ITTO970095A1 - TRAVELING ELEMENT FOR STEEL BUILDING CONSTRUCTION - Google Patents

Description

DESCRIPTION of the industrial invention entitled:

"Truss element for steel building constructions",

DESCRIPTION

The invention relates to a truss element for steel building constructions, and a modular system for setting up steel building structures, in particular bridges, as well as the construction itself considered, and a process for manufacturing the truss elements.

The load-bearing structures (frameworks) of bridges or other building constructions are often fitted with steel profiles. For this purpose, from the publication "Stahlbau, Grundlagen der Berechnung und baulichen Ausbildung von Stahlbauten" (Steel constructions, calculation foundations and constructive configuration of steel structures) by Petersen, C., Vieweg Verlag, the trick is known to connect individual truss elements together, by means of screw joints and with the aid of joint covers, which are superimposed on a head connection of the trusses.

Due to the considerable rolling tolerances normally present in the beams, the deformations produced by the welding processes, or other manufacturing defects, often the beams to be joined to each other have different thicknesses. Between the joint covers and the respective thinner beams then so-called interstices or mating gaps appear, which must be closed with appropriate tightening of the screws. If these gaps exceed, for example, a width of 2 mm, corresponding thickness inserts must be inserted after respective measurements on site.

Furthermore, it is necessary to prevent the screws used for the connections from undergoing bending due to the deviation of the support surfaces from planar parallelism. Nevertheless, the aforementioned interstices or mating gaps give rise to openings, non-planar supports or irregularities in the distribution of the pressures between the joint cover and the beam.

From DE 2904 324 A1 a load-bearing structure is known, the upper and lower beams of which are formed by I-beam sections. These are connected to each other by means of prefabricated reticular elements (trellis), approximately square in shape, which are fixed by screws to the beam profiles. For this purpose, the reticular elements have pins with threaded ends, connected in a fixed way with these, which correspond to holes specially prepared in the upper and lower beams.

In this regard, the joint head beams are each time connected to each other by means of screws, each time with the use of an upper plate and a lower plate, and also with the application of two side plates.

Also in this case, the particular problems already illustrated in the aforementioned "Stahlbau" publication occur in the connections with joint covers.

From document DE 3045704 A1 a demountable bridge or road above-elevated is known, which comprises individual bridge sections, consisting of a certain number of truss elements. The bridge sections, which end up joining each other, are connected to each other by means of suitable joint covers, and also directly, by means of pins with threaded ends. The joint covers are fixed with screws on the longitudinal surfaces of the respective bridge sections, and overlap the head connections. A transverse wall, present on the front side of each bridge section, also has holes, through which the corresponding screws are inserted, which connect the aforementioned sections to each other.

As in the case of the state of the art described above, here too problems are encountered in the area of the connections with joint covers.

Starting from these considerations, the purpose of the invention is to provide truss elements, with which it is possible to set up load-bearing structures quickly and economically. The invention also has the purpose of providing a modular system for setting up building structures, and of proposing a method by which the truss elements are manufactured.

The first task is accomplished by means of a truss element having the particularities set out in the characterizing part of Claim 1.

The truss element according to the invention has a truss body, which for example is formed by a rolled profile. In at least one place, the truss body is arranged so that it can be connected with another truss element, for example by means of joint covers or similar elements. Such a place is for example the end, or even another place of connection of a body of truss. This area or place is made with a greater wall thickness. Here, support surfaces are obtained in defined positions.

Increases in thickness can be obtained by adding material. In this regard, the material inserts initially have thicknesses greater than the dimensional tolerance envisaged for the truss body. Therefore the surfaces of. support, regardless of the concrete dimensions of the truss body, included within the tolerance range, particularly as regards their mutual alignment, can be brought with relative precision to the nominal dimension. Consequently, the truss elements can be prefabricated with the supporting surfaces finished in a finished way, and in such conditions they adapt to each other, on the construction site, without further processing. This reduces the mechanical stresses in the truss elements after assembly, which could otherwise occur, for example due to thermal expansion, in the welded joints.

