EP0188395A2 - Lattice girder, in particular for constructing a bridge - Google Patents

Abstract

The prestressed lattice girder is supported at several points (1, 2) so as to form, for each span, two series of elements (4a, 4b, 4c, ... 5a). Each element comprises an upper boom (6a) and a lower boom (8a) made of concrete, connected to one another by bars (10a, 11a, 12a) forming an N. The vertical bars (10a, 11a) are arranged close to the ends of the booms and essentially resist the tensile forces. The third bar (12a) resists the compressive forces and is upward in the case of the first series of elements (4a, 4b, ...) and downward in the case of the second series of elements (5a, ...) situated beyond a point M at which the shearing forces under a permanent load are substantially nil. Used especially for constructing bridges. <IMAGE>

Description

The present invention relates to a prestressed truss beam, at least partially made up of prefabricated elements, in particular for the construction of a bridge. This beam is intended to be in support at two or more points spaced so as to form one or more spans.

Lattice beams are known comprising, for each span, two series of beam elements arranged on either side of a point of the span where the shear forces under permanent load are substantially zero, these elements being assembled by tensioned reinforcements. , say prestressing reinforcements, each extending over more than one element.

Each element of this beam has an upper member and a lower member in concrete, extending in the longitudinal direction of the span. These elements are assembled end to end so that the upper and lower members of the different elements are in line with each other.

The upper and lower members of each element are connected at least by a group of three substantially coplanar bars forming an N.

When a load is applied to this beam, the latter undergoes shearing forces which are transmitted by the bars which connect the upper and lower members of the elements. Some bars undergo tensile forces and others compressive forces.

By carrying out a very detailed analysis on the one hand of the forces which apply to the bars arranged in N and on the other hand of the forces which cross the junction surfaces of the members, in the elements constituting the known lattice girders, the applicant found that it was possible to optimize the transmission of these forces, so as to increase the resistance of the structure and possibly to lighten it.

The object of the present invention is to provide a lattice beam particularly well suited to construction from prefabricated elements and having improved resistance to shearing force.

According to the invention, this beam is characterized in that two bars of N connect the two members together and extend transversely to the longitudinal direction of the span, near the ends of these members and are anchored to the latter so as to resist essentially tensile forces, while the third bar of N extends between the two aforementioned bars, and in that for the first series of elements, when moving on the span of a support to the 'other, said third bars are rising, while for the second series of elements located beyond a point where the shear forces under permanent load are substantially zero, said third bars are descending, so that these third bars undergo essentially compression forces.

Thus, the beam according to the invention comprises at least one group of two bars located on either side of the ends situated in the extension of one another of two neighboring elements. These bars which are essentially subjected to tensile stresses are anchored to the concrete members so as to resist such stresses.

On the other hand, the arrangement of the other bars known as "third bars" is such that they essentially undergo compression forces and have their opposite ends in abutment against the lower and upper members.

Other features and advantages of the invention will appear in the description below.

• - la figure 1 est une vue en élévation d'une poutre en treillis conforme à l'invention,
• - la figure 2 est une vue en perspective, à échelle agrandie d'un élément de cette poutre en treillis, qui est réalisé entièrement en béton,
• - la figure 3 est une vue schématique en élévation montrant les membrures inférieures de deux éléments voisins représentés partiellement et les répartitions des forces dans ceux-ci,
• - la figure ₄ est une vue en perspective à échelle agrandie d'une barre adaptée à résister essentiellement à des efforts de traction,
• - la figure 5 est une vue analogue à la figure 2, dans laquelle les barres sont en acier,
• - la figure 6 est une vue en perspective, d'une barre en acier,
• - la figure 7 est une vue en perspective de deux éléments parallèles reliés par une dalle et formant un voussoir,
• - la figure 8 est une vue à échelle agrandie de l'un des éléments de la réalisation selon la figure 7,
• - la figure 9 est une vue analogue à la figure 7, concernant une variante de réalisation,
• - la figure 10 est une vue analogue aux figures 7 et 8, concernant une autre variante,
• - la figure 11 est une vue en perspective d'un voussoir comprenant plusieurs éléments disposés obliquement les uns par rapport aux autres,
• - la figure 12 est une vue en perspective avec arrachements d'un pont composé d'éléments conformes à l'invention.

