Proceedings of the International Association for Shell and Spatial Structures (IASS) Symposium 2013 „BEYOND THE LIMITS OF MAN” 23-27 September, Wroclaw University of Technology, Poland J.B. Obrębski and R. Tarczewski (eds.) Nervi’s cantilevering stadium roofs: discipline of economy leads to inspiration Sigrid Adriaenssens1, David P. Billington2 1Assistant Professor, Form Finding Lab, Princeton University, Princeton, USA, [email protected] 2 Professor Emeritus, Department of Civil and Environmental Engineering, Princeton University, NJ 08544, USA [email protected] Summary: Although he came from a tradition that built magnificent palaces and churches, the Italian master builder Pier Luigi Nervi is arguably mostly known for his sports infrastructure. In this paper, we focus on his first (Giovanni Berta Stadium, Florence, 1930-1932) and last (Olympic Flaminio Stadium, Rome, 1957-1959) internationally widely recognized stadium with cantilevering roof. Through literature studies and structural analyses, we compare and contrast these two stadia on the basis of their structural and constructional efficiency, economy and design intent within the specific social-political context of the interbellum and post-World War II era in Italy. The interpretation of these studies shows Nervi’s clear progression from imitation, innovation to inspiration in his stadium design. More specifically shortage of construction materials and skilled labor did not constrain his projects but drove Nervi to design and build great structures. Keywords: Nervi, ferro-cement, prefabrication, stadium, cantilever, grandstand 1. INTRODUCTION 2. IMITATION: ARCHITECTURAL FORMS AND IDEAS ABOUT REINFORCED CONCRETE Although he came from a tradition that built magnificent palaces and churches, the Italian master builder Pier Luigi Nervi (1891-1979) is Nervi’s life has been intrinsically linked to concrete. One year before arguably best known for his sports infrastructure. In 1960, at the height François Hennebique filed his reinforced concrete patent in France and of his career, Nervi’s Olympic structures were lauded worldwide. The Italy, Nervi was born in Sondrio (Northern Italy) in 1891 [1]. He question arises: how did Nervi’s structures evolve from his early pursued a degree in civil engineering at the University of Bologna. As a architectural projects to the engineering inspirational masterpiece found young student, he studied and appreciated the correct relationship in the Olympic Flaminio Stadium (1957-1959, Rome)? The design and between structure and shape, a correlation he would later routinely construction success of this widely-recognised stadium was entirely his express in his cantilevering roofs over grandstands. Figure 2 shows an own. Nervi stated that the series of innovations he developed in the two abstract from the course notes of Professor Silvio Canevazzi, who taught decades leading up to the Olympic Games formed the basis for this him Meccanica applicata alle Costruzioni. The figure shows the ideal realization [1]. How did constraints, imposed by Italy’s socio-political shape of a cantilever with uniform thickness and variable height (a) context before and during World War II, influence Nervi’s evolution in under an edge point load, and with uniform thickness and variable height stadium roofs? (b) under a uniformly distributed load [2]. In his lectures, Canevazzi In this light we study the progression from imitation, innovation to instilled in Nervi the notion that engineers should develop an intuitive inspiration in his two most widely-recognized stadiums with understanding of structures and be critical of established theories and cantilevering roofs. The Giovanni Berta Sports Stadium (1930-1932, practices [3]. Florence) and the Flaminio Stadium are compared and contrasted on the basis of (i) structural efficiency (forked cantilever versus simple pleated beam cantilever system), (ii) material (reinforced concrete (rc) versus ferro-cement), (iii) construction techniques (in-situ versus segmented pre-cast) and (iv) their socio-political context (interbellum versus post World War II Italy). Although Nervi designed many more stadia, the Berta and Flaminio stadiums were selected for this study because both (i) were entirely designed and constructed in an Italian context by Nervi’s own family-run firm (see figure 1); (ii) have similar cantilever roof spans, seating capacity and ellipsoid plan; (iii) at the time of their completion they were instantly internationally applauded and (iv) they span Nervi’s professional career (Berta early career and Flaminio late Figure 2. Ideal shape of a cantilever under an edge point load with uniform career). thickness and variable height (a) and under a uniformly distributed load with uniform thickness