# Some Tough Problems (for me at least)

OK,

I take the "contact forces" to mean you have a single horizontal reaction at the ridge and a single vertical reaction at the connection to the wall. I would not think any joints to be moment resisting in this problem.

The snow should be modeled as a single concentrated load at the midpoint of the rafter acting vertically only. The problem should be easily solved by isolating a free body of one of the rafters. You find the point where the wall vertical force and the horizontal force at the ridge would intersect (somewhere in the right corner of the picture). Now, sum moments about this point and you will find the force required in the cross-tie.

Most wood framed houses are basically this same setup. There are pinned connections at the ridge, cross-tie, and at the rafter to wall intersection. The roof diaphram keeps the building laterally stable by distributing the lateral loads to the shearwalls at the ends of the house.

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I found a solution on the internet somewhere, I'm posting what I did,
http://www.freewebtown.com/qdn745/ISP4C.JPG [Broken]
and here's the link to the solutions (hard to see the diagram though).
physics.ucsd.edu/students/courses/fall2005/physics2aa/Solutions_to_Suggested_Problems_Week_9.pdf

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Pyrrhus
Homework Helper
Wow, i was pretty close, except for a few simplifying they did, like considering only one horizontal component at the ridge (haynewp got this one). I don't think they were pretty clear, though, anyway if you would have solved the system like i suggested it would have converged to the same solution, except it would be more work. Nevertheless, nice job!.

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If anyone's interested, I can post my answer for the last question tommorow.

You could even reduce the last problem down to a single step, your final equation F3=2.4m(170kg)(9.8)/0.8m can be gotten directly by summing moments about the top left corner of picture, where F1 and F2 intersect.

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Here'y my answer for the final question.
http://www.freewebtown.com/qdn745/ISP5.JPG [Broken]
For parts b and c, all you have to do is sub the value of the angle in the eqns.

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