Dodgy? Supporting a joist with a parallel joist?

[CompleteDonkey]

Hi all,

I want to replace the banister around my landing upstairs with a couple of stud walls so I can essentially create a separate enclosed space upstairs. Part of the stud wall that needs building cannot be built directly on top of the existing subfloor, as there would not be enough space on the landing (width-wise) in order to meet building regs.
This means that this part of the stud wall would need to be built where there is currently just mid air on the edge of the landing and this stud wall would somehow need to be attached to the side of the landing instead.

The joists run along the landing in the same direction that I want to build the stud wall and the carpenter I was just speaking to suggested essentially attaching another joist to the side of the existing joist to create a base to build the stud wall on.

My initial thoughts were that this was a good idea, however after thinking harder about it, wouldn't the weight of the wall create a rotational force on the joist that the new joist was bolted to? Surely the better option would be to fit some sort of support beam into the load bearing wall so that the stud wall was supported by the support beam alone.

I've attached a diagram, hopefully it makes sense.
Grey boxes are load bearing walls.
Blue boxes are joists.
Green boxes are stairs.
Red box is where the stud wall will need to be built.

I'd really appreciate some feedback, I don't want to get this wrong :/

Thanks a lot

 

[foxhole]

Sister joist as suggested, you are putting in a partition, not a load bearing wall .

 

[CompleteDonkey]

Thanks for the response Foxhole.
I've done a bit more thinking today and in order to keep things simple, I've decided to compromise on the sound insulation slightly and just go with a thinner stud wall.
I'm now thinking to just build a 2" x 2" frame directly on top of the joist, put 12.5mm accoustic plasterboard on one side, 9mm plasterboard on the outer side (to keep the weight down) and fill the cavity with some DFM.
Should still keep the noise down, it's a lighter weight solution and won't involve any screwing together of joists

 

[JobAndKnock]

You could always go to metal studwork rather than timber if you want to keep the weight down

When sistering joists it is always better to bolt the new joist to the old one using something like M10 or M12 coach bolts at maximum 600mm centres (don't screw as it won't be strong enough). End support into masonry is achieved by chopping out a pocket at the end and putting in some form of padstone (either cast, shuttered concrete or by incorporating a pre-cast concrete padstone), end wrapping (envelope wrap to ensure that no moisture can enter the joist by physical transmission) the sistering joist with felt and colours, sitting the sistered joist on it (minimum 50mm bearing) and then back filling the pocket with masonry and mortar. Doing this puts an end to any twisting moments in the doubled-up joist becsuse the ends are adequately supported

At the other end the only form of support I can envisage is going to be something like a coach bolt within 150mm of the joist end combined with something like a 90mm angle plate twist nailed into position between the sistered joist and the trimmer which supports the stairs

In all of this I am assuming that the joist which supports the top of the stairs is either of larger section or is sistered as is normal practice (I.e that it is a proper trimmer)

 

[dontbelieveawordofit]

i’d ply at least one side of it.