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    • CommentAuthorgreenman
    • CommentTimeJul 9th 2008
     
    In recent years it seems that building using exposed timber framing has seen a resurgence in popularity. I've also noticed that such frames are sometimes constructed using modern steel jointing techniques rather than traditional joint and peg ones.

    I suppose it's horses for courses (modern techniques in a modern building design, or traditional techniques in a traditional design) - a matter of taste and aesthetics, though I'm afraid I'm a traditionalist - but from a green building perspective, given that those shiny bits of steel take resources and energy to make, and that traditional jointing techniques ought to be able to do the job as well (or better), should these new products be used in a 'green' building project?
    • CommentAuthorTheDoctor
    • CommentTimeJul 9th 2008
     
    the modern 'technical' solutions are often able to outperform some traditional joints if you are going for serious spans, eccentric loads or other things that take the materials to its limit, and is not ALWAYS about aesthetics.

    I have a soft spot for both, to be honest.
    • CommentAuthorgreenman
    • CommentTimeJul 9th 2008
     
    I can see that a stainless steel component (for example) would be able to take higher loads than an equivalent piece of oak (for example), but given that the steel is then attached to a piece of wood anyway, that benefit must be questionable surely? Furthermore, where steel fails catastrophically in very high temperatures, oak frames have been known to char and smoulder for a considerable time before failing and even then they tend to give plenty of prior warning before collapsing (creaks and groans). Lastly, properly constructed timber joints (those made in 'green' timber at least) tighten as the timber dries - those 'technical' solutions I've seen have become loose over time as the timber has shrunk to the point that the components no longer fit each other.
    • CommentAuthorTheDoctor
    • CommentTimeJul 9th 2008
     
    the joint can be doing so much more than the straight piece of timber to which it connects.

    Also, the cutting, carving and jointing of the timber also weakens it, in the same way that a knot reduces the breaking strain of a rope by over 60%

    it all depends on what you are using it for. If it is a barn style building - old school, tried and tested is a winner in my book.

    If it is a large span timber roof deck over a swimming pool or a school sports hall - perhaps a few stainless steel fixings are just the job?

    good point about fire though.
    •  
      CommentAuthorrogerwhit
    • CommentTimeJul 9th 2008
     
    Posted By: greenmanproperly constructed timber joints (those made in 'green' timber at least) tighten as the timber dries


    How so? Think of a mortice and tenon in 'green' wood - both the jointed members will shrink roughly equally (the exact amount dependent on grain orientation) as the moisture content reduces. Moreover the morticed surface will shrink away from the tenon shoulder leaving a gap. How can this make the joint tighter?
    •  
      CommentAuthorfostertom
    • CommentTimeJul 9th 2008
     
    I never understood that - but that's what they say - just another myth? like fwd Saabs (when fwd was a novelty) would 'pull the car through the corner' - in fact the reverse is true - quite fancy tricks necessary to stop a fwd from ploughing straight on.
    Posted By: TheDoctorthe cutting, carving and jointing of the timber also weakens it
    true, slightly of the mortised member - but the tenoned end is reduced to a fraction - how can that have any strength?
  1.  
    Joints always shrink away from each other so its def a myth! We always cut the tenons to fit very very tight for this reason and then bang them in with some grease and a mallet and strapping to get as super tight as possible cause you know they will move later.

    The mortice is usually a third of the size and while not as strong it doesnt need to be because the strength is in the shape of the frame, not the joints themselves.

    This is prehaps where mechanical/metal fasterners come in because they allow more flexibility in framing shapes where the load can be transfered through the strength of the joint, ie portal frames. With traditional framing you cant do this because the joints are not strong enough.

    so metal has its place prehaps...
  2.  
    "I can see that a stainless steel component (for example) would be able to take higher loads than an equivalent piece of oak (for example), but given that the steel is then attached to a piece of wood anyway, that benefit must be questionable surely?"

    even when attached to timber, it is the metal fixing is taking the shear forces, which it can do many times better than wood. The metal is usually attached using the compressive strength of the wood.
    • CommentAuthorjoe.e
    • CommentTimeJul 10th 2008 edited
     
    Posted By: fostertomI never understood that - but that's what they say - just another myth? like fwd Saabs (when fwd was a novelty) would 'pull the car through the corner' - in fact the reverse is true - quite fancy tricks necessary to stop a fwd from ploughing straight on.
    Posted By: TheDoctorthe cutting, carving and jointing of the timber also weakens it
    true, slightly of the mortised member - but the tenoned end is reduced to a fraction - how can that have any strength?

