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Braced Frames - Hollow Strucural Sections

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It is possible to use large HSS in braces of braced frames.  The requirement on
h/t is for "unstiffened" elements of sections.   This requirement was added in
1994 to prevent braces from experiencing premature local buckling and "oil
canning."  When this happens, very large strains occur at the yield lines in the
local buckle and fracture initiates after a few cycles.

It is possible (and laboratoy testing has demonstrated this) to fill the tube
with a noncompressible material, such as cement grout, and use lower h/t ratios.
The grout fill stiffens the tube so that the faces are no longer "unstiffened
elements."  This prevents the local buckling from occuring.

Refer to the papers by Hassan and Goel, 1991 and Goel and Tang, 1989 upon which
this code provision is based.