CA1182266A - Service girder of wood as well as a process for the manufacture thereof - Google Patents

Abstract

Abstract In order to create a service girder intended for use in the field of application of the conventional squared timbers and usable for such tasks, having dimensions in the range of approximately 10 to 20 cm in height, which is more econ-omical in use, after consideration of all the important practical aspects, than the formerly used squared timbers, a service girder of wood, which is connected with an upper flange and a lower flange and a flat solid web linking these two flanges which is linked to the flanges by a plu-rality of tongues engaging in said flanges and bonded over the entire length of said girder, wherein the web consists of plywood or of a plate of three wooden layers bonded to-gether and wherein the wood fibers of the outer layers run perpendicularly to the fibers of the center layer, and the flanges are in each case composed of several wooden layers, of which the wood fibers extend substantially in the longi-tudinal direction of the girder, is further developed in such a way that each flange (1, 2) consists of at least three wooden layers (5, 6, 7) and the wood fibers (F5, F6, F7) of each adjacent ?ir of wooden layers are mutually inclined at an angle of about 10° to 15°.

Description

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Service Girder of Wood as well as a Process for the Manu-facture Thereof The invention is intended to create a new type of wooden service girder in order to make possible, in a field of application which has not yet been exploited in formwork, more rational working and financial savings.
The traditional wood-glue construction has concentrated its development primarily on providing wooden girders for those applications for which the edged timbers usual in the trade were unsuitable due to their restricted c.ross-~ . section, limited length and also -their exclusively straight :Eorm. Such applications are girders for grea-t spans and ~or great loads, for curved shapes etc. such as are used in the construction of halls, churches, small bridges, sports installations etc. ~ue ot the large dimensions and/
or effective span and their high load capacity these gir-ders conventionally have to be made from the timber dimen-sions usual in the trade by bonding the parts together.
Over the years an abundance of solutions to this problem has been put forward, but all such solutions are directed ~322~i toward the manufacturing conditions for larger girders.
Mostly they are of such width that due to the wide dimen-sions of the individual parts they can be made only in relatively uneconomical colu bonding processes.

Inter alia, girders have been proposed (cf. e.g. US-P~
1 377 899 or AT-PS 2~5 897) in which both the solid web and the flange are made of cross-laid wooden layers. In this process, however, on both sides of a plywood board, thic]~, solid wooden layers are bonded by an adhesive, which because of the great costs - the labour and materi~ls fo; the manufacture of the plywood and then the labour and materials for the manufacture of the outer layers - has become so expensive even fox large girders that this process has not been widely adopted.

For roof trusses or the like, certainly wooden girders with a solid web made of whole timber have been proposed (cf.
DE-G~MS 1 847 569) which because of the design of the web are cheaper than the design described above. But they are only suitable for use in conditions where protection against the weather is possible, whereas it is an indis-pensable demand on a service girder that it should main-tain a high degree of shape stability even when exposed to '5 the weather over long periods. To achieve this high shape stability, neither the web nor the flange of the girder should warp or otherwise lose its form due to swelling, shrinking or the like. This cannot be attained, however, either with unbonded solid webs nor with solid webs consisting of several 30 ` bonded wooden layers with fibers parallel to each other, i.e.
laminated-bonded solid webs, since even the strip-bonded webs have practically the same swelling and deformation pro-perties as the unbonded webs, i.e. those consistlng of inte-gral timber.
For the above reasons quite diffexent methods had to be proposed for the development of wooden service girders.

Thus for formwork with a height of at least 36 cm, so,called lattice girders were developed. In ~hese lattice girders, the flanges consist of rectangular beams, on which is formed a zig-zag shaped web, struts inclined alternately at an angle of from 45 to 60 being connected by bonded tongue-or tenon ~ointing.