    The tenons have the strength they need because the timbers are sized for their resistance to loading in the middle, or to remain in column under compression, which makes them over-sized for resistance to shear at the joints. So you can cut away loads of wood, and the tenon remains over-sized for its own shear load.

    I think that's linked to another reason for using metalwork at the joints. Traditional timber-framed buildings use massive timbers in quite basic structures. It's possible to design much lighter buildings, bigger spans, flatter arches and so on. But as the structural possibilities are pushed further, the joints become the weak points - less wood and higher loads - and have to be strengthened.
    •  
      CommentAuthorrogerwhit
    • CommentTimeJul 10th 2008
     
    On a smaller scale, there is a technique used in windsor & stick chairs where the legs socket into or through the seat, of drying the leg-ends (with a heat source) so that they're pre-shrunk, so that following assembly the 'wetter' seat shrinks more than they do and tightens the joint.

    In suspended timber floors, the traditional type of herringbone strutting is self-tightening as the joists shrink in situ. If substituted, a ply web doesn't do this - it gets looser.
    • CommentAuthorgreenman
    • CommentTimeJul 10th 2008
     
    Roger - as you say, windsor chair construction relies heavily on choosing wood at the right stageof seasoning. Chair legs were part (or sometimes fully) turned in green wood (by bodgers), and then the ends turned to finished dimensions when fully dried. A similar technique was (and is) used when pegging joints in green timber framed joints - the frames were (are) cut and jointed while the timber is green, but the pegs are made from seasoned timber with the result that as the green wood dries, it tightens onto the pegs.

    Traditional timber frame joints were designed and evolved over centuries. Different joints were used in different situations (and there were an awful lot of them) precisely so as to ensure that loads were safely carried or transferred. A simple mortice and tennon joint would have disadvantages in some applications, but there were many variations on it, allowing it to be effective in those applications.

    joe.e is right though - timber framed buildings were over engineered by modern standards. Having said that, look how many of them are still standing - anyone want to predict how many of our 'modern' building stock will last as long? If you're trying to make the timber go furhter though there's no doubt that traditional jointing techniques wouldn't be appropriate. Whether stainless steel is the answer though, I don't know...
    •  
      CommentAuthorfostertom
    • CommentTimeJul 10th 2008
     
    Definitive book on evolution of timber frame - how it reached a max of structural efficiency and then degraded with over-fancy notions. Beautifully illustrated with line drawings, very clear: The Development of Carpentry 1200-1700 (an Essex study) by Cecil Alec Hewett, David & Charles 1969, 7153 4694 6.

    Compares well with The Art of Japanese Joinery by Kiyosi Seike, Weatherhill/Tankosha 1977, 0-8348-1516-8.
    • CommentAuthorgreenman
    • CommentTimeJul 10th 2008
     
    Thanks Tom - I don't think I've read either of those.
    • CommentAuthorTheDoctor
    • CommentTimeJul 10th 2008
     
    <blockquote><cite>Posted By: fostertom</cite>
    Compares well with The Art of Japanese Joinery by Kiyosi Seike, Weatherhill/Tankosha 1977, 0-8348-1516-8.</blockquote>

    i may well get the Japanese one. I worked in Japan 'on the tools' as such for a year.
    Mainly working with ancient joiners that could through up a traditional tatami mat house from memory - no drawings.

    Amazing double and triple tenons.

    But beware! a Japanese saw cuts on the pull, not the push!
    (didn't know that, did you?!)

    the bamboo scaffolding takes a lot of getting used to for a lanky lump like me
    • CommentAuthorgreenman
    • CommentTimeJul 10th 2008
     
    Japanese (pull) saws are a very useful addition to the tool kit - you can't beat them for trimming very small amounts off end grain. Because they cut on the pull, the blade doesn't have to be braced so they are comparitively thin and therefore produce a really fine cut.
    • CommentAuthorjoe.e
    • CommentTimeJul 10th 2008
     
