Help needed with sizing of loft floor trimmer ???

[iss1210]

Hi, I live in a terrace house with high ceilings and so when i started this project expected to span the width of the house (4.6m) with a large timber trimmer, about 1.5 metres from front and back wall so to not interfere with existing windows heights ! So what i need is help with is sizing the timber to span the 4.6m width of house. The timber will then support floor joists which will hang between and only cover a span of 4.2. (ive boxed off at 3ft in loft as don't want the small angled area)
I know it might to tough to imagine but think of it like a capital 'I' with the top and bottom being the width of house and the upright being the 4.2 span between. I found that a 9x3 will span 4.6 metre but im sure that does not include hanging joists off it !!!
I have worked out that the 4.6m trimmer will need to take a load of 15kn which i calculated by multiplying the width 4.6 x 2.1 half the floor x 1.5 kn/m the imposed load.
So ideally i want to know what size of timber will support the floor and take the 15kn load
Any help would be greatly appreciated

[stoneyboy]

iss1210,
You really need to consult a structural engineer who will carry out the required calculations. Your local BCO will insist on having these.
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