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ArcelorMittal high strength steel sections reduce weight and carbon footprint of LEED Gold certified Torre Diamante in Milan
With a height of 130 m, Torre Diamante is the tallest of the three high rise buildings constructed in course of the extensive Porta Nuova Redevelopment in the city center of Milan, Italy. The „Diamond Tower“ is Italy's highest building with a steel structure and also the country's third highest skyscraper. The use of high strength steel sections contributed to the weight reduction of the whole building which resulted in reduced costs, less transport, smaller columns and a shallow foundation.
A structural solution at the service of efficiency and architectural design
With a base measuring 30 x 50m, 30 storeys and 4 underground levels, Torre Diamante is by far the highest construction in the newly built complex. Its name derives from its irregular geometry: part of the façade columns are diagonal which results in a special alignment and reminds of the shape of a diamond.
Due to the steel/concrete mixed construction with steel columns, composite flooring and a reinforced concrete core, which is lighter than a conventional reinforced concrete structure, and the load distributing finned walls in the foundations it was possible to avoid pile foundation and use a more economical shallow foundation with a 2 m base plate instead – a cost effective solution that also reduced the time of construction.
The ground plan of the 30 storey building is characterized by a central core, around which a column free area is arranged. This concrete core contains all infrastructure and access functions like elevators, staircases and electricity supply and consists of 3 vertical shafts linked with connecting beams in the flooring areas. Each one of these shafts acts like a vertical cantilever fixed in the foundations and is integrated with the other shafts by the horizontal connecting beams. These beams connect the core walls and ensure interaction of the 3 shafts as a cross-bracing system. The connecting beams avoid the relative vertical displacement of the single shafts and transmit shearing loads.
High strength structural steels: weight and cost savings guaranteed
For the tower's steel structure high strength steels were used, which, due to their higher yield strength compared to the conventional steel grade S235, permitted a total material cost savings of up to 50%. Since the cost of rolled sections in S460 M is just 10-15 % higher than of those in S235, savings of 30-40% could be achieved in the material. Further savings could be registered in the workshop: reduction of welding material, reduction of the surface for corrosion protection due to the use of smaller sections and cost savings for transport due to the light weight structure.
Not only due to the higher yield strength, but also due to more favorable residual stress patterns results higher buckling loads of rolled sections in S460 which has a beneficial impact mainly on columns in the small and medium slenderness range regarding the amount of steel used. This way, S460 steel permits slender and highly efficient columns that are an economical solution for highrise buildings
These advantages were used for the structural concept of Torre Diamante: The columns out of the HD360 and HD400 column section ranges were made of high strength steel S460M. Their ends are mill faced in order to ensure direct contact of the elements in the column splices, where they are bolted together. 700 tons of steel was used for columns with a total length of 2.540m, with 250 column splices and 7.200 bolts.
Creating the 'diamond' shape: the positive effects of high strength steels in the structural break
In order to create the building's characteristic shape, the façade changes its inclination on the levels 9 and 22. Due to this structural break, horizontal forces of several hundred tons are activated which have to be passed on to the bracing system, the central core. The structure for this load transmission is located within the flooring system and consists of a truss of heavy, welded H sections (for tension and bending) and tubular sections that act as compression struts. The welded sections consist of 500x50 mm flanges and 400x30mm webs. By means of steel beams cast in concrete and stud shear connectors, the truss is connected to the core.
On the ground floor, the inclined columns consist of composite cross-sections with exterior steel tubes and centered sections welded together. On the first underground level, the load is transferred into the composite foundations via wide flange steel sections with shear connectors, which are tied in the composite columns of the underground floor and the foundation.
The use of high strength steels for beams under bending moment also has a positive effect on their bending resistanc and it can be shown that S460 is the most economical solution for composite beams. In Torre Diamante, composite beams with up to 11 m spans were used, containing several openings for service installations. Under these circumstances, IPE and HE sections in S355 could be used as floor beams. The composite floors with the undercut geometry of the steel deck that contributes to the reinforcement have a thickness of only 15-20 cm. 1.800 IPE and HE floor beams with a total length of 13.520 m and 26.000m2 composite decking were used.