Due to the prefabrication with precise adaptation of the truss elements, the labor expenditure at the construction site is limited to a simple assembly process, without further adjustment work of the aforementioned truss elements, which leads to cost savings. In addition to the support surfaces, the connecting holes are also preferably already provided on the truss elements, which in individual cases can be provided with internal threads. The material inserts with precisely fitting contact surfaces make it possible not only for problem-free assembly, but also for a good distribution of forces on these surfaces.

After preliminary processing of the truss elements, the necessary rust protection can be applied in the factory. There is no longer any need for chip removal at the construction site. The anti-rust protective coating remains unaffected.

The truss body can have a substantially constant profile along its length, and can be obtained by joining single elements by welding. In the case of rather small truss elements, laminated profiles can be used. If necessary, the truss element, shaped for example as an I profile, can also have lightening openings in its core area.

While the truss body is subject to a relatively large tolerance in its external dimensions, the distances between the parallel flat reference surfaces preferably have a rather narrow degree of tolerance, so that the joint covers can be fixed on the reference surfaces without interposition. of special thicknesses. This can be achieved by keeping the dimensional deviations to values less than 1/10 mm.

The material coatings can be formed by individual elements, such as plates welded or fixed with screws, elements of profiled material or the like, or also by applications of material by welding. In each case, they are connected to the truss body, before the supporting and reference surfaces are obtained on these surfaces.

While the truss body is preferably a rolled profile, the bearing surfaces on the truss elements are preferably obtained by machining the thicker wall areas by chip removal. In these conditions, the removal of material is carried out only on the facing part, and therefore any weakening of the truss body is excluded.

The supporting surfaces can have a flat configuration, and in such conditions the joint covers are tightened on the same surfaces by means of high-strength screws. The joint cover is then in frictional connection with the supporting surface, and therefore in the condition of use a rigid connection with dynamic coupling is made, which in terms of stiffness is comparable with a welded joint.

However, it is also possible to achieve a form-fitting connection, due to the fact that grooves, undercut recesses or the like are provided in the material overlay. This is achieved by dividing the respective support surface into at least two areas located at a certain angle with respect to each other.

Furthermore, the aforesaid condition can be achieved by inserting, in the material coating or in the truss body, adjusting pins with a relatively large diameter, for which the joint covers have corresponding housing openings. The lateral surface of the register pin, insofar as this protrudes from the material filling, forms part of the bearing surface, and is located at a certain angle with respect to it.

Calibrated bolts can also be used for the form fit connection. The calibrated holes can be made on a milling machine with a degree of precision such that, in the assembly operation, no further common boring of the corresponding holes is no longer necessary. Screws of a material having a lower mechanical strength (structural steel) can be used when their diameter is correspondingly larger. The corresponding threaded holes (Π40) can be made with the CNC milling machine (computerized numerical control).

With the truss elements according to the invention, it is possible to provide a system of finished parts, which contains at least a reserve of truss elements adapting to each other. With such a modular system, corresponding constructions can be set up quickly and economically.

With the method according to the invention, truss elements are obtained, on which the connection places are already prepared. These have tight tolerances, and allow the assembly of a load-bearing structure without further processing of the individual truss elements.

The same truss elements, which for example can have lengths even greater than ten meters, are obtained from a body which is preliminarily manufactured with corresponding pieces of sheet metal cut to size. On its connection places, this is provided with material overlays, for example by applying welded plates, which are subsequently milled to the required size.

In addition or alternatively, fastening openings or threaded holes with the prescribed dimensions can be made on a milling machine, for example for calibrated screws. The milling machine preferably has a machining stroke which is equal to or greater than the length of the truss element to be machined. By means of this procedure, the necessary coupling precision is achieved in the area of the connection locations, and the rest of the truss element can have quite large tolerances.

Since the connections between the truss elements can be made relatively easily at the construction site, by means of a simple assembly process, it is possible to equip the desired building structure with light truss elements, transportable by truck or by rail. The industrial prefabrication of the truss elements reduces the overall costs of the building structure.

