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RE: Seismic bracing system- Knee-braced Frames

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I think that previous discussion about KBF systems omitted several important facts:

1)	Typical KBF system used by PEMB industry is intended for small loads. It doesn't work, i.e., it becomes uneconomical, when the eave height exceeds say 20 ft. (even less than that)
2)	Typical KBF system used by PEMB industry does not carry gravity loads, other than selfweight. 
3)	Typical KBF system has a brace located not far from the beam-to-column joint. Consequently, the KBF system behaves similar to moment frames, where the "triangle" at the top of the column transfers the moment across the joint. The shape of the deflected members and the actual deflections are almost identical to moment frames. However, the integrity of this system relies heavily on three details: beam-to-column connection, brace connection to the column/beam, and brace strength for axial compression (and tension) loads. So, these would be the details subject to additional seismic criteria. 
4)	If the brace is lowered so it connects in the middle of the column height the behavior of the system under lateral loads resembles the K-braced CBF, which, as mentioned by others, may have detrimental effects on the column (frame) integrity. This is not the Knee-braced system!

So, lets recognize that in addition to apples and oranges, there are pears... The KBF system is a practical system that can be correctly designed and detailed within certain height/load limits. 
Maybe we need Charlie to suggest another category for the next version of Seismic Provisions. Maybe to add a subset to OMF category, with special detailing requirements for KBF details mentioned above? 


Igor Marinovic, PE, Memphis, TN

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