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Thread: Queston from non-Engineer--Sister Floor Joists with lumber or angle iron?

  1. #1
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    Queston from non-Engineer--Sister Floor Joists with lumber or angle iron?

    I hope it is kocher for a non-engineer to seek DIY advice on this forum, as that is what I am doing.

    I am sistering floor joists in my 1929 home. The joists are 16” O.C. and are either 2x8x16’, 2x8x14’, or 2x8x12'. I’ve already done the sistering on one side of the house. I jacked the joists to make the floors approximately level, wedged sisters next to the full length of the existing joist, glued, and bolted with 3/8” bolts spaced at 1’ intervals in a W pattern. When I removed the shoring, I lost half of my leveling in one part of the house where the joists are 16’ long and there is a wall directly above. (I jacked up Ύ”, but lost half of that.)

    My question concerns the best way to remedy this. I’m thinking of either adding a second sister to the sagging joists, or adding angle iron to the bottom of the sagging joists. I’d like advice on which strategy is best and, if I do the angle iron, what size would be best.

    If I do angle iron, I don’t want to drill any more holes in the joists, so I would utilize the bottom holes I’ve already drilled (1-1/2” from the bottom of the joist, at 2’ intervals). So the bolts would be in a straight line 1-1/2” from the joist bottom at 2’ intervals. Because angle iron comes in 20’ lengths, I’m thinking of using 10’ lengths of angle iron centered on the midpoint of the joists. Regarding angle iron size, it seems to me that what matters is the size of the vertical part that is bolted to the joist and not the horizontal (bottom part). I’m considering 3 x 2 x 3/8 or 4 x 3 x 3/8 or 5 x 3 x 3/8.

    I note that the weight of a 10’ length of 3/8” angle iron is 60# (3x2), or 90# (4 x 3), or 100# (5x3), which compares with 26# for 10’ of 2x8 lumber. I’m concerned about adding too much weight.

    Any advice would be much appreciated.

    Thanks,
    Randy

  2. #2
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    Hi Randy,

    Welcome to the forum and yup, it is OK to ask DIY stuff here. Whether you get the answers you want is another matter.

    I suspect there is a a lot more going on with the joists. It is highly possible that the sagging old ones have been that way for so long they are now permanently bowed. The new joists probably spent better than 50-percent of their stiffness in fighting the old joists to bend back down when you removed the jacks. It may be a game of diminishing returns.

    Only the 12' is the correct size for those spans and spacing with 2" x 8". For the 16' you are going to need 2" x 12" before you bolt and glue anything.

    I am of two minds in how to approach this given that the original beams may be set. My first thought would be to jack it all up again and sister 2" x 12" to the other side of the stock beams, but again it will be fighting the set.

    The preferred approach to my mind would be to use a temporary steel beam and jacks to raise the floor without lifting the old joists. Lever the floor up so the nails come up out of the old joists. Then sister the new 2" x 12" to the old bent beam and remove the jacks and steel beam. Then nail the floor to the new "sister."

    Dave
    Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

  3. #3
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    Thanks Dave. Your preferred approach is innovative. I did an internet search on fixing sagging floors but did not come across that idea. But after sisterming 33 joists I'm not inclined to go that route.

    I am interested in reacations to the angle iron idea. I would not lose 2" of headroom as I would with an additonal 2x10. (BTW, wouldn't paired 2x8's meet code on a 16' span?)

    Randy

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    Hi Randy,

    Another approach might be to use P-Lam 2" x 8" beams, the plywood looking type beams. I believe they are rated at a far greater load carrying capacity than standard pine, or cedar 2" x 8". They cost more, but see how many you need and see if its in your budget.

    Also, something to consider is jacking the floor upward, slightly past level, bolt and glue the sister beams, then when you remove your jacks, you will be adding load to the new beam and it will drop to level. There's no telling how far past level to go in the upward direction, though. I've never tried this, just a thought.

    Walter

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    Quote Originally Posted by randym View Post
    I am interested in reacations to the angle iron idea. I would not lose 2" of headroom as I would with an additonal 2x10. (BTW, wouldn't paired 2x8's meet code on a 16' span?)
    Hi Randy,

    I hate to point out that checking the required beam size should have been the first thing you did before cutting lumber or drilling a hole.

    You would be losing an additional 4" of headroom as a 2" x 12" just barely makes it for a 16' span. Doubling the beam does not necessarily halve the deflection (Bend/sag) It would increase the load before failure characteristics though.

