Rafter Tie Connection Design with Purlins

[ReverenceEng]

Hello,

I am looking to learn how to calculate the rafter tie tension (for designing splices/etc.) in a wood frame roof with the inclusion of purlins/strongbacks.

I am working with an existing roof that has a wall-wall span of 45'6", so rafter span of 22'9". The roof is 3:12 pitch with 2x6 RR and 2x6 CJ. The RR are at 24" O.C. and the CJ are at 16" O.C., so our rafter ties are only every 48" O.C. There are purlins that dump back into the rafter ties above a drop beam. SEE SKETCH ATTACHED.

https://files.engineering.com/getfi...1-0319-44a4-b626-042a4ddd9fa1&file=Sketch.pdf

Owner wants to raise that beam, but getting it above the rafter ties would be hard due to all the "stuff" in the way. He wants to see if we can cut the rafter ties, install the beam bottom flush, and then (re) strap the rafter ties using a strap or tension device.

Problem is, ignoring the purlins when performing statics yields an unrealistically high tension force in the ties due to the very long rafter span and the low pitch.

I could make use "judgment", but perhaps there are some "best practice" for doing this or tips???


Any guidance?

 

[Ron247]

Even though your ceiling joists and rafters only align at every 4', it would be conservative to consider each rafter at 2' has one ceiling joist that ties the walls together. The rafters that do not align with the ceiling joists do have a ceiling joist on each side of them. So basing your numbers on 2' rather than 4' cuts your load in half.

As far as the outward thrust from the rafters trying to force the walls apart it should be much less with the inclusion of the interior load bearing walls. Any wall with what you call a purlin (we call them a rafter brace, stiffleg or strut) reduces the thrust. Estimate how much your vertical load is on the exterior walls due to tributary area and then solve that joint for the ceiling joist tension. I would probably bump my answer by another 30% to account for continuity and other harder to estimate items.

 

[Ron247]

One thing I forgot, you still have to work over to the CJ location where you are cutting the CJ and calculate its tension. If I am not mistaken, the angle of your strut to the wall determines whether the CJ in that center area is being compressed or tensioned by the strut. Been a long time since I did one by hand. The tension in the CJ is not constant across its length.

 

[txeng91]

I’d have to see the whole building section but it’s pretty likely that what your working with isn’t going to meet code, so I would be wary of messing with the existing system unless your going to make it legit. Usually it’s hard to get the numbers to work without having ties at each rafter because of the limited capacity at nailed connections. With your proposed detail your cutting a ceiling joist that theoretically has a decent amount of tension in it, so what’s going to happen between the time they cut the joists until they have your proposed connection installed?Maybe if they tempeorarily shored the ridge it would take the thrust out of the ties. If it were my project I would be torn between bringing the whole roof system up to code or not. Maybe a better option would be to somehow slide a beam up in the attic space above the rafter ties without cutting them and attach the beam to the rafters above with a pony wall and the ties below with metal straps. Also what is suppprting the ceiling finish if the ties are at 48”? Is it gypsum board sheathing directly applies to the ties or a drop ceiling gridb hanging from them?

 

[BAretired]

Under gravity load, the strut is in compression, so the strut puts compression into the central ceiling joist, reducing its tension. If the strut had been vertical, it would have no effect on the force in the central CJ.

Some care needs to be taken to transfer the CJ load to rafters which do not align. It may be simpler and more economical to provide a ceiling tie at four foot centers.

BA

 

[ReverenceEng]

Hi Ron - thanks for the insight!

I understand everything you said, but I am a little unclear on the first part where we will get "credit" for the CJ that are not aligned with the rafters and thus do not lap with the required number of nails. I agree that that other CJ that are not lapped might help with some thrust, but without the lap at the RR, can we really count on them to take that much load off the "real" rafter ties? After all, they are just toe-nailed into the top plates...

 

[ReverenceEng]

TX - I actually suggested the beam up high to them instead and they are mulling it over, so thanks for the assurance!

They were going to have to shore the roof in order to do this and release the tension in the CJ, easier said than done. It's only a 14' span of a 50'-60' long roof is where this occurs so I'm not worried about the building falling down, but we definitely don't want any walls to push out or a ridge to sag when the tension is released.

I have toyed with them also installing secondary ties just above the "new" beam (still within bottom third of total rafter rise) using of 7x19 wire rope with turnbuckles to tighten. Maybe even leave those in place in addition to strapping the CJ back together down low.? I think I saw a post where someone did this, and I too have seen wire rope, rod, and pipe used in this manner in the field.

Any comments on that approach?

 

[OldBldgGuy]

OP: First of all you should work with the proper terminology. As Ron247 says, that's not a purlin it's a rafter brace or strut. Just for your own info you should look at what a purlin is ( parallel to the ridge, carrying the roof decking, carried by rafters, beams, or trusses).
Cut those ceiling joists without replacing their tensile capacity at your peril. This is very similar to another current thread. Don't figure on a tie, cable or other, at a different location doing what is being done now. In the best case scenario a tie where the strut connects to the rafters might work, but that changes the bending in the rafters. You know how Alice went down the rabbit hole, and the whole world was different? You're peering down there.