Concrete work around the clock
The construction of the basement ceiling has meant long days and weekends at work for dozens of people. The plans have been revised as the work has proceeded, and reinforcements and new structures have been made along the way. Everything has been possible through flexibility, and the work has advanced at an amazing pace. The picture shows the cleaning stage of the southern end of the site, because the wooden casting moulds have already been dismantled.
‘We have tightened our schedule, working 12-hour days and throughout weekends since summer’, says Site Manager Kyösti Kontio.
The only place which still requires concreting as October draws to an end is the bull block of the bridge cover, shown on the right side of the picture with the steel reinforcements visible. A challenging task was beginning: the bull block would draw even more concrete than the 500 cubic metres required for the latest casting part of the basement ceiling.
‘A high load is focused on the bull block, as the steel arches of the interior bridge, which support the higher floors, are to rest on it. Some 800 cubic metres of concrete is used for the bull block alone. Even though the rectangular shape of the block is simple enough, the concreting still has to be carried out successfully in one go for every single square metre. The block is 10 metres wide and 105 metres tall, and counting the metal parts, the total area to be concreted adds up to some 1,100 square metres’, Kontio explains.
Anchoring cables tightened three times
The trick of the bull block lies in the anchoring cables running through it, which ensure the durability of the steel bridge which will support the library.
The anchoring cables are drawn between the southern and northern ends of the bridge. When the cables are tightened, the bridge arches reach their full bearing capability, and will not be able to separate from each other. This solution allows for the 100 metres of open space with no pillars in the first floor lobby of the library.
Both ends of the cables are drawn through the steel slabs which are a part of the housing structure complex. The above picture shows the housing structure complex at the northern end. The cables are still untightened, running through the holes of the steel slab. The slab has a total of 17 holes for the cable bundles. One bundle holds 31 cables, which brings the total number of cables to 527.
The total weight of the cables between the southern and northern end is 70,000 kilos, which is roughly equivalent to 70 average passenger cars. Sturdy solutions are required in order to ensure that the bridge structure can carry all three floors of the library, as even the traverse steel gratings that support the frame of the upper floors are supported by the arches of the bridge.
The cable bundles have been slipped into silver-coloured protective casings, which can be seen in the picture above, before the bull block was concreted. The cables can be tightened for the first time after the casting has been finished and the concrete has dried.
The first of three tightening rounds will be done from the southern end of the site. The second tightening will take place once the steel arch has been lifted into place, and surprisingly enough, the third after the entire building has been completed. This steel slab area at the southern end of the site is left open for a long time, so that the final tightening can be done here. A hole is also left inside the library behind the northern housing structure in order to allow the cables to be tightened even as the construction proceeds.
At the northern end, near the facilities of the future cinema, another hole is left so a two metre long and 50 cm wide jack can be placed behind the bridge housing structure and the anchoring cables can be tightened after the building is completed. At this point, some holes have also been left in the ceiling of the basement, which will allow supplies to be lowered to the basement for the work being continued there.
Floor castings beginning in the basement
Even though the ceiling has already been cast over the basement, plenty of work is still being carried below it. During the past weeks the underdrain and drainage systems have been built and radon pipes installed. Some places are beginning to look ready for the first floor castings: floor fillings are completed, and the insulating urethane plates are in place.
Runs for drainage pipes and air ducts, for example, are being prepared on the basement ceiling. The picture shows the bottom of the pipe channel on the eastern side of the site. Once this task is completed, concrete will be poured over the channel. At these times some 40 people are working on various assignments around the site.
The steel reinforcements of the bull block are beginning to hint at the curved shape of the exterior wall. The upright reinforcements show the line for the reinforcements of the socles of the exterior wall. Once the concreting is finished, E.M. Pekkinen will pass the baton on to YIT. The Pekkinen flag will come down from the flagpole in November. The curved shapes drawn by the architects are sure to present inspiration but also a challenge to the construction contractor.
Text and pictures: Liisa Joensuu/Tmi Magic Words