rafter cable ties

[paulmars]

I cant understand this math. ive been over and over it. Each time I get a little further, then I loose it again.

ref:
How to calculate rafter thrust? -Help! at http://www.eng-tips.com/viewthread.cfm?qid=105386

Can someone help me figure cable load?

4/12

24" rafter spacing.

No rafter ties at all. A few existing collar ties.

ceiling joists run parallel to ridge.

house is 27' x 30'(ridge).

7/8" roof planking and asphalt shingles.

thanks,
pa

 

[tony1851]

A gross simplification, but would this work, using the principle of moments?

It assumes that the rafter does not deflect, and there is no fixity at the wall plate.
Also doesn't acount for wind load (pressure and suction).

On this basis, if the rafter tie is higher up, the tension becomes greater, until at the top, presumably it would be infinite. This is why the collar ties high up contribute very little to preventing rafter spread.

Untitled

• tony1851
• 31 May 2014
• 1

 

[paulmars]

too blurry, so i cant read. I may not matter anyway. Since your asking if this would work. I dont know if it would work. I dont know if this formula would give me an accurate result.

anyone else?

Also collar ties are not meant to stop rafter spread. Does above formula factor in dead load?

pa

 

[noseall]

anyone else?

Get some of those green-backs out of your wallet and hire an S.E.

 

[RonnyRaygun]

It's simple trigonometry, so once you have worked out the vertical load on the rafters, you can resolve to find the compression in the rafter (hypotenuse) and the horizontal thrust.

So if vertical = opp, and horizontal = adj, adj=(opp/tan theta)

That will give you your horizontal thrust, then you've got to work out the size of the cable required, and how you are going to connect it to the end of the rafter.

Simple