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Thread: The OT thread V1

  1. #3171
    Quote Originally Posted by cockerpunk View Post
    anyone have any experience doing the calculations for wood construction? i am making a modification to the house, nothing major, but i need to figure out how strong it is. i've found a lot online, but in most cases the estimation of E and max fiber bend stress is pretty much a +/-25% guess. i mean obviously there is a major factory of safety in the calculations, but wow, i am not comfortable with the accuracy of anything i have read so far. mostly its people pulling crap out there asses, or at least it feels like it.

    also, running the numbers for what otherwise seem like structurally sound construction (such as the floor) is utterly terrifying. when doing a new calculation i often try out things i know, are fine in order to "gain my bearings" so to speak in terms of what normal values should be. and yeah, not instilling confidence here.

    pretty sure my modification will be stronger than the entire rest of the house.
    Wood design strengths can come from the southern forest production association which has their strength spec books published on their website. There is a ton of service factors that can be applied to the wood but since you are indoors you can Add a bit more to the design strength they provide. Bad news is wood from home depot should be considered a #3 or #2 at best so the allowable stresses are even more trash. If its permanent you cant take the additional 33 percent overstress factor either.

    As Fred said above deflection criteria of L/720 would be pretty conservative because it just doubles the standard ASD and lrfd live load deflection limits.

    Not sure if mechies get into these kind of deflection criteria but essentially it is as simple as it looks take your span length in inches and divide by 360 for live loads and 240 for dead loads or double up the 360 for 720 to have a stiffer floor span which is nice to do.

    Floor loads for a residential application is something i wouldnt know but im sure it wouldnt be too hard to find or Fred might have them off hand. Snow load might be pretty heavy where you are and you have to determine if any walls are going to put big point loads on the beams because of it as well.

    Simple bending calcs using a simply supported beam would work with double integration deflection method if you have an odd loading setup, or some software if you dont want to do all that work, skyciv isnt bad but the freebee is limited on what you can add into your loading, or Risa 2d educational is floating around but Risa always pissed me off.

    Bending stress =m/s is effective for the stress calc, which can dictate with wood when it is uncommon with other materials.

    If you wanted to get crafty you could count the decking into your calcs and run a quick one for the shear flow through the nails which would juice up your I a bit which is fair for real world applications but i typically havent seen it done, though i normally design support of excavation kind of shit when using wood not residential work.

    You can back calc in assuming a 2x10 is sufficient for the span if its already in place as such, which will help give you some bearing as to where your numbers should be around. Or if its something like tji floor joists they have a whole heap of info on their site, they are the wooden i beams, they are easy to identify if you have them.

    If you are doing a small area hand pick your floor joists and always grab the heaviest ones as long as they are not full of water. And remember the grading for wood is a visual process so if you pick some nice looking heavy boards you can rest easy knowing that you exceeded your design. Pretty obviously dont grab wood with knots and problems on the outer edge for I and for the love of God make sure you or your electricians and plumbers drill through the middle the joists and not at the bottom like they always try to do when they are rushing.
    Last edited by Florypb505; 05-07-2018 at 11:35 AM.

  2. #3172
    its mostly a comfort thing, so i am attempting to match the specification for common use floors so that mentally i can imagine doing the same thing on the floor and feel at ease. the specification here is L/360 for floors.

    my attic joists are 2x6s, span is 13 ft, and im loading is about 4 feet from one support. based on my mechanical engineering level of analysis (at L/360), that puts each joist able to survive about 250lbsf. so to hold up 1000lbs, id want to attach to at least 4 joists. my plan is to use 2x 2x8s as a beam to cross the joists, and attach to 6, including one of the joists being at the load bearing wall, and the far other side being on top of a non-load bearing wall (i figure thats better than nothing). main reason im going with 2x8s there is that because they are so stiff, they will more evenly distribute the load the joists, ie, even the joists 2 away will see significant load. the cross-joist span is 7ft.

    actually computing the load on any joist in such a system would require a system of equations, i think 6 equations and 6 unknowns. so im taking it basically as 2 different 2d problems. the joists strength, and then the strength of the cross beam.

    by all accounts this is way over engineering the issue, but by the math its only "reasonable" in my mind. this makes the 40% swing in things like the elastic modulus of wood ranging from 1,000 kips to 1400 kips for example freak me out. 40% is like a lot, and thus why i dont actually feel like i trust the numbers.


    in order to trust my numbers more, i took to measuring my floors, and found out they often do not meet L/360, and the only reason my refrigerator doesn't go through my floor is that there is flooring on it, to spread the load paths. this did not inspire confidence.

    i am considering now the analysis to link the joists together via top nailed 2x4s at some interval parallel with the cross member, in order to create some of that load sharing and more complex load paths. i would really rather not put actual flooring type coverings up there, because all the insulation etc.
    Last edited by cockerpunk; 05-07-2018 at 01:40 PM.
    social conservatism: the mortal fear that someone, somewhere, might be having fun.

  3. #3173
    hummm gonna double check my measurements, according to standards they should be 2x8s, i was pretty sure they were 2x6s, but if they are 2x8s then i can do whatever the hell i want, they will be strong enough.

    EDIT: brings up the idea .... what about adding some 2x2s to the top? not gonna be equal to a 2x6, but will be stronger than a 2x6.i actually have to add a one inch riser in a certian place anyway.

    DOUBLE EDIT: wow, because moment of inertia dominates the calculation, adding 1 inch to the thickness of the beams (assuming a lower joint efficiency than a single beam), does more for there strength even than adding 2 whole new joists. i guess i have my new plan.
    Last edited by cockerpunk; 05-07-2018 at 05:01 PM.
    social conservatism: the mortal fear that someone, somewhere, might be having fun.

  4. #3174
    But by adding in a stringer to the joist, you are creating more work and is much more costly. You can look into engineered joists and beams for your application. They have exactly the ratings you are wanting. Don't forget metal construction beams and posts.

  5. #3175
    Quote Originally Posted by Nobody View Post
    But by adding in a stringer to the joist, you are creating more work and is much more costly. You can look into engineered joists and beams for your application. They have exactly the ratings you are wanting. Don't forget metal construction beams and posts.
    rebuilding the attic is significantly more work then glueing 6 2x4s to the top of the joists.
    social conservatism: the mortal fear that someone, somewhere, might be having fun.

  6. #3176
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    If your existing span is indeed 2x6, and you want to get additional bearing capacity and put some sort of floor or platform in there, have you considered sistering in 2x8s, and then running 2x on top across the span between them? This would save you depth, and keep your bearing conditions consistent.

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  7. #3177
    its actually a point load that im using, which is why the classic 40lbs/sqft specs don't actually make that much sense. i have considered additional cross beams though in order to strengthen the joists. sharing load between them should improve stiffness farther.

    I did the math, and im content with the stiffness of 2x6s with 2x4s on the top of the them. it actually equals or is slightly better than 2x8s, which are plenty strong for the application
    social conservatism: the mortal fear that someone, somewhere, might be having fun.

  8. #3178
    Point loads for a girder are common which it seems is what you are doing is putting a girder under the joists

  9. #3179
    Quote Originally Posted by Florypb505 View Post
    Point loads for a girder are common which it seems is what you are doing is putting a girder under the joists
    over them, but yes. thats what im doing. 4x8 cross piece is a "girder" mounted to, and across the joists.
    social conservatism: the mortal fear that someone, somewhere, might be having fun.

  10. #3180
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    I have to assume based on the information at hand that you're eschewing the in ground lift and will instead suspend your miata from the attic joists via the Tom Kaye method.
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