trusses and interior non bearing walls

[peach]

I can argue this either way but:

Do you allow the trusses to touch non bearing wall top plate (not attached.. just touching)?

 

[Mark K]

The concern is that when the truss deflects the "non-bearing" wall could be an effective support point. If the support point is at a truss panel point additional load could be put on the element supporting the non bearing wall thus causing distress. Probably of more concern is that if the support point is not at a truss panel point it could cause bending in the truss chord thus leading to failure of he truss.

To prevent these problems the gap between the truss and the non-bearing wall should be enough so that it does not close when the load is applied to the truss. So you could have a minor gap with no load at the end of construction but during snow load you could have a truss failure.

 

[Francis Vineyard]

Check the manufacturer's instructions; may specify a half inch gap at this point for deflection. What is the argument for the other way?

 

[4thorns]

Mark, could you post any links or info regarding the changes in load distribution after the installation of walls below roof trusses?

 

[Rio]

The trusses shouldn't touch the non-bearing walls and are usually connected with a truss clip

 

[peach]

The biggest problem I've seen is framers wanting to nail the bottom chord to the non bearing wall (creating a bearing point), and I've seen walls pulled up an inch or so when the truss returns to where it wants to be.... never have I seen a problem with the truss doing damage if it deflects downwards. I've lived in the severe north snow load and never saw truss failure from snow (from deflection .. well never, actually)

 

[Mark K]

The changes in force distribution will depend on the location of the new support and the magnitude of the deflection. Appropriate links would consist of a tutorial in how to analyze a truss.

In many cases you may never see any problem or you may see the joist under the bearing wall deflecting more. I would expect that the likelyhood of a failure is greater for long span strusses.

The simple solution is to provide the gap recommended by the truss designer. There are no easy ways for a non-engineer to evaluate the impact of the truss setting on the non-bearing wall.

From a regulatory point of view the approach would be to either require the gap specified by the truss manufacturer or to have an engineer, preferably the truss designer, evaluate the impact of the non-conformance.

 

[fatboy]

This was an interesting thing to learn when I first became an inspector. As a framer around here, and even building the house I live in now, it was common practice to have the interior non-bearing walls, snog to the bottom chord, and to nail it off O.C. to secure for the sheetrock. I know it's not right now, but I'm still in the same house, 14 years later, with no ill effects, and yes, we have snow loads.

 

[brudgers]

Once the ceilings and walls are sheet rocked and taped the truss is pretty much bearing at the partition anyway. IMO, this is not really a structural issue for typical residential construction except when the partition is deflecting the truss upward or something like that. YMMV.

 

[SCBO1]

I agree with Mr. brudgers!

Some houses have a center bearing with I-beams below the floor for support, nails are typically used to hold the truss true at the desired on-center. It appears the current trend is to have a more open floor plan with truss design in our area of the woods. I would refer to the designed truss document for bearing information or call the truss designer if there's a concern. Truss designers want to sell trusses, they are very receptive to questions on something they have designed IMO. I have heard that a truss can bow upward taking a non-bearing wall upward, but have never seen it, could be non-fiction. Could be a rumor started by a framer that dislikes truss construction?

pc1

 

[Daddy-0-]

I agree with Brudgers also. I am much more worried about things like proper truss bracing and uplift where needed.

 

[peach]

me too... but I opened up BCSI 1-03 - and the truss industry doesn't allow it; the second top plate of the non load bearing walls is being removed; hopefully I won't find bracing issues tomorrow.

 

[DRP]

I have heard that a truss can bow upward taking a non-bearing wall upward, but have never seen it, could be non-fiction. Could be a rumor started by a framer that dislikes truss construction?

If the bottom chord is buried in insulation, all cool and comfy and the top chords are up in the high heat and lower humidity... and then if you add some reaction wood that shrinks lengthwise as it dries into those top chords... truss uplift.

Hogging is what we are describing with the truss bearing over its' center rather than on the ends. For you sailors its the same hogging that can occur when a wave passes under a flexible boat and the stem and stern settle down into the troughs. I've never seen a failure caused by this. It could I suppose but I suspect in residential construction it wouldn't get to that point. I did quizz my truss supplier on this issue on a truss floor that was running over a wall with no provision in the truss layout. He was unconcerned.

 

[pwood]

some builders have gone to a 1x4 top plate on interior walls with the truss clips to address this situation. it allows some compensation when slabs or framing is high under the interior walls.

 

[DRP]

Someone asked what this does to the load distribution. It depends. This is a paire of very simple diagrams from a very simple online program. The truss configuration and loads are the same in both cases.This would be the intended end bearing;

I've moved the right end bearing in to the next inboard panel point. Notice the stress reversals in some of the members and their relative magnitude, look at the reaction on the unintended bearing;

This has no relation to anything in the real world, just an oversimplified example. The truss could settle down on the intended stiff bearing and revert to design, or not.

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[RJJ]

I have to agree with Brudger on this. I have seen more problems with dry wall when left in a state of none attachment.