THE HISTORICAL BACKGROUND OF LAMINATED WOOD
At the beginning of the twentieth century, Otto Helzer, a Swiss carpenter master from Weimar, had the ingenious intuition to replace the mechanical connection means with a casein-based glue.
This is how glued laminated wood was born, as it is understood today, and the process was patented by the inventor, in Austria and Switzerland, in 1905.
Other historical precedents, recalled by some scholars, cannot strictly refer to current technology.
They, in fact, took up the construction technique of the stone arch, to create structures formed by wooden elements profiled according to curved surfaces, with staggered joints and locking keys between individual elements.
In France, Philipe Delorme was among the first to propose the use of coupled knife boards to use them in the covering of frandi lights.
Delorme, thought of transferring to wood what was known for stone constructions: the arch shape.
In practice, it proposes an arch with blades arranged like a knife held under pressure by wooden tie rods.
(Image Philibert Delorme, 16th century)
The first treatise in Italy dealing with lamellar arches dates back to 1700. In Florence, G. Del Rosso
illustrates the construction of wooden bridges with lamellar arches held under pressure by means of metal bands. Here is a passage from the text: "the arch is constructed in the following way.
Once you have made a cent of wood veneers, you will have a large fir ax hinged on it, two cents per arm. Above this you will glue a second one and stop with sticks of the same wood. Above this, another one is placed with the same method and consecutively as many others as are sufficient to make the thickness that one wants this arch to have, warning to divide well the joints of said slats, so that one does not fall straight above the other one;
the glue is of maximum strength and takes for everything; the sticks are many and as far as possible, the holes to be made with the gimlet do not match those below.
Finally, since the arch is thus finished, iron harnesses must be made at various distances that go well in the center, strongly tightened with its posts above, so that they cannot be separated and this arch will be of surprising strength. "
1797 - G. Del Rosso, Grand Duchy of Tuscany, proposed bridge made up of slats.
Chronology of authors and proposals that have marked the evolution of the concept of the reconstruction of a beam, with reduced basic elements, up to the Hetzer patent.
It is interesting to note how all the proposals refer to the arch; perhaps because it is particularly suitable for large lights.
The evolution of the technology of lamellar arches, the main construction technique that was referred to until the beginning of this century to cover large spans, has followed four main theories due to Delorme, Emy, Migneron, Wiebeking.
In the Emy system, the boards were arranged as they currently do and formed a real glulam arch as we are used to seeing today.
On the left G. Del Rosso, main section of a wooden ship, in the center, arched covering in laminated wood designed by colonel Emy in the 19th century, on the right the arch with the Emy system: construction detail. (Click on the images to enlarge)
The Migneron system provided for an arch with lamellae formed by cold curved beams. The Wiebeking system was also quite similar, which used cold-bent beams held under pressure by metal brackets and wooden wedges that prevented possible sliding. Also in the last century, the German engineer Moller imagined 'a structure similar to that of Migneron and Wiebeking' but with the introduction of wedges between the wooden elements that improved the inertia of the section.
In the image on the left, the bridge over the Rio Nuovo in Venice, built by Eugenio Miozzi in 1932.
The choice of wood, that is, a light and resistant structure, was determined by the need to contain the thrusts on a shoulder where another bridge was inserted. Even today the structure, despite the demanding environmental conditions, carries out its service.
On the right, a shed made entirely of laminated wood in the province of Brescia in 1939, belonged to the company "Legnami Pasotti".