The Physics of Christmas Decorations

[tonyski]

Greetings all,

I am using a wire cable to suspend some Christmas decorations across my driveway. Total span is about 30 ft.

One end attaches to the exterior brick wall of my house. the other end goes through a pulley that is suspended from a huge oak tree and then down to a come-along(hand winch, cable puller) attached to the base of the tree. The weight of the decorations is about 40-50 lbs which are evenly distributed along the middle eight feet of the 30 ft span.

I am concerned about the load on the anchor point on the brick wall. Am I correct in my assumption that if I used a piece of angle Iron and anchored it at two points and then attached the cable to the angle iron, the load on each anchor point would be half of the total?
Thanks in advance,
Tony

 

[m.e.t.a.]

Yes, in theory, the load on each anchor point should be half the tension in the wire—if the wire pulls on each anchor point equally. In practice, the load will likely be divided asymmetrically between the anchor points, the degree of asymmetry depending on the exact anchoring setup that you use. I am not completely clear on what your angle-iron setup entails, but my instinct is that it may not be the best way to divide the load.

There is a brief guide here showing some of the methods rock climbers use to divide the tension between their ropes and anchor points. Perhaps these principles can be applied to your situation? http://www.ukclimbing.com/articles/page.php?id=2592

 

[tonyski]

Thank you m.e.t.a..

I'll try to describe the angle iron setup. I envision using a 12-16 inch piece of angle iron anchored into the brick wall at two points. Then I'll attach the wire that spans the driveway at the center point. The wire is almost horizontal but comes off of the wall at about a 15 - 20 degree angle.

Do you feel that using 2 slings as shown in the article is a better choice than using a rigid bar across the two points?