Unbraced Column Length

[EIT2]

To consider the X-axis of a column braced, shall the X-axis brace be physically connected to the columns' Y-axis? If the beam that is physically connected to and parallel to the X-axis, is continually braced along its' own length, by precast concrete plank on each side of its' web, may I consider the fact, that since the beam cannot move laterally in the Y-Y direction of the column, this is also, sufficient enough to brace the columns' X-axis as well as the Y-axis?

Am I making my situation clear?

y
------Beam------x + x------Beam------
y

Thank you!

 

[CSEllc]

Let me re-phrase your question to make sure I understand it.

You have a column (lets say a wide flange).

Beams frame into the column flanges. The beams are continually braced by precast concrete plank. Are you braced in the X-X axis or "strong" axis. The answer is yes.

If the beams framed into the column webs, then the column would be braced in the weak-axis or the Y-Y direction and not necessarily the the X-X axis. If the concreted plank is not grouted around the column, I would not count on the column being fully braced in the strong axis. You do get some "bracing" from the floor system. I believe Dr. Yura has published some papers on this problem.

 

[EIT2]

CSE||c,

My columns are square tube.

 

[AlohaBob]

You are trying to determine an effective column length for allowable axial stress. See 3-5 AISC 9.

See also 5-135 Table C-C2.1. I suppose to use this you must imagine what would the buckled shape of your column look like if that were to happen. And understand the effectiveness of the connections at the top of the column and the base to make that column conform to this shape. G>10 begins to resemble the model "fixed".

This applies both in the xx or yy axes seperately, but in your case, a square tube, the comparison of effective length to least radius of gyration, they are the same. The column still will buckle in that mode of least restraint.
E.G. The same column height with restraint (e) sidesway uninhibited would carry only 1/4 the load of column (a) which is fixed in joint rotations as well as lateral translation.

Wow, how's that for structuraleze.

 

[DaveAtkins]

If you can justify transferring the force required to resist buckling of the top of the column (an old rule of thumb is 2 percent of the axial load) through the two connections into the adjacent beams, then yes, the column is braced. Also check the beams for weak axis bending -- the nearest weld of plank to beam may be 8'-0" away!

DaveAtkins

 

[EIT2]

Thank you all for your replies.

Let me add one more component to this scenario:

Would you consider these square tube columns X-X axis' to be braced if the supported beams are connected by the same through-column knife plates?

Thank you, again!