EP0575268B1 - Building material made of layered and glued wood - Google Patents

Abstract

It is characterised in that: - it comprises at least two assemblies (1, 2) which each consist of a laminated and glued layer formed by slats of rectangular cross-section which are juxtaposed side by side without being offset in relation to one another; - it has a structure which is such that, if a bending force is considered to be exerted on the upper part of the said material, at least the outer layer (lower layer) subjected to the greatest tensioning during this bending stress is formed by slats (2) glued to each other in the vertical direction (direction of bending) against the lower surface of the layer which is adjacent to it. <IMAGE>

Description

The present invention relates to a new type of wooden material of the glulam type, material which in the following description will be designated by the expression "beam".

Among the oldest materials used for the realization of various works, in particular to realize the elements having to resist bending, the oldest which is still widely used today, is wood.

Wood, as a building material, has the distinction of having very great variability in its mechanical properties since it it is a natural material. These properties are influenced, within each species, by geo-climatic parameters, by data silvicultural, or by processing characteristics. Moreover, the products derived from its transformation are limited in their geometry and in their volume. Lumber products rarely have a length greater than ten meters, with maximum sections of the order 180/300 cm, which therefore limits the area of use of such product.

Furthermore, to determine mechanical values characteristics, made available to engineers through standards for each type of wood product intended for the structure, lower values of resistance and modulus of deformation, obtained by destructive tests of suitable sampling, are considered to be reference. Almost unanimously, and for the resistance values, the 5% fractile (value for which 5% of the parts tested have lower characteristics), is the reference data. At this 5% fractile, is associated with an implementation safety factor (s). The quotient 5% fractile divided by the safety factor, (s), gives the values of admissible resistance, usable by the specialist for sizing of the structure (frame for example).

Such an operating system is therefore highly dependent on lowest resistance values, without exploiting the existence of values higher. By way of illustration, within the same species, a factor about eight, between the lowest resistances and the lowest resistances higher, is usual. Such variability therefore leads to power sort or classify wood according to characteristics that he presents. The oldest method is to make a classification by simple visual check, which obviously gives very poor results. Very recently, it has been proposed to carry out non-destructive tests, such as for example ultrasound or bending control, which results in clearly best. Despite such controls, there is still a strong variability, generating admissible operating values relatively weak, and above all, uncertain reliability.

To obtain products, such as long beams and of unrestricted section and which also have characteristics much more constant and reproducible mechanical properties of a beam another, it was proposed almost a century ago, to make wooden materials using the so-called "glued laminated" technique, which consists of cut the wood into strips of great length and thickness, and at reconstitute it by superimposing said lamellae, usually with wires parallel and joining them together by gluing. Such a technique has the advantage of offering very wide geometric characteristics and allows, thanks to butting techniques (end-to-end operation blades), to obtain very long assemblies (which can reach thirty to forty meters), the only limit being that of transportation problem.

All the techniques of reconstitution of wood by lamination, have both an advantage and a disadvantage.

As an advantage, it eliminates characteristic beams very weak, due to stress distribution effect on the lamellae more strong (average effect in the field of weak constraints). In this, it allows admissible resistance values, generally slightly superior to that of sawn timber of equivalent quality.

On the other hand, because of the weakness of the jointing under constraints of traction, which is always found in a taut external lamella (case bending), the average strength of glued laminated timber is less than that of the same unglued wood. Indeed, when the beam works in flexion, which represents 70% to 80% of cases, one of the two external blades is tense. When this blade breaks, in a butt or in a knot from the board, it transfers its stresses to the upper blade, and the beam breaks completely. We are talking about a serial operation, weakening the beam. In addition, if the bonding of the butt is poorly controlled by the producer, which is very difficult to control, the jointing is systematically an additional factor of variability, causing beams with weak characteristics, and therefore causing a lack of product reliability.

To solve this problem and to improve the resistance of beams, it has been proposed as is apparent in particular from DE-B-1 609 898, to change the inertia of the beams which are initially rectangular by adding material in the lower areas and upper of said beam, and more particularly laterally. The disadvantage of such a solution is that it increases the thickness since we add material, but above all lies in the fact that the sections are complicated.

Furthermore, the solutions proposed lead to elements which no longer have a rectangular section, the new shape obtained being able to be damaging from a technical point of view (complexity of production, assembly modification ...) and aesthetics, therefore architectural.

Now we have found, and this is what is the subject of the present invention, a new type of wooden building material obtained according to the lamination technique, which solves these problems and practically eliminates all risks of breakage of the beam in the event of rupture of one of the external blades of the stretched area when said beam works in flexion.

It should be noted that, unlike previous solutions, the proposed solution according to the invention solves this problem improvement of the resistance of the beams by the fact that one obtains a reinforcement while keeping the initial rectangular section, without add material.

• il comporte au moins deux ensembles, superposés et collés entre eux, constitués chacun d'une couche lamellée-collée formée d'au moins trois lames de section rectangulaire, juxtaposées côte à côte sans décalage les unes par rapport aux autres ;
• il a une structure telle que, si l'on considère qu'une force de flexion s'exerce à la partie supérieure dudit élément, au moins la couche externe qui est la couche inférieure et la plus tendue lors de ce travail en flexion, est constituée de lames recollées entre elles dans le sens vertical qui est le sens de la flexion, et collées contre la surface inférieure de la couche qui lui est adjacente.

