Loft Conversion

[Shytalkz]

Has anybody heard about plastic beams instead of RSJ's? Somebody was telling me about them there meant to be just as strong but are a lot lighter and easier to manage.

Yeh, right....

 

[^woody^]

They also tend to melt in a fire and bend in a hot summer.
The same company makes fire guards IIRC.

I'm not sure how well the business is going though

 

[allloft]

I would recommend that you get some plans drawn up for your conversion
to avoid any costly mistakes.You will need these as well as structural calculations for the council.I don't think you will find plastic rsj,s .Steel u/b or u/c are usually specified,normally sliced into smaller sections.

 

[Duzzer]

Take it that's a no then. Been trying to find info on them tonight i've found one called Silent Floor here are the benefits
* SilentFloor® system guarantee squeak free floors - no dips due to the unique joist design!
* Long prefab spans for wide floor areas and design freedom
* Lightweight and resource efficient and environmentally sound
* Resistant to effects of temperature and moisture
* Free of wood defects such as knots, shrinking, bowing, twisting and splitting
* High-tech proprietary web material and laminated veneer lumber
* Roof structure and framing solution
* Ideal for floor framing, floor joist replacement
* For replacing floor joists with wide spans that are rotten or squeaking.
Looked at pictures and it looks like manufactured wood.

On the subject of plastic in a fire there would be wood as well as RSJ's in a normal loft conversion.
Also new technologies in plastic would make them fireproof to a certain degree. I'm a Gas Eng and never thought i'd see part of the heat exchanger's made of plastic and they get very hot and don't melt.

 

[^woody^]

Whilst some plastics may not melt, I don't think that they retain sufficient shear/bending resistance when heated. Wood performs really well when just heated, and very good even when charred, and steel does well too up to certain temperatures

But I can't think of any plastic that would stay straight over such long runs - let alone if supporting a load too. It would have to be unfeasibly thick and heavy if anything.

A resident engineer can advise on the plasticity modulus of plastic!

 

[Shytalkz]

God knows, not something I've designed in (although I did get involved with an acrylic tunnel for a shark tank and that was hellishly complicated with loading and unloading of the water pressure); presumably as we type, some little boffin is determining elastic values for plastics, as well as elastic-critical, elasto-plastic, plastic, yawnzzzz...as to what they are though, best speak to someone from Arup, they like doing that sort of thing They sure as hell aren't going to get up to steel values though.

The Silent Floor is the trade name for the Trus Joist TJI system. That's wood, not plastic. Not really geared up for loft conversions, the joists are all pre-made, you don't go cutting them about like timber joists, if they don't fit.

 

[^woody^]

I wonder why box beams or similar in-situ beams from some timber and sheets are not popular ?

I would have thought taking the small, light bits of timber and sheet and knocking something up in the loft would be the easiest thing to do. But no-one seems to do it

 

[Duzzer]

Found some info on a thing called Fibre Reinforced Plastic Joists(FRP). So there is something out there. If it's carbon fibre surely the carbon fibre would be fire proof.

 

[Shytalkz]

I wonder why box beams or similar in-situ beams from some timber and sheets are not popular ?

I would have thought taking the small, light bits of timber and sheet and knocking something up in the loft would be the easiest thing to do. But no-one seems to do it

The principle that you talk about is fine, in theory; the reality of constructing it correctly means much more fiddling about than lugging up some standard joists and cutting them to sit on the bearings. That's why no sensible/practical SE's would suggest this as an alternative.

I-beams rely on the glued joints to resist horizontal shear at the top and bottom interfaces of the ply web and that can only be done with any certainty in a controlled environment. There are the screwed/nailed alternatives, but you wouldn't like the numbers of fixings that that involved: as the depth of the beam would not be much, the forces (remember SAy/Ib from your two weeks of structures lessons? ) would be commensurately greater than for a deeper beam.

The same principle you suggest can, of course, used to site-construct plybox beams in roofspaces, where they act as supports to both floor and wall. However, these are much deeper, have several solid flanges and still need a comprehensive nailing pattern. Knocking up a site-made girder truss out of timbers is more straightforward, as well as being easier for the average chippy to both understand and construct correctly.

 

[allloft]

Found some info on a thing called Fibre Reinforced Plastic Joists(FRP). So there is something out there. If it's carbon fibre surely the carbon fibre would be fire proof.

Have you ever seen the price of carbon fibre?
rsj s or u/b or whatever you want to call them are there to take the weight
of the new floor joists and usually to take some of the roof load i.e if you remove the king post.Silent beams or engineered joists are mainly used in new builds in place of 8x2 or similar timber joists,they are very good for this use but I have never used them in a loft conversion

 

[Duzzer]

Thanks for the reply's i'll be sticking to RSJ's just got to get head round getting them into loft as chimney breast runs through centre of loft. Not gonna get them in in one piece am I.

 

[Shytalkz]

They can be spliced, so you can get them up in sections; but that does need designing by an SE, not Woody .

 

[Duzzer]

That's scary cos my mate thats doing it his name is Woody

 

[Duzzer]

Question on the electrics there is still a fuse spur in airing cupboard that was for the immersion heater. Put a combi in couple of years ago. Would that be adequate to use and just use a radial circuit for about 4 sockets and 2 lights. My guesstimates say its ok.

 

[brig]

hi your exiting purlin does not look if it is doing the work it should be doing flat against the c/rafters , an angle at pitch. regards brig