BACKGROUND OF THE INVENTION Bearing girders of the pre-fabricated type are known, comprising as essential parts one or more metal reticulated lattices, possibly associated with a reinforced concrete plate, in which the lower portion of the lattices is embedded.

The function of the reinforced concrete plate, however, is not to increase the bearing capacity of the girder, nor does it have its own self-supporting function, nor does it guarantee an absolute constructional stability; it mainly if not exclusively serves to support the floors and to contain a concrete cast that completes the structure.

Consequently, only the reticulated lattices, together with the completion concrete cast on site, perform a bearing function and for this reason must be of a relatively great size and weight, which make the girder considerably heavier.

The size of the reinforced concrete plate, and above all its height, must be great so as to support the weight of the floor that it must support and so as to transmit the load thereof to the metal reticulated lattice.

Finally, the conformation of the reinforced concrete

plate, since it is not an active part of the bearing structure of the girder, is affected by the camber that the lattice transmits during the self-supporting step, and remains obvious even after the completion cast. This camber gives rise to fissures in the concrete that cause a loss of characteristics. For this reason their use is limited to lengths and loads which cause minimum cambers, or which determine the construction of girders with very heavy metal reticulated lattices.

Bearing girders are also known which are totally pre- fabricated and pre-stressed, consisting of metal reinforcements tied together and totally embedded in the concrete.

Such girders, however, can only be used in the construction of buildings which are totally pre-fabricated, with structures that remain such for always, with the girders and floor simply resting thereon. With such girders it is not possible to achieve the structural continuity of the girders and of the floors which rest thereon, nor is it possible to integrate the floors and girders, nor to integrate the girders with the pillars that support them.

Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain other purposes and advantages.

SUMMARY OF THE INVENTION The present invention is set forth and characterized in the main claims, while the dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

The purpose of the invention is to achieve a mixed-type steel-cement bearing girder, preferably of the pre- fabricated type, wherein the base slab cooperates in the self-supporting capacity of the girder, which can ensure

stability in the construction step and which, at the same time, can absorb part of the tensions generated by permanent or even accidental overloads, to which the girder is subjected.

Another purpose of the present invention is to achieve a mixed-type steel-cement bearing girder of the pre- fabricated type which has a limited weight with respect to a traditional bearing girder, at the same time ensuring the bearing capacity and stability under conditions of load, thanks to a reduction in the size of the base slab and a reduction in the size of the metal reticulated lattices.

In accordance with these purposes, a bearing girder according to the present invention is of the mixed type, made of steel-cement, preferably pre-fabricated, and comprises a metal reinforcement, advantageously made of steel, consisting of at least one or more reticulated lattices and of a base slab consisting of a mix made of pre-stressed cement in which the lower portions of the reticulated lattices are embedded.

Advantageously the mix of pre-stressed cement is integrated with its own metal reinforcement and inside it, in a longitudinal direction, the pre-stressing reinforcements, such as strands or cables, are inserted.

In one embodiment, carbon fibers and/or other additional mineral fibrous component of a known type are embedded in the cement mix, to achieve a fiber-reinforced concrete.

According to the invention, the pre-stressing guarantees on the one hand that the base slab is self-supporting and contributes to the overall bearing capacity of the girder, allowing to reduce the size of the metal reticulated lattices and, on the other hand, it creates a counter- camber to the girder which contrasts and partly impedes the formation of the camber due to overloads present during the

laying and completion casting.

Moreover, thanks to this reduction in the size of the bearing girder, the quantity of concrete used to complete the casting of the girder and to make a floor is also reduced.

The reticulated lattice of the bearing girder is substantially of a known type and comprises a core consisting of a plurality of diagonal rods connected to each other by means of one or more upper and lower cross- pieces.

When several reticulated lattices are involved, their reciprocal arrangement, apart from their size and number, are not restrictive for the field of the present invention, and vary according to the load conditions of the bearing girder.

In one embodiment, the girder comprises two reticulated lattices having the respective upper portions converging towards each other.

