Urban densification makes it possible to increase the profitability of urban services (transport, water and sewerage networks) and increase the competitiveness of cities, but the construction of new structures very often requires the demolition of existing structures to make way for new land, or else the artificialisation of land that has not yet been artificialised. In the context of a circular economy, it is much preferable to make the most of existing buildings.
Particularly in the case of high-rise buildings (HRB), the volume of concrete to be demolished and then rebuilt can be considerable. For example, in the case of a 40-storey, 140m-high high-rise building, the total volume of concrete per square metre of floor space is the equivalent of 20m of solid concrete! From the outset, this enormous quantity of material means that the CO2 equivalent per square metre of floor space is extremely harmful, especially as the material for a high-rise building comes mainly from the columns: the tower is self-supporting.
A much more eco-responsible approach is to rehabilitate existing buildings, particularly high-rise buildings, in order to increase density.
- THE PROBLEM OF RENOVATION: RECOGNISING AND UNDERSTANDING WHAT ALREADY EXISTS
The experience gained by setec shows that, unlike when building a new structure, it is necessary to be prepared to deal with the uncertainties associated with the renovation of a structure which has had a previous existence, which was not designed for a second life and the quality of which was not always well understood at the time it was built. To deal with these uncertainties, setec has set up a design process whose main stages are as follows:
- Gather as much bibliographical data as possible: execution documents, formwork plans and reinforcement plans, preferably in the form of as-built drawings, execution calculation notes and soil reports:
- Expertise in the existing structure, understanding how it works structurally:
- Identifying applied loads: applied loads are also codified in building regulations, which are constantly changing:
- The calculation tools used at the time: the approach to structural calculation that was used in the 70s and 80s is no longer adapted today. Sometimes the simplifications made in the “manual” calculations at the time do not assess the path of the stresses, which can result – 50 years later – in disorders that will have to be repaired to ensure that the structure has a new lease of life;
- The resistance of concrete and steel reinforcement or structural materials to be identified by specific campaigns carried out by specialist laboratories such as LERM, a Setec Group laboratory.
These stages make up the “diagnosis” phase of the structures, which is essential for understanding any restructuring/rehabilitation project.
2. REGULATIONS
Taking account of regulatory changes: the initial project was designed within a regulatory context that has changed, for example with regard to concrete:
- CCBA 68 and CM66,
- BAEL 80 then BAEL 90,
- NV rules,
- Eurocodes
The justifications for a renovated project must be carried out, as requested by the Inspection Office, within the regulatory context applicable; this concerns both the structural justifications and the actions applied.
50 years ago, the applied loads were often calibrated as accurately as possible, whereas today’s building owners are asking for greater latitude in the load-bearing capacities of their floors.
3. REHABILITATION PROCESS
Working on an existing structure is more complicated than working on a new one. Therefore, the rehabilitation of a structure not necessarily designed to be restored requires us to follow a precise and adapted methodology, which we have developed and consolidated on a number of our projects:
- Recognition and understanding of the existing structure as described above,
- Precise definition of the actions applied (equipment loads, operating loads, wind),
- Familiarity with old regulations and knowledge of the possibilities offered by calculation rules to ensure that the principles of justification are correctly applied,
- Structural design of extensions to limit added loads (“good choice” of material used for this extension: metal, wood, etc.), while trying to respect the geometry of the existing building (grid of the extension in line with that of the existing building, limiting work on the primary structures, etc.).
- Complete recalculation of the structure in its existing state and in its future rehabilitated state to identify, structural element by structural element, the structures impacted by the rehabilitation project.
- Specific techniques for reinforcing existing structures (reinforcement by concrete, steel or carbon moulding, jacking techniques for transferring loads, etc.),
- Specific load transfer techniques (headframes, shoring, etc.), deformation control, monitoring, etc.
4. CONCLUSION
Particularly virtuous in environmental terms, the refurbishment of existing high-rise buildings is essential, and will continue to be increasingly so, given the volumes of materials involved and their potential impact on the environment in the event of demolition or reconstruction.
Reusing an existing structure involves challenging the study process usually followed for a new structure. Perfect knowledge of the existing structure is absolutely essential if uncertainties are to be reduced. Once the uncertainties have been removed, knowledge of the existing structure means that it can be reused in the best possible way, taking advantage of the regulations.
Initiated at the beginning of the 2000s with the renovation of the PB12 tower, which became Opus 12 and for which setec carried out the structural design studies, setec is now working as a structural project manager on projects such as the Montparnasse tower in Paris, the Lightwell building or the Ariane and Hopen towers at La Défense, or the Aurore and Altiplano projects, which we invite you to discover below.
The first-generation tower built in La Défense in the early 1970s, the Aurore Tower, 90 m high with 27 storeys of superstructure, is being renovated and raised by 6 storeys to reach a new height of almost 120 m.
The renovation project was designed to be as environmentally friendly as possible. For example, the number of storeys added is the maximum possible without having to reinforce the load-bearing structures (facade columns, central core walls) of the existing building: only the floor slab had to be reinforced.
According to an estimate by the project’s environmental consultancy, this renovation, which respects the existing building, has saved the equivalent of almost 50 years’ energy consumption for a high-rise building of its size, compared with demolition and reconstruction. So Aurore’s new life will have zero carbon footprint!
Altiplano
Given its limited height, the PB10 project does not fall into the HRB category, but is a building under the French Labour Law, albeit on a particularly large scale: the project has almost 60,000 m² of floor space.
The project involves the complete restructuring of the PB10 building at La Défense and raising it by two to eight storeys in superstructure, depending on the wings. Built in the early 1980s, PB10 has a surface area of just under 40,000 m² on six levels and is perched on top of the Villon overhead car park, which is itself spread over six floors via a transfer structure.
Renamed “Altiplano” and raised by 2 storeys, the building now has almost 60,000 m² of floor space.