prequalified Flexible Moment Connections. What I have seen with these connection types, is that engineers typically calculate the capacity of the connection to ensure that it will resist the lateral load moment. For FMC to work as intended, the connection must yield beyond the wind/seismic load moment. I have not seen where anyone looks at this condition to ensure the connection yields. If it doesn't yield, there could be excessive moment introduced into the columns under gravity loads since the columns are only designed for lateral load moments.

 

[bookowski]

Right, but I was asking about buildings. I see people saying that they can't be used for seismic - what does that mean?

 

[WillisV]

AISC 360 R=3 FMC OK. AISC 341 R>3 FMC Not OK, true PR OK within limits provided by AISC 341.

 

[JoshPlumSE]

WillisV is essentially correct. With one additional limit: Partially Restrained connections with R>3 can only be used for ORDINARY Moment Frames. Unless you do Conformance testing. That's because none of the pre-qualified connections (in AISC 358) are currently Partially Restrained connections.

 

[CJJS]

The attached drawing shows a connection typically used for mezzanine framing. I have been reviewing calculations for mezzanine design. In general, the columns are designed for moment resulting from seismic forces only, neglecting any moment induced by fixity of the beam-column connection. I recognize that the assumption is that the connection yields under gravity loads and thus will only transmit moment up to a certain point (at least as much as the seismic load moment). However, how would you go about verifying how much moment this connection actually can transmit? If the connection is too rigid, the columns will likely be underdesigned. Any thoughts on this would be appreciated.

 

[RFreund]

Not sure if this will answer your questions but it was a good discussion regarding Flexible/Type2/Wind Only Moment connections:

http://www.eng-tips.com/viewthread.cfm?qid=303493

EIT

http://www.HowToEngineer.com

 

[CJJS]

Thanks RFreund. That was an extremely helpful thread. Do you know if there are "pre-qualified" flexible moment connections published? The connection detail I posted is being used as a FMC. How would you go about calculating the capcity of this connection to transmit gravity load moments from the beam? I am trying to justify that this connection is flexible enough and that it will not transmit more moment than the moment due to seismic loading.