What does the Dock "A" Cable do?

Rattlesnake Guy · 10-06-2010, 10:05 AM

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.

10-06-2010, 10:05 AM	#1
Rattlesnake Guy Senior Member Join Date: Sep 2004 Posts: 1,254 Thanks: 423 Thanked 366 Times in 175 Posts	What does the Dock "A" Cable do? 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.