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Discussion Forum

Bow string truss’s

Sphere | Posted in Construction Techniques on January 28, 2005 10:01am

Ok..we often tell new posters here to “hire an engineer”…I agree that that may be the prudent answer. Many times tho’ I feel semi qualified to recommend an old, proven method of spanning a distance utilizing a bowstring truss vs. a standard “W” truss..( i don’t recall the technical name for it, ..some guy’s name, I think)..Oh..Ok a Fink truss….

Has anyone here had much practice in utilizing this design? I know there are many variants that can be employed..in different scales..

I.E…I created the truss on a small scale to stiffen undersized/over spanned floor joists…in an old farm house with incredible results.

Some one recently posted some ? about a boat cover..my idea was a cable stay in a PVC pipe, under tension, secured by end blocks..

With all the recomendation here from us wood guys, what says the Engineers about this? It seems like a great truss, in it’s place. And the basic engineering ought to apply in many other applications…

fire away..I’m just thinkin out loud here….

 

Spheramid Enterprises Architectural Woodworks

” Let behind the eyes, that which one talks” Rumi….

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  1. UncleDunc | Jan 28, 2005 11:52pm | #1

    Well, I'm not an engineer (nor do I play one on TV), but that doesn't look like a truss to me. It looks like a suspension bridge with a roof. I don't think anything above the catenary contributes significantly to the strength or stiffness of the bridge.

    To my mind, a bowstring truss implies a straight or very nearly straight tension member, not a catenary curve. Do you have a picture of your home grown application?

    1. User avater
      Sphere | Jan 29, 2005 12:06am | #3

      sorry, I do notr have a pic. of my fabrication. basically tho' a 1.5'' member nailed perpendicular to the supporting joists ( or just joists I guess) and another 2x4 fastened at each end to the joists..bowed around the flat 1.5 member..creating a sprung situation, where the now lower chord is in extreme tension, and bearing it's load in the mid span of the over span..make sense?I fastened one end of the bow part with screws and glue, and jacked the other end in place and fastened like wise, the 1.5" spacer gave enough "push" to compensate for the long span, effectively shortening the OA elasticity of the joist.Maybe I am wrong, hence my query. 

      Spheramid Enterprises Architectural Woodworks

      " Let behind the eyes, that which one talks"

      Rumi....

      1. UncleDunc | Jan 29, 2005 01:49am | #8

        OK, now (I think) I understand what you're describing. I've seen that technique described for stiffening scaffold planks. Yes, I think bowstring truss is at least a plausible name for it. I don't know if it's industry standard jargon or not.Sorry I can't halp you with any numbers or forumlas. When in doubt, build a mockup and test to destruction.

        Edited 1/28/2005 6:59 pm ET by Uncle Dunc

    2. User avater
      BossHog | Jan 29, 2005 03:24am | #9

      "To my mind, a bowstring truss implies a straight or very nearly straight tension member, not a catenary curve."

      The attached file is what I've always understood a bowstring truss to be. It's a crude drawing I did real quick in "paint".
      You are only young once, but you can stay immature indefinitely.

      1. User avater
        Sphere | Jan 29, 2005 04:58am | #10

        correct..now flip it upside down..same tensile strength? 

        Spheramid Enterprises Architectural Woodworks

        " Let behind the eyes, that which one talks"

        Rumi....

        1. HeavyDuty | Jan 29, 2005 07:10am | #11

          now flip it upside down..same tensile strength?

          Don't think so.

          Are you thinking in terms of prestressed cast concrete where the 2X4 should have a curve to it before it's fixed at the two ends?

