ArcelorMittal high strength steel sections reduce weight and carbon footprint of LEED Gold certified Torre Diamante in Milan

Detailed information

A structural solution at the service of efficiency and architectural design

With a base measuring 30 x 50 m, 30 storeys, and four underground levels, Torre Diamante is by far the highest construction in this newly built complex. Its name derives from its irregular geometry: Part of the facade columns are diagonal, which results in a special alignment and gives the impression 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 characterised 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 three 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 three 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 S235 steel grade, 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. Further savings could be registered in the workshop: A reduction of welding material, a 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.

Higher yield strength in addition to more favourable residual stress patterns results in 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 high-rise buildings.

These advantages were used for the structural concept of Torre Diamante: The columns of the HD360 and HD400 column section ranges were made of S460M high strength steel. Their ends are mill faced in order to ensure direct contact of the elements in the column splices, where they are bolted together. 700 tonnes of steel was used for the columns with a total length of 2.54 m, with 250 column splices and 7200 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 facade changes its inclination on levels 9 and 22. Due to this structural break, horizontal forces of several hundred tonnes 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 500 x 50 mm flanges and 400 x 30 mm 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 centred 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 resistane, 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. 1800 IPE and HE floor beams with a total length of 13.52 m and 26 000 m2 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 and 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, and the entire Varesine project was already precertified for this Green Building certification.

In the LEED regulations for 'New Constructions 2009,' Torre Diamante scores in six categories:

Sustainable sites:
Concerning the building's interaction with its surroundings, the practically dust-free steel construction could be rated positively in this category.

Water efficiency:
The building presents 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 presents 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 facades and optimised 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 were awarded in this category: On one hand, steel construction is a very low-waste procedure and the steel material can be 100% recycled. 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 maximised due to the arrangement of the usable surface around the central core and the slender columns in the fully glazed facade. As far as hazardous substances are concerned, steel does not contain any volatile organic compounds (VOC), and 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 the case of Torre Diamante, the use of S460M high strength steels has to be mentioned here as it significantly improves the resource efficiency and environmental impact.

Beyond the LEED categories and credits, the tower has barrier-free access and offers column-free office spaces for the highest flexibility in use.

The advantages of the steel structure in the case of Torre Diamante lie 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 major interruptions to traffic. A total of 3800 tonnes of steel were transported in about 150 loads, and during the main period of construction there were only two deliveries per day.

Porta Nuova Redevelopment

In the course of this large scale urban redevelopment that unites three projects - Garibaldi, Varesine, and Isola - around 290 000 square metres 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 square metres of parks and green areas will aerate the recovered urban context, and 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 and was supported by architect Paolo Caputo and the engineering firms Jacobs and ARUP - the latter also designed the structures of the high-rise buildings.