In the appended drawings, given by way of nonlimiting examples:

• FIG. 1 is an elevation view of a truss beam according to the invention,
• FIG. 2 is a perspective view, on an enlarged scale, of an element of this trellis beam, which is made entirely of concrete,
• FIG. 3 is a schematic elevation view showing the lower members of two neighboring elements partially represented and the distributions of the forces therein,
• FIG. ₄ is a perspective view on an enlarged scale of a bar adapted to resist essentially tensile forces,
• FIG. 5 is a view similar to FIG. 2, in which the bars are made of steel,
• FIG. 6 is a perspective view of a steel bar,
• FIG. 7 is a perspective view of two parallel elements connected by a slab and forming a segment,
• FIG. 8 is a view on an enlarged scale of one of the elements of the embodiment according to FIG. 7,
• FIG. 9 is a view similar to FIG. 7, concerning an alternative embodiment,
• FIG. 10 is a view similar to FIGS. 7 and 8, concerning another variant,
• FIG. 11 is a perspective view of a segment comprising several elements arranged obliquely with respect to each other,
• - Figure 12 is a perspective view with cutaway of a bridge composed of elements according to the invention.

In the embodiment of Figure 1, the lattice beam is in support at two points 1, 2, the part located between these two points constituting a span 3. This span 3 comprises two series of beam elements 4a, 4b, 4c , ... and 5a, 5b, 5c, ... arranged on either side of a point M of the span 3 where the shear forces under permanent load are substantially zero.

In the example of FIG. 1, the elements 4a, 4b, 4c, ... are arranged symmetrically with respect to the elements 5a, 5b, 5c, ... with respect to a plane passing through the point M and perpendicular to the longitudinal direction D of span 3.

Each element 4a, 4b, 4c, ...; 5a, 5b, 5c, ... includes an upper member 6a, 6b, 6c, ...; 7a, 7b, 7c, ... and a lower member 8a, 8b, 8c, ...; 9a, 9b, 9c, ... made of concrete which extend in the longitudinal direction D of span 3.

Elements 4a, 4b, 4c, ...; 5a, 5b, 5c, ... are assembled end to end so that their concrete members 6a, 6b, 6c, ...; 7a, 7b, 7c, ... and 8a, 8b, 8c, ...; 9a, 9b, 9c, ... are in line with each other.

The upper members 6a, ...; 7a, ... and lower Sa, ...; 9a, ... are connected to each other by a group of three bars 10a, 11a, 12a; 10b, 11b, 12b; ... 13a, 14a, 15a, ... arranged substantially in a plane and together forming an N.

In the example shown, the elements 4a, 4b, 4c, ... 5a, 5b, 5c are inscribed in a rectangle, their members being parallel to each other.

According to the invention, for each element, two bars 10a, 11a; 1 Ob, 11b; 13a, 14a of N connect the upper members 6a, 6b; 7a, 7b and lower 8a, 8b; 9a, 9b between them and extend transversely (in the example shown perpendicularly) to the longitudinal direction D of the span 3.

In addition, these two bars 10a, 11a; 10b, 11b; 13a, 14a, are arranged near the ends such as 16a, 17a of the members. These two bars 10a, 11a; 10b, 11b; 13a, 14a are anchored in the concrete members 6a, 8a; ... so as to resist essentially tensile forces, as will be explained in more detail later.

The other bars which will be called "third bars" of the N, such as 12a, 12b, ... 15a, ... extend between the two bars 10a, 11a; 10b, 11b; ... 13a, 14a, their opposite ends being in abutment against parts such as 18a, 19a, forming a stop for the upper members 6a, 6b; ... and below Sa, 8b; ...