Such service girders were used in form work owing to the fact that these lattice girders because of their relatively low weight, compared with w~!ole timber girders of comparable dimensions, are relatively easy and simple to handle and have good form and dlmensional stability when exposed to the weather. But the lattice girders have a fundamental disad-vantage, that high support loads can only be applied at the noda] points of the lattice. Between these nodal points only greatly reduced support loads can be appliedl because they load the thi~n flange in bending. Because of the system employed, lattice girders could only be used econo-mically when made of wood with a minimum height of about 36 cm. For smaller hèights, the struts are relatively short, the tongue or tenon jointing is very close together and the dimensions for the flange and struts a-e very small, so that due to the natural knottiness of the wood they are unacceptably liable to fracture. For these reasons the manufacturing costs of these girders rise drastically for small heights in rela-tion ot the load capacity, ~or which reason there are no lattice girders on the market today with a height of less than 36 cm.

Then a whole timber girder was developed which had the necessary properties for use as a service girder and was ecnonomical to make for the dimensional area of about 36 cm in height which is needed. But in order to do this, it was necessary to develop a design completely different not only from the designs which were proposed for roof trusses, but also from the concept of lattice girders.

2~ii6 This servi~e girder which was created in the course of ex-tensive development work (cf. DE-PS 16 09 756) is subject to almost no warping or other changes in form even in severe weather conditions but still can be made rationally and economically. In this service girder the web is formed as a solid web, which is composed of several wooden layers, the wood fibers in adjacent layers of which extend perpen~
dicularly to each other. This plybonded solid web is connected over its entire length by a tongued connection with the flanges which are made of whole timber. Due to ~he engagement of the tongues of the plybonded plain web in the whole-timber flanges, it was surprisingly found that with this girder design, not only the web, but also the two connected flanges have a high form stability over their en-tire cross section when exposed to weather, without it being necessary to form not only the web but also the two flanges out of plybonded layers.

Deviating from the direction taken for roof trusses, which was to meet the many demands place~ thereon by more compli-,cated design of both the web and also of the flanges, the service girder described is characterized in that the flange is of whole timber so that its ma,nufacture is substan~ially more rational and therefore cheaper than the previously known form-stable girders from roof-truss designs and nevertheless this new girder possesses high form stability and dimensional stability even when exposed to the weather.
Because of these surprising properties the service girder in question could be widely adapted ~y the market for heights of 2~ to 36 cm.

Proceeding from these sucessful service girders it was then proposed that the flange should no longer be made of whole timber, but should be composed of two wooden layers, the wood fibers of which are parallel with each other and extend in the longitudinal direction of the flange (DE-GBM 72 22 ~99).

~13Z2 ~i But by this measure alone, no improvement is at~ained, because strip-bonded wood is not much distinguished from whole timber as regards its form stability and it fissure stability. In this known girder design it was therefore intended that the flanges should be made of a substantially higher-value timber quality compared with the web as regaxds its tensional, flexing and shear resistance. This naturally made the production of this service girder more expensive, but without substantially improving its technical functional capacity.

For heights of less than 20 cm today so-called squared timbers axe used exclusively in formwork. These squared timbers are cheap and are available without difficulty. But they naturally show an unfavorable ratio of weight to load capacity. Further they have the disadvantage that when used for long periods in the open, they warp easily and are therefore neither stable in shape nor dimensionally stable. It is therefore necessary to check such squared timbers when in repeated use before using them again for their suitability as to form and load capacity, i.e. the warped and damaged squared tim-bers have to be set aside in a separate work process. A further disadvantage of squared timbers is their short active life, which scarcely exceeds one yeax on building sites in form work due to losses and to cutting, as experience has show.n.
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The use of service girders having a solid web in the sphere of application of squared timbers, i.e. for heights of less than 20 cm, has so far not been studied at all by these skilled in the art. This surprising fact results in all probability from the fact that it could not have been ex-pected that using the known design and manufacturing pro-cesses such a small girder could be made of wood, while being adequately robust and resistant as well as economical, so as to be able to compete with the cheap squared timbers in this size range, which are sawn out of one piece.

