BACKGROUND Glued-laminated wood is a product that may briefly be said to consist of a number of wood laminations that are stacked on top of each other and are glued to each other. Such products have been used for several years, mainly as structural members in different kinds of building or construction work. The main advantages of employing glued-laminated wood structural members in general, and not least as supporting members in buildings, are on the one hand that employing the different adhesively joined laminations minimizes the weakening effect of defects within the wood. On the other hand such glued-laminated wood structural members have a significantly improved resistance to fire than both other types of wood structures and steel beams or corresponding structural members. Furthermore they have an aesthetically appealing appearance and are dimensionally stable.

When it comes to a glued-laminated wood structural member, such as in the shape of a beam, the fact is also that compared to a steel beam it has significantly higher strength in relation to the weight thereof. All the same, many attempts have been made during the last decades to strengthen glued-laminated wood structural members for the purpose of improving the strength with respect to specific loads. In particular this has been done with regard to the tension that in a load condition normally occurs at the underside of a structural beam of a construction work. An example of such a solution is disclosed in U. S. Patent Specification No. 5 026 593. In accordance therewith it is suggested to introduce and adhesively fix a layer of high strength material, preferably a thin flat aluminum plate, between the two lowermost laminations of a glued-laminated wood beam. Thereby the strength of the beam shall be improved specifically with regard to the tensile stress in this area. U. S. Patent Specification Nos. 5 725 929 and 5 648 138 disclose other solutions to substantially the same problem.

Common to said latter Patents is that instead of an aluminum plate they employ synthetic fiber materials of different forms for strengthening a glued-laminated wood beam in its upper and lower areas, and that they relate to a positioning of the fiber material sheets between laminations or segments of the glued-laminated wood beam.

Thus, all of the above mentioned publications concern the strengthening or reinforcement of glued-laminated wood beams specifically in the lower and/or upper regions thereof. However, it is a general fact that the dimensions of glued-laminated wood beams will be significantly larger than those of steel beams of equal strength. In spite of the intense research that has been done and is still going on, no acceptable solution has been found to this condition. U. S.

Patent Specification No. 5 050 366 does indeed disclose a suggestion for strengthening a glued-laminated wood beam by introducing a number of rods into the interior of the beam, but in this case the specific purpose is likewise to strengthen the upper and lower regions of the beam. The rods are positioned in grooves milled in separate or adjacent laminations. Apart from the fact that such a solution does not provide any uniform or evenly distributed reinforcement the milled grooves cause an undesirable weakening of the individual lami- nations that are provided with such grooves. To our knowledge, said product has not come to practical use in any greater extent, and this is probably also due to the difficulties of providing a rational and thereby economical production.

In summary, it can therefore be established that although conventional glued-laminated wood structural members in themselves have performed very well, they still suffer from the above discussed limitations that have probably contributed to the fact that this in many respects very attractive product has not become more widely used.

SUMMARY In the light of the above discussion, a basic object of the invention is to provide a simple method of strengthening or reinforcing a glued-laminated wood structure. More specifically the object is to achieve this by providing a method of producing a glued-laminated wood structural member by means of which a strengthening is obtained that is uniform and relatively evenly distributed in the structural member. Expressed otherwise, the aim is to increase the applicability of such wood structural members by reducing the difference in

dimensions between such structural members and for instance steel beams having equal strength.

The invention is based on the understanding that the strength of a structural member of a given dimension and of the specified kind can be greatly increased by providing a steel reinforcement in the interior of the member, said reinforcement extending over substantially the entire height and length of the member. In accordance with the invention this is achieved by stacking wood segments on top of each other and gluing them to each other in at least two essentially parallel stacks, by providing a steel plate between the stacks or between at least two of the stacks and by adhesively fixing the steel plate to each of the stacks being adjacent thereto, as is specified in the characterizing portion of claim 1. This provides an integral, composite structural member that with a marginally increased weight has significantly improved strength properties with regard to different load conditions.

According to an embodiment of the basic aspect of the invention a steel plate is chosen having a dimension essentially corresponding to the height and length of the stacks. Said steel plate is glued to the stacks as stated in the characterizing portion of claim 2. Hereby a structural member is obtained having a reinforcement that is evenly distributed over its entire height and length.