          1. User avater
            Sphere | Jan 29, 2005 07:37am | #12

            close...very closewow..now...I gotta draw..or think about drawing..basically, yer right..a tension member counter-acting a ductile member.force applied to the ductile flexor ought to impede the moment of inertia that is loaded on it's weakest point, hence a non ductile member..look I could be way wrong here, this is hypothetical..or..conjecture.what if a load, was pinpointed on a span..that span was reverse suspended and held in traction?...(ferget sway for now)...is not the bow replacing the energy? a king post is still the fulcrum or a point load..as well it should be, but the "web" of the bow applies counterbalance...or, compression on the strut..?bear with me..I can't draw what i want to xpress...nor am I mathematically versed in sines and cosines , but it sems to me that many of the q's here at BT can be explained in simple geometric formulas..Like" I wanna make a room in my attic".........beefing and floor stiffness has been hashed to death..I have a plan, no one ever listened ( maybe cuz it's pointless) but a plan that has worked...and I READ it in FHB..I am sure..I lost most copies in my shop fire in '95..but I am sure,,I read it...I did my farm house in '90 or 92..from that info. i saw in FHB.I am serious..I need to do something along that line, and we have a dearth of Eng. on this board...whyzzat? 
            EDIT: I cant describe reverse susp. I made that up in a visual..should read..compression point load. at or near amid center.Spheramid Enterprises Architectural Woodworks "We try for conversation, we die in consternation..they kill prophets for profits"

            Edited 1/28/2005 11:44 pm ET by SPHERE

          2. IanDG | Jan 29, 2005 07:53am | #13

            In Australia, when using the local hardwoods for framing it was the practice to use green timber -- the reason being that once dried, it was impossible to nail.

            I only saw one roof done that way and once it was pitched the purlins [3" x 8" timbers running perpendicular beneath the rafters at 1/3 to mid-span] were tensioned with a 1/4" dia. steel rod stretched from end to end over a 6" steel Tee plate at mid-span. The timbers subsequently seasoned in-situ.

            IanDG

          3. User avater
            Sphere | Jan 29, 2005 07:56am | #15

            and..that created the needed tension? 

            Spheramid Enterprises Architectural Woodworks

            "We try for conversation, we die in consternation..they kill prophets for profits"

          4. HeavyDuty | Jan 29, 2005 07:55am | #14

            I am no engineer and with my limited engineering background it's no match to your situation.

            Your best bet as Boss suggested would be to sit down with a real engineer and you can trade ideas. It's going to be pretty hard for someone to give you the proper advice over the net especially

            I can't draw what i want to xpress

            How come no bite from engineers? Must be weekend.

          5. User avater
            Sphere | Jan 29, 2005 07:58am | #16

            How come no bite from engs? easy, they have super models for wives...they are busy. someone spends the moo.. 

            Spheramid Enterprises Architectural Woodworks

            "We try for conversation, we die in consternation..they kill prophets for profits"

          6. User avater
            SamT | Jan 29, 2005 08:52am | #17

            If you look close at that bridge picture, you'll notice that the Bowstring is not curved.

            It runs in a straight line from "arrow" to "arrow". Then there are guys to keep the "Arrows" from slipping on the Bowstring, since they are not perpendicular to the force vector.

            SamT

             View Image

          7. User avater
            Sphere | Jan 29, 2005 09:26am | #19

            Granted that was a small pictorial/sample of my curiosity, I se the guys as stays..which are in equal stasis...neither comp or tens..NO?Ideally, I'd want that (your arrows) in slight tension..maintain it's own harmonic balance is more imp. ( the well known brige in Wa,foe exp., Tacoma Narrow's)But if, and a big if, we would stucture a cylinder with a cable stay under tension (like pre cast w/0 load/camber) and minimized the design load , could it not be done in another matrix of available , affordablr substance?The basic eng. principles are established..I just wanns know why we don't expound on the tension/compression relationship to a greater extent. Or am I outta here, and we have gona as far as we can with our limited faculties?( no, I ain't on drugs, just gotta stiffen my wood...(floors that is, floors}}? 

            Spheramid Enterprises Architectural Woodworks

            "We try for conversation, we die in consternation..they kill prophets for profits"

          8. User avater
            SamT | Jan 29, 2005 10:36am | #20

            Oops.

            I was cornfused fer a minit.

            On the previous sketch what I call Guys are the Stay stabilizing wires, under tension. The Stays are in fact the verticle members that run from the Bow to the upper Beam. I drew 'em fat so's they'ld look more like timbers.

            About the confusion, on this sketch I labeled the stabilizing wires both guys and stays, just call 'em guy wires like on TV antennas. We'll call the verticle timbers (arrows in previous) Stays this time.

            The Stays are in compression only.