Thus, in the past, the manufacture of service girders with solid webs was caxried out firstly in such a manner that the upper and lower flanges as well as the web are made individually, including the wedged tongue connections or the corresponding grooves, and these three parts are then pressed together individually after the subsequent appli-cation of adhesive. When llsing this method of operation about the same costs are necessary to make small girders with heights of less than 20 cm as for the manufac~ure of girders of larger height, so that this mo~us operandi be comes ever more uneconomical as the girders grow smaller.
On the other hand, a simple reduction of the height of the known service girders while maintaining the structural de-sign has in the view of those skilled in the art a number ~5 of functional disadvantages. Thus girders with a height of less than ~0 cm, due to the relatively small dimensions of the flange, are inevitably substantially more liable and sensitive to damage such as is inevitable during trans-port and nailing in rough building site work, During nai-ling, flanges with small size have a strong tendency to cracking and to splitting, so that with the larger sizes of f~nge unacceptably high wear and tear occurs on the sites. Moreover the disadvantageous influence of knots on : the strength of the girder is substantial as the sizes de-....... .
crease since knots in flanges of smallex cross section are substantially weaker than those in larger flange cross sections. The manufacture of such small flanges of whole timber would there~ore in practice require the use of knot-free wood of quality class O. But such wood is too expensive

3~ for mass production and is moreover hardly obtainable in the large quantities needed.

The invention is based, as wa~ stated earlier, on the con-cept that by using a new type of service girder a new field of application can be opened up.

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On this basis, it is the object of the invention to create a special service girder designed for the field of appli-cation o~ conventional squared timbers and which is usable for suchtasks in the size range of about 10 to 20 cm in height, which is more economical, taking account of all the important aspects in practice, then the squared timbers used until now.

This object is achieved with a service girder of wood, having an upper and a lower flange connected by a solid flat web which is connected to the flanges by a plurality of tongues engaging in the flanges and bonded over the entire length of the girder, wherein the web is made of plywood or of a plate of three wooden layers bonded to- ::
gether and wherein the wood fibers of the outer wooden layers extend perpendicularly to those of the center layer, and the flanges are each composed of a plurali-ty of wooden layers, the wood fibers of which extend substantially in the longi-tudinal direction of said girder, characterized in that each flange comprises at least three superimposed wooden layers and the wood fibers o each adjacent pair of wooden layers are mutually inclined at an angle of about 10 to 15 , .. '` .
By the features of the invention a service girder is created which is specially suitable for those fields of application in which traditionall.y only mass s~uared timbers having a height of 10 to 20 cm were formerly used. Owing to the three-layered, plywood bonded design of the web and to the flanges composed of at least three woodel~ layers which are laminated together, in cooperation with the wedged tongue connection over the whole length of the girder, a service girder is created which has a high transverse force, shear and bending resistance, which is absolutely stable in shape and dimensionally stable even in long-term use in the open, and also has the necessary robustness for long-term use on building sites.

i6 In the service girder of the invention, symmetrical con-struction of the flanges each consisting of at least three wooden layers with a cross-wise orientation of the fibres is especially important as it leads to improved strength. It not only ensures that the tensional strength is optimal over the whole cross-section of the flanges, but also the symmetrical and cross-fibred design of the flanges ensures greater load-bearing capacity because in contrast to integral timber or normal laminated wood used for flanges even with large transverse and/or pressure forces, there is no splitting of the wood of the flanges when the wedged tongues of the web are pxessed into the flanges. This crossed-fibre orientation also provides great resistance to crack formation or splitting in general and a high nail resistance of the flanges despite their lesser thickness. Because of the multi-layered design of the flanyes of the service girder of the invention, the otherwise weakening influence of knots is negligible even with small heights, so that flanges made from wood of quality class II
are practicable and can therefore be manufactured economically.