The particularities of some advantageous embodiments are set out in the subsequent claims. Some examples of implementation are shown in the drawing. This shows:

in Figure 1, a reticular (lattice) bearing structure, in a schematic side view;

in Figures 2 and 2a, a truss composed of individual elements, which largely corresponds to the truss shown in Figure 1, without the connecting plates, in a schematic side view;

in Figure 3, a truss element with an end connection zone, prepared for connection with another truss element, in a partial and schematic perspective representation;

in Figure 3a, an I-beam composed of two beam elements in end connection, in a schematic side view;

in Figure 3b, the I-beam according to Figure 3, according to a cross-sectional view, corresponding to the place of the head connection;

in Figure 4, another embodiment of the truss element according to Figure 3, set up at the end for connection with another truss element, and of a connecting element, with form coupling connection in two directions , in a partial and schematic perspective representation;

in Figure 5, a truss member set up for connection with a corresponding truss member, having an area suitably prepared for form-fit connection in one direction, in a partial and schematic perspective representation;

in Figures 5a and 5b, the form-coupled end connections for two truss elements, in schematic representations;

in Figure 6, a truss member connected to another truss member by a form-fit cleat connection, in a schematic cross-sectional representation;

in Figure 7, the lower chord of the truss (lattice) according to Figure 1 or 2, with joint covers fixed by screws, in a schematic representation in cross section;

in Figure 8, two truss elements to be connected together with corresponding form-fitting connection elements, in a schematic front view;

in Figure 9, a bridge resting on pylons, according to a partial cross-sectional view;

in Figure 10, the bridge according to Figure 9, in a partial side view, and

in Figure 11, the bridge according to Figures 9 and 10, in a partial exploded perspective representation.

Description of the examples of implementation

In each of Figures 1,2 and 2a a truss 1 is shown, which is composed of individual elements 2, 3, 4, 5, 6. Each of these elements has substantially a length, for example, not exceeding sixteen meters, and the their connection is made with the technique of screws. The length of the truss elements is chosen so that they can still be transported with relative ease. Such relatively small truss elements can be transported, for example, by lorries, and placed in position with the cranes normally used in the construction sites considered.

The truss elements 2, 3, 4, 5, 6 are preferably manufactured with industrial processes, and with their measurements they fall within the pre-established dimensional network of a modular system. They can be obtained by combining laminated profiles or even single welded plates.

In Figure 3 a single beam 2 is partially shown. Its end 7, shown in the figure, is prepared for connection with another truss element 2 of the same type. The truss element 2 has a body 9, which is formed by an I-laminated profile. Between an upper chord 11, substantially flat, and a lower chord 12, extending at a certain distance parallel to the first, there is a core 13, which connects the aforementioned upper 11 and lower 12 currents together.

The dimensions of the beam body 8, ie the distance of the upper chord 11 from the lower chord 12, vary within a tolerance of a few millimeters. In order to be able to apply joint covers 14 without problems on a head connection with another truss element of the same type, plates 16 ÷ 23 are welded on the flat sides of the core 13 and of the upper 11 and lower 12 beams.

In this regard, these plates 16 ÷ 23 have a thickness greater than the maximum dimensional tolerance foreseeable between the upper beam 11 and the lower beam 12. They form material inserts, on which support surfaces 26 ÷ 33 are obtained with suitable processes machining operations with chip removal, for example with the use of a CNC milling machine (with computerized numerical control).

The support surfaces 26 ÷ 33 are flat surfaces, which are located in a predetermined dimensional grid. Therefore, the distance A between the supporting surfaces 26 and 27 or 28 is defined in a defined manner. Correspondingly, the distance B between the surfaces 32 or 31 and the supporting surface 33 is also established. The same applies to the distances C between the bearing surfaces 28 and 32, as well as between the surfaces 27 and 31.

The distance D, which defines the interval between the surface 29 and the bearing surface 30, is also determined within a narrow tolerance by means of this chip removal machining. This tolerance is determined so that two truss elements 2 , which end up joining each other, can be connected to each other by means of screws with the use of joint covers 14, without any interstices appearing between the joint cover 14 and the supporting surface 33, or another of these surfaces. The dimensional deviations are so small that it is not necessary to insert shims or the like.

In addition to the support surfaces 26 ÷ 33, in the area of the plates 16 ÷ 23 there are also corresponding holes 36, which are located within a predetermined dimensional grid, coinciding with the grid determined by the holes 37 provided in the joint cover 14. In if necessary, the aforesaid holes 37 can be provided with internal threads. This is particularly true when joint covers 14 are to be provided for the connection only on one side. When these joint covers 14 are provided only on one side, in certain circumstances three joint covers 14 are sufficient for the head connection of the beams. On the other hand, eight joint covers are required in the case of I-beams 14. The threaded holes and the supporting surfaces can be obtained by milling with a single clamping operation of the piece.

