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Thread: sister a joist with steel?

  1. #1

    Default sister a joist with steel?

    Hi all,

    We're new owners of a 100 year old foursquare home. After pulling down some ceiling to gain access for the plumber (under the bathroom shower/toilet area), we found that whoever did the remodel really cut up the 2 x 10 joists for the pipes. Irregular large holes, and for the toilet pipe, they cut a huge inverted "V" that nearly severed the joist, leaving maybe 2 inches at the top.

    We're on a very tight budget, and have gotten some advice regarding sistering the joists to do the job ourselves. Originally we thought of sistering with wood, but how strong would sistering with 1/4 inch steel be? My husband works at a steel fab shop, and can bend it to have a lip go under the joist as well. We also thought about putting in a piece of wood to fill in that inverted "V" cut as much as possible also before sistering with the steel.

    Any thoughts appreciated!

    J House

  2. #2
    In the Trades Gary Swart's Avatar
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    I think you will be fine as long as the steel is long enough to go well beyond the cutouts on both ends.

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    DIY Senior Member Mike Swearingen's Avatar
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    When our home was built in 1977, our great room had a wide expanse, and the contractor had a 2-3 joists sistered with bolted flat steel. One end rests on the brick fireplace structure and the other end rests on the outer wall header framing.
    Never had any sag.
    Mike

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jhouse01 View Post
    ... how strong would sistering with 1/4 inch steel be?
    1/8" would likely be quite enough, and the real issue there is overall size and design or "engineering" rather than thickness.

    Quote Originally Posted by jhouse01 View Post
    My husband works at a steel fab shop, and can bend it to have a lip go under the joist as well.
    A "C" would be much better even if his break press can only make the first break and inch or two tall. Or, you might be able to use a piece of 2"x?" tubing with the top wall cut away.

    Quote Originally Posted by jhouse01 View Post
    We also thought about putting in a piece of wood to fill in that inverted "V" cut as much as possible also before sistering with the steel.
    No harm there, but that will probably not really add very much to the overall strength of the repaired joist.

    Can you possibly post a picture or two?

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    Jack of all trades frenchie's Avatar
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    Before the advent of engineered wood beams, when you had a really wide doorway and not enough room for really big headers, you'd use what's called a filch plate - basically a steel plate sandwiched between 2 pieces of wood.

    There are tables that will give you an idea of how much strength a steel plate will add, for a given thickness... sorry that I don't know much about this, I let engineers sort out the details for me.

    Anyways, the only reason sistering with steel is so rare that you hadn't heard of it, is that it tends to be pricier than wood, especially once you include the labour cost to get all those holes drilled. Wood's just generally easier / cheaper, is all.


    FYI, the bottom of a joist is in tension: gravity is trying to pull it apart. So filling in the wood won't do anything. A steel strap screwed into the bottom of what's there, would, though.

    The top of a joist is in compression, and filling in would help there.
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    Homeowner geniescience's Avatar
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    flat bar under the joist, or angle iron on the side and underneath. (NO need to bend it, as it is already bent when bought). Long and well screwed down. 1/4" solid, or a 1/2" cylinder.

    j house, is that what was on your mind?

    david

  7. #7
    In the Trades Bob NH's Avatar
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    "Sistering" a damaged joist with steel requires engineering, unless you are replacing the full strength and stiffness with the "sistering" piece.

    Replacing some part with a piece fastened on, such as a steel strap on the bottom, is really making it into a composite joist. The problem is that it is very difficult to fasten the steel to the wood with enough fasteners to transfer the loads to the steel. Also, the strap doesn't replace the shear strength of the beam so it would be necessary to insert a substitute for the "web" of the beam and fasten that web to the steel.

    If you look at the splice plates on both sides of the lower chord of a wooden truss you get some idea of the number of fasteners required to connect to a wood structural member. You would need to put that large number of fasteners in a steel strap to attach it securely to the wood.

