Size of trimmer and trimming joists for stairwell

[Calliman12]

Hi all,

Doing home renovation in my 1930's semi detached involving moving stairs so need to cut some joists and hoping for some advice. Please note I will be seeking services of a structural engineer closer to the time.

Currently upstairs joists run left to right. I plan of fitting a double winder staircase with the upper part resting on joist no. 7 (D)
As per the sketch i need to cut 6 joists back and fit a trimmer (C). Current joists are 2x4 and based on the dimensions shown, my questions are:

1. What size does the trimmer (C) need to be? (a stud wall will be placed over it to close off the bedroom which will be made smaller due to new stairwell)
2. Presume a wood-to-brick hanger will suffice on the exterior wall for for the trimmer (C) (exterior walls are double skin with small cavity 1930's house)
3. I wish to retain joist no. 7 (D) as I want the stairs to rest on this; is this ok or does it need replacing/strengthening?

Thanks for the input in advance,
C

 

[George_engineer]

Usually you'd keep the same depth as the joists but double, triple or quadruple them up.

 

[Calliman12]

Thanks George, I thought doubling up would be the easiest option.

When doubling up joist no. 7 (D) which I presume is already built into the wall (can’t see its end yet); would bolting on another joist to it be fine or does the ‘doubling up’ joist also need to be fixed to the walls on both ends?
The other end of no. 7 is a few meters away under flooring which is rather not take up..

Thanks,
C