After assembly, the joint cover 14 with its flat surface on the side of the truss element is clamped against the supporting surface 33 of the truss element 2, and against a corresponding supporting surface of another truss element, not shown. , contiguous to the first. A corresponding situation applies to the other joint covers 14, not shown here, which by means of screws are tightened against the other supporting surfaces 26 '÷ 32. With these supporting surfaces 26 ÷ 33, the joint covers 14 form connections with friction coupling, by means of which the truss elements 2 are rigidly connected to each other.

When not starting from a rolled profile, the manufacture of the truss elements 2, 3, 4, 5, 6 is divided, according to a brief summary, into the following phases: cutting of sheets for the upper, lower and lower beams. core, and also of sheets to be superimposed, or of greater thickness, for the terminal areas of the truss elements;

joining of the sheets by welding, to obtain an initial shape of the truss element;

light stress relieving annealing;

sandblasting e

finishing machining on a gantry milling machine.

The areas with greater material thickness, necessary for the execution of the support surfaces 26 ÷ 33 by means of the aforementioned portal milling machine, in which the greater thickness of the material itself, before milling, exceeds the thickness and distance tolerances of the remaining areas of the truss element can be made in different ways.

For this purpose, it is possible to provide material inserts in the desired connection points on the upper, lower beams and on the core, or also, as shown schematically in Figures 3a and 3b, to butt-weld, at the end of the element 2 of truss, portions 2 'with the same profile and with greater thickness of material. A corresponding weld bead, indicated by dashed lines in the figures, extends transversely with respect to the longitudinal direction of the beam.

Instead of the frictional connection between the truss member and the clamped cleat, the connection can also be made in the form of a form-fit connection. This is achieved by the fact that a certain number of adjusting pins, having for example a diameter of 10 cm, are inserted in the supporting surfaces 26 ÷ 32, with which corresponding holes are associated, provided in the joint covers. The screws then only perform a retaining function more, and can be designed with a corresponding lower strength.

Instead of the adjusting pins, in the surfaces 26 ÷ 32 of the truss element 2, 3, 4, 5, 6 they can also be provided with grooves, for example with a longitudinal course, to which corresponding ribs are associated, obtained on the joint covers 14. Two or more parallel grooves and corresponding parallel ribs can also be provided on the truss elements 2, 3, 4, 5, 6 and on the joint covers 14. Due to the lower clamping force required, it is possible to significantly reduce the number of necessary screws, and / or the diameter of these. Furthermore, screws of material with lower mechanical resistance can be used.

Figure 4 shows the thus varied embodiment of a truss element 2, and in the following only the differences with respect to the previous types will be considered. At its end 7, the truss element 2 is provided with welded plates 16 ÷ 23, of which at least some are provided with grooves 39, 40, 41, 42 with longitudinal or transversal course. These are adapted to ribs 43, 44 of corresponding shape, which are obtained in corresponding places on the side of the joint cover 14 facing the truss element 2.

In width and height, the ribs 43, 44 agree with the groove 42, as well as with a corresponding groove present in another truss element 2, so that after assembly they are housed substantially without play within the groove 42 under consideration. Both the support surfaces 26 ÷ 33, as well as the grooves 39 ÷ 42 and the ribs 43, 44, are made with small dimensional tolerances by means of milling processes. The lateral surfaces or sides of the grooves 39 ÷ 42, as well as those of the ribs 43, 44, therefore form areas of bearing surfaces, which form a certain angle with respect to the remaining bearing surfaces. They are used for the form-coupled transmission of tensile and compressive stresses as well as bending moments.

In the modified embodiment of the truss element 2, illustrated in Figure 5, likewise use is made of means for transmitting forces in a form-fit manner, as occurs in the case of the truss element 2 according to Figure 4. However, while in the case of the truss element 2 according to Figure 4 the grooves 39, 42 are provided with a transversal course, and the grooves 40, 41 have a longitudinal course, on the truss element 2 according to Figure 5 all the grooves are oriented in a transversal direction with respect to the same element 2. The connecting joint covers required here have corresponding conformations.

