Above ground construction starts with shafts

Worksite stories opened a new chapter in November. YIT took over the site, set up a village of yellow containers at the side of the park and brought many new characters into the story. Now, we will start construction above the ground. First, we will build the staircase and lift shafts, such as the northern stair shaft shown at the front of the picture. The building itself will be built from the north side towards the south.

The YIT’s site management team is excited about starting the building’s construction project. This is an one-of-a-kind project, in which we will build a unique building in the middle of Helsinki. For all those participating in the construction process, the building will become a historical part of their portfolio.

“The project’s challenges are related to the building structures that have not been often used in Finland. The frame structure is exceptional, but challenging projects teach us the most,” says senior site manager Tero Seppänen.

The library will become an urban space for millions of people and a new meeting place for them. According to Seppänen, this is a rewarding concept. The constructors will have the opportunity to bring added value to the lives of people. Team photo. From the left: site engineer Ville Nevala, site manager Janne Kurikka, senior site manager Tero Seppänen, site management intern Miika Luokkala, site manager Ardam Mansoor and site engineer Unto Miettinen.

At first, we compiled a shared schedule and then started the preparations for cast-in-situ structures. The worksite has five shafts, and during their cast-in-situ stage we will need around 30 workers on site. Tero Seppänen points at shaft number three, which is the site’s most important shaft.

The site will expand outside the current fences. After the turn of the year, new fences will be erected from the old magasin building towards Kansalaistori.

The building has been roughly divided into three sections: the northern A section, the middle B section and the southern C section. Each section has its own, more specific blocks. At first, we will start with the northern shafts, numbers 1 and 2, and the middle shaft number 3. Shaft number one will have stairs, shaft number two a lift and shaft number three both a lift and stairs.

“We have named shaft number four as ‘back-up place’. If work in other shafts will have to be put on hold for any reason, we will transfer the workgroup into this shaft. Three work teams are building the shafts,” says Tero Seppänen.

The southernmost shaft, shaft number five, will house stairs. The southern end of the building has an option for a city centre tunnel that would, according to initial plans, go from Länsiväylä to Sörnäisten rantatie, below the city centre. Through the stairs of shaft number five, people can exit the car tunnel by foot, if necessary.

Preparations for erecting the steel arches soon underway

These five shafts are standard construction projects, but the preparations for erecting the steel arches that support the entire library will be started in December. For these, we will first build temporary supports. Installing the actual arches will begin in January and if all goes according to the plan, the installation process could be finished in March.

Pictured are the stairway and lift shafts and the steel arches. The northern stairway shaft, pictured at the front of the image, is located on the side of a future film theatre. In the computer model, the film theatre’s stand is shown in violet colour.

“Due to the frame’s arch structure, certain movement margins must be taken into account. When the arches are burdened with the structures they support, they will bend down. Due to this, we will not make the permanent structures before the entire structural load of the arches has been built. We will leave expansion joints and movement margins for structures in several places.”

The finished building will also have a movement margin of a few tens of millimetres, as due to the arch structure solution, the payload – furniture, items and people – may move the structures. This movement is so small that the people inside will not notice it.

The tower crane placed in the middle of the worksite is in the way of the arch installations, which means that it will eventually be disassembled and replaced by mobile cranes. In the picture, the north-side end housings are shown on the left side of the crane and the cast moulds of the third shaft can be seen behind them. Both a lift and stairs will be built into the shaft.

A work team for preparing and erecting the arches will arrive at the worksite from Normek Oy. After installing the temporary supports, the arches will be mounted on them as separate pieces. Each arch has been divided into four sections. Finally, these pieces will be welded together.

With its curved walls, the form of this building is also extraordinary. According to Tero Seppänen, however, this does not slow down the building process or require much manual work.

“The curve forms over a long expanse of the structure, and it is possible to use elements for the outer walls. The wood panelling will be made over the elements. The frame also has many steel structures.” The floor levels are made of hollow-core slabs, which enable the quick building of large surface areas. Due to the arch structure solution, the hollow-core slabs will not be cast together immediately. The slabs will mostly be left without seams until the building’s total mass rests on the load-bearing arches and the arches have bent down equally to the load of the total mass. Only after this, the floors – which are also the roofs of the lower floors – will be cast.

Tero Seppänen takes things calmly. He has been working in construction for 20 years and has worked in site management for nearly as long. He knows that the project will proceed gradually and in smaller sub-projects. Several operators take part in the project and they all have their own roles. Architects and structural engineers manage their own duties and YIT is in charge of implementation.

For YIT, the unusual part of this project is that it does not start from a ‘clear table’ or as an unconstructed site. E.M. Pekkinen Oy has already carried through the bedrock excavation, civil engineering and basement construction works. Their work will still continue in the basement in the form of floor casts and sewer works, and the company will also finish the refill works of the sides of the building. The masonry works of the basement partition walls starts in January.

“When the partitions have been built in the basement, we will start constructing the machine rooms for building services. The brickwork walls will have openings for sewer and ventilation pipes and cable trays. In fact, the building services works are underway, as electrical piping has already been installed into the shafts,” says Tero Seppänen.

As the new constructor starts its work and the building starts to grow above ground, more attention will be paid to worksite supervision. Camera surveillance system will be installed, including motion sensors and alarms. The guards will be there within a few minutes.

Any communication of the building project of Central Library will be as open as possible. The Premises Centre, the library and YIT held a meeting together at the end of November, discussing each of their communication needs. In the future, information will also be available through YIT’s channels.

 

 

Text and pictures: Liisa Joensuu/Tmi Magic Words

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