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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RGuy,
My guess is that cable is not in compression, but rather has tension on it. It is under neath providing resistance to the top of the A frame as it pulls back. And as you mentioned is precisely for making sure the A frame itself doesn't buckle and fold back on itself. I would also guess that its importance is in direct relation to the Vertical stance of the a frame as it raises. More tension the closer to Horizontal it is, with less tension the closer to vertical it gets.