The schematic representations of Figures 5a and 5b show corresponding head connections, with the use of joint covers, between truss elements 2 with an asymmetrical I profile, approximating a U profile. Both on the core 13 and on the beams upper 11 and lower 12, the truss elements 2 are provided with a joint cover 14 on one side only. The same applies to the head connection according to Figures 5c and 5d, which show such a connection for an I-shaped profile of the symmetrical type. As can be seen in Figures 5a and 5b, the connecting plates, or joint covers 14, are fixed with screws from the inside of the bridge, and therefore are better protected against atmospheric agents. Blind threaded holes can be used.

Figure 6 shows a truss element 2, which is made with a box profile. For the connection with another truss element of the same type, joint covers 14 are used, fixed with screws to the same truss element 2. For this purpose, profile elements 51, 52 are fixed on the body 8 of the beam with a box-shaped profile, on two opposite sides, which as supporting surfaces for the joint cover 14 have recesses 53, shaped as open grooves. on one side.

In this regard, the supporting surfaces 53 are shaped in such a way that the profiled element 51 and the profiled element 52 have two surface areas in the form of strips, lying in a common plane, and two other parallel surface areas. between them, facing each other. Their distances are established so that the joint cover 14 fits without play between the aforementioned profiled elements 51, 52, and mates without interstices with the supporting surfaces 53. By means of suitable screws 54, 55, the joint covers 14 are tightened on the profiled elements 51, 52, so that between the same covers 14 and the aforementioned profiled elements 51, 52 there is a connection with shape and friction coupling.

Figure 7 shows a truss element 2, used as a lower chord of the truss (lattice) according to Figure 1, with form-fit fastening of a diagonal element (strut), which is connected by means of joint covers 62. This element 2 of the truss has a body 8 of a beam with a box-shaped profile, on the side surfaces of which plates 63, 64 are welded.

These plates, which like the profiled elements of the truss element 2 according to Figure 6 can be provided with holes 65, in which an internal thread is obtained, are machined to the required extent on their external side, and therefore have surfaces 66, 67 of support, which are used as reference surfaces. The joint covers 62 are tightened on the supporting surfaces 66, 67 by means of screws 68 inserted in the holes 65. As occurs in the previous examples of embodiment, by adding material, ie by means of the plates 63, 64, a wall thickness is obtained which makes The execution of relatively large threaded holes is possible. Therefore, the number of screws required for the connection can be reduced to a reasonable extent.

Figure 8 schematically shows two truss elements 2, to be connected to each other by form coupling. These elements, which for example are truss elements 8 with a box-shaped profile, for example on their upper and lower sides, or on their lateral surfaces, are provided with plates 71, 72, which on their external sides have flat surfaces 73 , 74, work to the required size. On their sides which are each time opposite with respect to the respective ends 7, the aforementioned plates 71, 72 are provided with grooves 75, 76 open on one side, which extend transversely with respect to the longitudinal direction of the truss.

Corresponding protrusions 77, 78 in the shape of strips are formed on the joint covers 14, used for connection, which in the assembly position engage within the grooves 75, 76. The joint covers 14 are likewise held here by means of screws on the truss elements 2.

Figures 9, 10 and 11 show a bridge set up with the use of truss elements 2. As can be seen from Figure 9, two longitudinal beams 52, 53, parallel to each other, are held on the pylons 51 of the bridge. The pylons 51 of the bridge are welded steel structures and, when necessary, they are prefabricated in part or in whole. For example, a 40 m high pylon (cross section 4 m x 2.5 m) can be formed by three sections (12.5 m x 4 m x 2.5 m), with respective complete cross sections, and an end part.

The front surfaces of the stubs are milled. The load is transmitted through the flat front surfaces. Likewise, the support surfaces for the joint covers, located internally, are milled. The processing is performed on gantry milling machines. The machining stroke is at least 12.5 m x 4 m x 2.5 m, i.e. it corresponds to the dimensions of the truss element.

Threaded blind holes are used for the connection, which enables better corrosion protection for the screws. The connection points of the pylons 51 of the bridge are previously machined for precise coupling on a CNC (computer numerically controlled) gantry milling machine, as has already been illustrated with reference to elements 2, 3, 4, 5, 6 of truss.

