Column Extension and KL/r

[jheidt2543]

I am wondering how others might handle this situation. In order to raise an existing roof a new column is set atop an existing column. The new column is an A36 W8x35 16' long and sits atop an existing builtup metal building column 17' long of 50 ksi, W 7-3/4" deep x 10" flange x 5/16" thick. The connection was made by placing a 1/2" thick cap plate on the lower column, stetting the upper column on the cap plate then fillet welding the columns to the plate. There are no cover plates on the flanges or in the webs. The column is loaded at the top by a roof truss reaction of 50 kips and at the lower column by a roof truss reaction of 50 kips on a bracket attached to the column flange. The column is braced in the strong direcion at the transition elevation (between the two column types) by the truss framing into it and at the top. In the weak direction light gauge wall girts frame in on both sides at 5' o.c.

What KL/r do we use? The upper column, the lower column or a combination. What if the lower truss load is raised to the top of the column? Then we have 100 kips at the top of a 33' column made up of two different materials. What is KL/r in that situation?

 

[DaveAtkins]

From what you describe in your first option, I would design the upper column for 50 kips, with an unbraced length of 16', assuming the new upper roof framing braces the top of the column in both directions, and the existing lower roof framing continues to brace the top of the lower column. If there is not adequate bracing at the upper roof, K = 2.0 instead of 1.0 for the upper column. The lower column should also be checked, since it has additional load.

Your second option would result in a column with an unbraced length of 33' in the strong direction (I assume the lower column continues to be braced in the weak axis direction). I would use "rx" of the W8X35 for calculating Klx/rx.

DaveAtkins