Horizontal Deflection, Wood Truss Design

[bigmig]

I was wondering if anyone has a code reference or rule of thumb they use for limiting the horizontal deflection of a prefabricated wood truss that has a sloping bottom chord configuration (i.e. scissor truss).

The issue is that once the truss deflects too far the joint in the gyp board at the ridge splits out.

Section 7.6.3 of the Truss Plate Insitute says 1. 1/4" limit, which is pretty confounding. There is no way that works for residential construction.
The walls do not provide horizontal support (the are basically 12 ft tall, 5. 1/2" wide noodles).

Thanks in advance.

 

[jayrod12]

That's 1 1/4" total, so 5/8" at each bearing point. You likely aren't even getting that theoretical load since the diaphragm will help stiffen that lateral movement up.

 

[Canuck65]

I have thought about issue for quite some time and have not yet identified a solution that I prefer. You mention the gyp board at the ridge splitting but what about the joint in the gyp board between the sloped ceiling and the wall, where the truss bearing point is supposed to be allowed to move?? I guess some sort of floating trim pieces could be installed.

The vast majority of the contractors that I know wait until the building inspector has passed the framing inspection and then they nail all the sliding connectors so that they are then fixed. (I am not saying this is correct, just that it is commonly done).

I also asked truss design companies if they could design a scissor truss assuming no horizontal deflection (or very little, less than the TPIC values). I didn't care if the truss became more expensive or reduced spacing, increased member size, etc... Flat out the answer was no. There was nothing they could do to make this situation disappear.

What we see specified often is to stagger the sliding connectors, put one on at the 'left' bearing point, next truss gets it on the 'right' bearing point, etc...

 

[dauwerda]

Sliding connectors on the ends of trusses? The only place I am used to seeing sliding connectors is at the top of partition walls, which allow for the downward deflection of a truss without dumping load into a partition wall. I have never seen sliding connectors placed on the end of a truss - how would out of plane forces on the wall transfer into the roof diaphragm if there are sliding connections? The wall and the truss simply deflect together, there is no need to put any type of sliding connection in.

I also don't see how this small rotation at the peak would actually result in drywall cracking, is that really an issue? Now, if you have any partition walls that tie into the bottom of the scissor trusses in the middle of the span, there are definitely potential cracking issues.

 

[jayrod12]

I've never seen sliding connectors at the ends of the trusses. They're always fixed connectors and the flexibility at the top of the walls to bow outwards accommodates the movement required to make the truss analysis valid.

To have it have zero deflection, the additional forces going through the truss are enormous and that's why they flat out say no when asked to design for it. But, a small amount of movement makes a big difference in those loads, so often you can specify a maximum if you'd like to make yourself feel more comfortable and they will stiffen up the trusses accordingly to get to that point.

 

[Canuck65]

We see the TC Scissor Truss Connectors by Simpsons Strong-tie being specified by the truss companies all the time for scissor trusses. They accommodate up to 1-1/4" horizontal movement while also resisting uplift. See attached summary of the fastener. The contractors can't pass a building inspection without something like this installed on one end of the scissor trusses.

 

[XR250]

how would out of plane forces on the wall transfer into the roof diaphragm if there are sliding connections? The wall and the truss simply deflect together, there is no need to put any type of sliding connection in.

We see the TC Scissor Truss Connectors by Simpsons Strong-tie being specified by the truss companies all the time for scissor trusses. They accommodate up to 1-1/4" horizontal movement while also resisting uplift. See attached summary of the fastener. The contractors can't pass a building inspection without something like this installed on one end of the scissor trusses.

Agree with Dauwerda. If you have a sliding connection, how are you resisting the out-of-plane wall forces? This has been discussed before and I can't find the thread. Let the walls spread. The closer you get to the gable end, the stiffer things get in the roof framing due to folded plate action in the diaphragm (nothing that can be easily calculated, however). I have never seen an issue with this in my 29 years of field experience.

 

[dauwerda]

Wow, that seems like a terrible idea in most installation cases - if that slip really were to be engaged, I don't know how any finishes (inside or out) would stay intact. So I definitely understand why a contractor would nail them all off to prevent sliding. If they don't they will be getting calls to make repairs to drywall and siding for as long as their warranty lasts.

I also don't understand why a truss manufacturer would require it, if the two end connections act more like pins, it only helps the overall strength/capacity of a scissor truss.
A truss designer would like nothing more than to do the design assuming pin-pin connections for a scissor truss(much like a rigid frame PEMB), but we all know this cant be achieved as the top of the wall will deflect out when a lateral force is dumped into it, so they must plan on it being a roller on one end when doing the design. But, if it does behave more like a pin, I don't see how that is a bad thing.

 

[jayrod12]

It puts a lot of compression into the bottom chords that weren't designed for it. It just invalidates their analysis.