            If the stay does not exactly bisect the angle of the Bow, there will be a horizontal vector to the force applied to it like on the left side of this sketch. You be needing a member to apply a counter vector. You can use a guy under tension outside the area of the Bow, or you could use a horizontal compressive member from one Stay to it's corresponding Stay on the other end of the Bow, or a diagonal compressive member up to the main beam.

            The problem with diagonal Guys like in the first sketch is that they have a verticle component in their force vector which counters some of the verticle force applied by the Bow to the Stay.

            SamT

             View Image

          9. User avater
            Sphere | Jan 29, 2005 11:01am | #21

            yooo...I can see that..load on the walls -is all..good.is the proportion/scale reasonable...? i don't think so..the ratio of bow to string seems a bit more than fabriction on site would allow, not to mention the loss of ciel. hgt...Now, if I flipped the design...ahhh crap...I see the similarity to a common queen post truss...better start looking for a quonset hut...or Rez'll be here with tires..But ya see my point? The actual bowstring is a workable design..just the marrification is harder.........and harder.Cest bon..mon ami. 

            Spheramid Enterprises Architectural Woodworks

            "I've..seen all good people turn thier heads, so, satisfied, I am on my way.."They kill prophets for profits"..And, that's...the truth.........phhatt

          10. DanH | Jan 29, 2005 04:31pm | #23

            The bridge picture was of a suspension bridge. The solid members above the cable were only there to support the roof, and did not comprise parts of a truss system.If it had been a truss, the solid members would have been arranged in triangles, if only to simplify the (pre-computer) stress computations.

          11. DanH | Jan 29, 2005 04:37pm | #24

            And, I might add, the main difference between the sort of bowstring unit you're thinking of and a suspension bridge is the cable anchorages. With a bowstring unit the cables are anchored to the ends of the truss, whereas with a suspension bridge they are tied to detached anchorages on each end, and the bridge deck (or whatever horizontal member is present) isn't under significant compression.One aspect of the engineering definiton of a "truss" is that it's a structure that can stand on its own with only point supports.

          12. User avater
            SamT | Jan 29, 2005 06:09pm | #25

            thanks

            SamT

          13. User avater
            Sphere | Jan 29, 2005 06:29pm | #27

            can I kidnap you for assistance building my shop?I am cereal...I will email ya. 

            Spheramid Enterprises Architectural Woodworks

            "I've..seen all good people turn thier heads, so, satisfied, I am on my way.."They kill prophets for profits"..And, that's...the truth.........phhatt

        2. User avater
          BossHog | Jan 29, 2005 03:36pm | #22

          "now flip it upside down..same tensile strength?"

          Sure - If it's DESIGNED to have the same tensile strength. You can't just flip something upside down and expect it to have the same strength as it did right side up.

          When you flip something over, every member that was in tension is now in compression. And vice-versa. Some pieces that worked in tension might not work in compression.

          And the connections might have to be designed differently.

          So the CONCEPT is sound. But the details are what's important.
          Ninety-eight percent of lawyers give the rest a bad name.

          1. User avater
            Sphere | Jan 29, 2005 06:28pm | #26

            Boss..I was hopin you woulda stepped in here. And i am thankful..The concept indeed is valid...execution or design values are almost nebulous..at this point.I just wanna ask, why is not "conceptually" designed in more structures? Are Eng. ambivilant to the concept? Hard to manufacture? Or just not a practical solution to a Fink truss?I am not tryin to reinvent the wheel, I just had a vision of my new shop, and the roof is still "up in the air"...LOL 

            Spheramid Enterprises Architectural Woodworks

            "I've..seen all good people turn thier heads, so, satisfied, I am on my way.."They kill prophets for profits"..And, that's...the truth.........phhatt

          2. User avater
            BossHog | Jan 30, 2005 12:13am | #28

            "I just wanna ask, why is not "conceptually" designed in more structures? Are Eng. ambivilant to the concept? Hard to manufacture?"

            You mean bowstring trusses? I've never built any, but I'm sure they would be an awful pain.

            For starters, a true bowstring truss is curved. That makes roofing options limited.

            Jigging something curved like that would also be difficult.

            And no engineering software exists for them. (As far as I know)

            Ripping lumber to a curve is also time consuming and wasteful.

            So I honestly don't see why anyone would want to use them. Probably why I haven't seen any in the 20 years I've been in the business.

