Purlin Support Above a Glulam Truss

[JohnnyB_]

Hello -

I have a large glulam gable truss that is supporting 14" LVL purlins @ 24" O.C. (see attached detail). In the plan view the purlins are spanning 18 feet from truss to an exterior wall for bearing on the opposite end of the glulam gable truss. I am calling out that full blocking is needed between each purlin at bearing points and in the middle of the span.

My concern is the 14" depth (needed for insulation purposes) of the LVL is too large and the flat bending occurring on the purlin, due to the 8:12 pitch is an issue. Has anyone had experience with a similar issue?

 

[BAretired]

I assume "flat bending" means minor axis bending. The gravity load for each purlin must be resolved into two components, parallel to the major and minor axis of the purlin. If midspan support is not adequate, use third point support, i.e. blocking at 6'-0" centers, each purlin supported by sag rods which continue up the slope to a ridge beam. The ridge beam carries the cumulative load from all of the purlins.

Alternatively, you could run the purlins down the slope, parallel to the truss, using a ridge beam and a couple of eave beams to support them. That would be much easier to construct, it seems to me, and it eliminates concerns about minor axis bending.

 

[JohnnyB_]

BAretired, thanks. That is exactly the direction I was looking for.

 

[BAretired]

You are welcome JohnnyB, but personally I prefer the second option.

 

[JohnnyB_]

BA, I like that and haven't given it much thought due to being stuck within my original design.

Generally I do just use rafters with a ridge and plate beam to support the load. I will take a look at the associated load conditions and see if that is a better option for my design.

Appreciate the input! It often helps to get a conversation going to drag myself out of the "weeds".

 

[BAretired]

Yes indeed. Hopefully others with more experience with timber framing will join in the conversation.

 

[JohnnyB_]

I agree.

 

[jayrod12]

I think BA's proposed solutions are the ones to fit the bill.

There's an argument to be made that the exterior sheathing and the interior finishes (assuming gypsum) are capable of bracing/supporting the flanges such that the purlins themselves don't see weak-axis bending. But for the cost of blocking it at 1/3 spans or 1/4 spans to help sleep better, seems like a cheap solution.

Flipping the orientation also makes a bunch of sense if it can work. What's the intent for getting an appropriate air gap between insulation and underside of sheathing?

 

[JohnnyB_]

Jayrod, the architect was going to use SIPs but the lead times were too long so she is planning on just spray foaming the entire cavity.

 

[jayrod12]

Interesting. Around here, spray foaming against the roof sheathing is only used when absolutely necessary. We've seen more issues with that then batt insulated cavities. Often we get separation of the insulation from the joists causing breaks in the vapour barrier. This leads to condensation forming and leaking down onto the ceiling finishes.

Perhaps the climate in your area is more welcoming to the spray foam.

 

[JohnnyB_]

This project is in SW Colorado with an arid climate. The location is approximately 9,500' in elevation.