3/28/2024 0 Comments Collar ties on roof raftersI did not consider any moment resistance at connections, which i believe is appropriate. All members are continuous, so there aren't any internal releases in the rafter at the ties, for example. Not exactly sure what you mean regarding the bending moments in the members. Inadequate rafter tie connections were the problem most of the time. Our area experienced a 1/50 year snow 3 years ago that lead to several failures/collapses. We're finding that residential building codes aren't adequate in several cases. With initiatives to improve insulation in roofs and increased frequency of severe weather events, we're seeing more snow on roofs and therefore more failures. I'll admit that i don't do any residential design that would include an unsupported ridge typical of non-engineered construction (scissor trusses aside), however, i'm often hired by insurance companies to investigate failures in residential buildings. Sure, if you analyze it and provide appropriately sized members and connection details, it will work fine. If they're installed somewhere along the rafter span, they should be considered in your analysis and the impact on the rafter tie considered. If you want collar ties to only help with uplift resistance at the ridge, place them as high as possible. So, to summarize, collar ties see compression loads. I've seen and fixed lots of roofs where the rafter ties are too high. Below are the moments in the various members under the same loading conditions noted above: As the tie placement moves further up the rafter, the bending in the rafter increases significantly. Similar to collar ties, traditional rules-of-thumb would have them located anywhere in the bottom third of the rafter span ( ). Improperly located rafter ties can be even more dangerous. Yes, they'll go into tension in the event of uplift, but if they aren't designed to resist the appropriate compressive forces, the member or connection may be damaged and unreliable when it's needed in the uplift condition. The frames at the bottom are for the rafter tie discussion to follow.Īs you can see, the collar tie goes into compression under load. Frames on the left have full snow on the roof, frames on the right have full snow on the left side, 50% snow on the right. Collar and rafter ties (where shown) are at the third points. Below are the axial loads in the members. The span is 24', c/c, 4:12 slope, 50 psf snow, and i can't recall the dead load but it's not significant relative to the snow. This puts the collar tie in compression and increases the tension force in the rafter tie (the moment arm between the compression and tension forces in the collar and tie, respectively, decreases). However, when you add collar ties, you are introducing a lower point where the rafters can react against each other, like they do at the ridge. Common rule-of-thumb practice is to locate the collar ties in the top third of the rafters. Their intended purpose is primarily to prevent roofs from splitting at the ridge in the event of large uplift forces as the connection to the ridge is traditionally tenuous (end and/or toe-nails). I'm not a layman, so let's get it on! :)Ĭollar ties are common in residential roof construction. The thread has since been removed (layman question i suspect). I posted an unpopular fact in a thread the other day that i was hoping we could discuss a little more. Shall be given without reservation for the public good.Īnd in fidelity to my profession, I shall give the utmost. Practical use the principles of science and Has been spurred by the engineering genius.
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