Partical Moment Connection
Partical Moment Connection
(OP)
I have modeled a basic steel moment frame. The fabricator has shown a bolted angle at top and btm of the beam. I figure this to be about 60% of fixed. I cannot model partial moment connections in the software I have.
I am trying to determine the negative effects this would have on my frame, since it was modeled at 100% fixed. What comes to mind is increased deflection and p-delta effects.
What is the best way to figure the effects for partial fixity connections in frames?
I am trying to determine the negative effects this would have on my frame, since it was modeled at 100% fixed. What comes to mind is increased deflection and p-delta effects.
What is the best way to figure the effects for partial fixity connections in frames?






RE: Partical Moment Connection
RE: Partical Moment Connection
RE: Partical Moment Connection
I am concerned about the increase in story drift I will now have.
RE: Partical Moment Connection
How much lower Ix is the tough part. You could do some hand calculations to get an estimate on the moment/rotation of the joint and somehow equate that to a flexural stiffness in the short section.
In any case, the less-stiff mini-beam would at least approach the exact solution and create a frame with less stiffness than your original fixed frame.
RE: Partical Moment Connection
RE: Partical Moment Connection
For bare steel PR connections:
Rotational stiffness, K = MCE/0.005
Where MCE is the expected moment strength of the connection.
Then, the adjusted stiffness of the beam is:
EIadjusted = 1/(6h/(L^2*K)+1/(EI))
See FEMA 356 for more info.
RE: Partical Moment Connection
If you accept that, you may consider that your beam end connection provides say 60% of full fixed end moment(FEM) for the beam; Is the applied moment (at the beam end) as determined by your analysis, greater or less than the 60% (say)of FEM?? If the applied moment is less than the 60%, then as far as that beam is concerned in that structure, its end is fixed.
Hope these are helpful thoughts.
RE: Partical Moment Connection
Then go to reanalyzing the members for the increased deflection and p-delta. Is this correct?
RE: Partical Moment Connection
(Sorry I'm new to PR connections.)
RE: Partical Moment Connection
You can always ask the fabricator to give you the connection calculations for that connection and verify it's adequecy.
However, as much as you hate to do it, if it is not your design intent.....REVISE AND RESUBMIT
RE: Partical Moment Connection
Theoretically I can place small beam segments at the ends of the actual frame beam. These segments will have about 60% of the actual beam's Inertia. Running the frame with lateral loads, I should get pretty accurate results regarding drift and p-delta etc. I will also have a redistribution of the gravity load moments due to the less stiff ends.
Next, I need to check the actual moment at the ends of the beams due to lateral and or gravity loads. This moment should be less than or equal to 60% of the actual beam capacity, and also less than 60% of the column capacity.
Finally, I will need to check that all AISC seismic provision requirements for detailing and design are also met (if I use an R>3 (IBC 2000)).
Does anyone disagree with this procedure.
RE: Partical Moment Connection
Like the saying goes...a building is not a bunch of beams and columns connected together at their joints. It is, rather, a bunch of connections that happen to be linked together with beams and columns.
RE: Partical Moment Connection
RE: Partical Moment Connection
Squeaky wheel gets you greasing.
RE: Partical Moment Connection
The questions everybody seems to avoid with PR connections are the additional story drift and the correct "k" factor. The standard nomograph only applies to fully fixed connections.
Don't give up stability for some cost savings.
RE: Partical Moment Connection
The effect on the beams.
For rough stimating if you have a simple supported beam the moment in the mid of the span is (W*L^2)/8 and a bem fixed both end is (W*L^2)/10.
That's mean if you try to save on the end connection you have to increase for stresses or deflections (not shown).
The economy increase (small requeriments) if there is some continuity, the maximun economy is found arroud four or five continuos span, more span have no impact on the size of the beam.
The effect on columns.
Like redhead state, you are changing the slenderness factor of the column k, the column is more slender for that have less allowance stresses at the end.
The global behavior.
The columns fixed at ground and the beam simple supported at the columns. You have the more expensive structure, the stablity, deflection is just column responsability. The beam act like they are and have no economy.
The columns fixed at ground and the beams fixed to the columns. Is the most economical, taking advantage of the continuity for the beams. Unloading the columns ang giving some responsability for delection and stresses to beams. Your structure is located in this place.
Changing the restrains, and you go to an expensier structure more than the save for the changes. The posibility of reinforce the beam, or increase the size of the columns for the lateral movement control. The worse could be the change of member size (if the material if already bought).
Go for the safest way make an analisys with the changes, i normally use STAAD III 22.3WM this have an option for member end release partial moments.
RE: Partical Moment Connection
RE: Partical Moment Connection
I've never seen a cost analysis breakdown per connection, seems like a lot of variables could be involved, but I have asked to see one from him. I cannot have x braces in this case nor do I have shear walls to use.
RE: Partical Moment Connection
RE: Partical Moment Connection