Bridges are seldom built indoors, but that is exactly what is being built in the new library. The building is supported by two steel arches that make it possible to have 100 m of open space without any pillars in the ground floor lobby. The end casings of the arches with their tensioning slabs arrived to the site in July as an oversized transport from Oulu where they were custom-built.
The casings with their tensioning slabs that arrived from Normek Oy’s plant were lifted into their places with the help of the hydraulic telescopic boom of the truck crane slightly before mid-July. The photo shows half of the casing entity of the northern end of the bridge hanging from the crane. Photo by Risto Sell.
“The casing entity of one end weighs approximately 100 tonnes. The lifting was done in two parts, one 50-tonne piece at a time. In other words, the concrete strip foundation will carry a weight equal to hundred small passenger cars,” says the site’s General Foreman Kyösti Kontio.
The two arches of the bridge are supported in the southern and northern ends to their respective end casings. The photo displays the casing entity of the northern end. For the time being, the casings remain closed. Underneath is the steel tensioning slab through which the anchor cables will run between the northern and southern ends. Here the cable openings are closed with duct tape.
In computer modelling, the arches of the bridge are displayed in grey all the way to their end casings. The cables coming from the tensioning slab are shown in green. The bridge resembles an arched bridge built over water or traffic lanes.
“The structure does not differ greatly from that of an ordinary bridge, but the speciality comes from the fact that it is built indoors. If it were a traffic bridge, it would need to be supported also from the middle. This bridge is supported only at the ends to the foundations and walls so it would not take the weight of car traffic. Still, the bridge does support three floors. The carrying capacity is obtained by the tension generated between the tensioning slabs. The tensioning requires skill in order to prevent the bridge from moving or making noise, which can happen with its counterparts in traffic use,” says Kyösti Kontio.
The carrying capacity is obtained with the help of a large-scale “spring assembly”, that is a large number of anchor cables. 17 cable bundles run through the tensioning slab, each bundle consisting of 31 cables. So, the spring assembly includes a total of 527 cables. When the cables running between the tensioning slabs of the northern and southern ends are tensioned, the bridge arches will obtain their carrying capacity and they will not be able to retract.
“The strands come from Germany. The actual work of placing and tensioning the cables is done by Finnish Tensicon Oy with extensive experience in different tensioning and anchoring jobs. Before inserting the cables inside the tensioning slabs, we will install casing pipes for them,” says Site Engineer Risto Sell.
The span from the northern bridge end to the southern end is 105 m. In the southern end in front of the Sanomatalo building, the end casings seem to be at a lower level than in the northern end, but the level is precisely the same. The optical illusion is due to the fact that there is empty space around the northern end foundation while the southern foundation is hidden in a hole. The casings are not empty but contain a honeycomb structure.
“The casings feature a thick plate at the top and bottom as well as horizontal plates. The contents are like puff pastry,” says Kyösti Kontio.
At the southern end, the 17 cable bundle openings are easy to spot for now.
First indoor walls take form
The external walls of the library basement are nearing completion. In mid-July, the wall sections were poured with the exception of one, so 21 out of 22 were finished. In terms of external walls, one section means a piece that is 11 m long and 6 m tall.
Gang forms of the internal walls are already being lifted at the site. Hanging from the hooks attached to the tower crane’s chains they are effortlessly manoeuvred into place. The basement ceiling, that is the floor of the ground floor, is also starting to take shape at the northern end of the site.
Internal walls are now being built around the loading bay that will be hidden in the basement. The walls of the loading bay have been moulded and even poured in part. The construction of the pump station has also progressed to the point where the pouring of the ceiling will soon begin. The wooden mould of the ceiling is shown in the forefront of the photo.
It is possible to enjoy summer even at the site. Workmen can sit in the swing outside on their coffee break, like carpenters Tommi Elo and Juha-Matti Komu here. When a robust man swings, no padding or canopy is needed.