Geomerty and limitations of scissor trusses 3

[RontheRedneck]

I thought this subject might interest some of you, and it was worthy of a new thread.

I often get people thinking that if there’s a 2/12 difference between the TC and BC pitches of a scissor truss that a truss will work. That's not always true.

The first issue is geometry. A 4/12 pitch is an angle of 18.4° and a 2/12 pitch is 9.5°. The difference between the two pitches is 8.9°.

A 10/12 pitch is an angle of 39.8° and an 8/12 pitch is 33.7°. The difference between the two pitches is 6.1°.

So as the pitches get steeper, the relative angle between the two chords is reduced. A shallower angle obviously means higher forces. A truss may not work because of that.


The 2nd problem is the limitation of the component saws that cut the lumber for the trusses. Once you get below an angle of 9° or so the saws start to have trouble making the cuts. (The limitations vary with the brand and type of saw)


The 3rd issue is horizontal deflection. With steeper pitches you start getting more of it. We're limited to a max of 1.25".

If you're looking at a short truss span - Let's say 20' or so - You can push the limits more. But the longer the span gets the more horizontal deflection you get.


The overall height can also become an issue. We can build trusses up to about 13' 2" tall.

If a flat BC truss exceeds that height we can piggyback it. But with scissor trusses piggybacks are more problematic. Sometimes of the BC pitch is steep there just isn't enough depth to piggyback a truss without the chords failing.


Hopefully that makes sense. If not let me know and I'll take another run at it.

 

[XR250]

Thanks Ron.

I have always used the rule of thumb of the inside pitch is 1/2 the outside pitch.

 

[kipfoot]

I also go with bottom chord slope at half the top chord slope. I've never heard the 2 in 12 difference rule of thumb. I also haven't worked on anything where I'd be okay with a 1 1/4" horizontal deflection at the top of all, so I won't try to push the limits of the 'cathedral' ceiling slope.

 

[dik]

Hopefully the outer slope is great enough that you don't get snow...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[ALK2415]

@ RontheRedneck
Thanks for your kind efforts, if you could adding drawings / pictures so the rest of us could comprehend your descriptive text

 

[RontheRedneck]

Regarding the rule of thumb that the inside pitch is half the outside pitch -

Yes, that's a good starting point. But often people want a higher slope on the inside, and want to push the limits.

Regarding posting some pics- I can do that. But it will be some time during the work week before I can get it done.

 

[ALK2415]

@ RontheRedneck
all the back and forth of plated-wood Trusses discussions, came after the church renovation/expansion. My question
1- did you done some repairing works for large span wood Truss ? like using FRP laminates (with epoxy).
2- did you use strands/cables (with some pretension) ? any cable configuration is the most effective
3- Are those repairing techniques effective timewise ?
i tried to add some related pictures for clarity

Note: Reasons of asking is to learn about wooden structures construction/practice
and thanks for your Reponses

 

[phamENG]

Thanks, Ron. Good to know. Especially the 1.25" number. It backs up my 'diagnosis' for some of the damage I've seen in cheep track homes with cathedral ceilings. With no effort to detail the differential movement between the cathedral truss and the flat BC truss, the walls tend to rip themselves apart.

adn26 - Ron is a metal plate connected wood truss designer. He has nothing to do with those behemoths you posted (except the one in the middle that looks to be a pack of trusses on a stand waiting to be erected). They're really fascinating, but not exactly relevant to Ron's expertise (at least as I understand it).

 

[ALK2415]

@ phamENG appreciate your clearing
renovation/repairing started this thread (discussions) therefore, i thought you all had experience/related with this kind of projects
like :
extension - expansion - repairing - rehabilitations ...etc
asking here to learn from you all
Wooden structures is a closed world to me
Note: I'm Not English speaking so forgive my bad language

 

[XR250]

With no effort to detail the differential movement between the cathedral truss and the flat BC truss, the walls tend to rip themselves apart

Never seen this issue - even though it should happen more often.

 

[RontheRedneck]

Sorry I haven't gotten back to this thread for a while. It's planting season.

adn26 - The framing you posted pics of is fascinating. But as someone said - Not my area of expertise.

You asked for pictures to better explain what I meant. So today I input a 28'scissor truss and played with it a bit.

The first example is 4/12 over 2/12. The difference between the two pitches is 8.9°.

Total horizontal deflection was .51" (For both live and dead loads)


Second example is 6/12 over 4/12. The difference between the two pitches is 8.2°.

Total horizontal deflection was .98" (For both live and dead loads)

Third example is 8/12 over 6/12. The difference between the two pitches is 7.1°.

Don't know if you can read the text, but the total horizontal deflection is 1.61". And you can see the error message that shows it exceeds the TPI limit of 1.25".


So as you can see, as the pitches increase, the horizontal deflection rapidly increases.

And while it sound on the surface like the 2/12 pitch difference between the TC and BC is maintained, the angle between the 2 also rapidly decreases.

Since you guys deal with math all the time it might seem easy to you. But this is a misconception I run across all the time.

 

[BridgeSmith]

Avoiding excessive deflection is probably the reason for the 'inside pitch half of the outer pitch' rule of thumb. Seems you've confirmed that it's a more reasonable limit than the 2:12 difference.

Rod Smith, P.E., The artist formerly known as HotRod10