Jahan writes:

8 cm wide by 3 cm U Channel and 3 x 3 inch x 2 mm tube

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That's a fairly complex engineering question (??)

Apart from in pure tension, the load limit is usually about when the
section will buckle - go unstable - or exceed elastic bending
(smallish) and plastic bend by large amounts to collapse.

Normally, when the service is not pure tension, closed sections -
SHS's - Structural Hollow Section - are much stiffer for the same
amount of material and will give a much higher load bearing.

The technical breakthrough of being able to economically manufacture
large amounts of Structural Hollow Section from good-specification
steel has been a transformation.
Other advantages with SHS's are eliminating rust-traps, with
hermetically-sealed internal volumes (no corrosion) and smooth
external sections advantaging paint systems to give good protection
against corrosion for long low-maintenance service.
Hence the return (?) of truss bridges.

Complex matters.

You'll be wanting to study Second Moment of Area and the beam and
column calculations / equations.
The Euler column and the Euler-Bernoulli beam (both derived around the
1750's - about 250 years ago) which serve well for most applications
of beams and columns.

Regards,
Rich Smith