I-Beam Calculations

Whst does your loacl building code say?

What Country?

What State?

What planet?

Building inspector wants calculations

UK

You will need to hire a competent person.

Surely, the Building Inspector would not accept an analysis taken off of a blog site

You will need to support the 2 walls. It is very convenient to have them the same width as the wall thickness. Or one - one size plus onother for the second size. Or one that takes both - this will be very wide like HEA or HEB. If you are limited in height go with HEB (thick wide flanges) for 6 Ft they probably require a 180mm height or higher.

If Building Control are asking for calculations, then these should be provided by a professional engineer. Two advantages: they won't get it wrong and they have insurance to cover any mishaps.

But from what you have described, the best solution is more likely to be two UC (universal column) beams on concrete padstones and dry-packed above the beams to at least 50mm depth.

As a civil engineer in California, I recommend a Structural Engineer.

Simpson Strong Tie makes some great fixed moment systems for these kinds of openings and they still need a SE design.

Charlie