Return to index: [Subject] [Thread] [Date] [Author]

Re: Use of Collar Ties in Light Frame Wo

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dennis,

I have some concerns with just adding additional collar ties because the problem area still hasn't been addressed. Let's just look at the geometry of things.

Neglecting any axial compression or elongation, if the ridge dropped 2 inches the outward horizontal movement had to be about 3/4 inch at each wall or a combined movement of about 1.5". A 2 inch deflection of the ceiling joist is going to cause an inward movement of about 1/64" at each end or a total of 1/32" between the ends. For there to be that much differential movement between the rafter and the cj, either the wood had to split at the nailed joint, or there wasn't any nailing. I wouldn't expect the nails to bend if the cj & rr are indeed face to face. The only way to tell what happened is to get eyes on the members by either cutting a hole in the ceiling or laying in the attic and maybe even using a magnifying mirror. I also carry a piece of 22 ga strap to try to slide between members of a wood joint if I can't see the nails. This movement could occur without forcing the wall out of plumb if the nailing to the top plate was inadequate. But if I remember right, you said there is continuous stucco from the exterior wall to the eave so a crack would be visible. However, the wall could have been pushed out of plumb before the drywall was added and the only way you'll detect that is to hang a plumb line at a couple of places along the length of each wall. If for some reason, only one wall moved, you may have a problem with the wall.

Considering the split in the ridge board, I'd also stretch a line along the bottom of the ridge board and verify the vertical displacement. If the vertical displacement is more, the geometry issues discussed above are even worse.

If the heel connection has failed or was non-existent to begin with, adding a collar tie one foot down from the ridge can be designed to resist all of the horizontal thrust, but then the moment in the rafter at the point of the collar tie becomes a possible concern. Now there also has been discussion about the "unknown" actions that help conventional framing work even though it may not calc out. One of those items may well be some diaphragm action. Trying to envision 3/4" deflection of a roof diaphragm under static loading is difficult for me. And if any rafter has experienced horizontal movement of 3/4" either the diaphragm has deflected that much, or there is inadequate nailing between the plywood and the rafters.... If indeed the diaphragm is working and has deflected 3/4" in each direction, it is beginning to take on the properties of a warped plane. I'm not accustomed to looking at plywood as a 3-d element, or even working in two directions at the same time, but as the ridge sags, the plywood will have to start working parallel to the ridge at some point while at the same time deflecting perpendicular to the ridge.


* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org