According to a variant, the reticulated lattices are arranged parallel to each other, connected by cross- stiffening brackets for bracing purposes.

In a preferential form of embodiment, the aforesaid base slab is reinforced by means of a plurality of transverse metal elements, such as for example stirrups, shaped rods, bars or other, which alternate for the whole extension of the bearing girder.

Advantageously additional cross-pieces are also embedded in the base slab and can be shorter than the slab itself.

BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the present invention will become clear from the following description of some preferential forms of embodiment, given as a non- restrictive example, with reference to the attached

drawings wherein: - fig. 1 is a side view of a mixed bearing girder according to the present invention; - fig. 2 shows an enlarged detail, partly in section, of fig. 1; - fig. 3 is a transverse section of fig. 1; - fig. 4 is a view of fig. 3 associated with a floor; - fig. 5 is a view of reinforcement elements of the bearing girder in fig. 1; - fig. 6 shows a variant of fig. 4; - fig. 7 shows an enlarged detail of fig. 1; - fig. 8 is a view from above of fig. 7; - fig. 9 is a front view of fig. 7; - fig. 10 shows a variant of fig. 7; - fig. 11 is a view from above of fig. 10; - fig. 12 is a front view of fig. 10; - fig. 13 is a simplified variant of fig. 4.

DETAILED DESCRIPTION OF SOME PREFERENTIAL FORMS OF EMBODIMENT OF THE INVENTION With reference to figs. 1,2 and 3, a bearing girder 10 according to the present invention is of the mixed, steel- cement type and comprises at the upper part a steel reinforcement 11 and at the lower part a base slab 13, made of cement, which is anchored at its ends 13a on two pillars 15,16.

The reinforcement 11 comprises two reticulated lattices 12 and 14 (fig. 3), each of which consists of a plurality of diagonal rods, respectively 18,20, which form triangular meshes closed at the ends 13a by means of vertical rods 21 (fig. 1), and of cross-pieces, upper 22a and lower 30a, 30b.

The reticulated lattices 12,14 have their respective upper portions 12a and 14a converging and attached to each

other, for example by welding, by means of a common central cross-piece 22b.

On the contrary, the lower portions 12b and 14b are embedded in the base slab 13.

According to a variant, shown in fig. 6, the reticulated lattices 112 and 114 of a mixed bearing girder 110 are arranged parallel to each other, and each of the respective upper portions 112a and 114a is provided with its own upper cross-pieces 122a, 122b.

In the simplified solution shown in fig. 13, a girder 210 comprises a single reticulated lattice 12, the lower portion 12b of which is embedded in the base slab 13.

According to a characteristic of the present invention, the base slab 13 comprises a cement mix 36 of the pre- stressed and self-supporting type.

The mix 36 is made by means of a cast of cement concrete, which is poured into a corresponding containing formworks with a thickness S in which the lower portions 12b, 14b of the reticulated lattices 12,112, 14,114 are arranged, with the relative lower cross-pieces 30a, 30b.

The pre-stressing is achieved in a known manner by means of a plurality of strands 37, in this case six, which are aligned in the containing formwork in a substantially intermediate zone of the thickness S, above the lower cross-pieces 30a, 30b. According to a variant, apart from the strands 37, carbon reinforcement fibers or other mineral component are used.

The mix 36 (figs. 3,4 and 6) defines laterally two supporting planes 39, lateral to said reticulated lattices 12,112, 14,114 on which the ends of corresponding strips 31 of a floor 33 are arranged, in order to contain a final cast of concrete 35, which drowns the reticulated lattices 12,112, 14,114.

The pre-stressing of the mix 36 guarantees the stability of the bearing girder 10 during the construction phase and limits the tensions caused by permanent and/or accidental overloads, to which the floor 33 is subjected once it has been made.

Thanks to this stability it is possible to use reticulated lattices 12,112, 14,114 of relatively limited size and also a limited quantity of concrete 35, so as to obtain a light bearing girder 10.