To achieve the above-mentioned symmetrical design of the flange in combination with the desired cross-fibre orientat-ion, each flange consists preferably of at least three wooden layers and with a more f~r-reaching layer design of any uneven number of wooden layers. With a design using three wooden layers, the wood fibers of the outer layers extend either parallel to the girder axis at an angle there-to of about 5 to 8, or at an angle of about 10 to 15.
The fact that symmetrical designs of the flanges, combined with the crossed-fibre effect, was not made obvious by the prior art can be seen especially from DE-~BM 72 22 899, from which the service girder design of claim 1 is distinguished. The known service girder in fact possesses a plybonded web and flanges, which consist of two wooden layers. But the significance of the symmetrical design of ~ z~

the flanges with an o~d num~er of layers while simultaneous-ly providing the cro~sed fibre effec~ was not recognized.
The use of laminated flanges in the known ~ervice girders results ln the fact that ~hese service girders ean only absoxb small transverse forces, ~ince the flanges are pressed on the tongues of the web causing -th-e~r wood to split. In addition, he laminate bonded wooden layers of the flanges, granted the ~ma~ size of thP girders at which the inv~nti~n aims, have almost the same inadequate deformation propertîes and the same lack of robustness = against nails and the like as in the case of flanges made of integral timber. With the known service girders there-fore, due to the layer design of the flange~, increased manufacturing costs have to be accepted, without their suitability for service girders inthe siæe range of about 10 to 20 cm being improved. This is clear evidence that the combination of the invention is not obvious.

Under the trade mark "Steidle-Compact" moreover a service girder has become known which is in~ended for use in ~he field of application below 20 cm of girder heigh~. This gi-der design comprises a plurality of binders and joi ~s ~ed one on top of another, the wood fibers of which extend : parallel to each other and in the direction of the girder axis. On the two sides of this girder design areas are milled out, whereby a "constriction" is effected in the center of the girder which is intended to create the effect of a double-T-girder. But due to the bonding of the individual wooden layers with the wood fibers parallel to each other, such girder designs have practically the same insufficient deformation properties as s~uared timbers, so that when used ~or long periods in the open, an adequate degree of dimensional and shape stability is not attained. Because of the construction described, thes~ service girders have inevitably a relativel~ unfavourable weight - to -load bearing ratio. In addition, ~anufacture is costly since to create the constricti~n after the bonding of the individual 26~
~o wood layers, an additional milling process is required.
Moreover this girder design is relatively unfavorable as regards the transmission of shearing loads. Thi~ develop-ment too makes it clear that the combination of the in-vention distances itself substantially from the prior art.

An improvement of the locking effect and thus of the shape stability and strength results from the fact that the tongues of the web each engage in at least two wooden layers of the flanges.

The woocen layers of ~he girder of the invention do not have to be boards, but can also be veneers so that a plywood gir-der results. It may be especially advantageous that the flan-ges are bonded together from layers on thin veneer. In principle, these pared veneers can also have the same fiber direction. Due to the peculiarity of the paring process, the fiber direction within the individual veneer layers is usually non-uniform to the extent that, expecially having regard to the larger number of layers inserted into the flanges, an adequa~e crossed-fiber effect is attained without the veneer layers having to be deliberately bonded at an angle to each other.

~:25 When constructing the flanges from board layers, the crossed-fibre effect provided by the invention in the flanges can not only be achieved by the corresponding angular placing of the wooden layers wit~ fibers substantially parallel to each other, but also by the use of the annual ring of the corresponding jointed boards, if they have a suitable fiber track.

The wood fibers of the wooden layers of the web of the service girder according to the invention, it is true, pre~
ferably run perpendicular to each other But the fiber direction of the center layer can also be in the range fro~
g5 to 90 to the fiber direction of the two outer layers.

