Steel beams to support garage roof

[dnb]

I have a pair of adjoining 1960s concrete garages. The centre wall has been removed, but this obviously leaves a row of concrete columns supporting the roof. The roof is the usual crinkly asbestos panels, but it wouldn't take much to cause me to change it for something lighter.

I would like to replace the concrete roof beams with steel so I can get rid of the centre columns. The span would be 5.1m so not what I would consider excessive. I'd like a rough idea as to what gauge steel I should use. Ideally, I'd like a 3" or 4" box section with 2"x1" "key" section on the bottom so it's roughly the same shape as the concrete beam it replaces. I can post up drawings if need be.

Does this sound sane and sensible?

Thanks for any help - I can do maths if anyone can point me at equations.

[dnb]

There's nothing in the surplus bin like that at work. I'm not going to call it a bodge if it works. Concrete garages are quite bodgelike in themselves after all

[dnb]

The current roof material is asbestos sheet. This ought to stay so I don't "destroy the character of the area" according to the planning department...

Not found anywhere on the web that provides tables of data. That's what I want really. Everything I have found seems to want me to pay for them to do calculations.

I'm not known for under-engineering anything.

Timber is not my prefered choice for a 5m span. I haven't enough height to get it thick enough without either lifting the roof (planners would become upset) or reducing the headroom (I would be upset because my Range Rover wouldn't fit in) So I turned to steel, but since I've never done this before I don't want to end up with it costing 5 times what it should and being 5 times stronger than need be.