Paper mill and bridge
Pier Luigi Nervi in Mantua
P. L. Nervi (1891-1979) wrote the history of the first great season of modern structural engineering, together with Robert Maillart (1871-1941), Eugene Freyssinet (1879-1962) and Eduardo Torroja (1899-1967), great masters who were the first to explore the expressive potential of the new material – reinforced concrete – producing inventions founded on structural experimentation with new static figures and the construction and building site technologies.
Beginning in the 20s, but above all between the 50s and 60s in Italy, a number of studies and advanced works into structural engineering were formed based on the theories of Gustavo Colonnetti (1886-1968) and Arturo Danusso (1880-1968) and, from the operational point of view, on lan, given the great international clamour that was caused, on Nervi and Riccardo Morandi (1905-89) , followed by Sergio Musmeci (1926-1981) , Silvano Zorzi (1921-1994) and Fabrizio De Miranda (1926) whose fame was internationally known. These studies made the structural engineer enter the world of architecture, where many other minor heroes appeared and gradually created regulated town planning and rational architecture, the reconstruction and the economic miracle (the Fanfani plan), the Rome Olympics (1960) and the “Italy 61” celebrations in Turin (1961) .
In Nervi’s vast production the Burgo Paper Mill is in fact one of the less “Nervi” examples, based on the structure/shell dichotomy which, through the composition procedure of mounting static and construction figures (the reinforced concrete and steel ‘bridge’ and the steel-glass ‘box’), solves the need for a complex industrial installation to manufacture paper.
The mill is still working today, and stands at the gates to Mantua like an enormous relict that was washed up on the banks of the Mincio River.
The questions involved with building design restore the profoundest reasons for formal and structural invention. The functional layout from 1960 planned for a shell to house the hundred meter long machine for producing the paper rolls, without any static structures along one side as there were plans to double the plant. This did not occur, but the functional layout inspired a design for a long lengthways span.
The idea of a building on an arched structure seemed excessively costly, and was replaced with the idea to statically and figuratively split the structure of the building with a suspended bridge to hold up the roof of the building containing the large machine, making it statically independent.
Consequently, the static structure was everted, which is the noteworthy and exceptional feature of the work together with the use of steel, unusual for Nervi who called on the assistance of the engineer Gino Covre, who was specialised in this field .
The outcome is a bipartite system formed of the shell-building and the bridge-roof, which breaks away from Nervi’s tradition of a single and monolithic style building.
The shell-building is formed of a reinforced concrete base, emphasised by an external brick face, which the large nave rests on that holds the paper machine, infilled with steel and glass to underline the hierarchy of the sections.
The base is on two levels where the paper is manufactured, culminating in the large machine on the higher floor (7 meters above the ground).
The nave is 30 meters wide, 250 meters long and 15 meters high (net height app. 11.5 meters).
Towards the outer wall there is a 4.85 meter tall interface (the aforesaid base covered in brick), detached by about 80 centimetres from the inner face, and forming a hollow wall (for technical and service passage), with the series of steel infill uprights on the top. Each upright is a vertical bracket that is slabbed at the foot, creating a sort of reverse curtain wall, a technical device that makes the building shell independent from the roof structure. The shell-building and bridge-roof infills are able to move separately due to their different deformations, without compromising their functions.
The boundary line between the two sections of the aforesaid bipartite system creates the discontinuity point, which determines the original construction weakness due to the induced condensation from the relative ‘thermal bridge’.
The uprights are 13.50 meters tall with a constant width of 25 cm, measured on the front, and are placed with a centre distance of 1.50 meters to withstand the wind load on the large façades, which have now been ‘renovated’: the section is formed of a hollow box element by joining two “C” elements, with a reinforced core (able to hold the rainwater pipes set with a centre distance of 4.50 meters), tapering in from 90 to 35 centimetres, towards the top and bottom ends, beginning at a height of 1.85 meters from the impost.
The bridge-roof has a suspended bridge design, formed of two pairs of reinforced concrete piers, twelve steel catenaries, eighty-four parallel steel suspended cables and the reticular grid of the steel roof.
The pylons are formed of an inclined upright held by a shorter compressed strut, with a lambda shape reaching a height of 47 meters. To reduce the economic burden of the formwork, given the irregular shape of the section design that requires tension, Nervi ordered specific research into structural prefabrication , making prefabricated forms buried in the iron-concrete which was done on the construction site, with 80 different types, mounted with the help of stiffening pillars, which are placed in series according to the concrete casting, and create the current external finish and the final impression of the amazing figures of the piers.
The pylons are joined transversally by reinforced concrete girders (one half way up and the other at the top), with two caissons at the top placed above the second windbracing girder: these caissons have an irregular pentagonal shape (measuring approx. 6 meters the base and 5 meters in height), with a steel core coated in reinforced concrete. They have a circular ventilation manhole measuring approx. 65 centimetres in diameter, and they house the anchoring for the four catenaries that the suspending uprights are connected to for the main roof girders.
The catenaries are polygonal in shape, with a parabolic envelope made of articulated steel bars formed of flat irons joined to each other and to the caissons by means of a hydraulic device (hydraulic jacks to control the tension). Each element in the polygon measures between 13.60 and 6.30 meters in length and they are joined to each other and to the vertical suspension cables by a complex node. The uprights are formed of a series of parallel cables measuring 45 millimetres in diameter, placed at intervals of approx. 10 meters, to hold the main longitudinal girders in the roof.
The roof is made entirely of steel, measuring 250 x 30 x 2 meters. It is formed of the main longitudinal girders hooked onto the uprights, joined orthogonally by a secondary framework with diagonal windbracing girders. It is fixed to the pylons by longitudinal and transversal blocking plates which prevent all translation. The covering pack for the extrados, placed on trusses with spacing elements for the best transversal inclinations, is made from Alusic fretted plate anchored onto Eraclit panels, with layers of bituminous felt roll between, and on the inside a false ceiling of corrugated glass resin panels characterise the intrados finish.
Reading Nervi’s projects means following, uninterrupted, the process from conception to construction : an intense research into the limitations of the theme and the architectural idea that leads to the birth of a new construction.
Nervi writes: “An architectural work cannot be considered such until it becomes a living reality of materials and organisms, which satisfies the functional and economic reasons for its design. […]. This means that an architectural work must respond to the numerous limitations and requirements that are achieved in the three macro categories of statics, function and economy. To satisfy these limitations, harmonising with the fundamental aesthetic idea or, better still, making them linguistic and expressive terms for it, constitutes the real essence of an architectural problem and one of the main reasons for the incomparable nobility of Architecture” (P.L. Nervi, Science or art of constructing?, Rome 1945, p. 12).
Is architecture today still statics, function and economy?
Acciaio Arte Architettura 53