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  1. #16
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    Quote Originally Posted by IanW View Post
    Interesting that Terry does a single angle, as is traditional, but I've always seen it done the other way round, with the tail on the bottom side of the shelf rather than the top. He [Terry's] explained his reason for doing it that way is because the top of the joint wants to pull out when the shelf is loaded & sags, so it needs the reinforcing there & not on the bottom. At first blush, this seems logical, but I don't think it's quite right. When loaded, the bottom of the shelf will be in tension the top is actually in compression (this is true when you bend any board), so I suggest the tail is probably doing more good on the bottom than the top. Someone with the relevant engineering expertise please tell me if I'm out to lunch with my reasoning...
    Terry's reasoning is correct.

    The top of the shelf is in compression and wants to pull out of the housing. Locating the DT edge of the housing on the top edge of the housing and the shelf tends to hold the very end of the board used for the shelf in position. In broad terms the half DT is fixing the top of the end of the shelf in the housing creating what is termed a fixed connection -- the same effect as can be achieved by driving screws into the end of an Ikea style bookshelf.

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    I still remember some of the structural engineering I studied almost 50 years ago.
    regards from Canada

    ian

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  3. #17
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    OK, I'll concede to superior knowledge - it does seem intuitive that the ends would want to pull away from the housing if there is any deflection in the shelf, even though the top of the 'girder' is in compression . I was unsure of my initial reasoning, which is why I begged for help..

    I still think that this may be one of those situations where the best structural solution is not necessarily the best practical solution. Having the top of the housing straight & putting the bevel on the underside clearly works, attested by numerous old pieces I've seen that are still sound. It's much easier to make the joint with a square edge on the top of the housing, imo. There is no necessity to remove any material from that side of the shelf, and as long as you've made a straight trench, the top of the shelf is automatically level & square. If you make your shelf solid enough for the expected loads, there should be very little deflection, so you are unlikely to get significant movement at the ends of the shelf. In any case, I'm only expecting stuff I make to last 100 years if it's lucky, so I think I'll stick with tradition on this one.....

    Cheers,

    Edit: I had another think about it after I hit 'send' & I think I'm still talking rubbish. If you make the bottom of the trench square, and your shelf material is of even thickness throughout, the same applies, the shelf should end up straight & level if you take the material off the top to form the sliding tail. I guess that is so easy to do in this age of 'lectric thicknessers, so it is just as easy to do it the structurally superior way. In days of old when shelves were hand-thicknessed (& often not very carefully, even on high-end stuff I've seen shelves left pretty rough on the bottom - out of sight out of mind!), it would have been easier to do it with the tail on the bottom...
    IW

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