The buildings designed and built over the last 80 years, since the end of the Second World War, can be broadly divided into two main groups. The first includes buildings where the load-bearing structure is retreating from the edge of the building envelope into the interior, and the second includes buildings where the building envelope is part of the load-bearing system. This division between the structural and façade plane, which extends into two layers, is a concept that began to develop in the late 1920s. One of the most famous architects who contributed to this development was Le Corbusier. In his Five Points of Architecture, he pointed out that the retreat of the structure in depth allows the façade plane to be freed, which in turn leads to the design of a continuous band of horizontal façade glazing.
Fully flexible content, structural shading around the entire perimeter
The concept of les fenêtres en longueur, or the longitudinal window, has radically changed the architectural appearance of buildings. Whereas before its introduction, façades were generally articulated by window openings and load-bearing façade slopes, the longitudinal window introduces a horizontal compositional and structural articulation expressed in horizontal bands. This innovation transforms the classical vertical composition of the building perimeter into a contemporary architectural image. centuries. Typical examples of such buildings in Slovenia are the Kozolec and the Metalka tower block in Ljubljana, both by architect Edvard Mihevc.
In 2018, the Motorway Company of the Republic of Slovenia (DARS) took the decision to consolidate the dispersed tenant locations of its administration in Ljubljana and Celje into one place. The area of the DARS Grič motorway base in the western part of the motorway ring around Ljubljana proved to be the most optimal. A public architectural competition was launched, with sustainability design criteria specifically highlighted and in line with DGNB certification criteria. The solution presented by the authors was recognised as the best, as it offered a restrained urban design and several innovative sustainable approaches. The long-term adaptability of the building’s contents was key, as the structure of DARS changes regularly in line with the dynamics of the company and its logistical challenges. DARS pointed out in the competition that the structure and the number of employees are constantly changing, and that the layout of the workspaces is also adapting, allowing the occupancy of the offices and the size of the individual departments to change regularly over the lifetime of the building due to the constant changes in structure and number of employees.
The selected solution proposed long-term flexibility, with the possibility to move the dividing walls between offices almost arbitrarily according to a uniform module of 62.5 cm, without any intermediate structural barriers. The load-bearing structure was retreated deep into the building, to the area of the communication and installation cores and to the façade perimeter itself. The 250 cm wide façade column system provides vertical stability and the central cores also provide seismic safety. Reducing the structural grid to only 2.5 metres might at first sight appear illogical, as a denser structure could hinder the functionality of the space. However, a solution has been applied where the columns are moved to the outer side of the thermal envelope and additionally set back from the façade by 60 cm. This way, they do not interfere with the basic system of partition walls between the offices.
The depth of the perimeter columns is adapted to the angle of incidence of the sun, and the façade serves as a passive shading device in addition to the structure. During the design phase, the depth of the façade elements was optimised for heat gains and cooling savings using parametric CFD calculations of the building thermodynamics.
The layout of the building is based on a uniform perimeter arrangement of cellular offices and common areas, meeting rooms, booths and open spaces in the interior of the building. The entire perimeter is fully flexible and allows for a number of functional divisions over the lifetime of the building. The internal part between the four structural and communication cores is designed as a system of common spaces lit through a central multi-storey atrium.
During the design process, a rough Life Cycle Analysis (LCA) of the building was carried out, which influenced the choice of materials. Parts of the building that allow for long-term use without the need for changes (50+ years) are designed with materials with a higher environmental footprint (steel and concrete). Parts that are expected to be renovated in shorter cycles (10-20 years) are made of degradable and environmentally friendly materials (e.g. These elements include in particular partition walls, flooring and dropped ceilings. The whole building, except for the load-bearing structure, is designed to be easily dismantled and disassembled, allowing for reuse and recycling. The façade is also fully recyclable, as the façade steel can be melted and used for new structures.
The interior of the building is illuminated by a central covered atrium, which extends from the ground floor to the fourth floor. In addition to providing good quality natural lighting to all work areas, the atrium acts as an air stack, promoting natural ventilation. Narrow windows on the perimeter of the building provide natural ventilation to the offices, while exhaust air is provided through a skylight above the front door of the offices. The air is collected in the atrium, where it is directed by the updraft through openings in the roof light. The night cooling and ventilation system will be experimentally controlled via the CNS, with motorised opening of windows and overlights.
The DARS office building is one of the few buildings where the steel structure is visible and positioned outside the thermal envelope, giving it an appearance similar to industrial buildings. Its interior, on the other hand, expresses warmth and dynamism, creating an interesting contrast between exterior and interior and opening up the debate on the relationship between the material and the humane.
We tracked the construction on our website from March 2022 to November 2024. The following summarises the key sustainability principles that have been taken into account in the design and construction:
an inclusive and socially responsible space
- In addition to the permanent office space, the office building contains a number of additional spaces for informal meetings, project work and short breaks between work, which enrich the workspace and enhance communication;
- A common area for breaks and snacks has been carefully designed, including an outdoor area with a lake and a number of benches;
- All areas are accessible to people with reduced mobility, and the width of the entrance doors to all offices has been increased;
- Toilets adapted for disabled people are located on every other floor of the building.
long-term flexibility
- The functional, installation and structural design of the building allows for free adaptation of the building’s current and future contents without functional constraints;
- The modular design of all building elements (façade, installations, movable partition elements, furniture) allows for quick adaptation of the internal partitions in the building in a single 62.5 cm module.