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- r _ Apart from the advantages already described, the service girder of the invention with symmetrically designed flanges, with the high tensional strength and favorable crossed-fibre effect~ possesses the ~ollowing advantages against the squared timbers which today are generally used for this purpose.
- Savings in labour - The service girders of the invention always fit at once when being positioned because they maintain exactly their original dimensions during the whole of their active life. Due to the uniform exactness of fit of all the girders there is an increase in productivity of the work teams laying the girders, who can be less skilled owing to the reduction in possible errors.
- Their lower weight with increased load capacity speeds up the positioning and removal as against the squared timbers.
- Savings in transport and storage space - by stacking the flanges and webs of the service girders of the invention so as to interlock, a truck or a store can accommodate 50 % more girders than in the case of the comparable squared timbers.
- More precise formwork and savings in materials - using the precisely dimensioned service girders of the invention inevitably a framework which is more precise, is produced than with sawn squared timbers, which moreover swell, shrink and warp to varying extents over their entire cross-section.
Against squared timbers the service girders of the invention, because of the more favorable load/cross-section or load/weight ratios, produce better exploitation of the raw materials of the wood which is important to the national economy.

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In addition, the service girders of the invention all)ow better control of consumption as against squared timbers, which are used for purposes which were not intended and are thus consumed.

Lastly the reproducibly adjustable and constant load capacity of the service girders of the invention increa-ses the safety of the formwork equipped with them when compared with squared timbers.

- Increased working life : - The life of the service girders of the invention is between 4 and 7 years, whereas the life of squared timbers in formwork on building sites has been shown by experience to be at best only 1 year.

- Economy - The improved possibilities for the control of the ser-vice girders as well as their long life and the acceleration of the work processes provide in the final analysis substantial rationalization and cost-savings which compensate for the increased manufacturing costs against squared timbers after,only part of their useful lives and accordingly the service girders of the invention `::25 can be used more cost effectively than squared timbers.

The sexvice girders of the invention can preferably be used for ceiling formwork, beam formwork, loose wall formwork, special formwork as well as for narrow pit and bridge cross-sections.

The prime costs are decisive for successful use of a service girder in the market. It is therefore addit-ionally the object of the invention to make available a completely new multiple production process in wood-bond construction by means of which the largest possible number of service girders of the invention can be made ~2~2~6 `

in one working process, e~g. 15 service girders in one passage through the machine.

To achieve Ihe above-defined object of the invention a process is proposed in which in the first step, the layer construction of the upper and/or lower flanges and the layer const.r~ction of the web are manufactured.
In the second step, in the flange plates grooves for the tcn~ues are milled which are arranged parallel to each other and spaced from each other by about the width of the flange, and the web plate~are divided into web strips respectively for individual girders and are provided with tongues on their edges. Then the web strips with the tongues of one edge are sunk into the grooves of a flange plate after application of a~ adhesive, perpendicularly to the plane of the plate, and simul-taneously or afterwards another flange plate with its grooves is placed on the tongues of the other web edges. ~t the end of this work process, the flange plates are pre~sed together and after the adhesive has dried the flange plates are cut between adjacent pairs of board strips, to produce respective girders.

Thus by using the invention for the first time it becomes possible to produce a larger number of sexvice girders in one work phase in a kind of "multiple" manufacturing process so that by comparison with one-off manufacture, only a fraction of the manufacturing time and thus a fraction of the costs of manufacturing accrue to the girders made in accordance with the invention. Thereby rational manufacture especially of the service girders of the invention is ensured, so that the service girder of the invention can be made available at a price acceptable to the market. Thus the service girder of the invention is in the final analysis more economical to use, taking into account all the aspects which are 6i6 I L~L
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important in practice, than the formerly used squared timbers, which are to be replaced by the service girders of the invention.

An especially rational mode of manufacture can be attained in that the flange and web plates are manufactured in large-scale presses as large-dimensional plates. These plates can be up to 10 m in length of the girder or to a multiple of the girder length.

Preferably the flange plates consist of at least three wooden layers, wherein the wood fibers of the adjacent wooden layers form an angle of 10 to 15 to each other.
The wood fibers of the outer wooclen layers can extend parallel to the girder axis at an angle of about 5 to 8 thereto, or can form an angle of about 10 to 15. Advantageously the web plates are composed of three wooden layers, the wood fibers of which form an angle to each other of about 90. But this angle can also be between 45 and 90, since in this range an adequate crossed-fibre effect is attainable for certain purposes.