Furthermore, we see that for the first series of elements 4a, 4b, 4c, ... when moving on the span 3 in the direction D, the third bars 12a, 12b, ... are rising, while that for the second series of elements 5a, 5b, 5c, ... located beyond point M, the third bars 15a, ... are descending.

These third bars 12a, 12b, ... 15a, ... essentially undergo compression forces, as will be explained in more detail below.

In the embodiment shown in Figure 1, the elements 4a, 4b, 4c, ... 5a, 5b, 5c, ... are assembled to each other by prestressing cables 20, 2 ₁ , 22, extending in channels formed in the concrete members of these elements.

The opposite ends of the cable 20 which extends in the lower members 8a, 8b, ... 9a, ... are anchored in bosses 23, 24 formed on the two lower members located at the ends of the span 3.

The cable 21 is anchored in a boss 25 formed on the upper member 6b of the element 4b. The other end of this cable is anchored in a similar boss made on the neighboring span (located on the left of Figure 1).

The cable 22 is anchored in a boss 26 formed on the upper member of the element 5b.

The other end of this cable 22 is anchored in a similar boss formed on the neighboring span (located to the right of FIG. 1).

In the example shown in Figure 1, the ends in contact, such as 16a. 17a upper and lower members of two neighboring elements such as 4a, 4b are located in planes perpendicular to the direction D of the span 3 and parallel to each other.

Furthermore, each bar such as 11 has adapted to resist essentially the tensile forces, is symmetrical with the bar such as 10b of the neighboring element with respect to the plane in which the ends in contact such as 16a, 17a, are located. two neighboring elements 4a, 4b. Thus, there are on both sides of the contact planes between the elements two bars perpendicular to the direction D.

On the other hand, the third bars, such as 12b (see FIGS. 2 and 3), which essentially undergo compression forces, extend along an axis A which intersects the axis B of each member such as 8b, in a point located at equal distances from axes a and b of bars 11 a and 10b.

In FIG. 2, a distinction is made between channels 2 ₇ formed in the lower member 8b for the passage of prestressing cables such as 20 and the channels 28 formed in the upper member 6b for the passage of prestressing cables such as 21.

In FIGS. 2 and 3, it can also be seen that the abutment parts 18b and 19b molded in one piece with the upper and lower members have bearing surfaces 29, 30 which are perpendicular to the axis A of the third bar 12b.

In the example shown in Figure 2, the element ₄ b is made entirely of concrete.

We will now explain with reference to Figure 3, the technical effects of the lattice beam and the elements that compose it, which we have just described.

This Figure 3 shows that the lower members Sa, 8b of two neighboring elements 4 ₈ , 4b are in contact along a surface 17a perpendicular to the axis B of these members. The bars 11a and 10b are arranged in parallel and on either side of this surface 17a.

The axis A of the third bar 12b of the element 4b intersects the axis B common to the lower members 8a and 8b at a point 17b located at equal distance from the axes a and b of the bars 11 a and 10b.

The vector F located on the axis A of the third bar 12b of the element 4b represents the compression force applied to this bar. At the point where the axis A meets the axis b of the vertical bar 10b, the force F decomposes into a force F, which biases in tension the vertical bar 10b and a force F ₂ which is oriented towards the point of intersection between the axis of the vertical bar 11a of the neighboring element 4a and the axis B the frame 8a of this element.

At this point of intersection, this force F ₂ decomposes into a force F, which biases the vertical bar 11a of the element 4a and a force F which compresses the frame 8a of this element.

Taking into account the aforementioned geometrical arrangements, the forces F, and F, which apply respectively to the vertical bars 10b and 11a are equal and the force F ₂ which crosses the contact surface 17a compresses the members 8a and 8b one against the other. the other and forms with the axis B an angle α which is relatively small since its tangent is half the tangent of the angle of the axes A and B.

The angle formed by the force F ₂ and the axis B is the maximum angle which the total force which crosses the contact surface 17a can form with this axis, this total force being the sum of the force F ₂ and the compressive force F ₅ of the frame 8b which, for a suitably prestressed structure must always be positive.