In general, the invention relates to a construction element, of the beam type, having a rectangular section, produced from layers of glued laminated wood, and it is characterized in that:

• it comprises at least two sets, superimposed and glued together, each consisting of a laminated-glued layer formed of at least three blades of rectangular section, juxtaposed side by side without offset from each other;
• it has a structure such that, if we consider that a bending force is exerted on the upper part of said element, at least the outer layer which is the lower layer and the most stretched during this work in bending, is consisting of strips glued together in the vertical direction which is the direction of bending, and glued against the lower surface of the layer which is adjacent to it.

In the following description, the invention will be described with reference to such an arrangement and to such a type of beam with rectangular section.

Furthermore, if in one embodiment, the material conforms to the invention comprises a single layer of glulam, glued thus in the vertical direction, the other layers being made up of lamellae glued horizontally, according to a variant conforming to the invention, a second layer of glulam reported in the direction vertical can be arranged at the top.

Finally, according to another embodiment, all of the material according to the invention, consists entirely of layers of glulam arranged vertically and joined together by gluing.

It should also be noted that according to one characteristic essential of the invention, the layer of glulam arranged in the vertical direction, must include at least three elementary blades, to benefit from the stress redistribution effect of the most weak locally on the adjacent vertical slats (parallel effect), the increase in the number of vertical blades making it possible to decrease the resistance variability of the beams produced, and therefore to increase their reliability.

Figures 1, 2 and 3 schematically illustrate the three embodiments of a glued laminated timber building material produced in accordance with the invention.

In the example illustrated in Figure 1, such a beam consists a conventional glued-laminated assembly (1), consisting of eleven parallel layers arranged horizontally and, glued in one direction vertical, of a layer (2) of a glulam comprising five lamellae and arranged in the most tense area, and therefore oriented in the direction where flexion is exercised.

In the example illustrated in Figure 2, the material according to the invention is in the form of a beam also of section rectangular, made up in its central part of a glulam conventional (1) comprising ten horizontal layers and, located on one side and on the other side of this central zone (1), of two layers (2) of glulam, glued vertically, and each consisting of five strips.

The variant illustrated in FIG. 3 of a material conforming to the invention consists only of layers (2) of glulam arranged vertically in the direction of bending.

Thanks to such a design according to which one positions, in the stretched part of the beam, at least one thickness (2) of glulam, glued in a vertical direction, so we find a plurality of lamellae (2a, 2b ..) operating simultaneously on the stretched surface. Therefore, if one of these slats breaks, the adjacent slats support the constraints, by redistributing them, and the system continues to resist. A such functioning is akin to a very favorable parallel system of a ultimate resistance point of view. This increases the resistance mean of the beam and considerably limits the existence of values weak resulting from a problem of jointing or a big local defect on a blade (knot).

In general, we can say that if the coefficient of variation, standard deviation to average ratio, of supply wood (the stop blades) is 25% for example, the coefficient of variation of conventional glued laminated timber will be 18 to 20%, and that of the timber produced in accordance with the invention of the order of 10 to 13%. Reducing this variability is directly linked to the gain in reliability and thus allows to increase the resistance values used by specialists for the sizing of structures.

Such a design makes it possible to obtain beams having values permissible resistance of 30 to 40% higher compared to beams made of classic glulam. We also get a evolution of the modulus of elasticity from 10 to 20% compared to standards from different countries.

In addition, the mechanical quality of the beam is further optimized in sorting the best blades that are preferably intended for blades vertical.

As said before, in the material conforming to the invention, it is necessary to have at least three blades in the glued laminated element arranged vertically in the stretched area. For small beam widths, for example less than 110 mm, it blades about 25 mm thick should be used per example for making the element placed in the stretched area. For some larger widths, conventional planks 33 mm thick or more agree. Of course, the reduction in thickness of the blades of the multi-bonded element placed in the stretched area, is a parameter optimization in the sense that it increases the number of vertical blades and thus it reduces the variability of resistance of the beams.

It should also be noted that the product conforms to the invention has another advantage which is that the values mechanical results, are valid for all sections. If the height of the section increases, in the case of a glued laminated product conventional, reducing coefficients are used to reduce admissible values, taking into account volume effects negative on the resistance of the beam (case of all serial systems). With the product according to the invention, there is no volume effect, for as much as the element placed in the stretched area, represents at least 15% of the section.

Such a material can be made from all types of wood and using conventional lamination techniques, techniques which will therefore not be described for simplification purposes. Properties mechanics of the beam will be deduced from the properties of the woods supply (fir, spruce, pine, larch, etc.).

Claims (5)

1. Beam-type structural member of rectangular section made of laminated and bonded layers of wood, characterised in that:

   it comprises at least two parts (1,2) placed one on top of the other and bonded together, made up of a layer of at least three laminated and bonded wooden slats (2a, 2b) of rectangular section placed evenly side by side;

   it has a structure such that, if a bending stress were to act upon the top of said member, at least the external layer (2), which is the lower layer under the highest load during bending, consists of the slats (2a, 2b) bonded together vertically in the direction in which the bending stress is exercised, and bonded to the lower surface of the layer adjacent to it.
2. Structural member as claimed in claim 1, characterised in that it comprises a single layer (2) of laminated and bonded wood, with the lower layer bonded vertically and the other layer (1), which is the upper one, being made up of horizontally bonded slats.
3. Structural member as claimed in claim 1, characterised in that it consists of two layers (2) of laminated and bonded wood with the upper and lower layers being bonded vertically and placed on either side of a middle layer (1) consisting of horizontally laminated and bonded wood.
4. Structural member as claimed in claim 1, characterised in that all layers of laminated and bonded wood (1,2) are arranged vertically.
5. Structural element as claimed in any of claims 1 through 4, characterised in that each layer of vertically arranged laminated and bonded wood (2) is made up of at least three elementary slats.

Images