The rethoric of Pier Luigi Nervi

Pier Luigi Nervi.jpgPier Luigi Nervi was born in Sondrio on 21 June 1891. After finishing high school in Ancona, he moved to Bologna in 1908. Despite a passion for aeronautics, mechanics and chemistry, he opted for a course in civil engineering at the Regia Scuola di Applicazione per Ingegneri of Bologna under the guidance of professor, engineer and entrepreneur Attilio Muggia. He graduated in 1913 after completing a design for a small villa. That same year, Muggia hired him to work at his Bologna-based Società Anonima per Costruzioni Cementizie, a company Muggia had set up in 1908 together with the engineer Leone Poggi.

Capture d’écran 2016-06-30 à 17.24.25.pngThe company held the license for the reinforced concrete structural system patented by François Hennebique for the regions of Emilia-Romagna, Tuscany and Marche. When World War I broke out, Nervi was enlisted in the 69th Sappers’ Company of the Bologna Military Engineering Corps, and in 1916, he was assigned to the Hydrogen Gas Airship Battalion in Rome, where he was responsible for the gas supply service. His engineering skills enabled him to develop, in collaboration with the chemistry professor Carlo Andreucci, some solutions for improving the safety of the metal devices used to compress and purify gas for airships, and he filed three patents for these designs in 1917 (Nervi was to continue working on these ideas until 1919).

Prompted by his experience working on the building sites of Società Anonima per Costruzioni Cementizie, which he continued to visit when he was on leave, Nervi developed a cementitious compound manufactured with the addition of metal fragments. In 1917, he filed a patent for this compound, to which he gave the general name of “sidero-cemento.”

The patent description is a valuable document as it reflects Nervi’s thoughts about the cementitious mixture, reinforced concrete and the skeleton in general. Nervi’s line of reasoning stemmed from certain static considerations about the nature of concrete. While manuals dating from the early 20th century reveal that the contemporary trend in reinforced concrete engineering was to focus on standardising the optimum arrangement of metal bars, Nervi examined the material composition in an attempt to overcome the mechanical shortcomings of the concrete itself. In doing so, he virtually envisaged a compound that possessed all the qualities necessary for constructing large buildings, even without bars. That this was his goal was confirmed by the definition he chose as a synonym of sidero-cemento: “reinforced cementitious concrete mixture.” The “reinforcements” to be added to the concrete mixture during the preparation and the casting stages took the form of ironworking waste, “pieces of wire,” or strips of hot-rolled iron known as “mojetta.” In the latter case, the compound assumed the characteristics of concrete interwoven with wire mesh. Nervi alluded to his own personal practice of identifying the proportion of metal “reinforcements” required for the works to be built.

978-2-940222-95-7.pngNervi’s dreams of producing a concrete that was reinforced within its very substance led him to use the term “metal fibres” to describe the fragments that gave the compound an “overall fibrous structure.” Both Nervi’s experiences on building sites and the teaching of Silvio Canevazzi had helped make him aware that although mathematical calculation instruments could be used to monitor the static behaviour of reinforced concrete structures, they could not prevent the cracking effects that had to be avoided at all costs in the reservoirs, pipes, tanks or boats that he saw Muggia building and which, he believed, he could eliminate with his “reinforced cementitious concrete mixture.” He termed these effects “secondary stresses dependent on elements that cannot be accurately evaluated when calculating the main stresses.” As a natural conclusion, his explanation of the invention ended with a list of works that would be made possible by “reinforced cementitious concrete mixture”: “reservoirs, dams, reinforced concrete hulls, etc.”

One passage in the patent description allows us a glimpse of the development prospects for his “reinforced cementitious concrete mixture.” Nervi juxtaposed two structural approaches: the Hennebique system of right-angled grid reinforcement and a future structure, to be built using his own concrete mixture and mainly subject to compression. This clearly indicated that his side- ro-cemento alluded to a structural vision that went beyond the trends emerging in building practice at that time. The ground had already been laid for Nervi’s research into reinforced concrete composition, which eventually led sidero- cemento to evolve into the ferro-cementate (ferro cementato) or ferrocement (ferro cemento) used to construct variously folded and waved membranes.