In order to accelerate erection, a special solution was chosen to connect the horizontal and vertical elements: only the so called „stubs“ that are connected to the core through the steel beams cast in concrete as well as the fin plates , the connection and anchor plates were welded on site. Almost all other main and secondary beams were connected with cover plates on web and flange. This way, one floor per week could be finished.
Sustainability concept and LEED Gold certification
The central issue in the integration of the redevelopment area into the existing urban context is the concept of urban sustainability which respects both the social and environmental requirements of the city. Responsibility for public transport and alternative mobility are included in the concept of sustainable infrastructure and the concept of environmental responsibility, apart from alternative energy sources, takes into account the satisfaction of the people. Torre Diamante is certified with LEED Gold, the whole Varesine project was already precertified for this Green Building certification.
In the LEED regulations for „New Constructions 2009“, Torre Diamante scores in 6 categories:
Concerning the building's interaction with its surroundings, the practically dust-free steel construction could be rated positively in this category.
The building presents a particularly low water consumption which is important in this category: rain water is collected and used for irrigation of the outdoor facilities.
Energy & atmosphere:
In this category in which most credits are awarded, Torre Diamante convinces with a significant reduction of energy consumption and the generation of energy from renewable sources. Electricity is gained with a heat pump system from underground water and from photovoltaic installations. The objective is to reduce the building's energy consumption by 14% compared to a conventional building. The highly efficient façades and optimized heat insulation system contribute to these savings.
Materials & resources:
Use of local, renewable and recycled materials for the preservation of natural resources are the main focus in this category. Thanks to the use of steel, a lot of credits could be received in this category: on the one hand, steel construction is a very low waste procedure and the steel material can be recycled 100%. On the other hand, the determined recycled content of the steel used in this building was 58%, exceeding the 20% required in this category. The origin of the steel is considered „regional“ since the producing mills are located within the requested 800km radius of the city of Milan.
Indoor environmental quality:
In this category, air quality, harmful substances and thermal and visual comfort play an important role. Views and daylight could be maximized due to the arrangement of the useable surface around the central core and the slender columns in the fully glazed façade. As far as hazardous substances are concerned, steel does not contain any volatile organic compounds (VOC), also the paint and coatings used were ecological and biodegradable.
Innovation in design or innovation in operations:
This bonus category includes criteria that exceed the parameters established by LEED. In case of Torre Diamante the use of high strength steels S460M has to be mentioned here, since it significantly improves the resources efficiency and environmental impact.
Beyond the LEED categories and credits, the tower has a barrier-free access and offers column free office spaces for highest flexibility in use.
The advantages of the steel structure in the case of Torre Diamante lies not only in the weight reduction of the whole construction that enabled the preferred shallow foundation, but also in the reduction of transport. The challenges of the inner-city location of the construction site included heavy traffic, reduced space for unloading and virtually no storage space. These circumstances called for a thorough production and logistics plan, the steel elements were delivered just in time without important interruptions of the traffic. A total of 3.800 tonnes of steel were transported in about 150 loads, during the main period of construction this were only 2 deliveries a day.
Porta Nuova Redevelopment
In course of this large scale urban redevelopment that unites the three projects Garibaldi, Varesine and Isola, around 290.000 sqm of territory around a former freight area, unused for the last 50 years, regain their urban functionality and significance. New residential, office, commercial, cultural and administrative buildings are being constructed. Around 90.000 sqm of parks and green areas will aerate the recovered urban context, full access to the city's infrastructure is given.
The urbanistic concept for the Varesine Area was developed by Lee Polisano of the architectural study Kohn Pedersen Fox, supported by architect Paolo Caputo and the engineering firms Jacobs and ARUP, the latter also designed the structures of the high rise buildings.
Kohn Pedersen Fox
Project architect: Lee Polisano;
Stahlbau Pichler (steel construction)
ATI CMB, UNIECO (General contractor)
- Engineering Firm:
Lorenzo De Simone © Fondazione Promozione Acciaio
Thanks to Fondazione Promozione Acciaio for the technical documentation.
(Shortened version & translation of the article „Torre Diamante – ein flexibler Büroturm im Herzen Mailands“ by B.Hauke, E. Manganelli, C. Piccolin, M .May originally published in German languague in „Bauingenieur Sonderdruck der VDI-Jahresausgabe 2012/2013“)