Misshaped Scissor Truss

[GaryMN]

I can't seem to figure out why I am having this truss problem, any suggestions would be appreciated! The ceiling plane is set to the inside wall 3 in 12, the roof plane is set at 4 in 12, The existing part of the home is hand framed, the new will have scissor trusses ( if I can make it work).

[MPDesign]

Gary open the dbx for the truss in plan view. then on the right side of screen change the max span to a shorter number. both top & bottom cord.

[GaryMN]

Michael I tried what you suggested and it didn't work for this situation. I changed the max span several times making the number incrementally smaller, it seemed to only change the truss web. The shape is what is odd

[rgfeher]

Gary - just guessing but it appears your ceiling planes are giving you this grief- check their baselines or redraw them and rebuild

[johnnyprc]

Trusses follow ceiling plans, but Randy is right that is most likely the issue. Obviously the problem you have is on both sides of the "heal".

From a technical point, you might as well show a I-joist or parallel cords with webbing since your pitch variation doesn't appear enough a differential. The truss companies I work with tell me they need at least 30% or so difference to make a scissor truss work.

[sutcac]

Just FYI, the scissor truss manufacturers around here generally recommend that the interior pitch needs to be 1/2 or less than the exterior pitch, i.e., if your exterior pitch is 4/12 your interior should be 2/12 or less. Having more of a pitch differential might also help your issue in Chief.

[johnnyprc]

Yep, Bryce - on what he shows that is right - meaning he is wrong. The truss company I work with here on 10/12 or 12/12 pitches can get a reduction of about 30% (approx.) - but for 6/12 and below its a 50/50 split.

[rgfeher]

Gary - hope you have this worked out by now

I messed around with a 4/12 truss with a 3/12 ceiling and got some weird things

I think Johnny & Bryce nailed the major issue - pitch difference and the way Chief constructs a truss

I basically got it to work after I changed the top chord to a 2x6 (which is the way it would likely be constructed with the lower section section serving as a collar tie)
Chief did not like a 3 1/2 top and bottom chord for such a slight scissor truss and got confused I guess - anyway that was my experience

I realize this looks like conventional framing vs a tuss - and the funny thing is the 4--3 truss stopped dead center of the room yet a 4--2 3/4 sailed right thru
I had no trouble generating a 4 -- 2 truss, so I quess Cheif is giving us more room with trusses than what an engineer would (on the 4 -- 2 /34 being greater that 50%)

As far as celing planes go - the baseline should be at the outside main layer (framing hopefully) and inside bottom should be set at the wall height (if you want to intersect @ the exterior wall)

Maybe Scott will do something on this as the behavior is somewhat erractic - I tried in X4 and got the same results using both 3 1/2 and 5 1/2 top chords

[perryh]

This is a bug I reported a while back. Conventional const. and trusses don't work together. This condition will delete trusses near the 2nd floor. Hope its fixed in X6. I did a video on this also. What I did is not to cut my section in the areas that wont generate trusses

[johnnyprc]

This is a bug I reported a while back. Conventional const. and trusses don't work together. This condition will delete trusses near the 2nd floor. Hope its fixed in X6. I did a video on this also. What I did is not to cut my section in the areas that wont generate trusses

where are your videos?

[perryh]

I only do a few short ones, if you search here, you should find it. I don't use you tube, I use Screencast, so everyone can see it. They are just problem specific only.

[perryh]

I only do a few short ones, if you search here, you should find it. I don't use you tube, I use Screencast, so everyone can see it. They are just problem specific only.

http://www.screencast.com/t/NhCQL9pEy