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all. The top beam is subjected to uniformly distributed load. FYI, the column is 5m in length. As you can see, the major axis of column is connected to the beam with moment connection on top. However, in the minor axis of column, there's nothing to connecting them.

For the major axis, the buckling length shall be 0.7*5 = 3.5m (K = 0.7 for bottom pinned, top fixed condition) ,

For the minor axis , the buckling length shall be 2*5 = 10m , right ?? (because in minor axis, it's cantilever , nothing to restrain the column on top) . However, the program detect the buckling length as 5.0m.

So, my question, for the minor axis, is it considered as cantilever ? Or since in the major axis, it's already gt the moment connection, in minor axis of connection, there's really doesn't matter whether the minor axis of column in restrained or not...

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I assume you modelled the structure in 3D structural analysis program, for such a case, if the two frames are pinned at the base of columns, but not connected on the top (two independent frames), the program shall issue a warning, because the frames are unstable in the transverse direction (pinned at the bottom, free on the top). You need to check your inputs for mistakes.

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    $\begingroup$ No, the column design is controlled by the minor axis, which has the least of radius of gyration. You need at least two horizontal members on top to connect the two frames to make it a stable structure (like a table), and if the members are simply connected to the column, then the effective length in direction on the minor axis is equal to 5.0 m (since K = 1 for pin-pin supported column). $\endgroup$
    – r13
    Commented Feb 14, 2021 at 0:45
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    $\begingroup$ If you do not wish to provide upper members, then you would have two independent frames (why?), in such case, you have to make the columns fixed at the base, so in direction of the minor axis, you would have fixed-free (cantilever) support conditions. $\endgroup$
    – r13
    Commented Feb 14, 2021 at 0:47
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    $\begingroup$ As said above, the minor axis has the least radius of gyration, so in direction of the minor axis, it is the weak plane of the column, that is prone to sway and buckle. It would help to review the derivation of allowable compressive stress of the compression member (column), the equation involves the term - slenderness ratio, "KL/r", and the larger the ratio, the smaller the allowable stress, that is the controlling factor for column design. $\endgroup$
    – r13
    Commented Feb 14, 2021 at 1:13
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    $\begingroup$ Yes, if you do not want to add members to the top, then you must keep the column base fixed, so in direction of the major axis (framing plane), the columns are fixed-fixed, and in direction of the minor axis, the columns are fixed-free. $\endgroup$
    – r13
    Commented Feb 14, 2021 at 1:20
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    $\begingroup$ Unlike cross bracing in the vertical plane, without the horizontal transverse member between columns/frames, the horizontal cross bracing can't form a completely enclosed triangle ( that is the most stable shape), as a consequence, the braces can undergo lateral movement freely (image a pair of scissors), thus it is unreliable. It surely offers some help than nothing, but the effectiveness is very difficult to evaluate. $\endgroup$
    – r13
    Commented Feb 14, 2021 at 4:25

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