According to another embodiment of the basic aspect of the invention a steel plate is chosen having a dimension being smaller than the height and length of the stacks, and is glued to the stacks, as stated in the characterizing portion of claim 3. Hereby a structural member is obtained where the strengthening or reinforcement is still evenly distributed over the main portion of its height and length, but where a possibility is simultaneously created of providing the steel plate or plates recessed in the member, as is stated the characterizing portion of claim 4. Such an embodiment is particularly applicable in an aggressive environment since the steel plates are protected from the environment or surroundings.

Further developed embodiments of the basic aspect of the invention are specified in the dependent claims 5-11.

In accordance with another aspect of the invention, as specified in claim 12, a wood structural member employing the principles of the present invention is provided. According to this aspect the structural member of the invention comprises at least two substantially parallel stacks of segments that are glued to each other and a steel plate that is glued to the adjacent stacks. Together they form a composite, integral unit.

Embodiments of this structural member suggested in accordance with the invention are specified in the dependent claims 13-23.

A further aspect of the invention concerns a use of a wood structural member designed in accordance with the basic principles of the invention as a supporting or load-carrying member of a construction work.

Further objects, features and advantages of the invention will appear from the dependent patent claims and from the following description of exemplifying embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS The invention shall now be described in closer detail in connection with the accompanying drawings, where: Fig. 1 is a partial perspective view of a wood structural member built up in accordance with the basic principles of the invention; Fig. 2 is an end view of the wood structural member as illustrated in fig. 1, Fig. 3 illustrates an alternative embodiment of a wood structural member according to the invention Fig. 4 is a partial perspective view of a further variant of a steel plate intended for use in a structural member according to the invention, Fig. 5a-c are partial, partly schematical end views of further embodiments of a structural member according to the invention, employing profiled steel plates,

Fig. 6 is an end view of a further embodiment of the wood structural member according to the invention, Fig. 7 is a side view of the embodiment of fig. 6, and Fig. 8 illustrates a stack of laminations or segments in the structural member according to figs 6 and 7, in a cross-section along the line A-A in fig. 7.

DETAILED DESCRIPTION With reference primarily to drawing figures 1 and 2 the basic principles of the invention will now be described by means of a generally applicable embodiment of a structural member produced in accordance with the basic principles of the invention. The differences compared to conventional glued-laminated wood structural members will simultaneously be described.

It has been known for a long time to form glue-laminated wood beams and the like by stacking several wood laminations or segments on top of each other and by gluing them to each other. As was stated in the introduction the basic object of the invention is to provide such an assembled structural member 1 having a unitary reinforcement being relatively evenly distributed in the member. In accordance with the first embodiment of the present invention that is illustrated in figs. 1 and 2 this is achieved by forming two separate stacks 3a and 3b of segments 2 positioned on top of each other and glued to each other. Then, a strengthening or reinforcing member in the shape of a steel plate 4 is positioned between the stacks 3a, 3b and is glued to their side surfaces facing each other.

The invention is not restricted to the use of only two stacks 3a, 3b of wood segments 2. For the purpose of providing a structural member 1 where the strength properties are even more uniform throughout the entire member 1 a greater number of parallel stacks may be used, as is indicated at 3a'and 3b'in fig. 2 and also in drawing figure 6 that is more closely described below. The greater the number of stacks the more and smaller segments are used in a member 1 of a given dimension, and thereby the effect of defects in the wood will also be reduced even further. In such designs having several stacks 3a', 3b'the invention makes it possible to selectively, for instance depending upon the actual load condition of a structural member 1, provide steel plates 4 between only two, several or all adjacent stacks, and this is indicated in

the embodiment that is illustrated in figs. 6-8 and is described in detail below. Although the embodiment having more than two stacks 3a', 3b', as indicated in fig. 2, illustrates a central positioning of the only steel plate 4, it lies within the scope of the invention to provide the steel plate or plates asymmetrically with respect to the center plane of the member 1, that is between optional stacks. This may likewise be done for instance depending upon the actual load condition.

