The empirical approach in the evolution of bridge and structure

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The empirical approach in the evolution of bridge and structure
design: the contribution of Pier Luigi Nervi
Alessio Pipinato*
Università degli Studi di Padova
ABSTRACT: this paper illustrates the result of a study concerning the design and modeling of
structures of the famous Italian engineer Pier Luigi Nervi (1891-1979). His approach in studying and
in building a large amount of structure, has a theoretical base, never investigated deeply. From an
analysis of the documents and from a comparison among diverse buildings by Nervi, it is possible to
recognize his innovative theory, in which structure and architecture are the subject of the art of
making buildings: the theory of practice.
INTRODUZIONE
Pier Luigi Nervi, (Sondrio 1891 – Roma 1979), has been recognized as a pioneer of concrete
construction. In 1913, after the degree in civil engineering, he worked in a Construction Society in
Bologna for ten years. In 1920 he founded his own firm with eng. Nebbiosi in Rome. Then the
crucial step in his life: the idea to found his own construction society, in 1932, with his cousin, the
engineer Bartoli. This firm worked till 1978. Among recognized well known constructions we can
mention: the municipal stadium, Florence; two hangars, Orvieto; six precast hangars, Orvieto;
Unesco site, Paris; Pirelli center, Milan; sport palace, Rome; Corso Francia viaduct, Rome; Vatican
building, Vatican City1-2.
AN EMPIRICAL APPROACH
Nervi was very famous also for his practical approach in structure design. They are well known his
words: “It is necessary that architecture play an important role, and that architects, engineers,
constructors, students, has the consciousness of the necessity that in designing, executing and
judging an architectonic opera, they have to consider abstract and material values that comes with
it. An important role in this process, is that particular sector that we can now define architectonic
structure”3.
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His theory is not based in some abstract concept, as a special language, but in the use of a single
material, such as concrete. Concrete is used in innovative ways in building system, with it can be
created something like a stone, or, better, an artificial stone, fused, with any shape, better than the
naturals one because of its tension resistance: so it’s something “magic” for that historical period.
The best property of concrete is that it is monolithic, and is also the material from which you can
obtain shining and specific statics solutions. Every beam is solid with the columns, every frame of a
building is a unique structure, in which solicitations of one part are strictly related to the other parts
of the constructions, and nearly they melt in the resistant organism4-5.
In this approach, the concrete building is seen not only as a structure, but as a unique system. Here,
his view of the building is very different from other builders of that period, but consequentially and
well connected with the aims of the modern movement, especially with some ideas of Richard
Neutra, where he thought that architecture has to express the most affordable relationship between
science, technique, industrialization and good taste6.
Nervi proposed to engineers and architects to apply structure practice in an aesthetic manner, where
new technologies concur to concentrate the stresses, reducing the resisting section, using less
materials, showing what was previously hidden.
This view, and this aim to put structure in the field of architecture, gave him some problems.
For example, he didn’t care about critic: he thought that the shape of a building and the system of
membranes under loads so with various solicitations as bending moment, compression, tension and
so on, are made and connected to support the forces with the more adapted material, and didn’t care
about any stylistic research.
FORMAL LANGUAGE
However, it is impossible to forget that Nervi used a real language in building. Of course, concrete
is the principle material used, but also we have to consider his thought: “ The idea of a resistent
system is a creative act, only in the part based on scientific data; the static sensibility even if a
necessary consequence of the equilibrium study and of the material resistance, is, as the aesthetic
sensibility, a pure personal ability, or, better, the consequence of the comprehension and
assimilation of the physics laws “7.
In this words, he tried to explain also that the independence of the designers in the conception
phase, is not only from other solutions just adopted, but principally based on a deep and strict
surveys on the optimal relationship between available materials and purposes.
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The membranes he adopted in his famous project of the two hangars in Orvieto, or the beautiful
roof in the Vatican City, are based more on the empiric and small scale observation than on
complicates calculations. He said, for example, while the building of the new stadium of Florence
was growing on, that he had a lot of problem with an helicoidal stairs, especially from the structural
point of view and for the dimension and position of the reinforcement steel: he solved that problem,
thinking on a previous and more easy project, and doing simple and rapid calculations. An
important lesson from a great master.
THREE FOUNDAMENTAL PROJECTS
a) Stretto di Messina bridge, Messina-Catania
This project was thought in 1969 by Nervi and his cousin Antonio Nervi, in the occasion of a
national competition for the Messina Bridge, a link between Sicily and Calabria, in the south of
Italy.
