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

 

[BAretired]

http://mathscinotes.wordpress.com/2010/11/29/the-mathematics-of-rafter-and-collar-ties/

BA

 

[XR250]

1/2 * vertical reaction at rafter support * run / rise

Very simple statics problem.
Do a free body diagram of one rafter (there is no vertical support at the ridge)

 

[paulmars]

huh?

ref: Do a free body diagram of one rafter (there is no vertical support at the ridge)

 

[BAretired]

There is horizontal support at the ridge. So you can calculate all that you need to know.

BA

 

[paulmars]

that helps, thanks.

ref: There is horizontal support at the ridge. So you can calculate all that you need to know.