rafter cable ties

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

 

You might need an engineer for that but (and it's only a thought) If the ceiling is running in same direction, the whole thing is held together with 3 or 4 nails where the joist attach to the outside wall. So what is the shear strength of 8 3" nails