Capture d’écran 2016-06-30 à 17.24.43.png“In solids subject to bending,” he wrote in the patent, “or in stretched membranes, the presence of the above metal fibres will not generally obviate the need for the presence of main reinforcements; despite this, if we can assume the concrete mixture’s ability to withstand a certain level of tensile and shear stress and an increase in its resistance to compression, the entity of those reinforcements could be reduced, and they could even be replaced altogether in certain cases, if the percentages are properly gauged. This concrete mixture system is therefore applicable in conjunction with Hennebique and Hennebique-like reinforcement systems currently in use and can also be used to partly or fully replace concrete constructions of all types (particularly in compressed solids, arches, non-reinforced concrete walls, etc.). What is more, its use may be extended to applications that are not yet feasible”.

Hyperstatic trusses for roofs and bridges

Capture d’écran 2016-06-30 à 17.24.57.pngNervi resumed his collaboration with the Società Anonima per Costruzioni Cementizie of Muggia when he joined Poggi to become one of the employees of the Florence branch when he was on furlough in 1919. The works built ranged from bridges to roofs for libraries, markets or cinemas and represented a crucial chapter in the spread of reinforced concrete throughout Tuscany. At the various building sites, concrete was used in accordance with the building customs of the early 20th century: cast in wooden formwork prepared in rudimentary fashion to obtain a rough structure that could be clad with compounds ranging from stucco to cement mortar, as required.

Nervi was involved in designing individual works and began to experiment with his own idea of structure. He tackled technical approaches to the building site, learned to invent and develop details and structures guided by the principle of economy and examined ways to improve the manufacture of concrete and disseminate the results.

Some works from the Florentine branch of the Società Anonima per Costruzioni Cementizie are characterised by the emergence of complex geometrical structures that give a glimpse into Nervi’s originality. Nervi invented his first significant reinforced concrete structure for the vaulted roof of the building housing the Gioco della Pelota court in the Alhambra complex in Florence.

This structure, designed by Adolfo Coppedé and built between 1919 and 1921, heralded some of the technical and static aspects that are so characteristic of his masterpieces of the late 1930s. The space is covered by special arched truss girders with an elliptical profile connected to extending pillars as well as to the roof. Parallel ribs were applied to the truss girders by means of diagonal bars, all enclosed by a curved shell so that the players would see a smooth surface. The unformed lines of the structural device, including the pillar stumps, protrude from the roof like an outlandish animal skeleton, as slender as the lines of a metal truss. They bear witness to the experimental aspiration and inventive courage of their creator. Furthermore, they offer an early hint of reinforced concrete as a material able to give visible form to an intuitive perception of a flow of static forces that can be harnessed to form appropriate geometric patterns. Through the very way in which trusses, ribs and shells are juxtaposed to create a figure with its own surprising logic, this work also reflects a desire to cast doubt on the role of the engineer and explore possible geometrical juxtapositions as a means of defining a structure that is able to symbolically encapsulate the entire creative potential of reinforced concrete. What is more, this roof also offered an intuitive and rudimentary glimpse of the aspect that ultimately transformed Nervi’s own concept of structure for large roofs. Evident here is the relationship between rib and shell that he was able to put together in ever more original ways, due to his capacity for invention in the field of static form and also his research into the very nature of the compound, which he had already demonstrated with the experiments summarised in his patent for sidero-cemento.

the rethoric of pier luigi nervi

During his work on the roof for the Gioco della Pelota, Nervi became aware that temperature changes caused problems determined by the different expansion coefficients of the reinforced concrete in the pillars, which had already been poured, and the wood of the arched truss girder formwork. Until it set, the concrete deformed with the formwork and was unable to follow the movement of the pillars. At the end of the 1930s, he devoted himself to the study of prefabrication systems, partly with the aim of removing this type of defect.

