How much tension under tarp tieouts?

[Mario Caceres]

I’m working on a tarp-poncho made out of silpoly 1.1oz PU4000. Knowing that one of the weakness of this fabric is its tear strength (The trade-off is a much higher aged HH rating of 3500+ which is very important for me when used in Poncho mode) I have done some catastrophic failure tests for tie-outs (for when it will used in tarp mode).

I found that rolling the hem over twice and sewing the tieout produce failure at 25 lbs.

When I reinforced the tie out section by gluing a  reinforcementt patch up of the same fabric (with the ripstop grid layered in the same direction of the main fabric) and rolled over (twice main fabric, once patch, so 5 layers of fabric at stitch line).  The tieout failed at at the stitch line at 40 lbs.

In this post David Gardner, a fellow BPL member, mentions that he  measured 25lb of tension at the ridge of his tarp. He stated that he pulled on the guy-lines tighter than he normally does but still I assume this is just under regular conditions. I have pitched my tarps under strong winds and when pulling the guyline did certainly felt less tension than when I was pulling the 40lbs in the test mentioned above.

My question is how much tension is reasonable to expect on a small tarp (5×9) assuming the whole panel is exposed to the wind? (i.e in lean-to configuration).

[J-L]

This thread may be helpful to read:

https://backpackinglight.com/forums/topic/100747/

Rather than going for a specific load, I would recommend comparing to a known “good” tarp tie-out. For example, if your tie-out is just as strong (or close to it) as one sewn using standard 30D silnylon (which you know works fine), then it’s probably good enough

Keep in mind though that wind loads are not static loads. They are dynamic, and probably closer to shock events. Most materials will fail earlier when 40lbs are loaded very quickly vs when 40lbs are gradually applied, and it is unknown how the 1.1oz PU4000 will fare over time

[Mario Caceres]

Thank you John. Unfortunately what works for silnylon will not work for Silpoly PU4000 as their properties are different. For example using the same construction techniques (I.e double rolled hem, no reinforcement) on regular 1.1oz silnylon failed at 72lbs (per the article you referenced) while the same construction technique on silpoly PU4000 failed at 25lbs. (per my own testings)

So basically silpoly PU4000 needs a beefier tieout construction. My problem is I don’t know how much to shoot for. So far I have gotten up to 45lb before the silpoly PU4000 failed which I’m leaning to think is enough, but I wanted to check with more experienced folks on this forum before embarking on the construction of the actual tarp/poncho.

Good point about gust winds. I’m planning to deal with that in the same way than David Gardner suggested in the article I mentioned in my original post. Basically a short loop of shockcord to absorb any gust winds. Still not sure what thickness I will use, but something that allows me to put tension on the tarp panels without being fully extended. According to this manufacturer their 3/16” shockcord is rated to 200lbs of tensile strenght.

[Author]

Posts