Replacing floor joists and supporting with an RSJ

[willfloor]

Hi

Can someone help?

I am in the process of replacing a downstairs floor as it was in extremely bad condition – sloping off to one side. Having removed it I have found there is just over a meter drop to the ground level. I would like to simply line it and put boards down so as to utilise this as storage space as we have none at the moment (2 up 2 down with no loft space). To that end would someone be able to advise me on what size I Beam I should use to support the floor joists?

Originally there were several timber trimmers, most of the pillars have deteriorated or fallen over (hence the slope) and would make the space unusable as storage anyway.

The room is 4200mm x 3380mm. As I would like to use the original floor boards that I have saved the joists have to run the longest dimension. The Clear Span of the Beam would be 3200mm as there is a 180mm of stone foundation wall present before the partition wall is built on it. On the other side a 2 thick brick wall which I’ll have to remove a couple of bricks to insert the other end and strengthen to support the beam(s); depending on how deep the beam will be it will be either 2 or 3 courses above the stone foundation wall on that side. Firstly how much more than the 3200mm span should I have to support the beam at either end?

As I would like to maximise head room below the floor I would like to insert the new joists into the I beam rather than them resting on top. Notching them so the top of the joist is flush with the beam so floor boards can be laid on top. I’m thinking along the lines of two options

1. Using 50mm x 100mm C24 timber at 600mm spacing (6 in the span) would give me a dead load of 0.5 – 1.25 kN/m2 when the clear span is 1610mm. If I was to use 2 x I beams equally spaced across the longest room dimension what size would they need to be to give the same support as the joists? I would like to use a beam that is 100mm deep – do they exist? If so how wide and how thick would it have to be?

2. Using 50mm x 125mm C24 timber at 450mm spacing (8 in the span) would give me a dead load of 0.5 – 1.25 kN/m2 when the clear span is 2400mm. If I was to use 1 x I beam at the centre of the longest room dimension what size would it need to be to give the same support as the joists? For this I would hope for a beam that was 125mm deep.

I would prefer to use the 1st option as it would give a bit more head room to crawl around in and I’m guessing more structurally sound as the resulting loads would be spread across 2 beams?

I would be very grateful for any help on this subject. I can't afford a structural engineer.

Thank you in advance

Will

 

[Teahead]

I work for a structural engineers, and reading what you have wrtitten its sounds like you can not afford to not get a structural engineer involved.
Your floor has to be designed for a live load in kN/m^2 based on the intended use of the room not from the size of joists being used. and if you are supporting a partition wall on this floor the floor would need to be designed to carry this additional load.

Can i ask what you do for a living. i expect people pay you for your services in one form or another. Do the job properly employ a S.E.
Small one man bands are not as expensive as you might think.

 

[noseall]

it's not exactly a pleasent place to be storing stuff?

 

[willfloor]

I work for a structural engineers, and reading what you have wrtitten its sounds like you can not afford to not get a structural engineer involved.
Your floor has to be designed for a live load in kN/m^2 based on the intended use of the room not from the size of joists being used. and if you are supporting a partition wall on this floor the floor would need to be designed to carry this additional load.

Can i ask what you do for a living. i expect people pay you for your services in one form or another. Do the job properly employ a S.E.
Small one man bands are not as expensive as you might think.

Thanks Teahead. The partition wall is supported by the stonewall foundations/footings. This foundation is much wider than the 1 course of bricks that make up the partition wall, hence the 180mm of foundation the beam can be supported on.

Its for the kitchen. As it was before it had 2"x3" joists at 600mm spacing supported on the outsides by the foundation tops, with 3 trimmers of 4"x4" timber. I'm not changing the lay out of the kitchen or the equipment within. Surely someone can give me advice about what size I beam I need?

As for a job the money isn't great hence the reason i'm doing it myself..... every penny counts.

 

[willfloor]

it's not exactly a pleasent place to be storing stuff?

I know mate but somewhere is better than nowhere

 

[IJWS15]

Do you need all the space for storage or only a portion??

Traditionally the joists would be supported by sleeper walls reducing the effective span and hence the joist thickness. You could put sleeper walls in parallel with the short walls with gaps for access through bridged by lintels (as you would an interior load bearing wall). Would restrict the usefulness of the space by breaking it up but you shouldn't need the services of a structural engineer to do it.

The space should be ventilated to avoid condensation problems with the floor and if it is you may want to insulate under the boards as you re-lay them. It also sounds like an old house and there is unlikely to be a DPC in the screed (might just be hard packed earth) and you need to consider this when storing things down there.

If you do it with an RSJ set it up so the RSJ is not likely to rise up through the floor if the joists dry out and shrink.

 

[Teahead]

Just run some numbers and your joist sizes will not work with the required imposed load of 1.5kN/m^2.

125 x 50 grade C24 joists at 400 ctrs spanning 2.1m will work

Steel beam would need to be 127 x 76 x 13UB. with joists built tight against web to restrain beam from buckling.

As mentioned above, when building joists into beam top of joists should sit about 10-15mm proud of top flange of beam, so that when the joists dry out and shrink the top of the beam will not become a high point in the floor.

 

[willfloor]

Thanks for the feed back Teahead, sorry haven't got back been a bit busy lately.

I was thinking of using 100x50 c24 joists hopefully utilising 2 100mm (or close to) steel beams so they were at 1.4m and 2.8m thus giving the clear span of the joists at 1.4m.

Could you let me know if this is possible and what size beam would be needed again? Its just that I have quite a bit of 100x50 C24 already and would rather not waste it.

Thanks for your help so far.

Will

 

[willfloor]

To clarify

I was thinking of using 100x50 c24 joists hopefully utilising 2 100mm (or close to) steel beams so they were at 1.4m and 2.8m thus giving the clear span of the joists at 1.4m.

So in effect Joist group 1 would go from wall to 1st beam, Joist group 2 would go from 1st beam to 2nd beam, and Joist group 3 would go from 2nd beam to wall.