Quote:
Originally Posted by Rattlesnake Guy
I play a mechanical engineer on TV but this one has me baffled.
We are making an "A" to compensate for a tree that used to faithfully hold up our dock all winter that recently died. 80+ year old hemlock who's rings amazingly document the various growing conditions over the decades on the edge of the water.
Anyway, I have always been puzzled by the nearly universal practice of the incorporation of a compression cable you often see running below the center line of the A from top to bottom. Offset below about 1 foot, creating a bit of a suspension bridge.
I have struggled with various explanations for what the function of this stressing is for. Does it improve crumple resistance? Why does stressing the frame in one way do that? Does it help twist? Is the cable tight? How tight?
My most reasonable explanation...does it compensate for gravitational frame sag which might reduce compressive strength and initiate collapse?
Thanks for any help in quieting my obsession as I install one and waiting 6 months to see if I was successful.
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It puts a tension member in the frame. If you pull up your dock without this line attached and site the A frame you will notice about 4-5inches of sag in the A frame. Then install the tension line and see that that sag reversed into a slight bow after tensioning it. Its sole purpose is to remove the sag, which could in fact cause the frame to buckle.
A structural truss always has two forces acting on it, compression and tension. Without that line everything is in compression and as an engineer, you know what happens when a balance of forces is not achieved.
The key element is that it is held away from the frame with a point member. Otherwise it is not doing anything.
A simple explanation: take a stick that is nice and straight, put your foot on one end and pull the other end toward you. The center of the stick bends away from you. Now try the same thing with a strung stick (like a Long Bow,
http://en.wikipedia.org/wiki/File:Longbow.jpg with the center of the stick bent toward you), it will not react the same way and you will end up hurting yourself trying to bend it the wrong way. Add in the three point connection for rotational stability and you have the same thing as your dock support A frame.
Hope that helps.