A frame question
A frame question
(OP)
See the attached diagram. A larger beam is supported by two smaller columns, which in turn are pinned at the bases. When the larger beam is loaded, you get a certain amount of end rotation which in turn induces moments in the columns. The problems I'm running into are that when the height H is reduced, it actually increases the moment in the columns due to their increased stiffness. And trying to upsize the columns to the required strength just increases the moment.
The pinned connections are actually anchor points to a foundation. If one of those anchor points can slide about 1/16 of an inch, it eliminates the moment in the columns. Is it an acceptable design solution to assume that this happens? For that matter, if the the columns yield in bending, it shouldn't hurt anything. Any comments or suggestions here?
The pinned connections are actually anchor points to a foundation. If one of those anchor points can slide about 1/16 of an inch, it eliminates the moment in the columns. Is it an acceptable design solution to assume that this happens? For that matter, if the the columns yield in bending, it shouldn't hurt anything. Any comments or suggestions here?






RE: A frame question
RE: A frame question
RE: A frame question
RE: A frame question
hokie66- I think what you're saying is what I was already doing.
civilperson- I was trying to avoid having to brace for lateral loads which is why the connections weren't already pinned.
RE: A frame question
RE: A frame question
RE: A frame question
For a moment connection the actual maximum moment in the column is located at the beam bottom flange, which is less than that by modelling at the member centrelines.
RE: A frame question
RE: A frame question
I have always done it to centerline.
RE: A frame question
RE: A frame question
RE: A frame question
The reasonining is a little different though.
When you design the connection you treat the beam as if there is a horizontal force at each flange giving a moment couple. The coonection formulii used in portal knee connection design are based roughly on this truss analogy.
now if you apply these two forces to the column with a support at the base you get a triangular moment distribution that is largest at the bottom of the beam and tapers off to zero at the top and bottom of the column.
RE: A frame question
Basically, the length of column across the depth of the beam is most likely connected with angles or plate such that the column will not bend along the depth of the beam.
If a member doesn't bend, there is no stress and thus no corresponding moment.
RE: A frame question
I have always modelled and designed to centreline, however, as csd and others have indicated, the push-pull of the beam flanges will result in the greatest bending moment of the column at the level of the bottom beam flange.
However, you mentioned that you are relying on frame action to resist lateral forces, may I asked how the frame performed with respect to drift?
RE: A frame question
RE: A frame question
Well actually if you want to be really specific you can provide rigid offsets to the face of column and bottom of beam. But it wont make much difference.
RE: A frame question
Perhaps the use of rigid offsets will make a difference and will result in better accuracy.
RE: A frame question
The difference may not be much, but I would expect the column moment to increase. Perhaps my intuition is wrong (again). I'll have to run some numbers.
RE: A frame question
In this case you are probably right.
RE: A frame question
RE: A frame question
RE: A frame question
even with the increased stiffness from the rigid offset I would still be very surprised if the increase in the moment from the rigid offsets is greater than the reduction by taking it at the bottom of the beam.
RE: A frame question
RE: A frame question
RE: A frame question
Now, if you change Ix where the column is attached to the beam, to 9040 in4, the moment at points E and F, representing the column at the bottom of the beam, is 39.0 k', a 13% increase. My analysis ignores shear flexibility.
RE: A frame question
1. Place plate elements in between the columns (beam elements), and perform analysis.
1. Change the support mechanism, let the columns end (fixed/pinned) at bottom of the beam, perform the analysis and compare results of the models.
RE: A frame question
This is actually a simplified version of a more involved assembly. Beam length is 162", height to beam centerline is about 39". Columns are pipe so rotating them doesn't help. Wind forces were pretty insignificant compared to gravity forces on the beam, so drift wasn't a problem.
Thanks for the help and the input on it.