huangyhg

a frame question
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?
are you checking the moment in the column at the bottom of the beam or at the beam center? it should be at the bottom.
as you are connecting the beam to the column flanges, design the column for the vertical load combined with the moment from the connection. this will not change with the deflection of the beam, but the eccentricity will increase slightly with column deflection.
model the ends of the beam as simple,(pinned), connections. use shear tabs to the web of the beam sufficient for the shear. takes the column out of the flexure region on the column interaction diagram.
csd72- i was checking moment at the beam center; that will help some to check at the bottom of the beam.
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.

looking at it some more, i think checking the moment at the bottom of the beam instead of the center will be enough to make things work out. thanks for the input.
your columns must have an interaction diagram with greater strength than the bending and axial loading imposed. the location vertically in the beam means nothing. extreme fiber at the top and the bottom are where yield will occur.
civilperson
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
just as a matter of interest why is the design moment at the bottom of the beam rather than at centreline of the beam?
i have the same question as civeng.
i have always done it to centerline.
for a rigid connection the column is stiffened by the beam above the level of the bottom flange and therefore will not fail in flexure there. critical flexure will then occur at bottom flange level, if it is a typical frame with maximum moment at the column-beam connection.

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