Random Neural Misfirings

Having taken the bjurts out to Flipside, I have updated these notes to reflect my additional experiences

I am a theme-camp lead for Burning Flipside and one point of dissatisfaction with my camp has always been our shade structures. For the past few years, we have made do with a few cheaply made carport canopies lashed together. These drip water in between when it rains, don’t cut the heat effectively when it is very hot, and are interspersed with poles, breaking up what should be a communal gathering place into cramped zones. I had been casting about looking for something better. Domes are an obvious candidate, but they are a pain in the ass to make, and a pain in the ass to assemble on-site. After a while, I ran across bjurts, a plan for a shade structure designed by a burner to stand up to the harsh conditions at Burning Man. These seemed perfect, except for one drawback—they’re kind of small. Sizes can be varied somewhat, but the biggest calculated plan is 17′ in diameter. Not big enough to replace three 10′x20′ carports.

I corresponded with Bender (the designer of the bjurt) about ideas for making bigger bjurts, and other possible modifications, and he wound up providing me with a standard set of connectors for an 8-sided bjurt, and also a connector kit to build a giant 16-sided bjurt. This 16-sider is unknown territory for Bender and me.

We wound up dimensioning the small bjurt with a 12′ diameter, the big one 24′. This allows both of them to use some tube sizes in common, which simplified ordering and fabrication.

After much cutting, drilling, grinding, improvising, and a little bit of intemperate hammering, Gwen, some friends, and I have gotten both the big and small assembled. In case anyone else is considering doing this, I am writing up some construction notes. I have also posted some photos of the construction process to flickr.

Bender bases his dimensions on the assumption that you will fabricate your bjurt using standard EMT (conduit pipe). On advice from John, I ordered tubing in quantity from US Wholesale Pipe and Tube. The tubing I received is stronger and harder than EMT, although not necessarily stiffer. It came mostly in 24′ lengths, which allowed us to come up with a cut plan that produced less waste than 10′ EMT would have—and was cheaper, and delivered to my door.

Measuring tips

Again on advice from John, we measured out our cut lines and drill marks using an 8′ piece of ½” angle iron. We marked all our various intended dimensions on the angle iron, laid that on the tubes, and transferred the marks to the tubes. This made it much easier to make collinear drill-hole marks, which is very important for building a bjurt.

Cutting tips

We did our cutting using a mixture of handheld pipe cutters and a jigsaw. The jigsaw has the advantage of speed; the pipe cutter, precision. Don’t cheap out on pipe cutters: we used a $10 model that is worthless, and a $25 model that works great—but some of the tubes we cut are too big for its 1⅛” maximum diameter. Using a vise-grip to hold onto the tube you are cutting with pipe cutters makes the work a lot easier.

Drilling tips

Drilling tubes requires a drill press. Period. Fortunately, you can get a serviceable one for about $60 from Harbor Freight. Drilling properly centered tubes (again, important for a bjurt) requires not only a drill press, but some kind of jig to hold the tube in place. It doesn’t need to be anything fancy: we put together jigs made out of a few plywood scraps to create a channel that held the tubes exactly in place, and clamped the jigs to the drill-press table. We elevated the far end of the tube with bricks and junk to hold it level with the jig. Drilling into metal is a bit tricky, as the bit tends to wander before it penetrates. I found that a “pilot point” drill bit from De Walt did a pretty good job. The drilling process produces a huge amount of metal tailings. I kept an old paintbrush and a small bucket handy, and cleaned my workspace every couple holes. Tailings would often wind their way around the drill bit and need to be cleaned out.

Assembly notes

For a standard 8-sided bjurt, connect 16 long poles together at their midpoints scissorlike, into 8 pairs. Connect one upper and one lower bjurt connector to one side of a pair. Then connect a short pole between the upper and lower connectors. Then connect another scissoring pair to the other side of the connectors. Continue until the walls are complete—they’ll stand on their own after only two pairs are together. Next lay the crown truss in the middle and attach 8 long poles to it. Lift up the truss so that it stands on the poles like a spider. Lift up the distal end of one pole and attach it to the “claw” on the upper connector. Attach opposite poles in the order N-S-E-W, and then fill in NE, SW, NW, SE.

One way in which I diverged from Bender’s original plans is with the perimeter poles: in his plans, short poles run horizontally from the top of one upper connector to the next. I realized that these are loaded only in tension, and so I have replaced them with a cable—much lighter and easier to pack (hopefully not to forget). This perimeter cable prevents the weight of the roof from pushing out the tops of the walls.

The 8-sider went together in only about 90 minutes, with me working alone, and despite what I feared were some inaccurately drilled holes, everything went together with a minimum of pounding and cussing. Even more amazing is the knockdown, which takes a only few minutes, and results in a compact, portable bundle of tubes. Subsequent deployments likewise take only a few minutes.