    Angle iron is not very good ins stiffness as a beam. Although bolting it to the beam may help as it will prevent twisting of the angle iron which is why it has poor beam-stiffness. BUT, it is anyone's guess what additional beam stiffness it will add. Ideally if one is thinking of using angle iron as a beam then the blade must be pointing down so it is in tension. However, I do NOT recommend using angle in any way as a beam.

    You have already created 33 monsters with the incorrect sized sister beams, think about it before adding more metal to the disaster. There are only so many ways you can fix this and you do not seem to be contemplating any of those.

    As I said, you may not always get the answer you want.

    Dave
    Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.
    Last edited by PinkertonD; 06-09-2011 at 07:56 PM.

  6. #6
    Administrator Kelly Bramble's Avatar
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    I'm with Dave on the Angle Iron (Steel). You might run the math on two plate strips sandwiching the joists. I'm wondering how much movement is given at the 3/8 bolts due to bearing area deficiency with the old or new wood.

    randym, your description is very good; however I would love to see a picture of what you have done.

  7. #7
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    Quote Originally Posted by PinkertonD View Post
    Hi Randy,

    You would be losing an additional 4" of headroom as a 2" x 12" just barely makes it for a 16' span. Doubling the beam does not necessarily halve the deflection (Bend/sag) It would increase the load before failure characteristics though.
    Thanks Dave. Yes, I should have been more careful. This all began because an electrician put holes in my joist much too close to the bottoms and I wanted to restore the joists. I think the sistering with same-size lumber does that. But I of course wanted to also correct sagging at the same time.

    Regarding whether paired 2x8's are sufficient for a 16' span, I have not been able to find a span calculator that goes beyond 12" O.C. (I assume doubling the 2x8's approximates 8" O.C.) I was able to determine the maximum span for 12", 16", and 24" O.C., and extrapolating my estimate of the non-linear relationship I figure the maximum span 15' 4" for 8" O.C. (To estimate the non-linear relationship, I related the maximum span to the number of joists (N) in a 48" section, which is 3 (24" O.C.), 4 (16" O.C.), 5 (12" O.C.), and 7 (8" O.C.). The increment to the maximum span is 22" (132” to 154”) for N = 4 and 16" (154” to 170”) for N=5, so I figure the increment is 10" for N=6 and 4" for N=7.)

    Is this a reaonable inference?

    Randy

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    Hi Randy,

    A bit confused by all the numbers but here are a couple of calculators. The timber size v span is a non-negotiable issue.

    http://www.awc.org/calculators/span/...rcalcstyle.asp
    http://www.wclib.org/SpanCalc/SpanCalc.asp

    Set the "Size" for 2" x 10" and weep, then try 2" x 12"

    Please do not assume that doubling up materials doubles up everything. Stiffness is what you are after not load bearing. Load bearing is important but the floor sagged because the joist system was not stiff enough. The actual loads are quite light from an engineering point of view, the deflection (sag) is the issue.

    As Kelly suggested maybe 1/4" x 3" or so flat bar either side might do it. Without sitting down and doing the Math it is hard to say. I am never one for a patch up job so I would go for the obvious solution and that is 2" x 10" and 2" x 12". Or as also suggested, manufactured floor joists. The span is the issue here and we cannot negotiate options.

    Dave
    Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

  9. #9
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    Hi Zeke,

    Any one of those options would work just fine in another situation, but Randy's first problem is/was that the joists were never tall enough to prevent excessive sag. Whether he likes it or not, and I suspect "not" as he has not returned, the joists need to be 2" x 10" and 2" x 12" for the spans he has. He has already wasted innumerable hours and dollars on a flawed first-solution and I suspect will waste many more hours/bucks patching it up. To me, it is bite the bullet time and do it right, now, rather than "how about this then," or "will this work."

    You can lead a horse to water, but you cannot make it drink.

    Dave
    Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

  10. #10
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    Hi all, thanks for contributing, I'm learning a lot.

    I'm finishing my 1960's basement and am concerned about cracking in a few of my joists and the main beam. I am paranoid of having to rip out my finishing work to make structural repairs down the road. I am considering adding 2x4 reinforcement along the bottom of the joists (forming a "T") and am looking into solutions to reinforce the beam (possibly metal banding). Below are a few facts and pictures:

    Joists = 2 x 10 SPF (not sure which as couldn't find stamp)
    Distance between joists = 16 inches on center
    Span between supports = 11 feet
    House = bungalow

    Any thoughts/clarity is appreciated!

    Thanks,
    Nat

    "Plans go wrong for lack of advice; many advisers bring success"
    Last edited by Kelly Bramble; 02-20-2016 at 08:43 PM.

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