Continuous Beam over Column - Lateral Bracing

[jdgengineer]

I know versions of this question have been asked on here in the past, but I thought I would post again for clarity. When you have a steel wide flange beam running continuously over the top of an HSS beam do you put in only web-stiffeners and design the stiffener as a "continuation" of the column to provide necessary beam bottom flange bracing, or do you put in the stiffeners and an additional brace (say perpendicular beam or diagonal kicker)?

Chapter 2 of AISC 360 seems to indicate that the stiffener only approach is acceptable (and this is what I commonly see). On page 2-21 of AISC 360-10 "...The required stability can be provided with stiffeners in the beam web along with a moment connection between the column top and beam bottom to maintain alignment of the beam/column assembly. A cap plate of reasonable proportions and four bolts will normally suffice".

However, commentary to J10.4 on page 16.1-418 says "If flange rotation is permitted at the loaded flange, neither stiffeners nor doubler plates are effective".

Does anyone have thoughts on this?

 

[JAE]

Vertical stiffeners extending up the web, full height, create a sort of column extension through the beam.
At the point of load application (at the bottom flange attached directly to the column) there is little, if any rotation as the column cap plate will not rotate all that much. Therefore, there is no flange rotation at the loaded flange.



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[jayrod12]

I would always brace the top of column. To me it seems like a cheap and easy way to ensure I can sleep well at night.

 

[KootK]

stiffeners in the beam web along with a moment connection between the column top and beam bottom

If flange rotation is permitted at the loaded flange, neither stiffeners nor doubler plates are effective

Effectively, the moment connection mentioned above allows your column to act as a torsional brace for the beam. Like a vertical roll beam. The frustrating part about this scenario is that it's quite difficult to estimate the stiffness of such a connection so that you could rationally quantify the performance of the joint with the AISC bracing appendix methods. Like jayrod, my default position is a kicker to the top of the column or bottom of the web stiffener.