    Japanese planes are intended to cut on the pull too; interestingly, Japanese chefs cut on the pull with their beautiful knives. I'm very fond of my Japanese saws, although a minor downside is that generally you can't resharpen them, you have to replace the blade. I think the really expensive ones can be sharpened, but only by a real expert.
    • CommentAuthorgreenman
    • CommentTimeJul 11th 2008
     
    The ones I've seen don't hve any set on the teeth either - are they all like that?
  3.  
    Metal has been used in timber framed houses for ages-I'm renovating a 16th century house which has original iron connecting pieces on the sole plates, and elsewhere. I don't think carpenters were purists then, they did whatever worked, but metalwork was a lot more expensive so they preferred not to use it if there was a choice.
    And on the subject of what worked, I think this whole green oak has been over emphasised- for better quality buildings, i'm certain the oak would have been left to season for a while. When you see a mediaeval or tudor moulded ceiling with the joints and mouldings still meeting exactly, you can be sure they used seasoned oak.
    • CommentAuthorgreenman
    • CommentTimeJul 14th 2008
     
    Paul

    I'm sure you're right that metal was used in some frames, but that doesn't necessarily mean it was better. In some cases joints were plated to repair damage because it was easier than dismantling the frame to install a new component. As to whether or not frame makers were purists, I suspect it was just the same then as it is now - some took more pride in their work than others, some were better at it than others, some were purists, others were progressive and so on.

    Your point about moulded timber ceilings is an interesting one. I don't know whether or not they used seasoned oak, but I don't think you can assume that they did. I don't believe that it's necessarily the case that just because a frame is assembled green means that the joints will open up significantly as they dry. For a start, there are degrees of greenness, particularly depending on the time of year when timber is felled. The rate of shrinkage of oak almost certainly isn't linear, so a timber that is 'half dried' will have done more than half its shrinking.

    Ok - I admit it - I'm biased!
    •  
      CommentAuthorfostertom
    • CommentTimeJul 14th 2008
     
    Considering using green doug fir for steel-flitched structures e.g. pr of 8x2s bolted each side of a 190x10 fabricated/galvanised flat. Green doug fir is much much cheaper than seasoned, and should be OK, I'd have thought, seeing the steel limits movement. Don't mind a bit of cupping and splitting - rustic, nice. However, doug fir seems to shrink extraordinarily lengthwise, which would pull the members unpredictably out of shape, as well as opening half inch gaps at the ends (which doesn't matter structurally as the steel makes the connection, but looks bad). Is this just softwood behaviour, or peculiar to doug fir?
    • CommentAuthorgreenman
    • CommentTimeJul 15th 2008
     
    Tom - I don't know anything about Douglas Fir. You say it seems to shrink extraordinarily - that implies that either you've seen it used elsewhere, or have some other source of information?
    •  
      CommentAuthorfostertom
    • CommentTimeJul 15th 2008
     
    Something I've been warned about but more I think about it, I think it's misinformation. Maybe it's the widthwise shrinkage of the piece it's against at the end, I will look into it further.

    About whether timber tightens on a mortise and tenon, or loosens, I got this from a joiner/framer today:
    It depends how the grain is, on the cross section of the mortised member. Which ever way the curve of the grain (the rings) run, the section will tend to go banana shaped the other way. In other words, the curve of the rings tends to straighten a bit as it dries (this is well visible, with the extreme movement you get with green timbers). So you can see that one edge of what was a rectangular cross section lengthens, the other contracts, as the edges go curved. If it's a through-tenon, the mortise will therefore tighten on one end of the tenon AND LOOSEN ON THE OTHER END (oops). Best practice is that grain should run such that it tightens on the root, not the tip of the tenon. In so doing, as the mortised face goes concave (cups) it also forces hard against the tenon's shoulders.
    •  
      CommentAuthorrogerwhit
    • CommentTimeJul 15th 2008
     
    Tom I have figures somewhere for radial & tangential shrinkage (either for every 10% change in RH I think, and alternatively for going from 'green' to 'air-dry' - for diff species, and I can't see anything outlandish about Douglas fir. A bit more than oak. (Figures not to hand this min). None of my sources mention longitudinal shrinkage!!!
    •  
      CommentAuthorrogerwhit
    • CommentTimeJul 15th 2008
     
    Posted By: fostertom
    Which ever way the curve of the grain (the rings) run, the section will tend to go banana shaped the other way. In other words, the curve of the rings tends to straighten a bit as it dries (this is well visible, with the extreme movement you get with green timbers). So you can see that one edge of what was a rectangular cross section lengthens, the other contracts, as the edges go curved.


