Big difference between industrial, commercial and public use catwalks. If in a theater and the catwalk could be loaded up with groupies, cast or crew then 40psf or even 100 psf may be light. Adding a few pounds or even tons of steel is small compared to the issues if a failure occurs and people are hurt.
NO GROUPIES. It is for a School with very little acces to anyone other than the maintainance people. The architect did not even want a guardrail but we provided one.
You may be (mildly) interested to know that on this side of the Atlantic, the British Standard specifies not a udl but a series of point loads on a cat-walk of 1kN (approx 220lb) at 1m centres.