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Another Collar tie/rafter tie question

Another Collar tie/rafter tie question

Another Collar tie/rafter tie question

(OP)
Is there any remedy for a roof framed with a "rafter tie at 46% instead of 33% above rafter bearing? The obvious fixes are move it down or ridge beam but the vaulted ceiling has already been installed. The building width is 25'-7,4/12 slope 2x6 at 16 with 2x6 ties on both sides of eaxh rafter. Tried to run it in RISA but the deflection is like 2" and the load from it being a reaction I'm sure is too high for a wall to take. Problem is I can't even get a span table rafter to run as a pass in RISA (with rafter ties at bearing), so I think something wonky is going on in the program or with me. Thanks for any input.

RE: Another Collar tie/rafter tie question

Your results are probably correct.

Could you install cable ties every so often, at the top of wall level? You would see them inside the space, of course.

DaveAtkins

RE: Another Collar tie/rafter tie question

How often are the collar ties spaced?

Mike McCann, PE, SE (WA)


RE: Another Collar tie/rafter tie question

(OP)
Love the cable ties actually did it in another project, they're trying to keep from doing that now though. They are every rafter (16" o.c.)

RE: Another Collar tie/rafter tie question

Since the building is roofed and sheet rocked at the ceiling, I see no way to fix this short of adding knee braces at the outside wall which I'm sure they would love too.

Mike McCann, PE, SE (WA)


RE: Another Collar tie/rafter tie question

(OP)
Ya. I wish people would consult first.

RE: Another Collar tie/rafter tie question

Quote (jplay)

the deflection is like 2"

Are you sure that this is a problem? The drywall along the collar tie ought not be an issue as there's going to be little deformation there. And you're going to have a natural crack controlling joint at the ceiling pitch breaks. All that you really have to worry about is the curvature along the rafter tail extensions.

Quote (jplay)

and the load from it being a reaction I'm sure is too high for a wall to take.

The wall should be treated as roller supports. Consequently, there should be no horizontal reactions to cause you grief at the top of the walls.

Quote (jplay)

Is there any remedy

If the 2" deflection is a major concern, you could get into the attic and truss the system by adding a king post with a diagonal web on either side. This would stiffen the back span of the cantilevered rafters and thus stiffen the entire system. This would not, however, affect the bending moment or curvature in the rafter tails. As such, I'd be inclined not to bother.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Another Collar tie/rafter tie question

I imagine in addition to the serviceability issue, you have a strength issue as well that cannot be fixed by internal trussing.
Any chance of adding in a structural ridge under the existing ridge.
I would ignore RISA if you are checking it with the collar tie at plate bearing. Use the code span tables for that situation.

RE: Another Collar tie/rafter tie question

The 2" deflection... is it vertical deflection, or lateral spreading of the walls? Or both???

Mike McCann, PE, SE (WA)


RE: Another Collar tie/rafter tie question

(OP)
KootK I did place one side as a roller and it's deflecting 2 1/2" and I think for a 9' tall wall, there will be some stiffness but even with that taken into account it would deflect quite a bit and push the wall no?
XR250 I won't be able to get them to put a ridge beam in, it'll be a tall one and 3 1/2" wide to it. It would be a 28' long one too, I mean if they came to me first we could have gotten that done but you know engineers aren't needed I've been building for 30 years I got this.

RE: Another Collar tie/rafter tie question

(OP)
Sorry was going out of hand with the rounding. 2.5" horizontal wall deflection (roller) 1.2" sag at ridge.

RE: Another Collar tie/rafter tie question

Ah, I thought the 2" was vertical deflection. I get how it's a problem now.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Another Collar tie/rafter tie question

Remember, the 2.5" deflection at the roller will actually be shared by both supporting walls--1.25" at each wall.

DaveAtkins

RE: Another Collar tie/rafter tie question

(OP)
I didn't think about that. Because the other side is pinned for analysis it would have to share, that's not AS BAD. Still pretty bad, but again I think the wall won't deflect that much because there is no stiffness for the wall incorporated into the model so we're coming down on the severity of the issue. They don't want to fix anything but I told them under higher loading this thing can sag/deflect outward. Hasn't happened over the past couple of months but there is potential in the future for sure.

RE: Another Collar tie/rafter tie question

Especially with winter comingbigsmile

DaveAtkins

RE: Another Collar tie/rafter tie question

There was a long string discussion on lateral spreading, flat or sloped bearing, roof diaphragm contribution to resist deflection, and ridge beams maybe six months ago. BA and many others contributed.

Might want to check it out. Lots or working structural elements to consider here.

Mike McCann, PE, SE (WA)


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