The girder 10 can be connected in known manner to a subsequent girder 10 before the final concrete cast is made. The connection is obtained by means of an integrating reinforcement consisting for example of segments of steel and other connection lattices so as to obtain a longitudinal structural continuity of the girder 10.

In the embodiment shown in fig. 3, each of the aforesaid supporting planes 39 has a length 1 equal to about half the width of the two reticulated lattices 12 and 14 in correspondence with the upper surface of the mix 36.

Moreover, the thickness S of the mix 36 is not negligible with respect to the overall thickness of the bearing girder 10 and, in the embodiment shown, corresponds to at least a third of the height H, that is, of the overall height of the girder 10. The thickness S ensures that the floor 33 has a high resistance to fire (FRT, or Fire Resistance Time).

The invention also provides to use in the base slab 13 a plurality of transverse metal elements 40, with a reinforcement function, which alternate in a known manner according to a pre-determined construction plan, for the whole extension of the bearing girder 10.

The transverse metal elements 40 comprise C-shaped stirrups 42 and W-shaped rods 46.

To be more exact, the stirrups 42 are arranged alternately in opposing positions with the respective open portions facing towards the inside of the base slab 13 in such a manner as to contain any possible warping of the cement during pre-stressing.

The base slab 13 is further reinforced, substantially in the peripheral zone, by a plurality of additional cross- pieces 34, which have reduced diameter and length with respect to those of the lower cross-pieces 30a, 30b, and which are embedded in the mix 36 without making the bearing girder 10 excessively heavy during the construction of the floor 33.

To be more exact, the pre-stressing strands 37, the lower cross-pieces 30a, 30b, the additional cross-pieces 34, the stirrups 42 and the shaped rods 46 are positioned on each occasion at a pre-determined distance from the lower and lateral surfaces of the base slab 13, so as to classify the girder 10 according to the norms in force, with the desired resistance to fire (R 60, R 90, R 120, R 180, etc).

It is clear, however, that the overall length, width and height of the bearing girder 10,110, 210, the size of the lattices 12,112, 14,114 and the size of the mix 36 are variables that depend on the load conditions to which the bearing girder 10,110, 210 is subjected.

The shape and structure of the end portions of the girder 10, 110,210 can also vary according to pre-determined construction plans.

In the case of figs. 7-9, the base slab 13 protrudes longitudinally at at least one end 13a with respect to the reticulated lattices 12,14 and is provided with a corresponding metal sheet 50 thanks to which the head zones of the girder 10 are shaped according to particular constructional necessities.

To be more exact, the metal sheet 50 is C-shaped or, according to a variant not shown in the drawings, L-shaped.

The central zone 52 of the metal sheet 50 is cubic, semi- conical or shaped like a truncated cone, so that it can be adapted to a corresponding pillar. The pre-stressing strands 37 are also butted on the metal sheet 50 (fig. 9).

According to a variant shown in figs. 10-12, the reticulated lattices 12, 14 protrude longitudinally with respect to the base slab 13 for a segment 55, which functions as a supporting element to rest the girder 10 on pillars and/or walls. Outside the segment 55 a cross-piece 56, also called hammer, is attached, arranged below the lower cross-pieces 30a, 30b in correspondence with the vertical rods 21 and which defines the height at which the girder 10 is supported. The segment 55 and the hammer 56 allow the girder 10 to be rested with safety and precision on the corresponding pillar without interfering with the reinforcements of the pillar and/or possible walls.

In this case the metal sheet 50 is substantially plane.

It is clear, however, that modifications and/or additions of parts can be made to the girder 10 as described heretofore, without departing from the field and scope of the present invention.

In fact, all the possible combinations of the girder 10, 110, 210 come within the field of the present invention, comprising parallel or converging reticulated lattices 12, 14, 112, 114, or a single lattice 12, and the base slab 13 made of pre-stressed cement, provided or not with the shaped or plane metal sheet 50.

It is also clear that, although the present invention has been described with reference to specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of bearing girder, all of which shall come within the field and scope of the present invention.