why design a joint to be partially restrained

huangyhg

why design a joint to be partially restrained?
what is the advantage of designing a joint to be fixed as opposed to being pinned?
what is the point of patially designing  a joint?i.e for instance allowing a column to be free to translate in the z direction while being fixed in the x and y?
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i am not sure i completely understand your question.  there are many reasons you might fix a joint.  probably the biggest is to create a moment frame for lateral resistance.  another is you may have a cantilever situation where one beam frames into another at the same elevation and you must "fix" the connection to cantilever it past.
as far as column translation, a column should almost never translate in the z direction except for the column shortening due to axial load.  one reason you might restrain translation in the y, but not x, direction is because a column's strong axis is the x axis and, as a result, can have a greater unbraced length and achieve the same axial load capacity as the shorter unbraced length in the y direction.
i am not sure this was asked in your post, but just fyi, there is also a great advantage to partially fixing moment connections in steel frames.  you can use top and bottom angles to create a semi-rigid frame, that will yield under gravity loads (and the beams and girders can be designed as pinned-pinned), but are still capable of resisting lateral loads.  the interstory drift is about 1.5 times that of a rigid moment frame, but this is a much less expensive and more easily fabricated connection detail.
you model it as close as you can to how it will actually behave in practice.
if you do not understand the fundamentals, then you should do some reading. this forum is no place to learn structural fundamentals.