Zang here, structural engineer for Suspended Animation
. We have a lot of experience in tying things to other things, and in particular I have a lot of experience in structural rigging -- including helping other camps tie off their tarps so they don't shred.
Your biggest problem won't be how to secure the grommets. Your biggest problem will be avoiding what's known as static rigging.
Static rigging is where you connect to the grommet and then make a single straight line to your structure. What this does is create a yank point on your tarp, and it will slowly degrade and ultimately shred. Using shock cord (a.k.a. bungee) helps with this, until some serious wind comes up and it hits the maximum stretch on the cord, and then you have a yank point again.
The solution is to borrow from nautical rigging, and go what what's know as running rigging.
Imagine you have a tarp with 6 grommets along the edge. The static rigging solution is to connect six individual lines, one at each grommet, and then connect that to your structure. The running rigging solution is to use a single line, tied off to the structure at one end. Then, run the line through a grommet and back again to the structure; run it around a pipe in your structure, then back to the next grommet, then back around the structure, and so on. The result should look something like this: WWW
It is important that you don't tie off the line at any point. You simply want to run the line through the grommet and back again; then a half loop around your structure and back again. This allows the line to shift a bit which is exactly what you want. When you tie off all the edges of your tarp this way all the force will be equally distributed across your grommets, and the shift in the line will help absorb the shocks. Put plenty of tension on the lines when you tie them -- slack in the lines will allow your tarp to flutter and it will yank free.
I strongly recommend paracord for this application. It's cheap, strong, light, and nice and stretchy.
Two final thoughts:
- I've seen structures use both bungee & ball ties, and running rigging. The bungee & ball solution failed catastrophically; the running rigging stayed solid. A lot of people swear by bungee cord but years of experience on the playa have proven the running rigging solution to be far superior. Trust me.
- Trying to explain this in text is a little hard, so if you're not 100% on how to do this drop me a line at zang (@) suspendedanimationcrew.com and I'll gladly explain in person.