PER UNA STORIA DEGLI ELEMENTI METALLICI NELLA COSTRUZIONE E NEL CONSOLIDAMENTO DELLE CAPRIATE LIGNEE

Emanuele Zamperini

Università di Pavia, via Ferrata 1, 27100 – Pavia, emanzamp@yahoo.com

ABSTRACT

Ferrous metals are often seen as antagonists of timber in buildings, since their gradual spread in structural elements has led to the progressive decrease of timber structures diffusion. However metal elements were and still are used in timber structures and have significantly influenced their evolution in time. On the basis of information found in treatises, historical handbooks and – for more recent times –technical journals, of archival sources, of modern historical investigations, and of direct survey, the paper attempts a critical historical reconstruction of the technological evolution of metal elements used in timber trusses between Middle Ages and the mid-20th century, making explicit its influence on the evolution of trusses typology. The paper also deals concisely with the diffusion of metal elements in strengthening works of existing timber structures, especially with reference to the last century of the analyzed period.

In Middle Ages carpentry joints and constructional types of trusses were generally designed so as not to require metal elements because of their high cost. Between 13th and 15th century, the evolution of iron industry allowed a drop in the cost of ferrous metals. In trusses the use of wrought iron in the form of nails and brackets grew, therefore a series of changes took place, including the gradual abandonment of “closed-joint” trusses in favour of “open-joint” ones. In 18th and 19th century, iron and steel industry revolution led to an exceptional production increase: new metal elements spread allowing the creation of the steel-timber trusses and of entirely metal ones. Since the second half of 19th century, the transition from empiricism to structural mechanics and a progressive loss of building traditions led designers to strengthening decayed timber structures using metal ties or rolled beams. In early decades of 20th century, further innovations in the field of hot and cold steel working allowed the invention of new elements and connection systems for timber structures (e.g. bolts, split rings, toothed rings, spiked plates) leading to the creation of new types of timber roof structures (reticular beams and portals).

Parole chiave/Key-words: nails, bolts, brackets, metal ties, timber trusses.