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Re: Unbraced length question

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In a message dated 11/12/98 11:21:50 PM EST, wish(--nospam--at)cwia.com writes:

<< Mike,
 The columns are connected to beams radiating out of the building and bearing
 on the columns. The beams are only tied by a radiused GLB which bear above.
 The links are mechanical - ie, welded column caps that bolt to the beam,
 radius glb and weld to the columns.
 
 Considering the radius in the GLB, there is not direct tie and I would
 expect deflection to occur within the GLB.
 The only way to adjust stiffness to insure different story drift would be to
 change column thicknesses and force each to take the same load. Therefore,
 the longest column will control. The difference in deflection using the same
 members is expected to be about 1/2" although some of it will be absorbed in
 the GLB.
 Do you think this is a problem worth worrying about? or should I just design
 for the taller unspupported member and keep it as stiff as possible (under
 code drift criteria of 0.005h)?
 
 Thanks for the response, Mike
 Dennis
  >>

I would probably use the same size for both columns and not worry about the
1/2" deflection (probably less expensive to buy more of the same size columns
and less expensive for fabrication if the connection dimensions are all the
same) assuming the glu-lam beam can handle the out-of-plane bending.  If we
get an earthquake large enough to cause displacements greater than 0.005H,
there will probably be just as much nonstructural damage if you have used two
different sized columns.

How much of a bending moment is induced in the glu-lam if you get 1/2"
deflection in the out-of-plane direction.  I assume the glu-lam is probably
lightly loaded for gravity load, and is sized more for architectural
appearance ( I picture the radius glu-lam beams extending out beyond the face
of the building or to the exterior wall of the building).  If the glu-lam is
oversized for gravity, I imagine it can handle the out-of-plane bending. 

Does the diaphragm extend over the entire length of the glu-lam beam or only a
portion of the glu-lam (portion of glu-lam is a flying beam to the flagpole
column).  If a portion is a flying beam,  try and provide at the edge of
sheathing over the glu-lam a drag struct perpendicular to the glu-lam to
distribute the cantilever reaction back into the diaphragm.

Mike