2x8 roof purlins on glulam rafter beam connection

[TroyD]

Just looking for some input on the attached roof framing sketch/concept. I'm a little unsure about how to resist uplift with these 2x8 purlins on edge bearing on top a glulam beam. Roof pitch is 2.5:12. The blocking between the purlins is held in place with framing angles. The purlin space will be insulated, with a gyp board ceiling. Typically these purlins are secured to the sides of the rafter beam with face-mount hangers, but this setup is different. I think I can satisfy the uplift forces with the truss screws angled up from the bottom thru the top ply of the glulam into the purlin. Any additional input or suggestions is appreciated.

 

[jayrod12]

Or use a hurricane tie down. Like a simpson strong-tie H3 or similar. That's what I use for all my joist/truss tie-downs. Not fully sold on angled screws.

 

[azcats]

You dragging a lot of shear thru the blocking and tension thru the joists? I'd expect to see toe-nails from blocking to top of beam and I'm wondering why so much lap on the joists? Totally agree w/ jayrod12 on the H3 or similar. I'd almost rather see the framing angle securing the joist to the beam. Or use an A35 w/ 3 way bend to cover everything (if the loads work).

 

[TroyD]

JR12,
I prefer the H series hurricane ties also, but I would like to avoid having the clips exposed. I see the H3 is quite small - once the 5/8" gyp board ceiling is attached, only about 3/4" of the clip will be visible. That might work. Thanks!

 

[CANPRO]

Do you have a lot of uplift? I wonder if you could justify enough tie-down capacity with your blocking that is secured to the glulam. You might need (2) angles each side of your blocking and some clips between the rafters and blocking, but at least then all of your fasteners are hidden.

 

[jayrod12]

I feel if he's planning on trying screws on the angle through the beam, that the actual uplift can't be unmanageable. If I remember correctly, the simpson strong-tie truss tie-down screw is only good for a couple of hundred pounds or something like that. Too focused on getting to the end of friday to look it up myself.

 

[dhengr]

TroyD:
See what kind of premium you pay for 18’ or 20’ long 2x8’s, and make the purlins 2 span continuous members, alternating over the rafters, ever other purlin line. Use the cut-offs for your blocking. Don’t lap the purlins by a full 2’, those long free ends tend to kick up and literally lift the roof sheathing above the free ends. Stagger the blocking by 1.5-2” so you can end nail them through the purlins, and toenail them to the gluelam. Now, look at what hardware you need to make this whole grillage stay down on the gluelam rafters.

 

[Prestressed Guy]

I will be the contrarian here. I actually like the Strong-drive SDWC and use them all the time. Each SDWC15600 is good for 590lbs of uplift and 177lbs of F1 shear. Given the blocking shear capacity is not an issue. On every job I have spec'ed them on I have initially gotten complaints from the contractor wanting to use H1 because they are cheaper to which I said tough, use the SDWC anyway. After about a week they called me back and said thank you for insisting and they would be asking for them on all future projects.

Change is hard and we have all been comfortably using the H1 for decades but sometimes you can make a better mouse-trap.

 

[Ron247]

Why not tie the rafters directly to the top of beam. the blocking does not have to be centered on the beam since it would only serve to prevent rotation. Why is continuous blocking there in the first place? I usually block one place every 3 or 4 spaces and let the decking prevent the rest of them from rolling over. Is it there for fire?

 

[TroyD]

Thanks all for the good input. I think a combination of these suggestions will work well for my situation. The allowable uplift values for the SDWC truss screws satisfy the roof uplift requirement. But I will add H1 hurricane ties at the end wall, where the end zone/overhang wind pressures are higher. The continuous blocking is not for a fire separation requirement; I just assumed it should be continuous and it would allow for more anchorage to the beam below. Also, I think they will need the blocking to get the purlins spacing set and to hold them in place until the roof sheathing is attached.