Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

Buy individually or both books together. Delivery is free!


widget @ surfing-waves.com




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthorcjard
    • CommentTimeAug 20th 2017 edited
     
    Having received a quote of 2 grand for 27 no 350x97x7200 I joists I started looking at the raw material costs for making them myself.. I've a table saw and can easily rip sheet materials up

    My pondering turned to the flanges.. 2x1 is about 30p a metre, and I need a thousand metres of it. 2x2 is about 60p a metre. Plywood that is 18mm, ripped into 50mm strips is about 30p a metre again so really, I'm wondering about the design of the flanges:

    Should I do them out of 2x1, and ensure that no cuts are coincidental, not in the OSB, nor any of the four 2x1 layers that make up a flange? Just staggering the straight ends

    Should I do it in 2x2, again, it has plenty of scope for not having coincidental joints across the two parts of the flange plus the OSB.. should I cut the end of the 2x2 at an angle such that I can mate two pieces together; a hundred mm overlap would let me get 7.2m out of 3x2.4m lengths..

    Should I do it in ply? Is it going to be stronger to make the web plus flanges (each requiring 4 rips of ply) from 18mm ply for a similar overall dimension but with double thickness web. Not sure; they make them like they do for a reason but then just look at those parallam beams you can spend a fortune on..

    Pic of possible flange layouts; top down view of a beam standing upright, resting on its long edge
      IMG_0100.PNG
  1.  
    We used 2 x 2 and 9mm ply.
      DSCF0268.JPG
    • CommentAuthortony
    • CommentTimeAug 20th 2017
     
    Is that a clear unbroken span?

    It is not easy to diy things like that signed off by an engineer and also tricky to prove quality processes and control though easy to achieve in practice.

    My advice is to halve the span and just use normal joists.
  2.  
    I cannot open your pic. Ours were of course non-load-bearing.
    • CommentAuthorMike1
    • CommentTimeAug 21st 2017
     
    Agree with Tony. You need a structural engineer's input.
    • CommentAuthorcjard
    • CommentTimeAug 23rd 2017
     
    I meant to say 6.8m, though the span is 6.3m. There's also no load on these joists other than a bit of plasterboard and shed felt - for an outbuilding so I'm not too bothered about structural (also why i'm not too keen on blowing 2 grand on them)
    • CommentAuthortony
    • CommentTimeAug 23rd 2017
     
    No regs diy then :)
    • CommentAuthorcjard
    • CommentTimeAug 29th 2017
     
    If your timber merchant offered, for the flanges, any graded lengths of timber up to 6 m (in 0.6m increments) and you were looking to make 6.3m spanning joists, what would be best to go for? Does it even matter where the joints land (so long as they don't coincide on both sides of the web)?

    Essentially, is there any sense in going for the longest length possible, or does it not matter, because all parts of the flange of a uniformly loaded beam experience equal tension/compression?
    • CommentAuthortony
    • CommentTimeAug 29th 2017
     
    Bottom chord one piece
    •  
      CommentAuthordjh
    • CommentTimeAug 29th 2017
     
    Posted By: cjardIf your timber merchant offered, for the flanges, any graded lengths of timber up to 6 m (in 0.6m increments) and you were looking to make 6.3m spanning joists, what would be best to go for? Does it even matter where the joints land (so long as they don't coincide on both sides of the web)?

    Essentially, is there any sense in going for the longest length possible, or does it not matter, because all parts of the flange of a uniformly loaded beam experience equal tension/compression?

    Since it's impossible to make a 6.3 m long beam out of pieces of wood that are all shorter than 6 m (actually <=) you will of course need at least one joint in both top and bottom chords.

    I don't think the flanges do have equal tension along their length, do they? I would expect the tension/compression to be greatest in the centre but let a proper engineer answer that question.

    I would splice/scarf the lengths together, rather than butt joint, although I suppose a half-lap might be OK. As well as allowing extra length to make the joints, I would allow even more extra length so you could at least cut off any poor looking ends. Putting flange timbers on both sides of the web would also reduce the risk of any weak spots.

    So I'd probably put a single joint fairly close to an end for each run of flange. Maybe a metre or so?
  3.  
    For a simply supported beam supported equally at both ends, the bending moment and flange tension/ compression are greatest at the centre of the span, and zero at the ends.

    The transverse shear stress is t'other way about - zero at the centre of the span - but the bending will be a worry long before it shears.
    • CommentAuthorcjard
    • CommentTimeAug 30th 2017 edited
     
    @djh - no idea why the pic in the first post is no longer showing, but that half lap was the basic idea - to use a 2x2, but shave it down to 2x1 for 4 inches at the end, do the same with the next bit, and then assemble them like a tetris piece, then put a glue+screw right through the centre of the joint, through the web and into the flange at the other side. Naturally I'd also flip the flanges so that one flange is jointed thus at this end of the beam and the other at that end of the beam. Similarly for the top flange, but oriented so that the 4 joints required are distributed 2 at each end of the beam and not on the same side of the beam either

    Real i joists are purely glued (with urethane glue), with the osb web TnG'd and the flanges finger jointed in a sawtooth pattern. I've tried to find a finger jointer machine but I've come to think that the simple lap design, glued and screwed as a homebrew in less controlled environments would exceed the strength of such a glued finger joint anyway. I intend that my beams will be fully glued and screwed every 300mm along each flange with partially threaded screws that clamp everything..

    One thing that did surprise me looking at the commercially produced beams I have, is that some of them really have very little offset between a joint in the flange and a joint in the web; the worst one I have is about 10mm offset. Clearly these guys are confident about the strength of the joint far exceeding the strength of the wood (and this seems borne out by tests of various woodglues I've seen)

    The biggest problem I'm having is in finding suitably sized timber. I've already rejected one lot of 2x2 that was supposed to be 47x50 but turned up as 44x44 because I'm aiming to model a 300x97 cross section JJI by putting a 2x2 either side of an 11mm osb3 web to generate a 320x111 ish cross section (slightly oversize to compensate for the homebrew environment). Turns out that timber that is the size advertised is ridiculously hard to get - I'm being told to order 3x3 and cut it up myself to get 2x2, so i might have to revert to the initial 44x44 idea and go up on the web height some to amke the equivalent of a 350D.. Doesn't increase the tension resistance though, though maybe bonding the web joints by glue/screw a flap of osb over the joints would help?
    •  
      CommentAuthordjh
    • CommentTimeAug 30th 2017
     
    Posted By: cjardThe biggest problem I'm having is in finding suitably sized timber. I've already rejected one lot of 2x2 that was supposed to be 47x50 but turned up as 44x44

    Yes, timber from different sources (countries) have different conventions about what nominal 50x50 actually is but I can't remember what those conventions are. It depends how many times its been planed/regularised of course, so treated is a bit smaller than untreated and so forth. It drove me batty.

    Just oversize everything a bit?
    • CommentAuthorcjard
    • CommentTimeAug 31st 2017 edited
     
    Seems to be 10% less in each dimension, so you're getting around 20% less wood than what you paid for!

    At least when they say it's 4.8m long, it's 4.8m long, not 4.32!
Add your comments

    Username Password
  • Format comments as
 
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press