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STRUCTURAL DESIGN OF RESIDENTIAL BUILDINGS

ARCHITECT STUDIO DANIEL LIBESKIND

CITYLIFE AREA, MILANO

SUMMARY

Citylife is a Company originally established by three main partners, Generali Properties SpA, Gruppo

Allianz Ras, and Progestim SpA (Gruppo Fondiaria SAI), to develop a €2 billion real estate investment,

comprising many residential buildings, three skyscrapers, a museum, and a park in the former area of the

Fiera Campionaria in Milano (northern Italy), which is 255.000 square meters wide. Three vastly

renowned architects were chosen by Citylife: Daniel Libeskind, Zaha Hadid, and Arata Isozaki. Giovanni

Franchi, P.E., was appointed chief structural engineer for the eight residential buildings designed by

Studio Daniel Libeskind. Their structures were completely designed in reinforced concrete. Mr. Franchi

was in charge of the preliminary, definitive, and executive phase of the structural design. The article that

follows is an exposition of the relevant information about the work done by Mr. Franchi and his team,

trying to highlight its main, interesting aspects.

1 – THE CITYLIFE AREA AND THE RESIDENTIAL BUILDINGS DESIGNED BY STUDIO

DANIEL LIBESKIND

The Citylife area is downtown Milano. It is a residential and business district currently under

construction. The project involves the construction of three skyscrapers: Il Dritto (The Straight One), Lo

Storto (The Twisted One) and Il Curvo (The Curved One). In addition, more than 50% of the available

area, 143.000 square meters, will be devoted to a park characterized by waterways that evoke the canals

of Lombardy (Navigli). A Museum of Design is also part of the project. The residential area will cover

about 148.000 square meters, with 1.300 apartments (housing about 4.500 people) in high-rise buildings

up to 20 stories. Finally, there will be parking for 9.400 vehicles located in underground facilities.

Work on the project, started in 2007, is planned to be completed by 2014.

The Citylife master plan follows:

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Figure 1: The Citylife Master Plan.

The Libeskind Residences are composed by eight buildings situated in the south-east part of the Citylife

area (left corner to the bottom in Figure 1) and will feature luxury apartments. They are divided into two

different lots, named Rb1 (five buildings) and Rb2 (three buildings). These residences will benefit of the

160.000 square meters park and of all the facilities of the futuristic urban planning system.

What follows are some examples of the architectural designs by Studio Daniel Libeskind:

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Figure 2: Studio Daniel Libeskind architectural renderings (from the outside).

Figure 3: Studio Daniel Libeskind architectural renderings (from the inside, luxury apartments).

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2 – THE STRUCTURAL DESIGN

Many are the difficulties to be faced and the obstacles to be overcome while trying to design the

structures of a building designed by Studio Daniel Libeskind! Mr. Libeskind, in fact, is renowned all over

the world for his daring shapes and geometries. First of all, then, to follow and to study those complex

geometries was the major goal that Mr. Franchi and his team had to achieve. For instance, in all the eight

residential buildings there are not even two levels whose floor shapes are the same; the introduction of

askew columns was another topic that had to be carefully evaluated; and the cutting-edge appeal of the

facades designed by Studio Daniel Libeskind was obtained by “jewel” shaped parapets, combining the

use of reinforced concrete and structural glass.

Indeed, most challenging and “intriguing” by far was the structural design of the eight penthouses, located

on top of each building. Their 10 meters high, 75 meters long and 20 meters wide, box-shaped structure,

subdivided into 12 volumes, had to be modeled with carefulness and great accuracy. To that end, the

MIDAS FEM software was employed, as it is showed in Figure 4 below:

Figure 4: The 3D dwg drawing of a penthouse and its related FEM model.

