Knee Braced Ordinary Moment Frame

[cec17]

Another access platform question here.

I am using knee braces for a maintenance platform, using a non-ductile R=1 system per ASCE7 15.4-1 "steel ordinary moment frame". It is SDC D. In reviewing AISC 341 Commentary E1.2 it states the following:

OMF Knee-Brace Systems. Knee-brace systems use an axial brace from the beam
to the column to form a moment connection. Resistance to lateral loads is by flexure
of the beam and column. These systems can be designed as an OMF. The knee-brace
system can be considered as analogous to a moment frame with haunch-type connections.
The knee brace carries axial force only, while the beam-to-column connection
carries both axial force and shear. A design approach for knee-braced systems is to
design the beam-to-column connection, the braces, and the brace end connections for
the forces required to develop 1.1RyMp/αs of the beam or column, or the maximum
moment that can be delivered by the system, whichever is less. Mp is the plastic
flexural strength of the beam or column at the point of intersection with the knee
brace. The column and beams should be braced out of plane, either directly or indirectly
at the knee brace locations, consistent with the requirements of Specification
Appendix 6.

My question is regarding the last sentence, "The column and beams should be braced out of plane...at the knee brace locations"

Does this imply that you are required to have beams at the knee brace landing locations, and also a knee brace in the opposite direction to brace the column? What if we are using some other lateral resisting system for the transverse direction? Also am I even beholden to this commentary since we are not under the detailing provisions of AISC 341?

 

[canwesteng]

With R=1 you aren't required to use AISC 341. I would be putting lateral restraint at the knee brace locations to comply with AISC 341 otherwise though.

 

[cec17]

Thanks for the response canwesteng. So I take it you would only use the OMF knee brace provision if it were used in both orthogonal directions, since the column requires out of plane bracing at the knee brace.

 

[Deker]

The commentary says that the column and beams should be braced "directly or indirectly," which implies to me that you can check the column (and beam if no brace is provided there) for an out-of-plane brace force that occurs simultaneously with the primary lateral forces. If you can determine using Appendix 6 that the column (and/or beam) has adequate strength and stiffness to brace the knee joint for the combined load condition, you shouldn't need to provide direct bracing.