The longitudinal trusses 52, 53 are formed by a certain number of shorter truss elements 2 (for example, by 16 m), joined together for example by end connections with joint covers, as shown in Figures 3 ÷ 5, and they cover for example a span of 48 m. The truss elements 2 have cores with a height of approximately 2.5 m. On the longitudinal trusses 52, 53 there are transversal trusses 54, which are formed by other truss elements 2 'with corresponding smaller dimensions.

The transverse beams 54 carry concrete slabs 56, on which tracks or a road surface can be placed. Inserts are embedded in the concrete slabs 56 having the fixing openings or the supporting surfaces previously machined to the required dimensions. The fixing openings are made on the milling machine after the inserts are embedded in the concrete. Even in the case of a steel plate, the fastening openings are previously machined on a milling machine. Such plates can also be placed directly on the longitudinal beams.

Both the longitudinal beams 52, 53 and the transverse beams 54, as well as between the transverse beams 54 and the concrete slabs 56, are provided with connections by means of screws. For the preparation of these connections, before the assembly of the bridge the truss elements 2, 2 'have already been previously provided, in the necessary connection places, with plates 61 (Figure 11), which have been milled to the required dimensions by means of a gantry CNC milling machine. Therefore the position of the bearing surfaces thus obtained is established with a narrow tolerance.

The construction of the bridge takes place on site with a pure assembly technique, using smaller parts, which are easier to transport and handle. The truss parts are already machined to the required dimensions in advance, with such precision that, during assembly of the bridge, parts of the same type can optionally be interchanged with each other, without any further adjustment work being required. In particular, it is no longer necessary to drill corresponding holes in the truss elements at the construction site. All the holes are already prepared with dimensional accuracy on the same truss elements.

In particular, for the setting up of building structures, consisting of steel elements, truss elements 2, 3, 4, 5, 6 were provided, on whose areas 7, intended for connection with other truss elements, were made a coating of material by applying welded plates 16 ÷ 23 or other profiled elements 51, 52. On these plates 16 ÷ 23 or profiled elements 51 52, preferably with a machining process with chip removal, surfaces are obtained 26 ÷ 33, 53 of reference or support which, within the necessary degree of precision in assembly, couple precisely with the support surfaces of other truss elements, or with the corresponding joint covers 14.

The truss elements 2 ÷ 6, prepared in this way, can be transported to the construction site, and here they can be assembled without any particular adjustment operations, in particular without the use of shims. In this regard, the relative dimensions A, 8, C, D are respected in particular at the connection sites.

Claims (28)

