7 metre span on to timber structural walls hanging 4 metre joistsA flitched beam generally consists of a steel plate sandwiched between two timber joists.
What you have described is a timber trimmer built up out of three joists. If they are well-bolted together, they will be fine to support your floor joists off hangers, assuming the span is not too great.
If the span of the trimmer is 7m, three 9x2s will not do, due to excessive deflection, and the timber may also be over-stressed.
On that span, you would realistically need steel.
Can the steel be joined in the middle using correct joiners then in cased with 9x2 in webEven with one or two steel plates, a flitched beam of steel plate(s) and timber would not be sufficient.
You really need to consider steel, and on that span, from back-of-envelope calculations, you would be looking at something like a 254x147 UB or a 203x203 UC section.
7m is just too long to do in anything other than a conventional steel beam. Ideally you need to get an SE to do proper figures for you. Yes. it will cost, but it will also cost when you end up with a floor like a trampoline.
You can support a steel beam on a structural stud partition if necessary, subject to a suitable bearing plate.
Can the steel be joined in the middle using correct joiners then in cased with 9x2 in web
Then looking to brace rafters to it and remove existing supportCan the steel be joined in the middle using correct joiners then in cased with 9x2 in web
Yes, you can have it spliced, but the connection (usually using top and bottom plates and a web plate), requires some drilling and bolting, and needs to be properly sized.
Some SEs don't like splicing in the middle because the bending stress is greatest there, but with just a floor load in your case, it should be practical.
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