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Tension Cable Backup Plan?

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Old 08-21-2016, 02:18 AM   #1
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Default tension cable backup plan?

My 30' came with a 3/16" (+ vinyl) 4000lb

tension cable

.

A snow load can weight 10s of thousands of pounds..

Not only do I notice that some 30' manufacturers supply 1/4" 7000lb cable, but I'd like to hang some stuff (bed? 1/2 a loft? potentially a basement access trap door foolishly built to weight 600 lbs?) from a 12' hoop I'll attach to all the rafters.

It seems prudent, since the cable is a single point of failure, to add a backup, so I bought enough 1/4 cable to string the cable around through a new set of holes just inside the snow load studs.

Is this a terrible idea?

Does anyone here understand the math behind calculating the tension on the cable given all the mechanical advantages?

Thank you,
AH

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Old 08-21-2016, 05:40 PM   #2
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Default Re: tension cable backup plan?

I would think the construction of a loft would not add much weight to the cables, rather most of that weight would be born by the studs and whatever vertical supports you add. I know some people sister the joists to the rafters but I have heard more than one manufacturer recommend against that.
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Old 08-22-2016, 01:29 PM   #3
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Default Re: tension cable backup plan?

Thanks for the reply, Jafo!

I could have a thousand lbs hanging from the rafters with no vertical supports in the center, only suspension wires. I guess I'm asking if you can think of a reason why tensioning another perimeter cable would be a *bad* idea.

Thanks!
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Old 08-23-2016, 04:17 PM   #4
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Default Re: tension cable backup plan?

How much math do you want?
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Old 08-23-2016, 06:01 PM   #5
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Default Re: tension cable backup plan?

Quote:
Originally Posted by hierony View Post
How much math do you want?
Great question. Whadaya got?

One could naively say that if you have enough rafters and dense enough lattice, you could tie the whole thing together with dental floss. But the closer you get to a circle, it seems the "pulling sideways on a taught cable" deflection angle approaches 0 and the force approaches infinity. I'm interested in where on that line my 3/16" cable is with a couple thousand pounds of load in the center. It does seem that the wedging of the cable into the lattice is probably the weakest part..?
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Old 08-23-2016, 10:03 PM   #6
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Default Re: tension cable backup plan?

I don't like differential equations much, but could manage if I were able to find that one reference book I once paged though. As long as I don't have to do very complex systems I don't mind a little engineering, but I'm more of a physicist and like relatively crude estimates :P Sometimes I miss important details...

I did rough conceptual work for the crown ring, moderately thorough work on the roof poles, somewhat decent conceptual design of the lattice, and generally neglected the tension cord. I realized center posts are very handy & really strong (basically doubles roof load capacity). But it would be good to revisit everything again. I'll write some stuff up once I go through it all and post an image of my calc's.

Re: dental floss. The lattice only carries the vertical load. All the roof poles are pushing in line with their inclination--you have to resolve that force into two components, one vertical/downward (which the lattice holds), and one horizontal/radially outward (which the tension band holds). Dental floss is decently strong stuff--to use it, you'd want to braid it into a pretty hefty cord (tensile strength is proportional to material cross sectional area, if I recall) :P Dental floss is nylon, which has an Ultimate tensile strength of 45-900 MPa; steel is ~400-900 MPa or 250-2600 MPa (depending on the reference table/steel/nylon). Assuming the steel is 10x stronger, your floss tension band would likely be ~3.6 times the steel cable diameter (2*sqrt(strength factor/pi) for the same total strength.

I discourage people from adding loads to their roof poles. Most of the time it will be fine, but then there's that one windstorm, or that one blizzard, or your climbing on your roof, and something breaks because of your additional load. Lofts aren't light--a bed frame with the fixings could be 200 lbs, plus the weight of a human or two, maybe three? Then throw in the dynamic loads of a person jumping on the bed--you could have 600 lbs force, maybe double or triple that on two to four rafters (highly dependent on your attachment details). That plus most modern yurts use a reduced number of rafters (bigger & stronger than traditional roof poles), which means it's harder to distribute the loads evenly (unless it's applied to something like the outer canvas). So I don't like the idea and generally suggest people talk to their yurt manufacturer/engineer for more of an official 'no'.
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Old 08-24-2016, 08:20 AM   #7
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Default Re: tension cable backup plan?

I would think vertical supports would be a must. Every loft I have ever seen pics of, the yurt had studs and the there was at least one vertical support from the foundation to the loft, somewhere near the center of the yurt.
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Old 08-25-2016, 12:52 AM   #8
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Default Re: tension cable backup plan?

Quote:
Originally Posted by hierony View Post
I don't like differential equations much, but could manage if I were able to find that one reference book I once paged though. As long as I don't have to do very complex systems I don't mind a little engineering, but I'm more of a physicist and like relatively crude estimates :P Sometimes I miss important details...

I did rough conceptual work for the crown ring, moderately thorough work on the roof poles, somewhat decent conceptual design of the lattice, and generally neglected the tension cord. I realized center posts are very handy & really strong (basically doubles roof load capacity). But it would be good to revisit everything again. I'll write some stuff up once I go through it all and post an image of my calc's.

Re: dental floss. The lattice only carries the vertical load. All the roof poles are pushing in line with their inclination--you have to resolve that force into two components, one vertical/downward (which the lattice holds), and one horizontal/radially outward (which the tension band holds). Dental floss is decently strong stuff--to use it, you'd want to braid it into a pretty hefty cord (tensile strength is proportional to material cross sectional area, if I recall) :P Dental floss is nylon, which has an Ultimate tensile strength of 45-900 MPa; steel is ~400-900 MPa or 250-2600 MPa (depending on the reference table/steel/nylon). Assuming the steel is 10x stronger, your floss tension band would likely be ~3.6 times the steel cable diameter (2*sqrt(strength factor/pi) for the same total strength.

I discourage people from adding loads to their roof poles. Most of the time it will be fine, but then there's that one windstorm, or that one blizzard, or your climbing on your roof, and something breaks because of your additional load. Lofts aren't light--a bed frame with the fixings could be 200 lbs, plus the weight of a human or two, maybe three? Then throw in the dynamic loads of a person jumping on the bed--you could have 600 lbs force, maybe double or triple that on two to four rafters (highly dependent on your attachment details). That plus most modern yurts use a reduced number of rafters (bigger & stronger than traditional roof poles), which means it's harder to distribute the loads evenly (unless it's applied to something like the outer canvas). So I don't like the idea and generally suggest people talk to their yurt manufacturer/engineer for more of an official 'no'.
I enjoyed your response. I hesitated to ramble ignorantly, but my estimation was take the load, multiply by 3 for the rafter angle, divide by 48 rafters, then myltiply by about 10 for the perimeter deflection of 1 inch between rafter and lattice. I suspect that every place the cable changes direction is a place where force is applied sideways, and therefore, multiplied in for that particular load.
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