The milling in the flange plates o~f parallel grooves for the wedged tongues can be expediently achieved using a continuous flow machine with a multiple milling device.
In this way, for example for a girder with a flange width of 6 cm, it is possible to machine 15 flanges simultaneously from 1 m of plate width. The division of the web plates into web strips can be done expediently on a continuous flow machine with multiple cutting device to which a continuous milling installation is connected for the milling of the wedged tongues.

The assembly of the flange plates together with the web strips is also performed advantageously using a continuous flow machine. The web strips are provided with adhesive in the area of the tongues and are sunk into the grooves of the first flange plate parallel to each other and per~
pendicular to the plane of the plate. Either simultan-eously or thereafter, another flange plate is positioned with its grooves on th~tongues of the web strips and the plates are then pressed towards one another, preferably in a horizontal press. The units which in this stag~ form a kind of sandwich are either hardened in ~e press under application of heat, preferably by means of high frequency, or they are transported from the press to a hardening store at room temperature.

According to an advantageous embodiment the glued joints of each flange plates are at a distance not greater than 15 mm from the nearer major surface of the flange plate. In this way, hot presses can be used for the manufacture of the flange plates, by means of which the bonding time of the glue is substantially reduced with the resultant quicker and more economical manufacture of the flange plates needed for the service girders of the invention. Against this in the prior art the bonding of the wooden strips is done by cold bonding, so that substantial hardening time is needed, and therefore it is uneconomical, especially for the present purpose.

After the end of the hardening process the plate unit is fed into a multiple bladed circular saw, so that in one working operation the flange plates are cut between adjacent pairs of web strips.

The result is the simultaneous production of a plurality of girders (a total of 15 girders per metre of plate width for girders with a flange width of 6 cm).

If necessary the girders can then be passed through a final processing section in the course of which chamfers or radii are provided on the outer edges of the flanges, fol-lowed by impregnation and automatic stacking.

3L113~ i6 In principle, it is important that the length of the flange and web plates should be limited to 6 m, so as to be able to manufacture service girders which are also 6 m long. But should greater lengths be required, the finished girders can optionally be linked up in a separate manufac-turing step to any re~uired length by a wedged tongue formation of the type already known.

Because of the minimum of three layers in the design of the flange and web plates, wood of quality class II can be utilised, since the existing knots therein only weaken the cross-section by a third. This is a very decisive aspect of theinvention.

For further explanation and better comprehension of the invention, embodiments of the service girders of the invent-ion are explained in more detail below with reference to the drawings.

Fig. 1 shows in a perspective overall view which has been partly cut away an embodiment of the service gir-der according to the invention, Fig. 2 shows in perspective view a part of a flange of the embodiment according to figure 1, Fig. 3 to 5 show schematically a plan view of three dif-fering embodiments of the course of the wooden layers of the embodiment shown in Fig. 2, Fig. 6 shows in perspective view a part of a further em-bodiment of a flange, Fig. 7 shows in perspective view a part of a solid web comprising three wooden layers, 2~
. l7 ., Fig. 8 and 9 show schematically the fiber course of two embodiments of the solid web according to Figure 7, and Fig. 10 shows, correspondingly with Figure 7, a further embodiment of a solid web.

As can be saen in F.igure 1, the service girder consists of an upper flange 1, a lower flange 2, as well as a straight solid web 3 connecting these two flanges, said web being linked with flanges 1 and 2 in each case by two tongues 4 which engage therein and are bonded with an adhesive over the entire length of the girder.

To make clear the layer design of the flanges 1 and 2 which in the present embodiment are identical, the upper flange 1 has been cut away in stages in Figure 1.

In the embodiment shown in figure 1, the flanges 1 and 2 each consist of a total of three wooden layers, namely 20 - the outer layers 5 and 6 and the center layer 7 disposed between the outer wooden layers.

~he solid web 3 also consists of three wooden layers, namely the outer wood layers 8 and 9 as well as the center .........
~5 layer 10 disposed between them.

