Our assignment
The Lilla Bommen office building was completed in 1990. Skanska implemented the project on a proprietary basis and was responsible for project management, financing, project planning, construction, letting and management of the property. The services of the experienced architect Ralph Erskine were retained to create a spectacular and functional building. He was assisted by White Architects, Gothenburg.
A tall building was a prerequisite, as was a flexible layout, good functionality and energy-saving and low-maintenance solutions. Ralph Erskine’s design proposal, incorporating complicated geometry, entailed additional costs, which he resolved by increasing the floor space by just over 20 percent.
The process surrounding the design of the building was protracted. The Building and Planning Committee was critical and had many questions and viewpoints before approving the design and color scheme. The building’s height was another sticking point, as it was important to achieve harmonization with other well-known high-profile buildings in the city, such as Gasklockan, the Maritime Academy and Hotell Gothia. A solution was finally agreed that satisfied the demands of all parties. The building’s east side was allowed to be higher and the building was thus given its asymmetrical and very characteristic finish.
Construction process
Those who participated in the construction of Lilla Bommen witnessed that it was an unusual project in many respects. Project planning was conducted in parallel with construction work; the time between design and production was often short. On occasion, production caught up with the design work, making it difficult to plan labor resources and material procurement.
Ralph Erskine’s fantastic and innovative architecture caused a few headaches in the actual building phase. The building’s complicated shape required close cooperation between the architect and construction engineers. Creative solutions and intelligent compromises yielded a fantastic end result.
Swedish record in piling
The ground under the building comprises clay sediment to a depth of about 100 meters and ground settlement continuously occurs in the area at a rate of about 10 to 20 centimeters per 100 years. Furthermore, the load imposed by the 22-story building on the foundation was many times more than we had previous experience of at Skanska. There were no building standards or previous experiences on which we could base our work.
The building would have to stand firm over time regardless of the ground settlement. A lot of time was therefore spent on calculating the necessary ground stabilization method and foundation design.
The foundation of the building was constructed using piles that were 95 meters in length. The load intensity used in the piling work for Lilla Bommen was far above anything we had previous experience of in Sweden. The permitted pile load is 1,100 kN, which is a Swedish record for cohesion piles in clay.
The bottom slab in the basement is approximately two meters thick to withstand the loads. The total volume of concrete used in the structure would fill four Olympic-sized swimming pools and the weight of the reinforcement used is equivalent to about 26 bull elephants.
Curtainwall façade
In addition to the foundation, the façade was also time and resource-consuming. It consists of a curtainwall panel system, resembling a curtain that hangs in the outside of the building.
The façade material comprises enameled sheet-metal. The high degree of airtightness of the external panel required the installation of a vapor barriers and air vents. Sound insulation and color fastness were also studied. Tests were carried out in a special climatic chamber on the construction site.
Hard winds at the top
Weather impacted the construction process. At times, strong winds suspended work at the top of the skyscraper. Another difficulty encountered was to find sufficient labor resources during the boom being experienced by the construction industry at the time of the project.
Lilla Bommen threw up many challenges, but the team of Gösta Backmark, Kurt Nilsson, Stig Andersson and Berndt Leth summarize their experiences in the following manner:
“The satisfaction and stimulation that this project provided made it worth all the effort! The feeling of seeing the results of all of the work overshadowed the problems and difficulties.”
The project in figures
• 86-meter-high building. Gothenburg’s tallest building when inaugurated.
• 7-year project period.
• 22 stories.
• About 1,000 workplaces.
• Permitted piling load is 1,100 kN. Swedish record for cohesion piles at the time Lilla Bommen was built.
• Bottom slab contains 185 tons of reinforcement, corresponding to the weight of 26 bull elephants. It is 0.8 – 1.8 meters thick.
• Some 7,000 square meters of concrete were used in the basement structure, which would fill four Olympic-sized swimming pools.
• The structure is constructed from hot-rolled and cold-bent steel profiles, together weighing about 1,600 tons.
• The floor slabs above ground comprise 265-milimeter thick hollow-core slabs of high-strength concrete with pre-stressed reinforcement, covering about 30,000 square meters, or just over four soccer fields.
