The advent of glulam has, no doubt, solved some deficiencies typical of solid wood (limitation of size and shape, mechanical properties extremely variable, dimensional instability, tendency to crack, etc..) But did not, of course, represented a point of arrival for technological research and innovation.
It has recently been proposed by engineers Claudio Cattich and Luca Gottardi, co-authors together with prof. M. Piazza a new technology for production of wooden beam armed, with which it fails to provide a method of production of a lamella armed industrially achievable in a series production and which ensures the obtaining of wooden beams armed, with a operating behavior similar to that of reinforced concrete (in non-cracked), high performance with regard to both the stiffness that the resistance (increases on the order of 100% compared to the beam not armed with the same section).
A beam armed of this type has, in fact, an allowable tension (equivalent) to bending up to 28 MPa and an elastic modulus (equivalent) up to 28 GPa, these mechanical features that allow effective savings in height in relation to a beam equivalent in glulam traditional with identical basic (up to about 40%) and which allow, in practice, to compare a beam armed two beams side by side in lamellar the same size.
In brief, innovation provides for the reinforcement of glulam with the insertion of one or more bars of steel or composite CFRP in special milling practiced in some lamellae and are joined to them with a suitable epoxy adhesive, an integral part of the same technology .
The main features of the structural member armed compared to the same element of plywood or solid wood can be briefly summarized as:
- up to 2.5 times stiffer and up to 2.5 times stronger than (the value 2.5 is not derived from theoretical limitations but it depends on design choices motivated and productive possibilities);
- fire resistance of the class R 30 or R 60 (the result is obtained by ensuring adequate distances from the edges of the armor section);
- bending failure with the formation of pseudo-plastic hinge (particularly interesting feature for applications in seismic zones);
- lower incidence of defects on the mechanical characteristics of the wood and resulting in less variability of the mechanical properties between different elements with the same section;
- limited effect of changes in moisture and duration of load on the rheological behavior of the global economy;
- savings in the section height of the individual and the member total saving of material;
- maintaining the appearance of traditional laminated wooden beam, in front of resistors and much higher performance;
- armor and its adhesive completely embedded in the wood, without any contact with the environment.
During the extensive campaign of experiments on beams reinforced with metal bars, has been noted as the breaking of the wood fibers in the tension zone has never led to the immediate collapse of the element, due to the non-elastic deformation field in the bars. To complete the research, both theoretical and experimental, has been taken into account armor consists of “bars” of CFRP. Also the experimental results of the specimens reinforced with carbon fiber bars were absolutely conform to theoretical predictions provided by the calculation model. However, the type of rupture surely fragile, the current difficulty of finding on the market bars CFRP with high diameters, as well as the cost (much higher than the metal bars) currently advises against the use: it is however possible that such bars or other innovative materials can become – in a not too distant future – convenient in applications of wood armed.
The overall behavior of the mixed section wood – steel is comparable to that of a section (not throttled) in reinforced concrete, with reinforcing bars “drowned” in a cementitious matrix instead of wood. The contribution of the complex reinforcing bars – adhesive can then be taken into account by an appropriate coefficient of homogenization n: the value of n may be assumed equal to the ratio between the elastic modulus of the steel and that of wood.
Depending, in good substance, the mechanical performance of the wood armed by the ratio between the elastic modules of the materials as well as, of course, by the percentage of reinforcement, it appears that a armed wooden element will also present a better behavior – compared to a similar element not armed – against humidity variations and long-term loads. In such cases, the behavior of the timber can be “interpreted” by a progressive reduction of the elastic modulus; the value of the coefficient n for beam homogenization armed “appears” therefore increased, since the elastic modulus of the material of the armor does not vary.