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Re: Is it frame or not

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I see no difference between a two cantilever column system and a moment
frame.

For a one story building it makes no difference whether the hinge forms in
the beam or the column.  The code allows a hinge at the top of a frame if
certain conditions are met.   If the hinge forms in the column you must
provide bracing at the location of the hinge to prevent an instability.  In
the case of the cantilever column system this bracing is provided by the
concrete foundation.  

Whether the hinge forms in the column or the beam you get one hinge per
column.  The amount of displacement is not increased by the fact that the
hinge forms at  the top of the column as opposed to the bottom of the
column..  Rather the amount of diaplacement is a function of the size of
the column or beam.

While with a moment frame you get some fixity at the bolumn base, but a
slight increase in column size will provide equal resistance and capacity.

I believe that the reason that we have the provisions for cantilever
columns was because there were problems with some buildings with cantilever
columns.  I believe if you look at the designs that had problems you will
find that the problem was not that they were cantilever columns but rather
it was because the columns were under designed.   After earthquakes there
is a tendency to respond emotionally and over look the theory.

Mark Gilligan




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