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Tensioning a bridge with liquorice and bearings


Liisa Joensuu/Tmi Magic Words

As autumn arrives, so have the preparations for a thriller of its kind: the bridge cover is drawn onto the basement ceiling. It is a part of the exceptional structure of the library, the indoor bridge, which will support the upper floors. Cables are used to create a tension to the bridge cover, in order to improve the load bearing capacity of the structure. The cable looks like spun steel wire, and it stretches over dozens of metres.

The next thing that happens is a plot twist that couldn’t be anticipated by laymen: after the cables are tightened, the floor is far from stable. However, to engineers, the following steps of the story are familiar: a slight clearance is designed into the bridge cover, in order to make the structure more durable during temperature changes. The clearance required for the bridge indoors is not large, but it should be prepared for. This is the cue for bearings. One of the directors of the play is construction supervisor Kari Suomala, shown in the picture above.

The vault, or ceiling, of the basement is cast supported by the yellow formwork panels, which will be removed once the concrete has dried. The tension cables, which look like liquorice ropes, have been installed between the moulds on their brackets, and the surrounding steel reinforcement will also be covered with concrete. The picture shows the passive ends of the tension cables. This means that the tension will be created in the northern end.

The first scoops of dirt and sand were dug on the plot in September last year. Once the basement was constructed, the site returned above ground. The basement has been hidden underneath the casting moulds and steel reinforcements, and a random passer-by might not guess that they hide the concrete structures, pillars, partition walls, lift shafts, underdrain system, pumping station, driving ramps, and loading yard, as well as many other things. The narrow ditch around the basement is being filled with sand until the level reaches ground level, because 80 per cent of the waterproofing bitumen felt layers and thermal insulations are already in their places.

The mouldings stretch from south to north as one large field. At the moment, the people working with the steel reinforcements are under a great deal of time pressure: castings are being made at an increasing rate, and a steady line of concrete trucks visit the site every day. The picture above shows the site management exchanging information on their observations.

The concrete castings are ongoing in the southern end of the site, which is the ground-supported part. 200 cubic metres of cast concrete is no big feat for any one day. The concrete is floated and smoothed using machinery and as handwork. In front of the Sanomatalo building, excavation machines pick out the parts of the grooved wall, which was used to support the ditch.

Bearings allow the bridge cover to move

When you hear the word ‘bearing’, you might think of rounded ball bearings, but that is not the case with bridge construction.

‘The clearance for the bridge cover is ensured using slab-like bearings, which are similar to those used in bridges for vehicle traffic. In the case of traffic bridges, reservations are also made for a large variation in temperature: the bridge cover will stretch in hot weather, and contract in sub-zero temperatures. Long bridges also have expansion joint equipment, which emits a small bang when it is driven over. Without the expansion joint equipment, a gap of up to 10 cm could form by the seams of the bridge. Such an equipment is not required for the library bridge, because it is placed in a space with a stable temperature,’ explains Site Manager Kyösti Kontio.

One of the more brilliant turns of the plot is placed here, underneath the end housings of the steel bridge which will be built: who would have guessed that slab-like bearings were slid between the housing structure and the footing before the footing was covered by the casting moulds and iron reinforcements? In effect, the housing structure rests over the bearings, and if necessary, can slide for approximately 5 cm. However, that would require an approximately 40 degree change in temperature.

The computer model shows the side profile of the housing structure. The slab-like bearing plates can be seen as the bright green stack in the middle of the picture. They sort of look like a set of chopping boards. A Teflon coating between the steel beam (shown in grey) and the bearings (green) makes the movement easier. Extremely hard grouting concrete has been cast between the largest bearing plate and the concrete footing below it (shown in blue). But there is no cause for concern: even considering the clearance, the library floor is not intended to sway like the sea.

Liquorice and Luke

The library construction site has a surprisingly international atmosphere. The crane operator is Cuban, and various other roles include people from Russia as well as Morocco.

‘The construction business was international when IT was in its child’s shoes. Our sites have employed people from around the world for years,’ Kontio says.

Luke Parsons from Australia is one of the newest faces at the library site. The young man moved to Finland because of love, after having lived in Australia with his Finnish girlfriend. Luke has previously operated many kinds of machinery at a coal mine. He found the job at the library construction through his girlfriend’s father.

‘Work is going well, but I’m a bit afraid of the winter coming. I’ve heard that it’s only a matter of how you dress. If I can make it through the winter, I believe there won’t be any problems surviving in Finland,’ he ponders.

As for the clothing, he has already received some instructions: it’s no good going to work in shorts and flip-flops in September.

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