    Yes, fine.

    Posted By: fostertomIf it's a through-tenon, the mortise will therefore tighten on one end of the tenon AND LOOSEN ON THE OTHER END (oops). Best practice is that grain should run such that it tightens on the root, not the tip of the tenon. In so doing, as the mortised face goes concave (cups) it also forces hard against the tenon's shoulders.


    You've completely lost me here!
    •  
      CommentAuthorMartian
    • CommentTimeJul 15th 2008
     
    If the timber used for both elements of the joint has the same moisture content , the shrinkage shoould be equal and therefore have no net result. If you want it to tighten as it ages, try using slightly drier wood for the tennoned member than the morticed one (although you will get a marginal step between the members as they dry). Alternatively draw-dowel the joint to hold the shoulders up tight and use wedges on end of the tenon. If you want a hidden finish, use blind wedged tennons and draw-dowel them too.
    •  
      CommentAuthorrogerwhit
    • CommentTimeJul 15th 2008
     
    Posted By: MartianAlternatively draw-dowel the joint to hold the shoulders up tight and use wedges on end of the tenon. If you want a hidden finish, use blind wedged tennons and draw-dowel them too.


    There's a lot of stuff going on there, some of it contradictory. Wedging the end of a tenon can be counterproductive. And I can't see much point in wedges AND pegs. For wedging, you need to ensure that the wedge grips the tenon near its root. This needs careful measurement to take effect, and the wedge cut to do this: merely touch the wood at the tenon's tip, and grip to the point of indentation (or beyond, if using hardwood wedge in softwood frame) near the root. This will minimise shrinkage gaps.
    •  
      CommentAuthorMartian
    • CommentTimeJul 15th 2008
     
    If you timber is not too dry, the best way to wedge tenons (in my opinion) is not on the outer edges. Instead, cut the mortice slightly wedge shaped, run a couple of saw-cuts about 3/4 of the way down the tenon and drive the wegdes down when the joint is assembled. The tenon then asumes a sort of dovetail shape and cannot easily be withdrawn. I agree that draw dowelling is overkill, but they look nice on large section framing.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 15th 2008 edited
     
    Posted By: rogerwhit
    osted By: fostertomIf it's a through-tenon, the mortise will therefore tighten on one end of the tenon AND LOOSEN ON THE OTHER END (oops). Best practice is that grain should run such that it tightens on the root, not the tip of the tenon. In so doing, as the mortised face goes concave (cups) it also forces hard against the tenon's shoulders.


    You've completely lost me here!
    A through-tenon runs from face A to face B (and beyond) of the through-morticed piece. If face A contracts by going concave and face B expands by going convex, then the flanks of the mortice will move together near face A and apart near face B, thus tightening on the root of the tenon at face A and going loose on the tip of the tenon at face B. Or vice versa, depending how the grain runs on the cross section of the mortised piece.

    Posted By: MartianIf the timber used for both elements of the joint has the same moisture content , the shrinkage shoould be equal
    Shrinkage isn't equal in all directions; by mis-matching that, either nett tightening or loosening can happen.
    •  
      CommentAuthorrogerwhit
    • CommentTimeJul 15th 2008
     
    Posted By: fostertomA through-tenon runs from face A to face B (and beyond) of the through-morticed piece. If face A contracts by going concave and face B expands by going convex, then the flanks of the mortice will move together near face A and apart near face B, thus tightening on the root of the tenon at face A and going loose on the tip of the tenon at face B. Or vice versa, depending how the grain runs on the cross section of the mortised piece.


    A through tenon doesn't have to project, it can emerge flush, but I won't quibble. I'm not following your thought at all Tom in what follows that.
    •  
      CommentAuthorrogerwhit
    • CommentTimeJul 15th 2008
     
    I'd say that your concavities & convexities are due to tangential shrinkage. But the wood also shrinks radially so the mortice shoulders will move away from the tenon shoulders whatever happens. Remember the grain of the meeting members may typically be at 90 deg so they're not in sync in that sense.
   
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