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It looks so neat! (thelemmy.club)
submitted 10 months ago by to c/wtf@lemmy.wtf
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[–] 26 points 10 months ago (23 children)

Hypothetically speaking, if he just cut a whole big enough for the pipe to go through how much better would it have been?

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  • [–] 74 points 10 months ago* (last edited 10 months ago) (9 children)

    Much better. Imagine you’re bending something. The maximum tension is going to be at the very bottom and the max compression is going to be at the very top (this is why steel I-beams are shaped the way the are - to put the most material in the areas doing the most work). If you can put the hole in the middle you’re not impacting the structural integrity of the joist too much. You’d still have to worry about shear forces so you’d not want the hole to be too close to either end. Look up “castellated beam” if you want to see some steel examples.

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  • [–] 26 points 10 months ago (7 children)

    Why not just use hangers? No way a metal bracket doesnt exist for hanging pipe from joists. DIY some metal wire and a screw to hold it if you have to, hang that sucker like fresh venison, but why cut existing structures?

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  • [–] 5 points 10 months ago (2 children)

    It's not structure, it's just extra wood

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  • [–] 6 points 10 months ago (1 child)

    I meant structure as in existing stuff, not 'structural' as in load-bearing. Extra wood would indeed still count as 'structure' within the context I meant.

    This is, however, not just extra wood. What was cut through in the picture is floor joists, which are what holds up the flooring and supports whatever is on the floor, including people and furniture. Cutting these joists severely compromised the integrity of the floor. Now instead of a squeak from stepping in that spot, it's far more likely that a loud crack followed by a fall through the floor will be the result of a heavy step.

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  • [–] 8 points 10 months ago*

    A joist hole can be no more than 1/3rd the total depth of the joist, I think that pipe is larger than that. Even if you placed it in the very middle you’d still be compromising the structural integrity.

    Best to just get some hangers and a fix it to the bottom; you’re still well clear of the ground. Put in some shims near one end if it’s a drain and you have to maintain pitch; I think this is a thick electrical conduit though?

    EDIT: nope that looks like 4” PVC drain pipe

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  • [–] 11 points 10 months ago (7 children)

    Assuming it's centered vertically and not too close to the ends, then the joists would still have essentially full strength. Because the top and bottom are seeing compression and tension, there's an area in the middle that's not so critical.

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  • [–] 6 points 10 months ago (6 children)

    No, no they wouldn't. In the UK there are very strict rules about notching or putting holes in wooden joists. Precisely because it weakens them.

    Full joist the load is spread through all the fibres of the timber. Drill a big hole in the middle and the load only goes through the top and the bottom fibres. So essentially it's as strong as those two blocks of fibers. So why not make the joist just that thin the the first place then? The answer is because it's nowhere near as strong.

    There are various steel, composite and wooden joists with a special frame construction that can have hollow sections. A standard piece of timber with a big hole drilled in the middle of it is not one of them

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  • [–] 29 points 10 months ago* (5 children)

    I was trying to keep it simple for the sake of answering the question, but I'm literally a carpenter and drilling solid wood joists is totally fine (at least in Canada) as long as you roughly follow what I said. Maybe I could've said "design strength" instead of full strength but again, trying to keep it short and simple.

    I've worked mostly commercial though so you've inspired me to look back in my books to make sure I wasn't way off base in my recollection.

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  • [–] 3 points 10 months ago (2 children)

    Thanks that exactly proves my point. As your diagram says. You're allowed to drill a hole at a max of 1/4 the width of the joist. So even if that's an 8 inch wide joist the biggest hole you can put in it is 2 inches wide.

    That looks like 4" waste pipe. So to drill a hole through a joist for that the joist would need to be 16" inches wide.

    So you can't just drill holes in joists in the "middle that isn't critical" As I said there are specific rules about where the size of holes you can put in joists because the size and location matters very much to the strength of the joist.

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  • [–] 6 points 10 months ago (1 child)

    Well, the joist loses perhaps 25 % of its strength to the big hole, as opposed to some 75 % to the bottom notch. It's still fair to call it "much better".

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  • [–] 1 point 10 months ago

    It also “won’t pass inspection” and it has the additional effect of “making your property worthless without repairs”.

    Additionally it might still “kill you” when the fridge slides over and tips on top of you when your floor is suddenly 28 degrees sloped downward because it cracked in half.

    Imagine dying pinned under your own couch because you did this thinking it made your house stronger

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  • [–] 5 points 10 months ago (3 children)

    A lot. Most of the strength from a beam comes from the top and bottom, because these are the parts that have to stretch or compress most when the beam is bending. The middle part is contributing relatively little for strength.

    That is why metal poles are often hollow, that saves a lot of material and weight without losing much strength.

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  • [–] 1 point 10 months ago (2 children)

    Every part of what you said is wrong.

    The middle indeed provides strength, when it is intact. It prevents the top and bottom from additional deflection under load as it takes on part of that stress. That is why there are very strict rules about when and how you can cut a penetration through a load-bearing joist (which, by definition, they all are).

    Additionally, metal poles are a cylinder and take an axial compressive load, for which a cylinder is a very good shape; if you tried to support a floor with a wooden cylinder as a joist and the load was lateral to its length, it would likely buckle - its whole shape is hollow so it has the same problems as a joist with the middle cut out.

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