To fill the large spaces between the Eucalyptus beams, we built another double reciprocal roof offset from the first using bamboo. This was easy as we already had the shape, pitch, and support of the wooden beams below. The bamboo beams are not yet connected to the wood, only to themselves. We will put in columns next, to the concrete bond beam, to raise the bamboo structure slightly and then we’ll weight down the bamboo to touch the eucalyptus. With the Bamboo structure slightly higher than the Eucalyptus structure, the cladding will create 12 shallow barrel arches, it should look a bit like a circus tent, or a hot air balloon, or a shell, we hope.

We hung 50kg earthbags from the tips of each of the outer reciprocal beams for a few reasons.

Firstly it raised up the back end of outer reciprocal beams, which are connected to the columns with a joint that allows them the see-saw, thus lowering the tips onto the inner reciprocal beams to create the double reciprocal structure. This allows us to put in 12 outer columns from the bottoms of the beams down the the concrete bond beam.

Next, it pre-stresses the structure. We plan to use ferro-cement to cover the roof but the weight of the mortar will compress the structure during application, so we are pre-stressing to achieve the final shape and removing the weights as we lift the mortar.

Finally, it is the first step in testing the strength of the roof. More weights will be added later.

We connected the tips of the outer beams with the inner beams using two 7″ nails in each connection, they will also be tied with thick wire.

With extra supports on beams 1, 5 and 9, removing the Charlie stick was as easy as opening the legs to lower it down. The next 2 hours involved a process of lowering one support at a time to slowly lower the first reciprocal beam onto the last so that they all support each other. On beam one we had a car jack which could be lowered easily. Supports 5 and 9 required a little extra love, which involved digging an inch deep hole next to the base and kicking it in so that the other two supports took up the slack. This worked well, at all times 2 of the supports were taking the weight of the beams, so we worked around each one, lowering them a bit at a time until the first beam sat in the notch of the final beam. Then, with a little more lowering all three supports came free and the roof is reciprocally supported.

We then used one of the supports and the jack to raise the beams where necessary, allowing us to twist and reposition each beam slightly, using the level lines that were drawn on the tips during preparations to get all the notches in the right places.

Raising the Inner Reciprocal beams took two days, it was a process of lifting the back end using a rope over the top of a column, and the tip using a pulley on the tripod. The first beam was held in place by a ‘Charlie’ stick, two poles connected at the top to create an A-frame, that can be closed or opened to raise point of the first beam. Positioning it was surprisingly quick, we got lucky. Line 3 (see previous post) on the tip of the beam had to be directly over the circle drawn on the floor, 636.6cm high… approximately. This was the height of the columns (5m) plus the pitch of the roof (106.6cm) plus a little extra (30cm) so that the final beam had space to slot under the first before we lowered the charlie stick.

We also placed support poles on beams 5, 9 and next to the charlie on beam 1 so that the charlie stick wasn’t taking all the weight and so that we could lower each a bit at a time to settle the roof.

We simulated the position of the inner reciprocal beams on the ground before raising them. The usual procedure is to approximately notch where the beams will overlap and then hope that they settle into place once raised. Instead we positioned them exactly as they would be so we could cut the correct angle on the notch. Using a metal sheet as a level, we marked a 1.5m circle which will be the size of the central sky light. We then placed 4 marks on each beam, which were determined using cad software.

1. 30 cm from the bottom – where the beam will sit on the column.

2. 487.5 cm from line 1 – where the notch will be cut

3. 40 cm from line 2 – where the beam sits in the notch of the next beam along

4. 40 cm from line 3 where we will cut the beam off later (we are leaving them long for now, its not much extra weight and it will help with lifting them

We then placed the base of beam 1 by the base of column 1, and using a plumb line we aligned lines 2 & 3 over the skylight circle. A temporary ‘charlie’ stick of 106.6cm was placed under line 3 to simulate the roof pitch.

Next we cut a notch at line 2 and placed the second beam in the notch, adjusting as necessary until line 3 of this beam was 106.6cm over the metal sheet, again using a plumb line. Finally we marked a level line on the base and tip of each beam so we could get the notch on top easily when reassembling. We continued around the circle in this manner until we were at beam 9, when we took it all down repeat the process from the beginning for the final three beams. With the roof in its final position it would be impossible to slot in the final beam, hence dismantling the first 9. When we raise the roof the charlie stick will be 30cm higher than necessary to leave a gap over beam 11 through which beam 12 can slot, under beam 1; more on this later.

 

In the other photos you can see our 6m long drill bit, a standard 1/2″ bit welded onto 6m or rebar, which we’re using to drill holes through the bamboo so we can pour in borax solution to preserve them. You can also see our new rope harness being tested, thanks James.

We laid out the inner reciprocal beams on the floor first and simulated their position in order to cut out grooves in the right places, safely. More photos are on their way.

We’re going to make a double reciprocal roof, like this one (wikipedia) but with extra columns supporting the inner reciprocal beams. This way if any element fails the weight can be taken by a neighboring column until things can be fixed.

In order to do this we raised the outer beams first, using a joint that allows the ends to move up and down (like a seesaw). The back end is held in place temporarily with bamboo whilst we raise the inner reciprocal beams. Columns will be added later down to the concrete bond beam instead of the bamboo, but we don’t know their length until the whole thing is built so they’ll have to wait.