When producing the stacks 3a, 3b, 3a', 3b', segments 2 of a corresponding size may be stacked on top of each other and glued to each other. However, in accordance with the invention the manufacturing can be made more rational by forming a single stack from segments having a dimension corresponding to the total dimension of the structural member 1, that is with reference to the width of fig. 2. When the segments have been glued to each other they are cut up to a desired number of stacks that are subsequently glued to each other or to an intermediate steel plate 4. It is obvious that with such a method the material that disappears through the kerf must be considered in connection with the dimensioning. Like by another method of manufacturing the dimensional increase caused by the introduction of the steel plate/plates must also be considered, except by the embodiment according to figs. 6-8.

The invention as such is not restricted to the use of specific material qualities or grades. With regard to the steel plate 4 this may be manufactured from a steel grade corresponding to the strength requirements that apply to the specific application. Thus, as an example high strength materials may be considered in connection with very high requirements for strength or where small dimensions are required, and a stainless steel may be employed in the case where the member 1 shall be used in an aggressive environment. The segments 2 may be manufactured from materials usually used within this field. In order to meet specific strength requirements it is likewise possible to employ particularly strong wood. In situations where very high demands are set for strength and/or resistance to external influence it may thereby be interesting to use the compressed wood manufactured by Lign Multiwood AB. For the application of the structural member according to the invention in for instance an outdoor environment, this is very well suited for a through impregnation. In particular when employing the above mentioned compressed wood, each segment 2 can be impregnated throughout prior to being glued together. This may be done by employing a very effective and

also environment-friendly impregnation method that has likewise been developed by Lign Multiwood AB.

The sides of the stacks 3a, 3b are glued to the steel plate 4 by means of flexible glue of a suitable composition, and in order to increase the adhesion against the plate 4 the plate surface may when required be treated in various manners, such as by blasting, anodizing, metal plating or other surface treatment.

In accordance with an alternative that is indicated in fig. 1 the plate 4 may be provided with through openings distributed across its surface. Said openings likewise contribute to strengthening the bond between the stacks 3a, 3b and the steel plate 4 and thereby to strengthening the entire member 1.

Drawing figures 3,4 and 5a-c illustrate further alternative embodiments of a steel plate as suggested by the invention. Said embodiments all present examples of how the holding together of the structural member 1 can be strengthened when required, although, in the normal case, a planar, possibly surface treated or finished steel plate 4 is mostly quite sufficient. Thus, fig. 3 illustrates a profiled steel plate 4a having the general shape of an I- beam, whereby the web 6 of the I-beam shape is glued to the stacks 13a, 13b and the upper and lower cross pieces 7,8 of the I-beam shape are glued to the uppermost and lowermost, respectively, segments 2 of the adjacent stacks. The cross pieces 7,8 could be narrower than the width of the stacks 13a, 13b, as seen in the drawing figure, but they preferably have a width corresponding to the total width of the structural member 1. Such a design provides a good holding together of the member la and is particularly useful in the cases where, for esthetical reasons, a visible wood surface is not required around the entire member 1 a.

Fig. 4 illustrates a variant of a steel plate 4b having V-shaped punched portions 9a, 9b distributed across its surface. After having been punched said portions are bent outwardly substantially normal to the plate 4b surface, forming gripping teeth 11. The gripping teeth 11 are alternately bent outwardly to different sides of the plate 4b and accordingly only half of them are visible in the drawing figure. In connection with producing the structural member, and in particular in connection with gluing the plate 4b to the adjacent stacks the gripping

teeth 11 are pushed into the corresponding stack for providing a mechanical interlock between the stacks and the plate.

Drawing figures 5a-c illustrate a further example of the use of a profiled steel plate in a structural member according to the invention. In the illustrated embodiments the profiled steel plate 4c, 4d, 4e serves the double purpose of on the one hand increasing the strength of the plate and thereby of the structural member and on the other hand of strengthening the adhesion between the stacks 23a, 23b, 33a, 33b, 43a, 43b and the steel plate 4c, 4d, 4e by increasing the available glue surface. In particular fig. 5a symbolizes the use of a"corrugated" plate 4c, for instance a so called slit plate, which is a type of profiled thin plate that has very high flexural rigidity. In this case the sides of the stacks 23a, 23b being adjacent to the plate are profiled in the corresponding manner.

