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RE: PEMB question - unbraced length

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In general you are correct...
 
Most use the secondary spacing for the Ly (weak-axis) unbraced axial length of the member, Lx would be similar and be the distance between objects preventing lateral displacement in the major axis (i.e. columns).
 
For flexure some may use the secondary spacing for the outside flange Lb unbraced bending length, but you are correct in that the flange brace spacing would be the Lb for the inside flange... might be some minor differences as well as to the starting and ending points used in those lengths (i.e. top of frame versus horizontal splice location , etc.)   Also if the members are shallow enough and the secondaries deep enough or inset into the member there will be cases where those members may be considered to be torisionally braced for bending by the secondaries without the use of a flange brace.
 
I personally like to try and think about whether the intersecting member can "torsionally" restrain the beam/column and if it can not then I don't consider it to be a Lb brace (i.e. keep the compression flange from moving out of plane from the member's major axis, either laterally or torsionally). 
 
To further complicate issue you can throw in unbalanced flanges and design the section each direction based upon which flange is in compression, account for slender material modifiers (Qs, Qa, Rpg), and design for shear with or without stiffeners, and with or without tensile field action (if stiffeners present and the member and stiffeners meet the criteria in AISC)...
 
HTH,
Greg Effland, KC MO USA
-----Original Message-----
From: Yi Yang [mailto:YI(--nospam--at)summit-sr.com]
Sent: Wednesday, September 01, 2004 2:13 PM
To: seaint(--nospam--at)seaint.org
Subject: PEMB question - unbraced length

For those pre-engineered metal building experts out there, what would be considered as the unbraced length for both axial and flexural strength of the column / rafter in a metal building frame, if there is no flange braces attached to the bottom / inside flange of the rafter / column?  Am I correct to assume that only the top / outside flange is considered braced for flexural strength, and the weak axis is braced for axial capacity?  Thanks for any comment.
 
 
 
 
Y i   Y a n g,   P. E.             
STRUCTURAL DIVISION
SUMMIT ENGINEERING INC.
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