Other peculiar aspects of the structural design, in no particular order, were:

- The need to drain a substantial part of the Rb2 lot ground from a widespread gasoline stain,

encountered 14 meters below foundations level, required Mr. Franchi and his team to design deep

foundations for two out of the three buildings in the Rb2 lot (in accordance with the suggestions

by a panel of geophysicists);

- The architectural requirement to build balconies with a structure that cantilevers up to 3,5 meters,

required Mr. Franchi and his team to employ two different structural floor systems, the U-BOOT

and the COBIAX, to design light-weighted two-ways reinforced concrete slabs; as can be seen in

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Figure 5 that follows, the U-BOOT system is characterized by modular, squared off formworks,

made of recycled polypropylene, while the COBIAX system is characterized by modular,

spherical formworks:

Figure 5: Examples of the U-BOOT system and of the COBIAX system.

Due to the spherical shape of its formworks, the COBIAX system is both stiffer and lighter than

the U-BOOT system, but it is also more expensive. Following a serious costs-benefits analysis,

Citylife decided to employ the COBIAX system for the structure of the penthouses, where its

stiffness and its lightness where absolutely required and irreplaceable, while preferring the U-

BOOT system anywhere else;

- Furthermore, the extremely varied and complex shapes of the balconies, and the need for them to

cantilever 3,5 meters, prompted Mr. Franchi and his team to perform a vibratility analysis, to

make sure that no discomfort could be felt by those people enjoying the wide spaces offered by

the balconies. Two standard methodologies were employed: the Richer-Meister-Lenzen method

(RML) and the Wiss-Parmelee (WP);

- The tallest of the eight buildings has got 3 underground levels, 15 levels above ground, and the

penthouse (for a total of more or less 20 stories). The architectural need to keep the columns very

slim and to have large spans between columns, required Mr. Franchi and his team to prescribe the

use of high-quality concrete (minimum strength standard deviation), up to C 45/55;

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- Besides, Mr. Franchi and his team prescribed the use of a light-weighted concrete for the structure

of the penthouses: its mix-design had to be studied so that, while still achieving good strength (C

35/38), the density didn’t exceed 2000 kg/m3;

- The first floor underground in the Rb1 lot is characterized by the so-called “Loop Street”: it is an

almost circular street, in which vehicles can transit and which connects the five buildings, better

showed by Figure 6 that follows:

Figure 6: The so-called “Loop Street”.

While the sketch by Daniel Libeskind might be a work of art, no columns where thought to

support the ceiling of the “Loop Street”! A request not easy at all to satisfy!

Even more so, if you think that five bridges, whose structure is a composite one (steel beams with

a reinforced concrete slab), link the center of the park to each building, weighting right on the

ceiling of the “Loop Street”. Figure 7 shows two of those bridges:

Figure 7: The park of the Rb1 lot and the bridges.

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A hard work ensued to find an agreement with Studio Daniel Libeskind on how many columns

could be added to the original idea, their shape and dimensions (see Figure 6, on the right).

Finally, a few, askew and rhombus shaped columns were accepted and designed!

Of course, even though Milano is at low seismic risk, the structural design of Mr. Franchi and his team

abided by the strictest anti-seismic requirements and the greatest anti-seismic safety was guaranteed. The

Italian Code (DM 2008) and, when not in contradiction, the Eurocodes n.2 and n.8 were followed.

3 – CONTROL OF THE COMPUTATIONS

Control of the computations has been achieved through two distinct procedures:

a) Control of possible operator’s errors,

b) Accuracy of the computations.

Control of possible operator’s errors:

In order to minimize possible human errors, the following actions have been implemented:

a1) For each FE model, two people with distinct responsibilities, one for the FE model implementation

and one for the FE model control, have been appointed,

a2) A checklist to be followed by the implementer of the FE model and another checklist to be followed

by the controller of the FE model have been formulated,

a3) A list of structural “simplified analytical” schemes have been formulated, in order to verify, with an

approximate engineering estimation, the final results,

a4) The person in charge to control a FE model had also to verify its results according to the previous list.