Bender recommends what he calls “hitch clips” (and what McMaster Carr calls “safety pins”—seriously) as fixtures. Unfortunately, they’re expensive. Cottered clevis pins are a cheaper (but not cheap) option, and one that I’ve discovered I can’t recommend: the cotters can accidentally drop out. In the interest of optimizing time and money, I used bolts and locknuts for the permanent connections, and hitch clips for the connections that need to be made in the field. This worked well at Flipside.

The monster

The 16-sider is big. Really big. Like, “half the size of my house and barely able to fit into my backyard” big. The tubing for it weighs about 350 lb; the tarp (once it is created) will weigh about 100 lb. The big one was much more than twice the amount of work as the small one. Just keeping track of the number of fixtures we’d need taxed my feeble math ability.

The most important difference, aside from the number of sides, is that the roof poles need to be twice as long. The wall modules are the same (in our case, constructed of 5′ and 7′ 16-gauge tubes of .75″ diameter), but for roof poles we used 1⅛” 16-gauge tubes. Because the roof poles alone will weigh about 150 lb, in addition to being too long to transport, we are using half-poles. Two half-poles of 7′ are joined by a 1″ diameter wooden dowel on the inside, and a 12″ x 1.315″ reinforcing sleeve on the outside; these fasten to the half-poles on both sides of the joint. Ensuring collinearity on these was daunting. Our workflow was as follows: Insert dowels and drill out one set of half-poles at both ends. Drill out the reinforcing sleeves at both ends. Drill out one set of half poles only at the end opposite the joint. Attach the sleeves to the poles that have the dowels. Mate a half-pole with sleeve and its counterpart. Align the hole in the sleeve with a collinearity mark on the tube within, and drill through the interior tube. This ensures that the holes at the opposite ends of the joined-up pole are collinear.

Test assembly was also daunting, but we managed using three people. It would definitely go a lot faster with more hands, but I think that with practice, two people could manage fairly quickly.

Update Having done this several times now, I feel you really need a minimum of three people, and a fourth person is very helpful.

The first steps in assembly are similar to the small bjurt: set up the walls, and connect the medial half-poles to the crown truss lying on the ground. Because we were using a perimeter cable with a turnbuckle instead of perimeter poles, the cable should be attached at this step with the turnbuckle opened up to create some slack.

The next step involved some trial-and-error for us. This is what seems to work best: stand the crown truss on the half-poles so the poles are nearly vertical. Pull out a half-pole and attach its mating distal half-pole; the outside tip should be lying near a wall corner. Finish attaching all the roof poles starting with N-S-E-W and continuing to fill in with sets of four separated by 90°: you want to make sure that as you get near the end, the structure is standing on four poles each separated by 90°, rather than having all the support clustered on one side.

Now attach the canopy. In our case, because the canopy was laced to the roof along two roof poles, this was the slowest step. It also required a ladder to lace near the peak. Having only one ladder was a chokepoint in our worfklow. I would not recommend attempting assembly under windy conditions.

Then attach the roof poles to the walls following the same pattern used to mate up the half-poles. After getting a few poles attached, it helps to raise the peak of the roof by grabbing some of the unattached poles and walking them in: this is to position the poles so that when they get raised up to the wall connectors, they’re in the right spot. Getting the last few poles into place may take some elbow grease. Once everything is in place, tighten the turnbuckle and stake down every corner. Candy-cane rebar works fine. I can imagine needing to begin staking it before every roof pole is attached, but I would recommend only staking down corners where the roof poles have already been attached, as parts tend to shift.

Fabricating the cable during initial assembly is tricky. Fortunately, you only need to do it once. This is not what we did, but it is what I’d recommend: Prepare one end of the cable. Go through the above steps and attach the roof poles to the walls without the canopy. Measure the perimeter. Work quickly: the tops of the walls are being pushed out by the roof poles, and for this reason, it may make more sense to measure around the base. Detach four roof poles from the walls at N-S-E-W so that they support the roof from the ground and take pressure off the walls. Prepare the other end of the cable. Using a turnbuckle gives you some margin for error, but don’t rely on that, and don’t forget to account for the length of the turnbuckle when finishing the cable. Attach the cable to the walls, with the turnbuckle loosened up to create slack. Re-attach the four roof poles and tighten the turnbuckle to confirm fit. For a cable, we initially used a 3/32″ stainless cable rated to 1200 lb, but my worst-case calculations were that up to 1700 lb of wind force could be exerted on the canopy, so I wound up getting a 1/4″ cable rated to 7000 lb. In hindsight, this is probably overkill, since something else would probably fail before the smaller cable would anyhow, but I don’t mind the added security. The turnbuckle also needs to have a high load rating. I found one at McMaster Carr.