Relative deflection in wood roof trusses 2

[RontheRedneck]

Had an interesting situation pop up. Thought it might be worth mentioning here in case any of you ran across a similar situation.

The building is a large one. Has some rather shallow 60' plus span scissor trusses on it. The building has a hip roof, which further complicates things.

Either the architect or EOR figured some bearing would be needed on the shallowest trusses, so they put a bearing wall in near the hip end.

The last truss that hits the interior bearing has about 1/4" of deflection. The next truss that's clear span has 3" of deflection. (All deflection numbers are for total load)

The specs call for a total load deflection limit of L/240. On a 60' truss that's roughly 3".

So technically, the trusses work. They meet the deflection criteria in the specs. But no way in hell is that a good idea.


I was not involved in the job until yesterday. A guy asked me to look at the job and see what I thought. I suggested referring it back to the architect and EOR to see what they want to do. That's not a problem we can solve.

 

[manstrom]

A good reason to:

-Put a max deflection limit on trusses
-Realize that L/240 over 60' is pretty damn big
-Not do 60' trusses that thin.

A good truss designer would have flagged this and brought it up to the SER. A good SER would have noticed this during shop review.

IMO, a 2.75" deflection over 2' does not meet code minimums, so yes, this is more than a serviceability problem.

 

[Celt83]

I agree that it is the design professionals duty to consider and try to avoid conditions like this but disagree that this would/does not meet code minimums.

The deflection limits set by IBC are member by member not system limits. Footnote i of Table 1604.3 defines l as the length of the member between supports. The IBC compliant member deflection check of the plywood spanning between two trusses is the local deflection of the plywood only removing the initial end deformation of the supporting truss. Just like a stair stringer spanning between floors the deflection of the stringer is the normalized deflection without consideration for the initial member end positions.

 

[BAretired]

One solution is to provide stiffening members perpendicular to the trusses, forcing the deflection to occur more gradually over a number of trusses.

 

[BridgeSmith]

I'm probably way out of my depth, here, but I'll wade in anyway.

It definitely seems that having 2 3/4" of differential deflection between 2 trusses 24" apart would cause problems for the sheathing and roofing materials. If would be especially bad if most of the deflection is due to snow load, since the sagging would would occur rather quickly, just when having a waterproof roof is most important.

One mitigating factor could be lateral end restraint, provided the ends of the trusses are adequately connected to the exterior walls, such that the lateral spread is limited. The trusses and connections to the wall would have to evaluated, detailed, and constructed properly, to validate the assumed reduction in the deflection.

Barring a significant reduction in deflection from end restraint, I would be looking to add a beam along under the ridge, from the last truss (the one with minimal deflection) that supports enough of the adjacent trusses to reduce the differential deflection between trusses to something that the roofing materials can accommodate.

 

[BridgeSmith]

One solution is to provide stiffening members perpendicular to the trusses, forcing the deflection to occur more gradually over a number of trusses.

That's probably a better alternative than the beam I proposed.

 

[phamENG]

Are strong-backs 'a thing' in roof trusses? I specifying them all the time on floor trusses, but have never considered them on roof trusses, nor have I seen a 'stock' detail for them.

 

[DoubleStud]

I guess I am still confused with this discussion. Is the problem the 3" deflection or the relative deflection? So what deflection of the common trusses is acceptable for you?

 

[BridgeSmith]

Is the problem the 3" deflection or the relative deflection?

Primarily, the 2 3/4" relative deflection, but since the 1/4" deflection at the adjacent supported truss would be very difficult to change, we're focusing on how to mitigate the 3" deflection at the unsupported trusses.

 

[RontheRedneck]

A few comments:

Manstrom said: "A good truss designer would have flagged this and brought it up to the SER. A good SER would have noticed this during shop review."

If you read the thread carefully, we discovered the issue. And the information was passed along to the EOR the first time truss drawings were sent in for review.


phamENG said: "One technique I've seen is to double the first truss off the wall."

The truss with 3" of deflection is already 3 plies just to get it down to where it will work.


phamENG also said: "Are strong-backs 'a thing' in roof trusses?"

I have never seen them. But it's an interesting idea.


BridgeSmith said: "One mitigating factor could be lateral end restraint, provided the ends of the trusses are adequately connected to the exterior walls, such that the lateral spread is limited."

With long span very low pitched trusses, the horizontal forces would be enormous. I don't think there's any practical way to restrain that much force on a 2X6 stud wall.

 

[BridgeSmith]

Then some deep blocking/diaphragms between the trusses may be the more practical way to limit the deflections.

 

[Mark_J]

+1 for camber and a sheetrock detail so ceiling can move up and down independent of adjacent stiff wall and corresponding wall covering. Maybe the roof should be made of steel instead of wood?