“Every my effort has been directed to eliminate every weak point of the great suspended bridges,
that is the insufficient lateral stability of the decks in the cares of the induced horizontal actions
from the wind. The relationship between the width of the deck and the length is yet next to a value
limit in the greater suspended bridges realized; in the present case it would have been a lot under
such value, so the traditional outline of suspended bridge with parallel ropes had necessarily to be
abandoned. Reflecting on the problem of an intrinsic cross-sectional stability of cables and
consequently of the inside of the deck, I convinced myself that such stability would have been
obtained spontaneously in the case of two cables oblique rather than parallels, with a parabolic
course whether in the vertical or in the horizontal projection. In order to obtain this design, it is
indispensable that the supports of the ropes to every extremities are considerably distanced one to
each other, except in the way in which you unite the deck’s cables and the central section."8
Nervi calculated the structures. As we can notice from the picture, he formulated a gigantic plan,
with four towers, totally different from the simple previous bridge made by Fiat, dealt for years as
the bridge from the minimal impact in that place. A bridge that runs to 70 meters of high, nearly
touched houses of two plans and poles of the public lighting system of 5 meters of height, and
significantly bigger than big boats.
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Fig. 1: Pylons are made in two parts: a basement concrete made, with a quarry structure interrupted
by horizontal plane, shaped like an hiperbolic paraboloide; steel towers, fixed with eight rope to the
basement, reinforced by four reticular anular structure.
Fig. 2: View and plant: in the central zone, the tow-rope are directly linked to the deck, making an
unique rigid structural organism.
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Fig. 3: Cross section: a steel reticular structure, and rigid as much as to obtain a sufficient torsion
resistance.
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Fig. 4: View of the model: except for the central zone, the link among the connections of the structural
ropes and the deck, is obtained by two orders of cables variously inclined, which constitute the
diagonals of a resistant spatial structure.
Fig. 5: Lateral view of the model.
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b) Corso Francia viaduct, Rome
In occasion of the Olympic Games of Rome, in 1960, a great part of the city, called EUR, in which
Nervi built also the famous Sport Palace.
The Corso Francia viaduct, was built to link directly EUR to the city centre, through the Flaminia
street. It was made of two streets of 10,5 meters, with a length of approximately a kilometre. The
two tracks are first rectilinear and separated from a continuous opening of 5 meters of width,
interrupted every 48 meters from pedestrian passages. In the final part of the curve, each side is
branched off, selecting the traffic currents according to the direction. The curves follow the
development of one clotoide9. The aim of the project, was to realize the deck and the columns with
the building unification and the partial precast.
Fig. 6: Original study for the deck and piers type.
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Fig. 7: Piers section type. The basement elements – the columns and the consoles -,substain a series of
“V” precast beam, on which are placed the precast slabs of the street platform. Down, on the left, the
scaffolding system. Down, on the right, the geometry of the column: the section, crossed at the base, is
linked to the upper part through a series of lines.
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Fig. 8: On the left, scaffolding and axonometric of precast slabs. On the right, axonometric view of a
precast beam. The structure is well proportioned to obtain during the various phases and on exercise,
no tension on the edges.
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Fig. 9: View of one of the final column: the viaduct has a 2% longitudinal slope and so the height of the
columns varies from 8 to 3,50 m. For the columns casting, the same shape were used.
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c) Risorgimento bridge, Verona [figg. 10-18]
This bridge is made of a continuous deck, three spans. The laterals span are 34,50 m large, while the
central one is 62 m. The deck is built on two central pylons, linked with steel hinge, and on the side
on concrete wall. One of the hinges is fixed while the other three are mobile to allow the structure to
move under exercise and thermal actions. The deck is composed by six longitudinal beams, with
variable height, linked together every 11,50 m to the lateral spans and every 10,30 m to the central
ones. The variable height of the beam is function of the internal stress variations; the upper deck is
20 cm thick; the inferior deck is variable. Therefore, all the bridge structure is the consequence of
the “crystallization” of the forces.
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Fig. 10: Particular of the lateral span; see the uniformity of the concrete that shape the structure as a
skin.
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Fig. 11: The central supporting pier.
Fig. 12: Photo-view of the bridge.
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Fig. 13: Frontal drawing view of the bridge.
Fig. 14: The principals cross sections: the different high of every section and the different thickness of
the inferior deck are the principals characteristics.
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Fig. 15: Static scheme.
Fig. 16: The construction phase.
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Fig. 17: The reinforcement design.
Fig. 18: Longitudinal and cross variable sections.
NOTES
1: P. Desideri, P. L. Nervi, G. Positano (1979), “Pier Luigi Nervi”, in Serie di Architettura,
Zanichelli Editore, Bologna;
2: A. L. Huxtable (1960), “Pier Luigi Nervi”, Masters of world architecture, Braziller, New York
3: P. L. Nervi (1963), “New structures”, Edizioni di Comunità, Milano
4: Aa. Vv. (1945), “Scienza o arte del costruire”, Edizione della Bussola, Roma
5: P. Desideri, P. L. Nervi, G. Postano, note 1.
6: D. Neutra (1998), “The Neutra genius: innovation and vision”, Modernism, vol. 1 no. 3
7: P. Desideri, P. L. Nervi, G. Positano, note 1.
8: P. Desideri, P. L. Nervi, G. Positano, note 1.
9: P. L. Nervi, note3.
*: Dr. Ing., Università degli Studi di Padova, Italia