            I'm not completely worthless. I can be used as a bad example.

          3. User avater
            Sphere | Jan 30, 2005 12:23am | #29

            good points...thanks fer yer input. 

            Spheramid Enterprises Architectural Woodworks

            "I've..seen all good people turn thier heads, so, satisfied, I am on my way.."They kill prophets for profits"..And, that's...the truth.........phhatt

  2. User avater
    BossHog | Jan 28, 2005 11:59pm | #2

    So was that a question? I'm not 100% sure what you're after.

    I don't doubt that you personally have some considerable skill in "redneck engineering". Lots of guys like you and me that grew up on farms are used to figuring stuff out on our own and making do with what we have.

    But to me, there's a difference between making do with something on our own and advising someone else to do something over the internet.

    Like let's saw you do something creative with the floor system in your own house. If you make a mistake, you take care of it.

    But if you do that in someone else's house (AND make a mistake) that becomes a whole different ballgame.

    .

    Don't know if you've been around long enough to remember this thread. It was started about building your own trusses.

    http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=21664.1

    Talking about building your own trusses can get pretty testy at times. (-:

    I could ramble on quite a while about stuff like this also, but I gotta get outa here soon...

    As the Buddhist said to the hot dog vendor, "Make me one with everything".
    1. User avater
      Sphere | Jan 29, 2005 12:09am | #4

      besides the best tag line ever...yer right, that's why i am looking for an Eng on the question..I thought of you first, but this not a common design..........lemme see if an Eng. pops up outta the woodwerks..ok? 

      Spheramid Enterprises Architectural Woodworks

      " Let behind the eyes, that which one talks"

      Rumi....

  3. Piffin | Jan 29, 2005 12:13am | #5

    Looks like a suspension bridge to me.

     

     

    Welcome to the
    Taunton University of
    Knowledge FHB Campus at Breaktime.
     where ...
    Excellence is its own reward!

  4. User avater
    CapnMac | Jan 29, 2005 12:33am | #6

    Always wondered why it was called a "bowstring," it's the bow that curves, not the string.  A bowstring is either straight, or in a "V," not like a cable catenary at all.

    IIRC, a catenary curve mimics, but does not follow the moment curve along a simple span.  Which is what the curved portions of arched trusses are "doing."  They are "filling in" where the moment is greatest, which give the strenght, etc., yada yada yada.

    But, that's just what I remember from many too many moons ago.

    Occupational hazard of my occupation not being around (sorry Bubba)
    1. User avater
      Sphere | Jan 29, 2005 01:01am | #7

      Kidder and Nolen's def was sorta like a moment of inertia/mass/ elasticity of the tension member x the tensile strength of the supporting web..I thinkgone looking in my old cloth bound eng. books I guess..but my brain tells me..( always suspect) that the modulus of elasticity, is directly proportional to the load..and fiber stress is directly related to tension vs. compression..as an inverse...ooops..I lost myself..never mind...LOLMoment of force...is the LL+DL @ rad/circ inter section? IOW, the chord is still all tension, but the "rafter' or arc negates some thrust, or can inversely inpart up lift, with a solid connection from apex of the arch to the now defined base of the said arch..So if the chord is not a true straight line ( sag, from load) and the bow is "Strung" or tensioned, to exert a vertical force in midspan of the chord..it effectivly halves? or reduces the the span?ex. 20' 2x8...bearing 5" each end..####2" running perp @ midspan, a 2x3 edgewise anchored @ 2' from bearing, stressed to BOW to the other anchor point also 2' from bearing. that effectivily shortens the OA span?..The compression of the bowed member adds uplift (if you will) to the center of the joist...........?dammm..I know there is a formula..just all my books are still boxed.Thanks Bud........scratch yer head please? 

      Spheramid Enterprises Architectural Woodworks

      " Let behind the eyes, that which one talks"

      Rumi....

  5. andybuildz | Jan 29, 2005 09:04am | #18

    and here I thought this thread was about Icurous.
    David Darling....you know him, right?
    If not, go Google
    ####

    The secret of Zen in two words is, "Not always so"!

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    I honor the place in you of love, of light, of truth, of peace.

    I honor the place within you where if you are in that place in you

    and I am in that place in me, there is only one of us.

     

     


     

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