According to the invention, the wood fibers of the adja-cent wooden layers of the flanges are so arranged that in each adjacent pair the fibres of respective layers are mutually inclined at an angle of about 10 to 15~o Having regard to this condition, it is possible to align the individual wooden layers with respect to the girder axis in various ways. Three preferred embodiments of the example of the embodiment according to Figure 1 are shown in Figures 2 to 5.

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Thus the wood fibers in the embodiment shown in Figure 3 (F5 and F6) (continuous lines) of the outer wooden layers 5 and 6 run respectively parallel to the girder longitudinal axis. The wood f'bers ~7 (interrupted lines) o the can-ter wooden la~er 7 run however at an angle of 10 to 15 to the girder longitudinal axis.

In the embodiment shown in Figure 4, the wood fibers F7 of the center wooden layer 7 run parallel to the girder !0 longitudinal axis, while wooden fib~rs F5 and F6 of tne outer wooden layers 5 and 6 form an angle of 10 to 15 to ` the girder longitudinal axis.

In the embodiment shown in Figure 5 again the wood fibers F7 of the center wood layer 7 as well as the wood fibers F5 and F6 of the outer wooden layers 5 and 6 form an angle of about 5 to 8 to the girder longitudinal axis. The arrangement of the wooden layers is selected so that the wooden fi~ers of the outer layers and the center wooden layer in each case intersect.

Despite the differing orientations of the wood fibers of the individual wooden layers to the girder axis~ all three embodiments fulfill the condition of the invention that ` 25 the wood fibers of adjacent pairs of wooden layexs form an angle to each other of about 10 to 15.

In the embodiment ~shown in Figure 6 the flange consists of a plurality of wooden layers in the form of individual veneer layers. The wood fibers of adjacent pairs of layers of veneer also form in each case an angle of about 10 to 15 to each other.

The solid web shown in Figure 7 consists of two outer wooden layers 8 and 9 as well as a center layer 10 bet~
ween them. The wood fibers of the adjacent layers are arranged so that they are perpendicular to each other. In ~32~

a preferred embodiment of ~his ex~mple, the wood fibers F8 and F9 (solid lines) of the outer wooden layers 8 and 9 are so arranged that ~hey extend in the longitudinal direction of the girder. This embodiment corresponds to S th~ example shown in ~igure 1 and is schematically illustrated in Figure 8. In this case, the fibers of the central layer 10 extend perpendicular to the longitudinal axis of ~he beam thus producing a crossed~fiber effect bet-ween the tongues and grooves.

However, it i5 also possible to construct the solid web in such a way that the wood fibers F10 (interrupted lines) of the center layer 10 run in the longitudinal direction of the girder, whereas the wood fibers F8 and F9 of the outer wooden layers 8 and 9 are aligned perpendicularly to the girder longitudinal axis. This embodiment is shown schematically in Figure 9.

In accordance with the invention, the solid web can con-sist of plywood, i.e. of a number of layers of veneer, the wood fibers of which run respectively perpendicularly to each other. Such a design of solid web iscchematically shown in Figure 10.

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Claims (20)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Service girder of wood, having an upper and a lower flange connected by a solid flat web which is connected to said flanges by a plurality of tongues engaging in the flanges and bonded over the entire length of said girder, wherein the web consists of plywood or of a plate made of three wooden layers bonded together and wherein the wood fibers of the outer wooden layers extend perpendicularly to those of the center layer, and the flanges are each composed of a plura-lity of wooden layers, the wood fibers of which extend sub-stantially in the longitudinal direction of said girder, characterized in that each flange comprises at least three superimposed wooden layers and the wood fibers of each adjacent pair of wooden layers are mutually inclined at an angle of about 10° to 15°.

2. Service girder according to claim 1, characterized in that the wood fibers of the outer wooden layers of each flange extend parallel to the longitudinal axis of said girder.

3. Service girder according to claim 1, characterized in that the wood fibers of the outer wooden layers of each flange form in each case an angle of about 5° to 8° with the longitudinal girder axis.

4. Service girder according to claim 1, characterized in that the wood fibers of the outer wooden layers of each flange form in each case an angle of 10 to 15° with the longitudinal axis of said girder.