Fig. 5b illustrates an alternative profiled plate 4d having right-angled bends, and corre- spondingly profiled sides of the adjacent stacks 33a, 33b, whereby the a kind of finger joint is formed with the intermediate plate profile. Although this is not visible in the drawing figure the"fingers"of the stacks are in this case formed by every other lamination or segment of the stacks, which facilitates manufacturing.

Fig. 5c illustrates a variation having a wave-shaped plate profile 4e and correspondingly profiled sides of the stacks 43a, 43b.

Finally, figs. 6-8 illustrate an embodiment of the invention that is particularly suitable to use in the cases where the structural member lb shall be applied in an outdoor environment or in another aggressive environment that may have a negative effect upon the steel plate. From another aspect said embodiment is appropriate for use in cases where a visible steel plate would be considered disturbing the esthetical appearance of the structural member lb. In this embodiment the structural member lb is formed by four substantially parallel stacks 53a, 53b, 53c, 53d between which are received three steel plates 4f, said steel plates having a generally planar shape. Figs. 6 and 7 illustrate that in this embodiment the steel plates 4f have a dimension that is smaller than that of the assembled structural member lb as partially shown in fig. 7, both with regard to its width or height and its length. Thus, the steel plates 4f do in all directions terminate inside the outer surfaces of the structural member 1 b.

In order to recess the steel plates 4f in the stacks the latter are provided with cut-in portions or recesses 54 illustrated in fig. 8, which shows a cross section, along the line A-A of fig. 7, through the stack 53a being positioned furthest to the left in the structural member lb of fig. 6.

The stack 53a is an outer stack and is, like the other outer stack 53d, provided with only one cut-in portion 54 in one 55 of its side surfaces that is facing inwardly. On the other hand the middle stacks 53b, 53c are provided with one cut-in portion in each side surface. The dimensions of the cut-in portions or recesses 54 are adapted to the overall dimensions of the plates 4f and they are centered in the respective side surface 55, that is they are at all sides surrounded by a remaining portion of the side surface 55.

Thus, in the illustrated embodiment the height of the recesses 54 extends across all of the segments 2a-c of the stacks 53a-d, that is they terminate in the uppermost 2a and lowermost 2b segments respectively and are accordingly formed to be continuous, with respect to the height in fig. 8, in the middle segments 2c. Fig. 7 also illustrates that they are formed such that in the longitudinal direction they terminate at a corresponding distance from the ends of the structural member lb. Furthermore, they are each formed having a depth corresponding to or slightly exceeding half the thickness of the steel plate 4f, such that the two recesses 54 in the facing side surfaces of two adjacent stacks together form a pocket adapted to receive a steel plate 4f therein with a minimum of play. Although the illustrated embodiment discloses steel plates introduced between all of the stacks, it shall be emphasized that here also the number of steel plates and their positioning and thus also the number of recesses and their positioning may be varied optionally depending upon the desired properties.

Characteristic of structural members manufactured in accordance with the principles of the invention is the evenly distributed, continuous strengthening or reinforcement thereof, which is obtained by the steel plate introduced between stacks of wood laminations. Thus, in summary a structural member designed in accordance with the principles of the invention provides the following advantages over a typical conventional glued-laminated wood member: * A significantly increased strength of the structural member-performed test show that a member according to the invention becomes approximately five times stronger than a conventional glued-laminated wood member having no reinforcement.

A A significantly reduced dimension of the structural member, with maintained strength.

. A possibility of"tailoring"the strength properties of the structural member by an alternative positioning of the steel plates as well as by choosing different steel grades and/or differently profiled steel plates.

* Makes it possible to obtain the significant, evenly distributed strengthening without any appreciable increase of the total weight of the member.

. Makes it possible to obtain the significant, evenly distributed strengthening while maintaining a rational and economical production.

. Makes it possible to obtain the significant, evenly distributed strengthening while maintaining an aesthetically appealing appearance when the steel plate is recessed in the member.

Increased area of application through reduction of the dimensions.

. Increased area of application by being suitable for through impregnation.

Presently, the invention is regarded having its primary use in connection with construction work, which is here regarded as including residential buildings, office buildings and industrial buildings as well as sports halls and bridges.

It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.