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A unique steel arc supports the library


Liisa Joensuu/Tmi Magic Words

A bridge structure unlike any seen in Finland so far will support the frame of the Central Library building. Ramboll Oy, who are responsible for the structural design, came up with the steel bridge to solve an architectural challenge: the lobby of the ground floor of the library was to contain 100 metres of open space, with no pillars or columns.

Johan Rosqvist of Ramboll Finland (left) was in charge of the structural design, and Erkki Pekkanen of Ramboll CM Oy (right) managed the construction assignments. Both men were happy with the cost-efficient and sturdy structural solution, which consists of two steel arcs and bull blocks. The bridge structure is shown blue on the screen. The support span of the bridge is 105 metres, and the structure looks like an arched bridge usually built over water bodies or traffic routes, but it supports the entire weight of the three floors of the library, which corresponds to three times the load of a normal bridge.

Foundations for the ends of the arcs as well as moulds for the basement walls are currently being prepared.

“From a structural design perspective, the Central Library is an exceptionally challenging project. The architecture includes curved lines, and several materials such as wood, concrete, and steel are used in the building. The diversity, long support spans, and the changing of the materials made structural design quite difficult,” says Johan Rosqvist.

The arc is intended to support the upper floors, and in practice, the traverse steel gratings that support the frame of the upper floors are supported onto the arc. In the beginning of the project, the structure was different. The bridge-like steel housing beam is a more cost-efficient solution, suggested by Ramboll. It helps to ensure that the Central Library building can actually be used for 150 years, as intended.

If the arc solution is pictured as a bow, the actual bow structure is placed in the basement ceiling. The bow structure contains a total of 527 strings. This kind of string type has never been used in a building in Finland on this scale.

“Structurally, the twine is so challenging that we have ordered all of the parts from abroad. This is the largest bow that has ever been imported to Finland,” says Erkki Pekkanen.

Another great challenge in the structural design was the tunnel that might be built under the city centre. Preparations for a tunnel are made under the Central Library by leaving an area without any columns in the basement, and by making preparations for later excavation work. It may be possible at some point in the future to drive into the Eliel parking facility next to the library.


Preparations for the arc structure are being made at the Central Library construction site, as foundations are being laid for the ends of the arcs.

This picture shows the foundations for the north end of the arc. The south end will be in front of the Sanoma House.

Mining work for the basement part was completed in early February, and soon after, formworks were being compiled for the moulds of the basement walls. When the footing moulds are finished, the iron mountings are installed for the concrete casting.

The footing moulds are to be finished by mid-April. The mould and casting work are carried out concurrently and in small batches. The concrete trucks come to the site a few times per week. The first castings were started towards the end of February. On average, castings will be made on two or three days per week.

In early March, a tower crane will be brought to the site for lifting the iron mountings and moulds. The crane will be placed on the west edge of the excavation. This picture shows a rectangular mould for the foundations of the crane being boarded.

“In the over 50 years of history of E.M. Pekkinen Oy, the Central Library is the first site where a tower crane is used. We could not manage this project using a car hoist, because the excavation will be six metres deep. The hoist would have to be placed on the edges of the excavation, which is impossible due to the limited amount of space available. The tower crane can reach the entire Central Library basement area, and the boom reaches above the opposite KPMG building,” says general foreman Kyösti Kontio.

The basement of the library is built below groundwater level, so the structures must be absolutely waterproof. That is why black injection hoses and green expansion waterstops can be seen on the edges of the excavation. Injection hoses can be used even later on during the construction to pump epoxy or micro concrete into the seam between the wall structure and rock, should it be too weak. Expansion waterstops fill every conceivable little gap when concrete is cast over the rock.

A seepage tunnel is also being built in the excavation. That is where the orange plastic arcs belong.

Water is sucked from tiny gaps in the rock to ensure that the moulds are ready for casting.

When the protective covers used during blasting were collected, they formed a pile way higher than the average person. The covers were sent to other sites to prevent pieces of rock from being thrown around during blasting.

The resulting edges of the rock are far from smooth, so they are cut further using a hydraulic hammer.

A model element of the future facade of the library can be seen between the Sanoma House and the remains of the old brick storehouse next to the Kansalaistori square. The yellow board wall with a sheet metal frame can be seen by anyone, as it is located outside the construction site.

All age groups are interested in the Central Library. One of the youngest is a six-year-old gentleman who has admired the site through the peepholes in the Sanoma House end. He had the following questions regarding the site: Why were there cars, even on a weekend? Why was there ice on the bottom?

Even several days later, general foreman Kyösti Kontio found at least one icy patch (in the picture). His reply was:

“Water is easily collected in the excavation hole, due to rainfall as well as melting snow. When the weather is cold, the water freezes. Once the basement of the library is built, the site will no longer be at the mercy of the weather. When it comes to the cars, the red VW is a site car that is here every day. During weekdays, people may use it to go get supplies that are missing, for example. The drill rigs and other machinery are also left on the site for the weekends, because the site owns them, and the men can’t drive them to their front yards. The machines also only move really slowly, so there’s no taking them to the rush hour traffic, either.”

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