In order for the vertical bars 11a and 10b to resist the tensile forces F, and F ₃ , these must be firmly anchored in the concrete of the lower members 8a, 8b and upper members 6a, 6b.

For this purpose, it is preferable that these vertical bars 11a, 10b, be anchored in the concrete of these members beyond the axis of these.

The boss such as 19b formed on the frame 8b can collect the compressive force F exerted by the third bar 12b.

FIG. 4 shows by way of example, a bar 10b made of concrete and capable of withstanding the tensile forces. This bar 10b has a rectangular section and has at its two ends steel plates 31, 32 perpendicular to the axis of the bar and which project on either side of the concrete body of the bar.

These plates 31, 32 are interconnected by steel wires 33 under tension, embedded in the concrete and the ends of which include bulges 33a in contact with the plates 31, 32.

The prestressing of the bar 10b is obtained by exerting before the pouring of the concrete equal and opposite forces on the steel plates 31, 32 in a direction which tends to move them away from each other in order to tension the son 33. These forces are maintained after the concrete has been poured until it has acquired sufficient strength.

The concrete of the members 6b and 8b is then poured onto the ends of the bar 10b so that the plates 31, 32 are embedded in the concrete of the members.

To improve the anchoring of the bar 10b in the concrete of the members, it is preferable to provide the ends of this bar 10b with steel rods 34, 35 projecting from the concrete perpendicular to the axis of the bar, as indicated in figure 4.

After casting of the members, these rods 34, 35 are embedded in the concrete thereof and extend parallel to the axis of these members.

The holes 36, 37 formed near the opposite ends of the bar 10b, perpendicular to the axis of the bar and parallel to the steel rods 34, 35 are intended for the passage of prestressing cables such as cables 20, 21, 22 shown in Figure 1.

In the embodiment of Figure 5, the beam member comprises upper and lower members 6b, 8b which are made of concrete, as in the case of the elements described above. On the other hand, the bars 10 "b, 11" b, and 12'b are made of steel.

The vertical bars 10 "b and 11" b have their ends firmly anchored in the concrete of the frames 6b and 8b, while the bar 12'b undergoing essentially compressive forces is in abutment against the parts forming stops 18b and 19b of the frames .

One 10 "b of the vertical bars is shown in detail in FIG. 6. This bar 10" b is a steel tube which, filled with concrete 50, has at its opposite ends a crown also made of steel 38, 39 projecting on either side of the tubular body of the bar. These rings 38, 39 intended to be embedded in the concrete of the members 6b, 8b provide excellent anchoring of the bars 10 "b and 11" b and allow them to withstand the tensile forces.

Furthermore, the bar 10 "b is crossed by holes 40, 41 allowing the passage of prestressing cables.

The beam according to the invention, instead of being constituted by a single row of elements 4a, 4b, 4c as indicated in FIG. 1, can comprise two or more parallel rows of elements.

In the embodiment of Figure 7, there are shown two elements 4 'and 4 "parallel and identical whose lower chord is free but whose upper chord 6', 6" is part of a slab 42 extending perpendicular to the elements 4 'and 4 "by connecting these and overflowing on either side of these elements.

This achievement forms a segment. Using several segments of this type, assembled end to end in the extension of the members, a bridge can be made.

Figure 8 shows in detail one 4 'of the two elements of the segment shown in Figure 7. The element 4' is identical to the element 4b shown in Figure 2, except that the upper chord 6 'has a thickness weaker than the member 6b of the element 4b of FIG. 2. In addition, the ends 10'b and 11'b of the vertical bars 10b and 11 protrude beyond the upper edge of the member 6 'so as to be embedded in the slab 42 which is poured between the two elements 4 'and 4 ".

Figure 9 shows a segment identical to that of Figure 7, except that the lower members are interconnected by two cross members 43, 44 of concrete arranged in a cross, which further improves the mechanical strength of the segment.

In the embodiment of FIG. 10, the segment is also identical to those of FIGS. 7 and 9, except that the lower members of the elements 4 'and 4 "are connected by a concrete slab 45 parallel to the upper slab 42.

