Collar Tie loading and detailing 1

[Bowsers]

Hello
A bit about me, as I’m going to be asking a lot of newbie questions going forward, and as I know and learn more, I hope to be sharing my knowledge with others. Currently I work in a small office, with the principal out much of the time, and no one to ask these questions.
I never took a wood class. I was primarily doing soil structure interface, and now am doing more wood design. Wood connections are easily the thing that are my weakest, and what I’m working on upgrading professionally.
I have a question on a solar project I have recently completed for a residence. I was plan checked, and the city department would like me to do 2 things.
1) Upgrade the collar ties and
2) Upgrade the thrust connection on the roof to the existing walls.
My boss suggested detailing the collar ties as 2x with double ½” sheathing on either side, using 16d nails, and bending them over, creating a double shear condition.
Questions:
What is C_delta? (table 12.5.1A), min shear area?
What is L/D (table12.5.1C)
Also (this is just me complaining/observing), the Greek letters make it difficult to ask questions.

For the collar tie I am designing for a 2.2 k load (output from risa, ASD forces)
The NDS doesn’t have a table for double shear connection for nails, would I double up the capacity of a single shear connection?
Based on this, using a nail diameter of 16d common, and a side member of 1” (1/2” x 2), and G =0.49, referencing table 12N, I come up with a lateral value of 137#. 2.2k/.137k = 16.05 -> use 17 nails.

For detailing:
Nail is 16d, D = 0.162 in
Min edge = 1.5d =0.25”
Min end = 5D = 0.81”
Min space = 4D = 0.75”
Min row = 2.5D = 0.45”
So I can use 2 rows of 9 nails, on a 2x4

 

[kissymoose]

• C_delta is the connection geometry factor, see section 12.5.1. This factor accounts for the spacing of dowel type fasteners larger than 1/4" diameter.
• For l/D - did you read the table footnote? It states in the table: "The l/D ratio used to determine the minimum edge distance shall be the lesser of: (a) length of fastener in wood main member/D = l_m/D or (b) total length of fastener in wood side member(s)/D = l_s/D."
• In general, rafters ties are used to prevent the wall from leaning out (thrust) and collar ties are used to prevent the ridge from opening up with wind uplift. They are both just horizontal members that tie rafters on either side of the ridge together. Rafter ties are in the bottom third of the attic space, collar ties are in the upper third of the attic space. Because of this, collar ties cannot be expected to prevent thrust. Even if you can size a collar tie to take the tension, and provide enough connections between the collar tie and rafter, you still will end up with a large point load on the rafter creating a moment that is larger than normal gravity loads. This is why the rule of thumb is to keep rafter ties in the bottom third. So for your case, this 2200 lb load in the collar tie raises a red flag for me. Why is this axial load so high? Are the rafter ties not engaging? Be sure to check if your rafters can take this point load.
• Nails can be driven through two members, bent over, and driven back through the members. This is called clinching, and it does provide double-shear capacity, you just have to meet minimum nail penetration on the return part. If you’re using (4) layers of ½” sheathing on either side of a 1 ½” member, you don’t have enough length in a 16d nail to clinch it. Did your boss mean to use just (1) layer of ½” sheathing on either side? This would allow you clinch the nails and meet the minimum 3D penetration requirement.
• As for the city comments, I’m not sure why they are asking for collar tie reinforcing. Maybe it’s a case of the city not knowing exactly what they’re talking about and trying to check their boxes, it’s certainly a common occurrence around me. Solar panels will add dead load that would only help with uplift capacity. For dealing with thrust, check that the rafter ties/ceiling joists have enough nails to the rafters and are continuous or connected adequately at splice locations. In a vaulted ceiling situation, check that the beams are strong enough to support the new loads and meet deflection requirements.

 

[Bowsers]

Ah, I guess the detail provided wasn't clear in the labeling. That's why I provided the section -- (1) layer of shtg on ea side of 2x.

Regarding l/D -- in this case, the minimum edge distance would be the board thickness (1.5") divided by the diameter of the nail (0.168") = ~9 dimensionless. The result of this equation being a dimensionless number is throwing me off.