Spent the morning at Opotiki Primary School with years 6 and 7. We built three bridges with varying degrees of success. The girls built quickest and most accurately but that may be because Jasmin (Granddaughter)had seen it done and knew what trouble you get into if it doesn't fit together cleanly. The other groups seemed to assemble the parts in random order so they didn't fit together well. Before tomorrow I will make the markings firmer so we can see whether they are properly assembled. It would be good to have a square to test things with too.
So, having run a trial, it is necessary to sort out the preparations and make an instruction set.
You will need:
15 pieces of wood, all the same crossection. 10 of them need to be exactly the same length.
The ten should be marked in the middle and the other five marked at each end as shown here.
The end marks are set with three members with a mark close to each end and two with marks close enough together to allow the long pieces to be alternated as shown here:
Once this is sorted out, the groups of three pieces should be fastened together so that the marks are hidden. This can be a temporary fixing. I used two large cable ties to avoid damaging the wood.
When this is done you will have 5 H shaped pieces and the markings should be invisible. If you lay the Hs out with a little clearance, and if they are fastend reasonably square and secure, it is possible for one person to do the assembly. The important thing, once you start, is to make sure it is always working as an arch, ie only supported on the ground at one end and by the cross of an H at the other. Your hand needs to be in the middle of the cross to support it and really, you need to twist the cross piece to maintain contact between the tips of the H and the next cross piece. I will add a pic when I have someone to take it.
Lift the crosspiece at the end of the set in one hand and the centre of the next H in the other, slot them together, put the end on the ground, step over the end crosspiece and repeat.
Once the bridge is erected it could reall do with some plan bracing. For real strength it also really needs more than friction holding the ends together. Have a look at the model bridge here.
Here, the model brige has many more parts (which makes it stronger and more stable but rather more expensive). It has beem built in two parts in cantilever and the two are being lowered together to meet. This requires some pretty slick setting out. I can't really see how it could be done with a full size bridge as there needs to be some inclination for friction to hold the bits together. Unless they used fixings. In the full size bridge, they lashed everything with rope made from bamboo fibres.
If the bridge is put together carefully, it is remarkably stable even without diagonal bracing. Our bridges were made with 50x40mm timber 1120mm long. These are very light so the stability of the arch is less than with heavier pieces. That can be overcome by adding distributed load.
The finished span is about 2.5m and the timbers would certainly not span that far with a load like this:
If you want this to carry serious load, the ends of the long pieces should be cut on a bevel so that they meet neatly. You would also need some half length pieces to finish the second ring and all the lieces would need to sit firmly on a base that shared the load and stopped them slipping. A timber plank on a concrete diagonal face would be good. The span can then be adjusted slightly by pushing the ends up or down the slope.
In New Zealand for a conference, some meetings and a visit to my Grandchildren. This time daughter Liz asked whether I couldn't build something with the kids at school instead of just talking to them.
After a bit of puddling around, I remembered the Rainbow Bridge, and ancient Chinese type. It is made of interlocking logs to form an arch and provides a good demonstration of how you can make long bridges from short pieces with arches.
We went begging for materials and RD1in Opotiki lent us a pile of small battens. THe first trial worked well. Lashing the battens with 300mm cable ties and usng a bit of rope to prevent (or atleast reduce) sway and arch spread. I now need to dismantle it and mark it up for the kids to rebuild it tomorrow (in Opotiki) and Tuesday (in Torere). More pics as the kids get going with building.
THis Image from Nick O'Riordan of Arup in San Fransisco. Pure Magic. He says: You may find the attached image, from a large painting exhibited at the De Young Museum here, an appropriate grace note to Dromgoole’s poem 'The Bridge Builder'.