Accuracy of the computations:

Accuracy of the computations has been guaranteed by the careful checking of some fundamental

quantities like global equilibrium equations, global mass matrix, etc.

4 – THE CONSTRUCTION WORK

Although Mr. Franchi (with his team) has never been appointed supervisor of the construction work, not a

single change has been made to the structural design, during the execution phase, without his consent.

Since the opening of the construction site, many have been the necessities to introduce something new or

different, and Mr. Franchi and his team have promptly evaluated, approved, or rejected all of them. What

follows are some pictures taken on site:

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Figure 8: Pictures taken on the construction site.

5 – MAIN FEATURES AND KEY-CONCEPTS

In no particular order:

- Box-shaped structures,

- Combined use of reinforced concrete and structural glass,

- Daring shapes and geometries,

- Different floor shapes,

- Light-weighted two ways reinforced concrete slabs,

- Vibratility analysis to ensure no discomfort,

- High quality concrete,

- Light weight (low density) structural concrete,

- Ceiling of the “Loop Street” supported by a few askew and rhombus shaped columns,

- Five connecting bridges,

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- Eurocode 2 and 8,

- Control of possible operator’s errors,

- Accuracy of the computations.

6 – CONCLUSIONS

With no doubts, this has been a great cultural and working experience! Not only it provided Mr. Franchi

and his team with always new and stimulating structural engineering challenges, but it was also a unique

managerial learning opportunity. Mr. Franchi is still young (born December 8th, 1976) and was

surrounded by professional engineers who are even younger. It is not an exaggeration to say that it was a

feat for such a young team to work together so untiringly and so effectively!

Of course, they were never left completely alone: both Prof. Arch. Ph. D. Civil Eng. Alberto Franchi (Full

Professor of Construction Science at the Politecnico of Milano University) and Prof. Arch. Ms Sc. Civil

Eng. Paola Ronca (Full Professor of Reinforced Concrete Design at the Politecnico of Milano

University), principals of the engineering firm AMiS AgenziaMilanoStrutture s.r.l., of which Mr. Franchi

is President, supervised all the operations and contributed to the overall success. Mr. Franchi and his team

would like to express their deepest gratitude to them.

Milano – August 10, 2012

Giovanni Franchi, P.E., and the AMiS Staff

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GIOVANNI FRANCHI, P.E.

He was born in Kitchener-Waterloo, Canada, December 8th, 1976. He graduated in Materials Engineering

both at the Politecnico of Milano University and at the Universitat Politécnica de Catalunya, Barcelona, in

2002. He became Professional Engineer (P.E.) during the first session of 2003, and subscribed to the

“Albo degli Ingegneri della Provincia di Milano” in May 2003. Since January 2004 he has been both an

ASCE member and an ACI member. He did some research work, commissioned by the Consortium CIS-

E at the Politecnico of Milano University, about the use of stainless steel in reinforced concrete structures

and in precast concrete structures, about the use of innovative non-invasive checking techniques, to

evaluate existing structures, and about the properties and uses of High Performance Concrete (HPC). He

hold a course on the use of structural glass in the Professional specializing Master “Sustainable Buildings

and Infrastructures” at the “F.lli Pesenti” Master School, Politecnico of Milano University. Nowadays, he

is in charge of all the online courses offered by the “F.lli Pesenti” Master School, and teaches a course on

business and investment evaluation in the Professional specializing Master “Project Management of Civil

Structures and Infrastructures”, at the same school. He has been President of the structural engineering

firm AMiS AgenziaMilanoStrutture s.r.l. since 2005. He is chief structural designer of civil and

infrastructural works worth tens of millions Euros.

For main projects and realizations, visit: www.agenziamilanostrutture.it

Lives in: Corso Indipendenza, 5 – 20129, Milano

Contacts:

@: g.franchi@agenziamilanostrutture.it

M: +39 3402628944

F: +39 02 36536941