5. Service girder according to claim 2, 3 or 4, characterized in that each flange consists of an odd number of wooden layers.

6. Service girder according to claim 2, 3 or 4 with three wooden layers per flange, characterized in that the thickness of the outer wooden layers of each flange is no more than 15 mm.

7. Service girder according to claim 2, 3 or 4, char-acterized in that the tongues of the web each engage at least two wooden layers of the asscciated flange.

8. Service girder according to claim 2, 3 or 4, char-acterized in that each individual wooden layer of the web comprises several parts.

9. Service girder according to claim 2, 3 or 4, char-acterized in that each flange comprises layers of thin pared veneer.

10. Service girder according to claim 2, 3 or 4, char-acterized in that the height of the assembly is between 10 and 20 cm.

11. Service girder according to claim 2, characterized in that a) each flange consists of an odd number of wooden layers and comprises layers of thin pared veneer, b) the thickness of the outer wooden layers of each flange is no more than 15 mm, c) the tongues of the web each engage at least two wooden layers of the associated flange, d) each individual wooden layer of the web comprises several parts, and e) the height of the assembly is between 10 and 20 cm.

12. Service girder according to claim 3, characterized in that a) each flange consists of an odd number of wooden layers and comprises layers of thin pared veneer, b) the thickness of the outer wooden layers of each flange is no more than 15 mm, c) the tongues of the web each engage at least two wooden layers of the associated flange, d) each individual wooden layer of the web comprises several parts, and e) the height of the assembly is between 10 and 20 cm.

13. Service girder according to claim 4, characterized in that a) each flange consists of an odd number of wooden layers and comprises layers of thin pared veneer, b) the thickness of the outer wooden layers of each flange is no more than 15 mm, c) the tongues of the web each engage at least two wooden layers of the associated flange, d) each individual wooden layer of the web comprises several parts, and e) the height of the assembly is between 10 and 20 cm.

14. Process for the manufacture of a wooden service gir-der comprising a multi-layered upper and lower flange and a multilayered flat web which is connected to the flanges over its whole bearing length by tongues, characterized in that a) the layer composition of the upper and/or lower flange and the layer composition of the web are established, b) in the flange layers of each flange grooves for the tongues are formed parallel with each other and mutually spaced at a distance corresponding approximately to the width of the flange, c) the web layers are divided into web strips and are provided with said tongues on the longitudinal edges, d) the web strips with said tongues of one edge are inserted after application of an adhesive into the grooves of a flange plate parallel to each other, and perpendicularly to the plane of said plate, and simultaneously or subsequently another flange plate with its grooves is mounted on the tongues of the other edge of the board, and the flange plates are pressed together, and e) after hardening of the adhesive the flange plates are respectively divided between adjacent web-strips.

15. Process according to claim 14, characterized in that the flange plates each comprise at least three wood layers, of which the wood fibers respectively form an angle of about 10 to 15° to each other and an angle of about 5 to 8° to said grooves.

16. Process according to claim 14, characterized in that the flange plates each comprise at least three wood layers, the wood fibers of which form an angle to each other of about 10 to 15° and alternately an angle of 0 or 10 to 15°
to said grooves, the wood fibers of the out layers being laid parallel to said grooves.

17. Process according to claim 14, characterized in that that web comprises three wood layers, the wood fibers of each layer forming an angle of 90° with the or each adjacent layer, the wood fibers of the outer wood layer being aligned parallel to the axis of said girder.

18. Process according to claim 14, characterized in that the web is made of plywood.

19. Process according to claim 14, 15 or 16 for the manufacture of service girders having a three-layered upper and lower flange, characterized in that the bonded joints of each three-layered flange are set at a distance no greater than 15 mm from the nearer major surface of the flange.

20. Process according to claim 17 or 18 for the manufacture of service girders having a three-layered upper and lower flange, characterized in that the bonded joints of each three-layered flange are set at a distance no greater than 15 mm from the nearer major surface of the flange.