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Boss, Scissor Truss ?

Framer | Posted in Construction Techniques on September 23, 2006 09:18am

This is a post from another forum;

>>You can get scissor trusses in pretty much any pitch you require
the 3/12 rule of thumb applies to standard scissor truss configurations…

if you want 10/12 over 9/12 for example the key here is to allow enough depth at the heel(over the wall) ie heel height…..to still allow the truss to work….and to limit the live load deflection

same applies to parrallel chord trusses (same inside and out side pitch)

with 33 feet as a span and depending on snow load conditions…
you would require a minimum of 1’4″ heel and up to 2′ heel could be possible..
then you could increase your interior pitch….>>

Boss,

I thought that you couldn’t do that. In this thread (http://forums.taunton.com/tp-breaktime/messages?msg=51248.33) a while back you wrote this to a drawing I made using a truss with the same pitches instead of a three pitch difference.

Is this guy wrong? I’m just curious because this was always one thing I never liked about having a low pitch for a cathedral ceiling using trusses.

If they can be built up to 33′ with the same pitch as the outside I’m sure it would be designed and cost a lot more to build.

What are your thoughts on this?

Joe Carola
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Replies

  1. Framer | Sep 23, 2006 09:26pm | #1

    Sorry about the first drawing. Here's the drawing.

    Joe Carola
  2. User avater
    BossHog | Sep 23, 2006 10:41pm | #2

    I'm not 100% sure what your question is. But I'll take a shot at covering scissor trusses in general and see if I can clear it up.

    On "regular" scissors trusses (Trusses with a conventional heel on the ends) the rule of thumb to have 2/12 or 3/12 difference between the TC and BC doesn't always work. The reason is that the differences in angles decreases as pitches increase.

    For instance - A 1/12 pitch has an angle of 4.8°. That's obviously 4.8° more than a flat BC. (or 0°)

    The angle for 2/12 is 9.5° - That's a difference of 4.7° between 1/12 and 2/12. The angle for 3/12 is 14° - That's a 4.5° difference between 2/12 and 3/12.

    Now skip up to 11/12 at 42.5° - That's only 2.5° between 11/12 and 12/12. So a truss that works at 4/12 over 2/12 may NOT work at 12/12 over 10/12.

    If you're trying to get a scissor truss to work and are having trouble, there are lots of things you can do. Adding webs is the first. That decreases the bending forces in each piece and makes it more likely to work. Increasing chord lumber grades also helps.

    After that you're looking at increasing lumber sizes and/or raising heels. Of course - Doing any of the above things also makes the truss more expensive.

    .

    Moving on to parallel chord scissor trusses - We don't do many of these. The reason is that most of the time the plans show the fascia lining up all the way around a house. If you do a 2' heel on a scissor truss to make it work, you either have to raise the whole house or have some of the fascia lines off.

    Also - The span will be severly limited. As I said in the other post you referenced, you quickly get into problems with horizontal deflection. And the forces in a truss like that can get real high real quick.

    One other limiting factor is that you can piggyback a truss like that. Occasionally that is a limiting factor in the bottom chord pitch of scissor trusses.

    As you guessed - Parallel chord scissor trusses cost a lot more than conventional trusses. Every piece you add takes more lumber, labor and plates. Cost is always a big issue on residential stuff.

    .

    What the poster said in the other forum you mentioned may be true for them. A lot depends on the loading requirements where the truss goes.

    Like down in Florida they use a 40# design load at a 25% stress increase. That give them a lot more design flexability. Up here we use 40, 45, or 50# at a 15% increase. I've heard that there are places up north where you use loads up to as much as 100 PSF. That would kill a lot of scissor options that would work around here.

    .

    I don't really know if I answered your question or not. Let me know if I didn't, and I'll cover whatever you want to until your question is answered or you fall asleep from boredom.

    (-:

    Moroccan Proverb: The world has not promised anything to anybody.
    1. Framer | Sep 24, 2006 01:26am | #3

      Boss,Thank you for that responce.It wasn't boring at all. I'm the one who gets boring with my posts.I didn't realize that a parallel chord scissor truss can be made ,although expensive and everything else you've described. I also didn't't realize about the 2/12 -3/12 pitch difference doesn't always work either.You did answer my question as to whether or not a parallel chord scissor truss can be made or not.Sorry my response is short but I have to go to my Aunt and Uncle's 40th wedding anniversary.Joe Carola

      1. User avater
        BossHog | Sep 24, 2006 04:34am | #4

        Thanks fer the response. Alway happy to know that a post I've put some effort into has been helpful. Sometimes there's no response, and I always wonder...
        Alimony, n.: Disinterest, compounded annually. [Walter McDonald]

        1. davidmeiland | Sep 24, 2006 07:05am | #5

          You guys with the deep knowledge on a specific subject sometimes get that deafening silence. That's the sound of the rest of us trying to come up with some intelligent sh!t to say and falling short.

          1. User avater
            BossHog | Sep 24, 2006 02:49pm | #6

            "That's the sound of the rest of us trying to come up with some intelligent sh!t to say and falling short."

            I guess that's sort of a compliment, but I think it's also bad in a way.

            I certainly don't know everything about trusses. I haven't worked in some markets in this country, so things like seismic stuff in California might make a difference in some of the stuff I said. (I really don't know)

            I know there are other truss guys here, and I wish they'd speak up. They may have different opinions, input from a different market, or a better way of explaining things.

            No one here has a corner on the market of knowing everything about any subject in particular.
            I figured out a way to make Death Row a little more fun.
            Musical electric chairs.

          2. Stilletto | Sep 24, 2006 02:53pm | #7

            I find your opinions and advice on trusses very helpful.  It's nice to have a guy with your experience on board. 

              

             

          3. davidmeiland | Sep 24, 2006 04:53pm | #8

            That's what I was trying to say.

          4. Stilletto | Sep 24, 2006 05:50pm | #9

            Thats what I thought,  I just repeated it in different words.  

             

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