The message . com
from "jim_in_sussex" contains these words:

In days of yore there were a fair number of posters on uk.d-i-y who'd
toss their penny's worth into a posting like this. Where are you all
now? Not killed off by 2 Jag's building police state I hope?

I had a couple of attempts at drafting an answer for the op before
giving up on the basis that my info was 40 years out of date and my
practical experience of the relevant calculations of similar vintage.

Just to now add my tuppenceworth.

AFAIK new RSJs have not been available for decades and any secondhand
stuff would now be at least getting on for 40 years old (just like my
information). The modern equivalents are Universal Beams and Columns
which have thinner sections and are wider relative to their depth than
RSJs. As the name suggests UCs are intended to be used vertically but
where depth of a beam is an issue ISTR that it might be more
advantageous to use a column as a beam.

On a practical note I have a 30 foot by 8 foot barn where the original
hayloft has been replaced and that floor is now supported by 2
transverse 8" x 4" RSJs with a clear span of 18 feet. That shows no sign
of collapsing and the deflection is not immediately apparent to the
naked eye.

I have a 1965 Collins Architects Diary that has tables for both RSJS and
the Universal Beams and Columns available at the time. That suggests
that 8" x 4" RSJ would have a safe distributed load of 4.5 tons for a 16
foot span (3.6 at 18 feet) but that buckling might be an issue without
adequate lateral support.

Tables for Universal Beams only go down to 8" x 5.25" but that is
significantly stronger than the same depth RSJ at 5.7 or 4.7 tons
(depending on weight of beam) SDL at 16 Foot span.

--
Roger Chapman