partical moment connection

huangyhg

partical moment connection
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?

depending on configuration and stiffness ratio of beam-to-column, you can expect the fixed condition to yield a moment at the connnection of somewhere between wl^2/12 and wl^2/15 (possibly a bit lower, but not much) for reasonable span ratios.  the variation between these values and a 60 percent value will have the effects you mentioned, but if you bracket those in hand calcs, you might find they are insignificant.  the effect will be greater in the   
normally we bother about end-moments in moment frames. with partially released moments at ends, the mid-span moments will go up and sections will need a check at mid-span too.
  what about for estimating the decrease in joint stiffness for the lateral design of the frame. i would imagine that some programs allow internal springs that would help account for partial fixity, but i do not have this available.

i am concerned about the increase in story drift i will now have.
if your software doesn't directly allow for partial restraint connections, the only other "trick" you can do is set a joint a small distance from the beam/column intersection joint (perhaps 6" away) and for the small 6" beam length, enter a smaller beam with lower ix.  
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.
jae, i am using risa 3d, are there any other tricks you know of for modeling pr connections in this program?
fema 356 section 5.5.3 provides a simplified method for adjusting frame stiffness to account for partially restrained connections.
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.
when you consider that a beam is connected to a column, and that the column is not entirely rigid itself, then no matter how stiff the connection, the beam end will never be entirely 'fixed'.
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.
  so i need to check that the moment capacity of the connection (at 60% of fixed) is actually greater than the moment that is required (if was completely rigid).
in other words, 60% of the capacity of the connection has got to be greater than the 100% fixed moment required.
(sorry i'm new to pr connections.)
jae's suggestion will work.  since you designed it as fully fixed, i would choose a beam for the short section that had a moment capacity of the 60%. i would also use a really short section, say 6", to imitate the connection. (oh, crap!!! did i just say 6" was really short??  dammit, i've judged myself again!).  the pia of it is splitting each of those beams to meet your condition. to be accurate, you'd have to apply it to all of those beams, not just the worst one.  it has to redistribute the stiffness matrix taking all those connections into account.  i think the end restraints stay with the member too, so if your fixed on one end, and pinned on the other, once you divide your member, the original