and variable height (b) (credit Library of School of Engineering and Architecture, Bologna) After graduation in 1913, Nervi spent approximately ten years (interrupted from 1915 to 1918 by World War I) with the Societa Anonima per Costruzioni Cementizie under the mentorship of his Professor, Attilio Muggia, who was the only licensee for the Hennebique patent in Central Italy [3]. This invaluable experience with one of the best organizations in Italy in reinforced concrete work, provided a solid foundation for his future work, but also restricted his freedom as a budding structural innovator. In 1923, when he was about to get married, Nervi formed his own design and construction company Soc. Ing. Nervi & Nebbiosi with co-founder Rodolfo Nebbiosi, the financier and Figure 1. Nervi and his sons Antonio, Mario and Vittorio at the entrepreneur of the team. The company’s portfolio showcased a wide Flaminio Stadium (1960, Rome) (book Pier Luigi Nervi: Architecture as range of utilitarian projects such as industrial plants, tanks and a Challenge, 2010) warehouses. In 1929 Nervi started working on the design of the Giovanni Berta Sports Stadium (which is now called the “Artemio 1 Franchi” Florence Municipal Stadium), a project that was divided into The roof (shown in figure 4) consists of a thin reinforced concrete shell many subprojects. spanning continuously between 15 curved reinforced concrete cantilever beams with an expansion joint at every third beam. The roof structure During the construction of this stadium, Nervi broke up with his region between the exterior and interior columns is referred to as the business partner Nebbiosi and founded a small family-run company Soc. ‘backspan’ and the part between the interior column and the roof tip is Ing. Nervi & Bartoli with his cousin Giovanni Bartoli (1932). The referred to as the ‘cantilever’. The cantilever measures 14.6m and the realization of the Florence Municipal Stadium, a reinforced concrete backspan is 7.4m long. All dimensions used for this study are taken or grandstand with a cantilevering roof, was crucial for Nervi’s success as scaled from the general cross-sectional drawings [7]. The depth of the the head of a young design-and-build company. The grandstand was overhang beam varies from 40cm at the cantilever tip to 2.6m at the highly appraised by Italian and international critics [4]. The bold design interior support before it forks into a tension and a compression chord of for the cantilever roof and the sculptural expression of the winding 55cm in depth. With no indication of the shell thickness and the roof external staircases appealed to a new generation of young rationalist beam’s width at hand, these values were conservatively estimated to be architects. 10cm deep and 40cm wide respectively. To prevent torsional buckling of the slender beams, Nervi introduced two restraints, namely two additional longitudinal beams. One large beam, located approximately at the overhang’s midspan, runs longitudinally between the 15 cantilevering roof beams and one smaller one between the tops of the exterior columns. The bifurcating roof system is integrated in the supporting frame below. This series of frames also support a floor. The floor is assumed to have a slab thickness of 25cm. At the ground level the frames consist of two columns (interior and exterior) spaced 9.85m apart. Reinforced concrete is assumed to have a density of ρ=2400kg/m3. Support reactions at the frame’ footprint: By modeling the roof as a cantilever beam supported at point A and point B and the cantilever tip at C under self-weight, the resultant of all loads is found at 7.4m from support A. This location falls well in the span between the two bottom frame columns A and D (figure 5). Nervi wrote “… the fact that the resultant falls within the two columns eliminates the necessity of costly foundation anchorages.” [5] Our analysis of the entire grandstand frame with overhang shows that the entire self-weight and any effect of Figure 3. Overall view of the Florence Municipal Stadium showing the overturning causes only compression forces on the frame’s footprint. oval playing field, straight race track in front of the covered uncovered Nervi would have considered other loading combinations (including grandstands and Marathon tower (book Aesthetics and Technology in wind, snow, temperature and maintenance) in his detailed calculations. Building,1965). Uplift forces on the cantilever due to wind are not as crucial since they would be counteracted by the self-weight of the roof. The self-weight is 2.1. Socio-political context of the Florence Municipal Stadium the dominant load case and thus suffices to conceptually demonstrate Nervi’s design intent to arrange the topology of the overhang