Siltasaari 10

The Siltasaari 10 project will breathe new life to the old Elanto department store and two other buildings in the Hakaniemi district of Helsinki. The extensive basic renovation and construction project will preserve the old building stock, even though the interiors will be remodelled from top to bottom. This is also reflected in the project’s carbon footprint. Sweco was responsible for the structural engineering.

Real estate investment company Antilooppi Ky is working to revitalise Helsinki’s Hakaniemi district, which holds significant cultural and historical value. The expansive renovation and construction project will combine the former Elanto department store and two other properties to form modern office complexes and shared use facilities. The Siltasaari 10 project also includes premises for outlets and restaurants, as well as parking spaces for cars and bicycles.

“Renovating old buildings for present-day use is never a simple task,” says Sweco’s Structural Engineer Anssi Vuorenmaa. The oldest building covered by the project is more than 100 years old and contains varying layers of repairs from multiple decades. As a result, the complex, amounting to a gross floor area of roughly 30,000 square metres, needed to be laser scanned for the BIM. Still, the details have been further specified during the design process. “For example, the underlay and overlay joists were not visible before dismantling.”

Old and new floors blend to form a seamless whole

The project began with the demolition of the Siltasaari 6 building to the third floor level. Concrete elements will be used to construct a new building on top of the remaining section, with the same exterior height but one less floor on the inside. “This ensures more space for building automation systems and an increased room height for offices,” Vuorenmaa says.

Sweco’s structural engineering specialists planned the demolition work and took charge of moisture management at the worksite. All organic materials containing potentially detrimental elements were removed. “The repairs were planned to ensure high-quality conditions that meet modern requirements.”

At the same time, the structures were updated to match present-day norms. “Stiffening structures were added at the top of the remaining old section to convey the horizontal stresses and wind loads affecting the building all the way to the foundation.”

Elanto building retains its exterior but gains an updated interior

The oldest building of the project is Siltasaari 8, which was completed in 1913 and is also known as the old Elanto department store. The protected plaster exterior was restored. “We retained the building’s historical appearance, while modernising the interior and functionalities,” Vuorenmaa says.

The load-bearing capacity of the old structures was ensured with new steel frames and cast-in-place concrete. “It would have been difficult to bring concrete elements into existing spaces.”

The old attic floor and roof structure were dismantled and replaced with svelte steel structures. This also made the attic area fully available for ventilation machinery. “The roof remains unchanged from the outside all the way to the roofing materials,” Vuorenmaa says emphatically.

Renovation saves materials

Sweco’s structural engineering specialists and the client conducted a detailed review of which building sections to retain and which to replace. Among the preserved sections are the facade and most frame structures of the Paasivuorenkatu building.

“The condition surveys indicated that the facade cladding and windows will remain viable for 10–20 years,” Vuorenmaa says. On the other hand, the old ventilation machine room was renovated to be twice the former size. “Modern technology takes up more space, but the building frame remains the same size.”

The Siltasaari 10 building has been submitted for LEED Gold certification, and in-service energy consumption is managed by means of building automation and state-of-the-art insulation. “For the purposes of HVAC engineering, we estimated the energy consumption and emission levels of the old and new structures.”

Renovation also saves materials compared to constructing new buildings, which is reflected by the carbon footprint of the entire project. “For a project of this size, renovation can save a significant amount of steel and concrete frame structures,” Vuorenmaa explains.