Olympic Stadium’s magnificent underground facilities are pleasing to the eye and can withstand heavy loads

A renovation project spanning several years expanded the historical Helsinki Olympic Stadium, primarily underground. Post-tensioned in-situ structures were utilised in the cover structures above the underground sports and logistics facilities to ensure that the facilities will withstand the weight of both overlying soil and dozens of trucks during international mega-events.

Well-known to everyone in Finland, the Olympic Stadium was completed in Helsinki in 1938, although it did not get to host the Olympics until summer 1952 because of World War II. An extensive renovation of the stadium commenced almost 80 years later under the supervision of the Finnish Heritage Agency. It was also necessary to build new structures in order for the stadium to function as an international event venue and multipurpose arena for decades to come.

“Because of the protected classic architecture, the construction of new structures focused underground, along the outer arch of the stadium,” says Jonas Heikkilä, Department Manager for Prestressed Concrete Structures and Post-Tensioning at Sweco. Sweco was responsible for the main structural design in this large-scale renovation project, including the steel structure design of the covered stands and special design of the new underground facilities.

The new underground facilities cover approximately 20,000 square metres, and most of them were implemented by using post-tensioned in-situ structures instead of prefabricated units. Besides building construction, this solution is also used in bridge construction. “It also facilitated the curved lines of the canopy above the north entrance above ground.”

Post-tensioning met the stadium’s space requirements

There was a need to build versatile storage facilities, a multipurpose room suitable for meetings, a logistics and a sports centre below the outer arch of the stadium. By utilising the advantages of post-tensioned structures, Sweco was able to minimise the structural heights and make the facilities spacious.

“In the sports centre, for example, there was no need to install support columns anywhere else besides the edges of the sports areas, thanks to the massive post-tensioned T-beams,” states Business Unit Manager Aleksi Keskinen from Sweco. The stadium’s vast yard and recreational areas were built on top of the cover slab structures. One of the most peculiar design solutions involved hanging the team facilities from the post-tensioned beams above by using tension bars. “This solution allowed the weightlifting facilities below the team facilities to be made spacious.”

The stadium’s challenging space requirements were known from the beginning. “The exceptional geometry and extremely long spans made post-tensioned structures the only cost-efficient option,” states Design Manager Atte Heiskanen from Sweco. The facilities also had to withstand heavy loads, such as an overlying soil layer more than a metre high. “During international concerts and mega-events, there may be several dozen trucks parked on top of the logistics centre’s cover structure.”

The prefabricated columns connected to in-situ vaults stand up to 10 metres high, and the sturdiest beams are 23 metres long, 1.5 metres wide and over 2 metres high. All in all, the post-tensioned structures required more than 700 tonnes of reinforcements and more than 100 kilometres of tendons containing strands coated with grease to allow the concrete to withstand heavy loads.

Quality was ensured with site-friendly plans

Everything was played safe at the prestigious site, and quality assurance was carried out continuously in multiple phases. For example, the post-tensioned cover structures were epoxied and waterproofed, even though the tendon structures are relatively waterproof as is when designed correctly. The designers also had to carefully consider formwork and the order in which the reinforcements were installed because the massive beams intersected in many places. “The reinforcements were designed, prefabricated and brought to the site, ready for installation,” says Keskinen.

Heikkilä is satisfied with the fact that very few changes were required on site, even though the project was extensive and demanding. “The structural designs were high in quality and, above all, site-friendly, which was absolutely necessary. The beams were so large that the fitter actually stood inside the mould during the installation work.”