planning the lifetime of a building
- Building elements are divided into different levels, depending on their life expectancy, the need for lifetime adaptations and exposure to wear and tear. This is followed by the functional and economic quality of the materials. More expensive materials are used where their expected lifetime is longer, and cheaper materials where more frequent replacement is expected;
- the structure and façade are designed to be fully universal, for all potential uses of the building with a life expectancy of 100 years and more;
- internal partitions, dropped ceilings and installations are designed to be flexible, with a lifespan of between 20 and 50 years;
- Wear surfaces, flooring and wall cladding are designed to last 10 years;
- The building is designed in such a way that the cost of materials used, maintenance and cleaning costs and regular replacement cycles are coordinated and optimised through Life Cycle Cost Analysis (LCCA). This means that more expensive materials are used only where their higher quality is justified in the long term.
the environmental footprint of building materials
- the construction of the building and the furnishing of the equipment are carried out using only building materials that do not contain substances harmful to health (formaldehyde, volatile hydrocarbons and other carcinogenic gases);
- different levels of building elements according to their lifetime, the principles of material selection also differ according to their environmental footprint;
- materials with a very long lifetime, which are mainly embedded in the building structure (steel and concrete), have a relatively high carbon footprint. They are permanently integrated into the building and no future adaptations, removals or modifications are foreseen. The use of steel makes sense as steel supplied on the Slovenian market contains between 60% – 80% recycled content;
- mid-life materials incorporated in dropped ceilings and partition walls are made of biogenic materials (wood, wood composites) with a negative environmental footprint;
- short-life materials on all wear surfaces are made entirely from recycled materials (floor and wall coverings);
- all materials used were verified during installation against the Environmental Performance Declaration (EPD) and entered into a comprehensive Building Environmental Footprint Analysis (LCA).
passive shading, natural ventilation and night-time cooling of the building
- the building’s external visible structure, with its density and depth, provides effective shading of the building during months that cause unwanted overheating of its interior. In winter, the low sun provides heat gains and reduces the need to heat the building. The relationship between the need for shading, the potential heat gains and the depth of the façade grid has been separately verified for all four sides of the building by a parametric thermodynamic analysis of the building (CFD). The building thus achieves an energy class A1, with a heating demand of only 6 kWh/m²K;
- passive night-time cooling of the building is also possible in summer, as all the rooms used for permanent work are grouped around the façade perimeter and each has at least one window for fresh natural air. Air is extracted through an overhead door connecting the offices to a common corridor and a central atrium, which functions as a large air stack with a roof dome for ventilation. The dome is electronically controlled and the windows are manually opened. This allows employees to cool the space when they go home at night, reducing the need for mechanical cooling. Open windows cut off the supply of heat, cold and air from ventilation units to each room, preventing energy loss.
heating and cooling the building using radiant systems and concrete core activation
- The buildings are primarily heated and cooled by ducting the heating and cooling medium in visible stepped concrete slabs at a depth of approx. 6m throughout the façade envelope. This ensures a stable and uniform maintenance of the intended internal temperature of the building. The radiant systems ensure a comfortable indoor microclimate without the need for air movement. In addition to natural ventilation, fresh air is supplied to the rooms via floor convectors, which are also designed to adjust the final room temperature according to the needs of the individual occupants within a level of +/- 2°C.
stormwater management
- Stormwater from the green roof and the external paved areas is collected in a large outdoor pond and underground storage for watering the external green areas;
- The outdoor pond, in addition to the many trees planted as part of the horticultural landscaping, also lowers the air temperature in the southern part of the building by evaporating water;
- Rainwater harvesting is planned through several retention plantings which, in addition to the flow pond, significantly reduce the impact on the public municipal stormwater system;
- The water level of the lake is maintained by rainwater as well as underground drainage.
circular economy
A significant part of the items and appliances have been designed specifically for the needs of the office building, with an emphasis on local sourcing, knowledge and performance, including:
- A freestanding luminaire specially designed for DARS by Lumenia, Kandrše;
- partition walls between the corridor and the offices, made by Gonzaga, Nova Gorica;
- lowered wooden ceilings made by Varion, Logatec;
- complete office furniture made by Gonzaga, Nova Gorica, and
- uniquely designed sofas made by Donar, Ljubljana.
environmental certificate
- the building is verified according to the internationally recognised DGNB system.
Authors of architecture Blenkuš Matej, Cimperman Katja, Valenčič Grega, Klobčar David, Gazvoda Matevž
Authors of landscape architecture Kučan Ana, Javornika Luka, Gligić Lara, Krnač Tomislav, Mohorič Danijel
| Subscriber | DARS, d.d.d. |
| Project year | 2020 |
| Year of implementation | 2024 |
| Other engineers and architects | Žvan Uroš, Avguštin Aleš, Peharda Filip, Blažek Peter, Vrčon Blaž, Dobrila Tomaž, Fojkar Andrej, Muhič Simon, Ercegovič Rok, Robert Smodiš |
| Consulting, FIDIC Engineer and Supervision | AXIS, d.o.o., Rus Ivan, Bajželj Gregor, Proctor, d.o.o., Hribar Stane, Hribar Tomaž |
| Retrieved from | CGP, d.d., Motoh Devis, Bobnar Gregor, Gortnar Jernej |
| Client representative | Branko Ribič |
| Photo | Kambič Miran |
| Publications | How office buildings support corporate sustainability goals, CER Partnership for Sustainable Economy, Ministry of Natural Resources and Spatial Planning [June 2025] Delo newspaper, “Dars’ cube with piano nobile”, article by Saša Bojc [15.2.2025] AXIS website [20.1.25] Outsider, online publication [24.12.24] 6th Consultation Concrete and Sustainable Construction, paper“Potentials of Building Perimeter Structures”, Cankarjev dom Ljubljana [21.11.2024] |
| Prizes | Nominated for the Golden Pencil 2025 Award, Slovenian Chamber of Architecture and Spatial Planning; ARCHITIZER A+ WINNER, in the Sustainable Commercial Building category; 1st prize in the public competition; |
