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Flitch beams

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  • Flitch beams

    Can anyone comment on the characteristics of flitch beams? After reading the JLC article on LVLs, PSLs, and I joists (and a paragraph on flitch beams), I still have some questions.

    1) will the steel actually increase the distance a member can span past the usual limits, or just allow more loading of the same span (e.g. if a 2x8 floor joist can normally span about 11' (depending on spacing, grade, etc...), can a 2x8 with a steel plate on one side sill only span 11', but maybe with greater o.c. spacing, or can it span much further (say 20')?

    2) Is fastener placement supposed to follow the areas of the greatest tension and compression (e.g. top and bottom, more at the ends)

    3) will flitch beams work with a steel plate only on one side, or does the steen need the strength of the wood to hold it vertical

    4) are there published tables of flitch beam spans, or do people always have them engineered?


  • #2
    Re: Flitch beams

    Andrew, I haven't read the JLC article yet, but in my experience flitch plates are usually used in repair or remodeling work, not designed in initially. The steel increases all the specs on the original beam--span, load, etc. A flat flitch plate will not increase span to 20', though.

    Fasteners are placed to allow transfer of loads between beam members and minimize horizontal shear between bolt holes. A single row of bolts along the center of the beam would work, but the wood would be inclined to split along this line. More bolts are placed near the ends because that is where vertical shear is greatest.

    Placing the steel along the side of the wood beam would usually be ok, as the gauge of the steel is generally thick enough that it would not bow between bolts. The beam would theoretically have a tendancy to bend eccentrically under load (twist) but I don't think you would see it.

    As far as I know people always have flitch beams engineered.

    All that said, for the same effort as bolting on a flat flitch plate you can bolt on c-channel and get much more strength for the same amount of labor. Or just put in a steel beam and pad it out with dimensional lumber to accept floor joists.

    Do you have a specific project in mind or just musing?



    • #3
      Re: Flitch beams

      The specific problem that I need to solve involves finishing an attic with existing 2x4 ceiling joists. The area that is a problem is over a living room and is about 12'x21' with the joists running in the 21' direction. I tell you, it's a little bouncy in the middle of the span now (to say the least!) Options I've come up with are below:

      1)put in a steel beam to split the span in half so I could then use 2x10's the span the 21' (10.5', 10.5')

      2) use flitch beams running in the long direction to avoid using a beam.

      Problems with solution 1 include having to chop a lot into the existing ceiling and room below to put in the beam and a couple headers and a new footing and post. Also a problem is the top plate space where the ceiling joists sit on one side... small- only about 3-1/2" on the outside edge, 5-1/2" on the inside edge.

      Any good ideas?


      • #4
        Re: Flitch beams

        Andrew - Using flitch beams as floor joist would be Way to expensive. You would be better off with engineered lumber for a 21' span eg. TJI.

        The option of running a beam to spit up the run of the floor joist is common & practical. A flitch allows you to use the same dimension Lumber as the FJ & keep it Flush with the floor.

        If relocating the Flitch would mean less interior work, Than split it up differently - 9' & 12"
        8' & 13' etc..

        There is a company that has manufactured flitch beams & the engineering to go with them -

        Check them out
        There is a span table there & you can figure out your header requirement



        • #5
          Re: Flitch beams

          Why not put in a glulam with the bottom flush with the bottom of the 2 x 4s (which are of course cut out to accomodate the glulam), and then add your 2 x 10's (or even 2 x 8 if Doug Fir)hung on hangers on the beam? When we do this we often first add a block to the plates so that the new joists are elevated. PS. I know nothing about flitch beams.


          • #6
            Re: Flitch beams

            I have thought of both solutions proposed by Mike and Jim. Here are the problems to both.

            1) I can't run a TJI all the way to the top plate because of the narrow angle at the top plate. This would require cutting the TJI at such an angle that the top of the web would be cut back behind the inside edge of the top plate.

            2) Running a new flush beam (glulam) is a possibility, however, this will require serious modification to the room below. There is a window on onse side and a doorway on the otherside of where the beams would sit. I would have to put in larger headers on both, then add in extra king studs (probably) and on the doorside, pour an footing for a new post (not an easy task with only about 18" or crawlspace).

            With all this said, I'm beginning to think that the glulam option might be my only hope, despite all the extra work it will involve.

            Any other ideas?

            BTW, the attic will have a knee wall, so the finished floor doesn't need to extend all the way to the top plate. That is to say, I could put in a glulam under the knee wall and hang 2x10s off one side and the existing 2x4 off the other.


            • #7
              Re: Flitch beams

              Andrew, could you do this: temporarily support the ceiling from below with two parallel walls running in the 12' direction. From above, sawzall out the 2x4's to provide a pocket for a 9 7/8" LVL running the 12' direction, with the bottom of the LVL flush with the bottom of the 2x4's. Run 2x10 joists from the wall plate to the LVL, attached to the LVL with joist hangers.

              Or...use the same system, but tear down the existing ceiling to make it easier on yourself.


              • #8
                Re: Flitch beams

                OK... Next problem- Assuming I go with Mike's suggestion, how do I keep the walls from thrusting outward as I will have effectively cut the ceiling joists that are acting like collar ties. I don't think joist hangers are rated for force in a horizontal direction.

                Would a metal strap from one joist, under the LVL or glulam to the other joist keep the whole thing from spreading apart?


                • #9
                  Re: Flitch beams

                  Joists hangars are rated for some horizontal load and probably enough to do the job. A more direct way is to use a metal strap.

                  In a similar situation, I've thought of drilling a hole through the LVL and passing a metal strap through to the other side. As long as that would put you near the center third of the LVL (top to bottom) I think that would be fine. But verify this with someone else if you want to do it.



                  • #10
                    Re: Flitch beams

                    Andrew, that's a good point, and well worth an hour of an engineer's time. One option that I would consider is using custom 1/4" steel brackets (or the ones sold to hold up porch girders). Through-bolt these through the LVL and the joist, probably every third joist or so. Glenn's idea of the metal strap would also work. As always, any engineering advice you get on this forum is worth exactly what you paid for it!


                    • #11
                      Re: Flitch beams

                      Andrew - Here's a few minutes of an engineers time. Use LVL joists to span the 21' direction. Yes, I know this is more expensive... but you will save on labor, straping other modifications to the lower superstructure. Not to mention, you will not be changing the load path.
                      Cutting the top plates of the wall and reversing the direction of the joists will require thrust to be addressed as you mentioned. Also, post loads from the additional beam will need to be considered. Seems like alot of work.

                      Joist hangers are NOT designed to handle any horizontal loading.