"Carl Ijames" wrote in message
news
"Jim Wilkins" wrote in message news
....and then bolt on a 4' piece of 3" channel with the bottom
flanges aligned so the crane trolley can roll the entire length.

Wouldn't an asymmetrical section tend to twist and buckle as its
deflection increased?

These gantries are set up temporarily outdoors on the bare ground and
need a leveling adjustment to keep the load from moving by itself,
either a tripod whose legs I can move in or out, or a large forged
turnbuckle hung from a braced post.
..
In both cases the channel ends can rotate to some extent, more so with
the tripod where the channels hang from chain. A stiffer top
connection that constrained channel rotation wouldn't adapt to the
uneven ground, plus it would add an unknown amount of cantilever
loading that reduces the column strength of the tripod legs which are
8' chain link fence posts or 2" x 10' EMT for lightness. A stronger
tripod of 2" x 10' water pipe legs is too top-heavy to set up safely.

-jsw