In 2004 or was it 2003 when our cheap shit pop-up shade structure was whirled instantly into a ball of lousy plastic and shards of sharp broken aluminum,it occurred to me that we needed something slightly better.
Where to find such a thing? We sought out help using the intertubes whereby we found a previous burners’experience to swipe from. Alas,I kept lousy notes and do not have this person(s) name or intertube earl. [July 2009 note:mykl found the original inspiration site: MaxIcon's "PVC on the Playa" page]
2007 was the third time we used this thing,and it’s held up pretty well. We never lost it though,it survived 70mph blasts on a couple of occasions.
The thing is made out of a bunch (you count ‘em) of 10-foot one-inch white PVC pipe,with one length of 1-1/2″chopped into foot long pieces for use as couplers. There is actual cleverness in the structure. The parts list as best I can recall is:
* 16? 20? 10-foot lengths of PVC pipe,1" * 1 10-foot length of PVC pipe,1-1/2" * Bunch of used bicycle inner tubes * Bunch of 2-foot lengths of rebar,5/8" ideal,1/2" will do * Big washers welded to some rebar,see photos * Tarp for the floor,see text * Tarp for the skin,see text * Bunch of trucker tie-downs * Two ratchet type load tie-down straps
The [pictures] tell the story. (Or if it doesn’t,tough luck,that’s all you get.) Here’s the high points,if they aren’t obvious from the photos. The rebar locates the hoops,and does a fair,but not adequate,job of holding the structure down to the earth. Anywhere but the playa they would be more than enough!
The bicycle-tube rubber bands and the slip couplers are the major tricks. The slip couplers hold absolutely,and have never moved around. There’s no need for pins,through-bolts,etc. Friction is plenty,yes,even on the playa. In fact,the major reason for failure in PVC structures are weak points caused by stress at drilled holes and metal pins! With the slip couplers,stress is distributed evenly over much area.
The rubber bands are the second secret;they tighten under stress! The also distribute load,and allow the structure to flex such that load is distributed over the skin,the hoops and other rubber bands. Rigid bolts and pins concentrate force on a small area.
The major problem with a quonset hut is when wind comes at a 45-degree or so angle to the hut;it applies side and upward load on the skin from the inside. (The hut’s radial asymmetry is the source of the problem,that a full dome doesn’t have.) This lifts the hoops off the rebar and the whole thing loses strength at that point. The hut is fine when the wind is broadside to the skin,or end-on where the wind passes through (obviously). Broadside wind just makes it hunker down and even increases the grip on the rebar that side. Domes are all “broadside”so they don’t generally lift off unless you let the wind inside (oops).
The solution to the lifting problem consists of two parts.
* One, trucker tiedowns (giant rubber bands with metal hooks each end,better than bungie cords and cheaper) on the four corners of the skin tug the skin tight to four rebar driven into the playa. The rebar have washers welded on to hook to0,but you could bend the ends instead. They simply apply rubber-band type force to the skin edges,near the openings,that increase in downward force as the wind lifts.
* Two,at each end of the structure,a ratchet strap pulls the top of the hut down the ground,to another rebar-with-washer,boy-scout-tent-style. While the straps are not flexible at all the structure is. This addition in 2008 totally eliminated the tendency of the end hoops to lift off their rebar at worst-case diagnonal winds;the structure sort of hunkered down,flexed,then popped right back. This isn’t shown in the photos;but see the last one,picture s36. The ratchet strap runs from the top where the hoop meets the longitudinal pole,and runs to the ground at 45 degrees to a rebar.
THEORY OF OPERATION
It wasn’t until after a discussion with a friend about a three-section PVC rib structure that I fully realized just how this two-section rib works. His three-section ribs exhibit a tendency towards turing into an “S”shape,if you picture the rib from the top down. A description of the two-section rib should illustrate.
When you stick two pieces of PVC into the slip joiner,bend it,and drop it onto the rebar anchors,the bending force concentrates at the joiner;the rib isn’t hemispherical,but more or less paraboloid. The PVC pipes are two levers in opposition,with a long section (outside the joiner) and a short section (inside the joiner). The fulcrum for the two levers is the outside edge of the joiner. Bent,the long section pries the short section with a LOT of leverage,so there is a very large amount of force inside that joiner! It doesn’t break because everything is squishy and deforms for maximum contact and distribution of force.
(Of course it’s not really that simple;there is no single fulcrum,the long and short levers and the position of the “fulcrum”is distributed over the length,the non-linear distribution of forces of course causing the parabola shape.)
The bottom line is,it acts like a single section of pipe,only better. One long piece of pipe bent in the same way would also form a paraboloid —there’s the same long-lever vs. short-lever thing going on —only the forces would concentrate in the very center of the pipe and it would shear-fail and pinch. The joiner and two sections is therefore actually stronger than a single piece;it’s thicker right where it is needed,and the forces on the end of the sections distribute inside the joiner.
The structure is of course multiple ribs in parallel,with a skin on it. The ribs are bent as described,and each end sits on a rebar anchor. Now imagine wind loading the structure on one side,from an angle (the worst case more or less). The skin pushes the rib sideways;it begins to fold over. However,pushing on the rib sideways has the same effect as pushing downward (the bending,above) just a different direction. The long-lever pries on the short-lever in the joiner,and transmits this torque to the other half of the rib,in the lee of the wind.
Each rib is also connected to all of the other ribs via the skin. Skin flexes and ribs bend to equalize tight and loose spots. The structure sort of shrugs to one side under severe wind load;each rib is twisting,pivoting at the jointer.
The ridge line further strengthens the structure by preventing the ribs from falling to one side or the other;it substantially ties the lateral wind load on the skin to all of the ribs,and at the point of maximum leverage (the apex). Further,since the joiners are short and fat they are very strong;the rubber bands flex yet tighten under load.
A three-section rib puts the joiners at the worst-possible location;the low-force long-lever arm. Since there is so little friction inside the joiner,the PVC pipes are free to simply rotate at the joiner;the wind side load simply bends the section on that side away from it;with the bottom anchored to the ground,the higher end pushes on the top section,which pivots over it’s length,and transmits some of the force to the section in the lee of the wind;it bends in the opposite direction,creating the “S”shape.
Since there is so little friction in the system,the ribs just lightly stick where the most recent amount of force left it —in an “S”shape. In a two-section rib,the forces distribute in such a manner that the ribs store the energy from the wind and release it when the wind subsides,restoring the shape.
There is some permanent bending in the two-section ribs;tey all take a faint “candy cane”shape at the top. Three successive years of use doesn’t seem to show any side effects.