    The easiest way to repair a situation that has been described would be to get some good quality kiln-dried 2x4s and 2x6s, a good supply of carpenters glue, and a framing nailer with lots of nails.

    Cut and fit pieces of the reinforcing material long enough to extend about 3 ft past the damaged area on both ends, and glue and nail them on the sides of the damaged joists. The reinforcement should extend from the edge of the joist to the pipe.

    If the joist is cut through to the edge, put the reinforcement on both sides. If there is less than 3" of solid wood at an edge, add a reinforcement on one side.

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    DIY Senior Member Hube's Avatar
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    use 10 ga, mild steel.
    make the depth at least 6" to 8" with a 1.5" break IN for under the joist, and a 1" break OUT at the top. Both breaks are at 90 degrees
    Bolt in place.

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by Hube View Post
    use 10 ga, mild steel.
    make the depth at least 6" to 8" with a 1.5" break IN for under the joist, and a 1" break OUT at the top. Both breaks are at 90 degrees ...
    That is a good shape, but there is not enough room for the 6" to 8" ...

    Quote Originally Posted by jhouse01 View Post
    ... whoever did the remodel really cut up the 2 x 10 joists ... for the toilet pipe, they cut a huge inverted "V" that nearly severed the joist, leaving maybe 2 inches at the top.
    At best, there is possibly 3" available at the bottom of that joist.

  10. #10
    Engineer chassis's Avatar
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    Steel is a great material for this application. Almost any size/shape of steel you can get into the space will help. The key is to make the steel as long as possible, along the joist.

    There are many possibilities for what you can do, the limitations are only your specific installation details; mainly what obstructions are there?

    I can envision a 3" I-beam under the full length of your 2x10 joist. The I-beam would be fastened to the 2x10 with u-bolts every 24" or 36" or so. Or you could use a 2" c-channel, again u-bolted to the bottom of the 2x10. Lots of different ideas, maybe if you post some photos it would help us to give you more specific input.

    Another idea is to use a 6" flat 1/8" plate bolted through the 2x10, flat against the 10" face. The steel in this situation would be a shear plate. Extremely effective, but you will need through-bolts (carriage bolts) every 18" - 24" or so. Just depends how much access you have.

  11. #11

    Default pics of joist

    thanks all,

    here's pics to clarify the situation. After deciding we couldn't alter the angle iron to sit on the "sill" (not sure if that's the correct term) to help bear weight, we're back to the idea of 2 x 6 so we can notch it out at the end. I like the idea of adding a steel plate for strength. (think it's called a flitch plate)

    http://mandolinbridge.net/2.html

    Now the idea is to sister with wood on either side, maybe the flitch plate, and use bolts--a handyman suggested 1/2 inch. Handyman also thought the 3 2x4's on the other side of the joist aren't providing much support if any, and suggested we take those off & replace with 2x6 on the other side.

  12. #12
    In the Trades Bob NH's Avatar
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    Here is some rationale for what works to reinforce a joist.

    The strength of a rectangular beam or joist varies as the square of the depth, and the stiffness to resist deflection varies as the cube of the depth.

    Therefore, a 2x8 at 7.5" deep is 4.6 times as strong and 9.8 times as stiff as a 2x4 that is 3.5" deep. To replace the strength of the joist with 2x4s you would need 4 of them, and they would have to be secured to the joist adequately to carry the load across the cutout area. You would still have a soft floor because you would need 9 of them to replace the stiffness.

    A reinforcement is not effective unless it is attached in a manner that transfers the load to the reinforcement. A 2x4 reinforcement with an area of 1.5 x 3.5 = 5.25 square inches, and an allowable tensile stress of about 1200 psi, has a tension capability of 6300 pounds.

    A 10d nail has a design load capability of maybe 150 pounds. Therefore, to develop the tension capability of a reinforcement to replace that material at the bottom of the beam where the notch is cut out would require 6300/150 = 42 nails ON EACH SIDE OF THE NOTCH.

