Bringing up to new code with Collar ties

[Apexpredetor]

If I understand correctly, all rafters not using ridge beam need to have collar ties installed every other rafter to resist uplift of the roof, correct?

If I am analyzing an existing structure for adequacy for additional load am I required to suggest/mandate collar ties to be installed?

If I am suggesting reinforcement (sistering identical members to gain capacity) must I also spec a collar tie to be code compliant (new code).

These houses are like 100 years old, 2x6 rafters on 24" (possibly 16", my notes got wet in a downpour so I went with 24" to be conservative), approx. 15ft slope length, and 7/12 pitch.

Thanks all. I appreciate it

 

[XR250]

Analyzing a 100 year old structure for "adequacy" is tough as there were no codes back then and the construction was generally pretty minimal for rafters and floor joists. However, I have yet to witness a failure unless they were damaged by moisture or insects. If it has stood for 100 years, why would you consider it inadequate? Are you tasked at bringing it up to today's code?
If you are going to sister the rafters, you might as well add collar ties.

 

[Apexpredetor]

This work is for a NonProfit Weekend & Summer Camp for children (HAs some church association, but I don't remember or care about that. I'm just doing it for the kids to have additional monies). My kids don't go the the camp, but I would still liek to help out. Total of four buildings getting panels plus ground array.

Their budget is very tight. I was thinking about just paying for the renovations and donating my services just to get it to be safe, but I am still wondering if I MUST specify the collar ties or not, or in what scenario I must bring teh structure to new code?

Thanks all.

 

[Archie264]

Apexpredetor,

The collar ties are more to resist downward forces than uplift. I'm guessing there isn't too much snow in your area for the walls not to have been pushed out. Or, perhaps there is some type of attic and/or ceiling joists that attach to the rafters effectively serving the same function as collar ties?

I'm guessing also that the rafters are actually 2" x 6", or close to it, not the modern 1.5" x 5.5" and if so that helps your cause in terms of the adequacy of the rafters themselves.

As Excel pointed out, sometimes it's hard to make these old structures work on paper and yet this one's stood for 100 years. So is it really worth messing with? And I agree with him also in that if you do then adding collar ties would probably be worthwhile.

 

[TomDOT]

It looks like someone has to mess with it, as apparently the owner intends to add solar panels to the roof.

 

[Archie264]

Oh, I missed that part of it. That's a bad idea, in my opinion, but, if it has to be done then a more in-depth analysis and possible retrofit might be in order.

 

[Apexpredetor]

I was under the impression that the collar ties resist upward forces trying to open the roof like a clam shell, and rafter ties (lower on the rafter) ties the bottoms in to resist that outward thrust (wall movement). Maybe just a difference of phrasing here.

Structure has 2x6 ceiling joists, unfinished above.

If I analyze it for stress and deflection I am ok with stress, but being that they are only 2x6 I fail in deflection (L/180) by quite a bit.

Thanks for all of the opinions. I really want to help these guys out best I can.

 

[XR250]

Collar ties are generally for the clamshell affect. Rafter ties are for resisting outwards thrust.
Collar ties CAn be used in conjunction with rafters ties to reinforce a roof by using the collar ties in compression. They have to be designed not to buckle, however and there is additional outwards thrust generated on the rafter ties.

 

[Jerehmy]

if you sister without collar ties, the original rafters are still going to take the majority of the load as they will be significantly stiffer than the rafters without collar ties.

a modeling program would obviously take this into account, but if you're doing hand calcs just keep that in mind-

 

[a2mfk]

Uplift on 100 year old houses? I would not be worried about the collar ties in that respect... But they act like a horizontal chord in a truss does in resisting the horizontal thrust in gravity loading situations. The lack of these has resulted in pushing walls out-of-plumb, but my experience with them has been that they usually work OK. Why? Residential roof dead load is pretty small, and roof live load is rarely applied unless the roof is being recovered.