Metal trusses: a construction tradition

The metal truss is born from the wooden one through a process of specialization of the structural components.

Metal trusses, as a construction tradition, derive from wooden ones and developed in the 19th century, coherently with the spread of the use of cast iron and steel in the construction industry. In turn, they gave impetus to the development of steel lattice girders, also the result of calculation and construction science.
They are therefore a fine example of the historical evolution of a construction element with an elementary shape that has transformed over the course of history in step with the availability of new building materials and the refinement of techniques.

In general terms, the truss is that triangular shaped structural element used to form the main framework of the pitched roof, on which the secondary framework and therefore the roof covering are placed.

The idea of ​​a hut, of two elements placed side by side that support each other by mutual contrast, resting or planted on the ground that supports their weight and absorbs their horizontal thrusts, represents an ancestral concept of shelter, of protection from the rain. If it is not an archetype as old as man, it is very close to it (we cannot forget about caves and dolmens), so much so that if you ask a child to draw a house, it is quite probable that we will obtain something similar to a square with a triangle on top.
It is precisely this scheme of the non-deformable triangle which over the centuries has changed in terms of materials, defines its structural concept and gradually streamlines its elements, until arriving at metal trusses which mark a sort of terminus, from here we move on to reticular structures composed of non-deformable triangular meshes, but no longer, or not necessarily, triangular in their perimeter shape.

Why is the triangle so important?
The triangle is an undeformable structure. As long as it remains inside the triangle it is called a truss, if there is a mesh of triangles we enter the realm of trusses, if the triangle does not have a base (the chain) the horizontal forces are discharged on the supporting walls and we talk about trusses.

In the traditional wooden truss, the one formed by a chain, two struts and a monk, the latter does not divide the triangle in two, because it does not touch the chain, it serves to balance the struts and is stressed only by the light traction of its own weight. If it weighed on the chain, this would have a load (in addition to its own weight), a greater deformation due to pressoflexion (traction instead limits the bending of its own weight), the monk would work in compression and we would no longer have that perfect machine which is the truss normally defined as Italian, or Palladian, which in fact refers to the treatises of our late Renaissance. The French tradition is different, it is influenced and derives from the construction technique of the Gothic churches of the Middle Ages, but this does not mean it is detached from the evolution that will lead to steel structures.

When the span to be covered increases (normally beyond 10-12 m), the Palladian truss becomes composed, because other elements intervene (braces, double monk, counter-chain, reinforcements on the struts, etc.), but always within the main triangle, without ever touching the chain and always with the aim of limiting the bending of the struts, or to create them with two shorter sections instead of one long one. 

A detail to be highlighted in the evolution of this structure from wood to iron is that the sections of the wooden trusses components are more or less all the same, regardless of the loads and stresses, to facilitate the fitting, i.e. the connections (dovetail, Jupiter's dart, etc.) and the relative metal bracketing.
It goes without saying that some elements are always oversized, especially the monk. This will change with the advent of steel and cast iron which are children of calculus.

Even in its most basic form, the truss is an isostatic lattice structure that serves to discharge only vertical forces onto the structures that support it, while absorbing the horizontal components deriving from the load of the roof on the inclined elements.
If at first glance wooden trusses seem to be very distant objects from steel lattice beams, the typological evolution of metal trusses makes this boundary much more blurred.
This proximity helps to “confine” the development and diffusion of steel trusses within a rather defined historical context that begins with the industrial development of the second half of the 800th century and ends recently with the transition to covering industrial sheds with flat structures (metal lattice beams, pre-stressed reinforced concrete, laminated wood, etc.).

This does not mean that they are construction elements that are no longer in use or out of fashion, only that they had their moment of glory in an era in which there was a need to cover rather large spans (those of the warehouses that are now part of our industrial archaeology) with relatively light roofing layers, arranged on pitched structures.
For this purpose, wooden trusses were less suitable, more expensive for large spans, too heavy for sheds with a thin shell that favors large openings to have as much light as possible inside. In addition, the presence of industrial processes could expose wooden structures to a greater risk of fire than that of a church. On the other hand, it should be admitted that the typical and valuable essences for wooden trusses, in particular larch, are not so flammable and resist fire quite a lot, while the thin metal structures may not catch fire, but collapse much sooner due to the heat caused by the fire.
However, the most important factors for the adoption of the metal truss remain the span, the cost, the lightness and its image, the result of calculation and in keeping with the age of steam, coal, cement and steel. To realize this, just compare the two images: the tie rods of the Barcelona market are almost invisible, the monks of the trusses of the Doge's Palace in Venice are much bigger than the fire extinguisher.

We are therefore witnessing the progressive replacement of the wooden truss elements with metal ones, starting with the one that works in traction, the chain, which is also the piece that requires cutting the largest and most valuable wooden logs, in spite of a much thinner metal tie rod. This is a progressive functional specialization of the components that can guarantee greater resistance with lower self-weight, which is particularly important for extended spans.

The study of mixed typologies in wood and steel can prove particularly useful in the restoration of wooden roofs where it is necessary to replace some elements, or to reinforce the structure as a whole by placing other supports alongside historical elements that maintain a clear typological distance from the former.

The protagonist of the story about the transition from wooden to metal trusses and then to lattice girders is called Jean-Barthélémy Camille Polonceau (1813 – 1859), a French railway engineer (like his father and uncle) who in 1837 developed a mixed wood and steel system in which a system of metal tie rods replaced the struts and monk beams. From this scheme he then arrived at schemes of completely metal truss that bear his name and that we still identify simply as Polonceau.

Modern materials guarantee greater durability than wood, especially on the nodes. The use of cast iron for the compressed elements (the buttress that replaces the strut) and steel for those that work in traction (tie rods) highlights even more the different paradigm of modern construction techniques based on calculation, even when wooden struts continue to be used, because in that position and with that type of compressive-bending load, a steel element would be neither much slimmer nor much lighter than wood.

The Polonceau may perhaps represent a transitional element and in its figure there remain three well-defined vertices within which a stiffening network develops. Another example of evolution in mixed structures is that of Amand-Rose Emy, again around the 40s.

From the double-strut structures in the shape of an inverted V by Polonceau and other experiments with metal and mixed frames, we move on to the English typologies that end their metamorphosis towards the lattice beam, up to conformations that resemble elongated pentagons rather than triangles.

Cover image: Lightweight Steel Truss System – https://www.roofseal.com.my/