    Bolts are not any better because the limit of strength is the bearing strength of the wood. There are special timber fasteners that are designed to carry greater loads but they are not practical for this application.

    The best repair to achieve both the strength and the stiffness of the original joist is to GLUE a reinforcement both above and below the cutout so the repaired joist has properties of an I-beam. The purpose of the nails is to clamp the parts together until the glue cures. As described above, you don't want to put in as many nails as would be required without glue.

    That joist with the notch looks like it is cut almost all the way through at the top. Those 2x4s that look like headers will not do anything but provide local support to the floor if they are cut off where they intersect that joist, and they will probably have to be removed to allow you to properly reinforce that joist.

    If you were to remove the floor above you could reinforce the area by putting a 4x8 ft piece of 3/4" exterior grade plywood on top and bottom of the joists, running the direction of the joists, glued and screwed, to make it into a box girder. I can't tell enough about the details but that might be the easiest way to do it. However, it would wreck that tounge-and-groove floor that appears to be installed in the area.

    It is certainly possible to replace the support with a shallow steel beam but it would be expensive unless you can get steel for scrap prices or less.

    If you want to do it with steel, then someone skilled with a welder could fabricate an open truss around those pipes to carry the loads. Someone would have to design the truss and attachments.

    You would have to be very careful about starting a fire when working around that old wood. Someone should stand by with a fire extinguisher and the wood should be shielded and wetted before welding in the area.

    If you are planning to put ceramic tile on the floor I suggest you reconsider because of the compromised stiffness of the structure.

  13. #13
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jhouse01 View Post
    After pulling down some ceiling ...
    Are the 2x10s your ceiling height, or was all of that added lumber already there? If the ceiling height is actually a few inches below the 2x10s, I would try to lower the plumbing and restore/repair/reinforce the framing.

  14. #14
    Engineer chassis's Avatar
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    Strange setup to be sure. With 5 pieces of lumber up there, I'm thinking you may have all you need. Only suggestion would be to add a 3" steel angle ("L" shaped piece of steel) as long as you can, along the bottom of the joist on the side with the notch cut out. Use 3/8" carriage bolts every 18", with washers under the heads. I would also add epoxy between the steel angle and wood joist. A 3" strip of steel installed vertically (as in a shear plate) would work if you don't have room for the L-shaped angle.

  15. #15
    Jack of all trades frenchie's Avatar
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    Bob -

    I'm not sure I trust site-glued connections, for structural work. It's not like you're in a factory, working with clean new lumber and tight temp & moisture control...

    On the other hand, you can home-build a completely glued boat. So maybe it's doable. But running beads of PL on a dirty old joist... I'm not feeling that. At the very least, I'd want to run a belt sander or a planer on any surface that was getting glued, and use screws instead of nails, to get some real pressure... nails will hold something in place, but not clamp it very much.


    Use of the word "sistering" led me to assume the OP means to run something the full length of the joist - like when you sister a joist with another.

    As I understand the way a filch plate works, the fasteners simply keep the metal sandwiched between the lumber, so it can't twist or turn sideways because then it would suddenly be very very weak... The fasteners basically hold the filtch-plate-beam together.

    But the plate and the lumber are all supported at the end of the span. So the load's not being transferred to the fasteners, like it would be in a mending plate...

    Of course, I am assuming the OP knows what sistering (as opposed to patching) means; and that everyone on this thread knows a filtch plate from a mending plate.

    Oops?


    Chassis: epoxy doesn't bond to metal. That's why Bob was talking about plywood. AFAIK, there's nothing that will glue metal to wood.
    Master Plumber Mark:

    there is nothing better than the
    manly smell of WD 40 in the air
    while banging away on brass with a chisel and hammer...

    it smells like......victory......

    do not hit your thumb...
    __________________
    Just so everyone's clear: I'm the POODLE in the picture ("french", get it?) The hot woman is my wife.

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