Article 009: Space structures (2).

In the second article of this series on expo 2008, we talked about space structures and now we are going to carry on with this topic. I am aware the the ordering of the articles may seem somehow illogic, but as I am very busy, the order is determined by the availability of time and resources to write them. If I have more time in the future, perhaps I can review the articles and order them in a more sensible way.

As you could see in the 7th article, the plaza’s space structure is already in place.

Picture 1: Cities on water. Space structure completed.

We crossed this area while they were lifting the penultimate section of the space structure, which was almost at its final position.

Picture 2: Cities on water: Lifting a space structure.

Picture 3: Cities on water: Detail of the nylon stripes used to lift the space structures.

Picture 4: Cities on water: Detail of the space structure leaning on the main metallic structure.

Just one last structure remained on the ground, waiting to be lifted. Notice, on the bottom right corner of the picture, that the bars do not end at nodes, they are loose. These will be the bars that the workers will join once the structure is lifted.

Picture 5: Cities on water: Assembled space structure laying on the ground.

After lifting the space structure to its final position, the workers assemble it with the rest of structures already in place. Being work at height, this operation is not free of hazards. The workers either work from a platform or directly on the structure itself. If the work is done from a platform, the workers are securely attached to it by a harness. If the work is done on the structure, they wear two harnesses, attached to two different points. Every time a worker moves on the structure, he opens one harness and moves while the other harness remains fixed. This is a tedious manoeuvre, but one that keeps workers safe at all time.

Picture 6: Cities on water: Worker joining sections of the space structure.

Picture 7: Cities on water: Worker on a platform. Detail of harness.

Picture 8: Cities on water: Worker on the space structure.

I commented in my previous article that assembling this structure is very straightforward: note the alphabetic labelling on the bars that eases the worker’s task.

Picture 9: Cities on water. Space structure, bar HJ

Picture 10: Cities on water. Space structure, bar X

Picture 11: Cities on water. Space structure, detail of a node assembled.

Picture 12: Schematic view of a node.

Once the space structure is in place, it will undertake a number of controls. The first one is performed by staff of the installer company, who will verify with a dynamometric key that every screw is tightened with a defined torque.

Picture 13: Dynamometric key.

As this is a little bit obscure for those not familiar with construction engineering, I’m trying to explain it in plain English. When we tight a nut with a key, we just tight it as much as we can. This may be good enough from some joints, but it is clear that it is not a scientific method: if someone is not strong enough, the joint might eventually get loose. Or, someone too strong and reckless, might wear  off the thread.

To avoid subjectivity in the strength applied, engineers invented the “dynamometric key”, which locks when the pre-set strength is reached and therefore ensures that al the elements are tightened to with precision.

You have seen something similar every time you have your tires changes at a garage. They set them with a “pistol” that locks when the tire is too tight. This is not a dynamometric key, but it will do as an example.

Anyway, as I was saying, the installer company verifies that 100% of the joints have been tightened with the right strength. First they apply a dynamometric key, calibrated to a point just below the specified and checking that it locks. Then they recalibrate the key just above the desired point and verify that it doesn’t lock.

The client, in this case Expo, makes a statistical control of the joints to verify that the controls preformed by the installer company are correct and, once this last control is satisfied, the structure is painted on its final colour.