Another unusual structure with moulded lines provided a rational configuration for a special device that Nervi designed in 1921 for the gas holder of the Società Mineraria ed Elettrica del Valdarno, in Castelnuovo dei Sabbioni. This was one of the first attempts to apply reinforced concrete to this type of system, which was patented by Otho Robert Verity and usually made out of metal. The new approach resulted in considerable savings, demonstrated by the international repercussions of this minor work. Nervi separated the ribs of the superstructure from the thin shell of the gas bell – some 10 centimetres thick – to allow the parts to expand freely under the effect of the different temperatures to which they were subjected. In order to build the shell, he combined the usual metal reinforcements with a metal network made out of a mesh with sides of approximately 3 centimetres and a mixture of fine aggregates in order to “achieve maximum homogeneity.” The gas holder offered him an opportunity to rave about the unstoppable advance of reinforced concrete over “iron constructions” in all “branches of constructive technique.” The competition between reinforced concrete and ironwork and sheet metal was to have decisive consequences on his view of reinforced concrete, with its geometrically structured configuration.

More truss girders, this time with a parabolic profile and stiffened by horizontal trusses, were used to form the three 25-metre spans of the bridge over the River Cecina, along the Massetana provincial road between Pomarance and Saline di Volterra. The bridge was built between 1920 and 1922, again by the Società Anonima per Costruzioni Cementizie. The details of the notching in the connections between the arched girders and those of the deck, or in the refined slenderness of the bars, now almost elastic, again revealed Nervi’s pleasure in making static function expressive and effective through structural geometry, with every part pared down to an essential skeleton in order to use less material. These connections gave rise to the “nodes” that became so crucial in Nervi’s structures. He himself explained the principles he followed when developing a structure: “maximum economy” and “minimum dead weight.” The considerable disparities that arose between the work of calculation and that of bridge construction taught Nervi about the unpredictable reactions of the parts of a reinforced concrete structure combined in a monolithic body.

the rethoric of pier luigi nervi

The piers, beams and deck were constructed out of formwork made from rudimentary wooden boards; the concrete was concealed by a layer of grey smoothed cement mortar, as was the norm for this kind of engineering work.

Nervi used I-beams hollowed out in accordance with Vierendeel geometry in order to build the bridge over the River Pescia, near Pescia, which was only 3.50 metres wide and subdivided into four spans (the longest measuring 20.60 metres), again built by the Società Anonima per Costruzioni Cementizie between January and May of 1923, and intended to carry the trucks and wagons transporting products produced by the Fratelli Marchi fertiliser factory. The beams also formed the high parapets, which were again clad with smoothed cement mortar. Studies for the bridge over the River Cecina revealed Nervi’s idea of a horizontal, yet trussed, girder to be set up beneath the deck. His method of calculating the moment diagrams steered the design for the structure of the bridge over the Pescia, including its piers and deck, toward the desired “minimum dead load,” low building costs and the use of minimal quantities of concrete and steel. Quintessential examples of this paring back of each part to its essentials are the cantilevered connections between piers, deck and beams used to stabilise the piers with their reduced cross-section or the application of hollowed out I-beams along the neutral axis area. In some ways, the cantilevered connections between piers and beams are equivalent to the diagonals in the truss girders of the Gioco della Pelota roof, given their static role in the general definition of the structure. To an even greater extent than in the spans of the bridge over the River Cecina, Nervi built restraints between the reinforced concrete parts in order to define a monolithic body with all the hyperstatic characteristics that were to become the trademark of his structures. The following is almost a statement of principle, a manifesto of the hyperstatic structure to come as well as a specific declaration of intent about the bridge over the River Pescia: “Due to the slenderness of the piers (designed to offer the least resistance to the free flow of water) and hence the weak resisting moment, it was not believed necessary to separate the beam from the piers, given the intrinsic deformability of the piers.”

Other bridges built in Tuscany by the Società Anonima per Costruzioni Cementizie bear witness to the design variations put to the test in structures built under Nervi’s guidance. The bridge over the River Sterza at Salitone, for example, reveals a new and more sophisticated expression of the new role that Nervi sought for the bars in the spans built for the bridge over the River Cecina – now the arches become a powerful support structure to which the deck is anchored by vertical ties.

Slender components, hollow material, rigid connections and an increase in the number of restraints transformed the constituent parts of the Gioco della Pelota roof or of the trussed bridges with the nodes that rendered them hyperstatic, into a quest for the ideal essential form, beyond the rules of mathematics.


Excerpt from The rethoric of Pier Luigi Nervi
By Roberto Gargiani & Alberto Bologna
Published by Presses Polytechniques et Universitaires Romandes (PPUR)

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