Stiffness of Partial Moment Connections
Stiffness of Partial Moment Connections
(OP)
I typically try to avoid specifying full moment connections and instead list the factored moment that I require from a connection in an effort to decrease the steel costs.
However, I have a situation where my factored moment is only 10% of the moment capacity of the weaker member being connected. If the connection is designed for this factored moment, can I really expect the stiffness of the connection to behave as if it were connected for 100% of the moment capacity? If not, is a full moment connection required to assume that deflections can be based off of a fixed connection type?
However, I have a situation where my factored moment is only 10% of the moment capacity of the weaker member being connected. If the connection is designed for this factored moment, can I really expect the stiffness of the connection to behave as if it were connected for 100% of the moment capacity? If not, is a full moment connection required to assume that deflections can be based off of a fixed connection type?
Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein






RE: Stiffness of Partial Moment Connections
Much depends on the type of moment connection that you end up with. If it's a classic welded or bolted moment connection the less weld or fewer bolts probably isn't a big deal. If you end up with an extended end plate connection on the other hand, the stiffness impact could be substantial. AISC's design guide on that issue may have something to say about connection stiffness.
I do think that your concern is valid. I rehabbed an excessively flexible facility a while back that had moment connections at every beam and column joint. The designer spec'd 10 kip-ft at each moment connection with no mention of stiffness or expected connection details. He got 1/2" wide flange column cap plates attached to beam flanges in a four bolt pattern, sans stiffeners. I checked the capacity and the 10 kip-ft is there and it jives with the lateral loads. However, there are signs of lateral building movement all over the place.
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.
RE: Stiffness of Partial Moment Connections
I don't have any reference material to back this up.
It has come up on this forum a few times and usually results in a discussion of sorts about semi-rigid connections and how they should be analysed and designed.
thread507-321048: Partial Moment release
RE: Stiffness of Partial Moment Connections
For my current situation I have multiple small moment frames supporting glass spandrel glass therefore I am more concerned about deflection than moment capacity. For my situation I have decided to use full moment connections. When I have more time I will look into this a little deeper in the hopes of coming to a more agreeable solution.
Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein
RE: Stiffness of Partial Moment Connections
RE: Stiffness of Partial Moment Connections
We also need to consider the rotational stiffness over the range of expected moment demand. Going from a 8" x 1" plate with 6 bolts to an 8" x 1" plate with three bolts would have little impact on the rotational stiffness up to the capacity governed by the three bolts in shear. Going from a 8" x 1" plate with 3 bolts to a 8" x 1/4" plate with 3 bolts will result in a very large decrease in rotational stiffness throughout the entire range of moment capacity.
RE: Stiffness of Partial Moment Connections
RE: Stiffness of Partial Moment Connections
EIT
www.HowToEngineer.com
RE: Stiffness of Partial Moment Connections
I'll try a more nuanced version:
1) If you wind up with a flexible, non-conventional moment connection detail, expect joint stiffness to take a significant hit.
2) If you wind up with a conventional moment connection detail, expect joint stiffness to take a modest hit. I suspect this would be linear-ish as Bookowski suggested. Your moment/rotational stiffness ratio would remain roughly constant so you could expect about the same ballpark level of joint stiffness for whatever level of connection moment is specified.
Asixth's method is sounding better all the time.
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.
RE: Stiffness of Partial Moment Connections
RE: Stiffness of Partial Moment Connections
I made a very quick model of a single story frame with two columns and one beam, pinned bases and fixed beam/column joints. It's a very simple frame with lateral load only. One has the full stiffness for the entire length of all three members. The other has a 6" section at both ends of the beam with a I/Ig = 0.1 to approximate a connection stiffness of 10% of the beam stiffness. This is to assume the moment demand is 10% of the beam capacity and the flange plates are optimized. That much of a rotational stiffness reduction in the 6" at each end resulted in a frame drift of twice that for no stiffness reduction at the connection. This is a crude model with no vertical loading, so second order effects are non-existent.
RE: Stiffness of Partial Moment Connections
I agree that a connection with 10% stiffness should be considered in your analysis - but that's probably not a likely connection. Again maybe you want to specify some lower bound, i.e. flange plates have X% of beam flange area or X% of capacity etc. Personally I would rather have real forces on the drawings and impose a lower bound some other way just so that anyone looking at the drawings in the future (including myself) can make sense of the forces shown.
RE: Stiffness of Partial Moment Connections
EIT
www.HowToEngineer.com
RE: Stiffness of Partial Moment Connections
If I make the entire beam have a I/Ig ratio of 0.1, then the frame drift is increased by a factor of 6, instead of a factor of 2.
RE: Stiffness of Partial Moment Connections
http://www.ejse.org/Archives/Fulltext/2010/2010v1/...
RE: Stiffness of Partial Moment Connections
RE: Stiffness of Partial Moment Connections
There are few methods for checking, try reading EN1993-1-8, there are calculations to perform to check the predicted behaviour... If not, you could menage to model a simple beam with equvivalent springs that will mimic your connection and another model with fixed edges. If moments and forces do not differ too much, you can state that it is OK to assume fixed connections.
Live long and prosper!
RE: Stiffness of Partial Moment Connections
See link that I posted earlier to paper on the topic, doesn't seem very sensitive until you get to very reduced numbers. Also ran a little test model and found similar results, also posted above.
RE: Stiffness of Partial Moment Connections