3/20/2024 0 Comments Roof rafter collar tiesAssuming 10psf dead load, the weight of the roof itself, and 10psf live load, the minimum allowed design live load. If this is a cathedral area, I assumed the rafter design you drew is to be spaced 4' apart so it doesn't get too busy up there. Notice how many have a steel tension rod or cable either fitted or retrofitted (that answers another question dug). If its just a rectangle with tall lightweight walls you'll be visiting a hammerbeamed roof later. If a house had 6' thick stone walls or wings on each side of a hammerbeamed greatroom, I would say go for it. There was a lengthy presentation on reasons why not to build a hammerbeam. One timberframe engineer described them as a perfectly acceptable truss with the bottom chord cut out. Great in a heavy stone walled church, dubious in a lighter framed structure. The best advice I can give on hammerbeams is don't unless you have some way of buttressing the walls, they do thrust, much more than most designers have given them credit for. One more thing- If you were going for a modern look in a structure (I am not) would a tensioned cable work instead of a rafter tie? Just a thought. Overkill? Not good enough? All opinions appreciated! Seemed to make it easier to make a better connection, and also strengthen the weak point between the rafter tie beam and top of the wall. So, thinking about how to connect this I pondered this Also it would work out better (with them raised) from a design standpoint because there will be a raised floor (15 in.) under this section. which is quite a ways away, will only mill up to 16 ft. The reason I want to raise the ties is because I am trying to create a partial timber frame look and plan on using rough cut 4 by 8's or 10's. So I thought, what if I doubled the rafters at those points, thereby doubling the strength of that weak point? From what I can envision this is what might occur in a worst case scenario, greatly exaggerated of course. This got me thinking about what could potentially happen if I raised my rafter ties a few feet. I case none of you have seen this it is an excellent post by DonP that clearly illustrates forces at work regarding roof thrust. I have been thinking a lot about this lately and have come up with a few ideas I'd like to run by all of you. This both supported the roof and redirected a large portion of the thrust down to the tie area. It acted as a brace and a compression strut. Notice the legs running from the collar beam down to the tiebeam. One approach was the French Jambe de force "leg of strength". As they evolved they developed some unique ways of redirecting the forces. I've seen pictures and analysis of the kneewall posts bowing outward and splitting when they crossed the line. In old european timberframes a high posted cape ran into this. Strictly speaking a kneewall situation already throws this into an engineer's hands so go carefully as you raise ties, there is more going on there than most realize. This is wandering off into more than anyone wants to know but it came to mind, so what the hey. If you look at the bottom of the rafter span tables there is a formula to figure how much to reduce the table rafter spans according to how high you raise the rafter ties. "Collar ties or ridge straps shall be connected in the upper third of the attic space.Ĭollar ties shall be a minimum of 1x4, spaced not more than 4' on center" In the '06 and newer code collar ties are now required, I suspect this is in the aftermath of the midwest tornados in ('04?). They are primarily used to resist uplift forces on the roof. QuoteA collar tie is usually in the upper 1/3 of the attic space. I'm not asking for any engineering advice, just your valued opinions. I realize that rafter ties attached to top plate and rafter is a stronger connection, but how big a difference would it be? I am using 2 by 10 rafters, and snow load here in So. This would be for aesthetic reasons, I have seen some done this way on this forum, and I kind of like the look. Like many here, I am modifying my plans here and there and am considering collar ties apx. by 20, leaving 18 by 20 where I want an open cathedral ceiling. Just wondering.Īnyway, I got my deck built for 20 by 30 1 & 1/2 story and am still trying to hammer out some design decisions as I go along. First off, are rafter ties and collar ties the same thing? I thought rafter ties were to prevent the walls from spreading and collar ties were for strengthening rafter spans, but I'm not sure.
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