CLAIMS 1. Truss element (2, 3, 4, 5, 6), especially intended for the construction of bridges, with a truss body (8) which at least in one place (7) is set up for connection with at least one other truss body (2, 3, 4, 5, 6), by means of connecting joint covers (14); and in this regard the truss body (8), at least in the place (7) to be connected with another truss element (2, 3, 4, 5, 6), has surface areas (26 ÷ 33), which they are used as support surfaces for the connecting joint covers (14), characterized by the fact that the support surfaces (26 ÷ 33) are obtained in areas (16 ÷ 23), in which the thickness of the material in the body (8) of the truss is increased compared to the remaining part of the same body (8) of the truss . 2. Truss element according to claim 1, characterized in that the areas (16 ÷ 23) are formed by means of material inserts. Truss element according to claim 1, characterized in that the truss body (8) has a substantially constant profile along its length, and / or is shaped as a rolled profile. Truss element according to claim 1, characterized in that the dimensions of the truss body (8), as regards the distances between its outer surfaces, are precisely established only to the nearest of a few millimeters. 5. Truss element according to claim 1, characterized in that the supporting surfaces (26 ÷ 33), provided in the areas (16 ÷ 23), are shaped as reference surfaces, the mutual distances of which are established within a narrow tolerance. 6. Truss element according to claim 5, characterized in that the distances (A, B, C, D) of parallel reference surfaces are established within a narrow manufacturing tolerance, whereby the joint covers (14 ), which connect the truss elements (2, 3, 4, 5, 6) together with the maximum dimensional deviation of the support surfaces (26 ÷ 33), match on the two support surfaces (33) of the two elements (2 ) of truss, without undergoing any permanent deformation. 7. Truss element according to claim 6, characterized in that the manufacturing tolerance is established with a degree of precision greater than one tenth of a millimeter. 8. Truss element according to claim 1, characterized in that the areas (16 ÷ 23) are formed by filler elements which are fixedly connected with the body (8) of the truss. 9. Truss element according to claim 8, characterized in that the facing elements consist of plates (16 ÷ 23). 10. Truss element according to claim 1, characterized in that the fill elements are portions (51, 52) of profiled material. 11. Truss element according to claim 8, characterized in that the facing elements (16 ÷ 23; 51, 52) are welded to the truss body (8). Truss element according to claim 8, characterized in that the facing elements are fixed with screws to the truss body (8). 13. Truss element according to claim 8, characterized by the fact that the supporting surfaces (26 ÷ 33) are obtained by machining the removal of chips on the overlay elements (16 ÷ 23; 51, 52), after the connection of these with the truss body (8). 14. Truss element according to claim 1, characterized in that the supporting surfaces (26 ÷ 33) have a flat configuration. 15. Truss element according to claim 1, characterized in that the supporting surfaces (26 ÷ 33) have at least two areas forming a certain angle between them. 16. Truss element according to claim 1, characterized in that, in the areas (16 ÷ 23) with greater thickness of the material, it has holes (36) for housing screws. 17. Truss element (2, 3, 4, 5, 6), especially intended for the construction of bridges, having a truss body (8), which at least in one place (7) is set up for connection with at least one other truss body (2, 3, 4, 5, 6); and in this regard the truss body (8), at least in the place (7) to be connected with another truss element (2, 3, 4, 5, 6), has external surfaces (26 ÷ 33), which are used as support surfaces, characterized by the fact that: fixing openings are made in the supporting surfaces, e the fastening openings are already made with a degree of tolerance that allows the assembly operation without further adjustment work. The truss element according to claim 1 or 17, characterized in that the front surface areas of the truss element are milled to the required dimension, and that the truss element, which is intended for the construction of a pylon or of a box-shaped truss, has a cross section coinciding with that of the truss or pillar to be set up. 19. Prefabricated parts system for the construction of load-bearing structures, in particular bridges, having: truss elements (2, 3, 4, 5, 6) according to one or some of claims 1 to 18, and in this regard in the truss elements (2, 3, 4, 5, 6) the distances (A, B, C, D] of the support surfaces (26 ÷ 33), parallel to each other, are established with concordant values, and connecting elements (14), which each have a support surface complementary to a support surface (26 ÷ 33) . 20. Prefabricated part system according to claim 19, characterized in that the connecting elements (14), in the assembly position, are held by means of screws on the truss elements (2, 3, 4, 5, 6). 21. Prefabricated part system according to claim 20, characterized in that the connecting elements (14), in the assembly position, are held by frictional coupling on the truss elements (2, 3, 4, 5, 6). System with prefabricated parts according to claim 20, characterized in that the connecting elements (14), in the assembly position, are held by form coupling on the truss elements (2, 3, 4, 5, 6). 23. Prefabricated part system with at least two truss elements according to claim 17, characterized in that it contains calibrated screws, which for the form-fitting connection must be inserted into the fixing openings, already made previously with precise dimensions . 24. Prefabricated part system with truss elements according to claim 1 and / or 17, characterized in that the truss elements (2, 3, 4, 5, 6) are previously machined to the required dimensions, so that truss elements of the same type can be interchanged within a building structure. 25. Building structure with truss elements according to one or some of claims 1 to 18. 26. Process for manufacturing a load-bearing structure, in particular a bridge, in which truss elements are prefabricated, and in this regard a truss body, in a place provided for connection with other truss elements, is provided with at least one area with greater thickness of material, which is subsequently subjected to processing with chip removal, to create support surfaces, the mutual distance of which is within the range of, a narrow tolerance, and in which, in predetermined places, fixing openings are already made in advance with precise dimensions. 27. Process according to claim 26, characterized in that the areas with greater material thickness are made by adding material onto the beam. Method according to claim 26, characterized in that the machining is carried out with the CNC (computer numerical control) milling process.