Hope my reply is not too late to be useful to your problem.
My personal preference would be to use a clamp, I.D. = O.D. of the
existing tube+3mm dimensional tolerance. Provide a gap of about 10mm or
0.5" between flanges to facilitate tightening. The clamp pipe to be
reinforced externally with a T section at 180 degrees. (increases
stiffness significantly). Bolts to be tensioned nominally, so as to be
effective, but not collapse the inner pipe. (inner ribs with tightening
to effect combined section behaviour as proposed by you could dent the
I presume the web tubular would have been fully welded to chord
W-sections at the flange. In that case the K factor would have been 0.8
for inplane buckling and 1.0 for out of plane buckling. Further, in a
truss work with slender members (large Kl/r) the bending stresses would
be quite small, and critical stresses would occur at the end
connections, where the clamp doesn't exist.
The original section alone should be checked for stresses, with Kl/r of
the clamp, including T-stiffener and bolting flanges. (For calculating
Kl/r, ignore inner pipe). Provide T-stiffeners in out-of-plane
direction, and bolt flange in the in-plane direction. Clamp pipe should
be considered only for stiffening, not load sharing. (Hence nominal
tightening of bolts). Should pass.
To select the T-section, first find the required 'r' which will make the
original section safe. Design the clamp tubular, T and the flange clamp
to provide the requisite 'r' in the two directions.
Engineers India Limited
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