In the embodiment of FIG. 11, the segment comprises an upper slab 46 and a lower slab 47 which connect the upper and lower chords of four elements 4A, 4B, 4C, 4D similar to those described above but which are arranged in oblique planes relative to the slabs 46 and 47. The upper and central member 6BC is common to the two central elements 4B and 4C, the latter being connected by progressively deviating from the plane of symmetry of the segment with two lower members 8AB and 8CD which are common to the external elements 4A and 4D. These checkers are each connected to a frame 6A and 6D forming part of the upper slab 46.

FIG. 12 represents a bridge resting on vertical piers 48, 49 produced by means of segments V ,, V _b V ₃ , V. comprising two parallel elements linked together by an upper slab and a lower slab, as in the case of FIG. 10, and of segments V ,, V ₆ , V, also comprising two parallel elements connected together by an upper slab, but the lower members of which are connected by crosspieces arranged in a cross as in the case of the segment shown in the figure 9.

The segments V ₁ , V ₂ , V ₃ , V ₄ which have a stronger lower chord than that of the segments V ₅ , V ₆ , V ₇ , are arranged near the stacks 48, 49.

In the case of the embodiment according to Figure 12, the upper members are as previously arranged along a straight line parallel to the longitudinal direction of the span.

On the other hand, the lower members assembled end to end define an arc.

In such a bridge, the forces are distributed as explained with reference to FIG. 3, namely that the vertical bars of the elements essentially undergo tensile forces, while the oblique bars (called third bars) in the above description undergo essentially compression forces.

Of course, the invention is not limited to the examples which have just been described and numerous modifications can be made to them without departing from the scope of the invention.

Claims (15)

₁ . Lattice beam, in particular for making a bridge. this beam being supported at several points (1, 2) spaced so as to form one or more spans (3) and comprising, for each span, two series of beam elements (4a, 4b, ... 5a) arranged on either side of a point (M) of the span where the shear forces under permanent load are substantially zero, these elements being assembled by tensioned reinforcements, called prestressing reinforcements, s '' each extending over more than one element, each element (4a, 4b, ... 5a) comprising an upper chord (6a, 6b, ... 7a) and a lower chord (Sa, 8b, ... 9a) made of concrete, extending in the longitudinal direction (D) of the span, the elements being assembled so that the upper and lower members are in line with one another, the upper and lower members of each element being connected one to the other by at least one group of three bars (10a, 11a, 12a; 10b, 11b, 12b, ...) substantially coplanar forming an N, characterized in that two bars (10a, 11a; 10b, 11 b, ...) of N connect the two frames (6a, 8a; 6b, 8b, ...) between them and extend transversely to the longitudinal direction (D) d e the span (3) near the ends of these frames, and are anchored to the latter so as to essentially resist tensile forces, while the third bar (12a, 12b, ...) extends between the two said bars, and in that for the first series of elements (4a, 4b, ...) when moving on the span from one support to the other, said third bars (12a, 12b, .. .) are rising, while for the second series of elements (5a, ...) located beyond said point (M), said third bars (15a, ...) are descending, so that these third bars undergo essentially compressive forces. 2. Beam according to claim 1, in which the bars, essentially resistant to tensile forces - (11a, 10b), located on either side and in the vicinity of the ends (16a, 17a) in contact with the members (6a , 6b; 8a, 8b), belonging to two neighboring elements (4a, 4b) are substantially parallel to each other, and in which the third bar (12a, 12b, ...) of each element which essentially resists compressive forces and which extends between the two bars of this element (10a, 11 a; 10b, 11 b; ...) essentially resistant to tensile forces, at its opposite ends in abutment against the lower and upper members of this element . 3. Beam according to one of claims 1 or 2, characterized in that the axis of the third bar (12b) of an element (4b) intersects the axis of the lower member (8b) of this element in a point located at equal distance from the axes of the bars essentially resistant to tensile forces (10b and 11a) and intersects the axis of the upper chord (6b) at a point located equidistant from the axes of the bars essentially resistant to tensile forces - (11b and 10c). 4. Beam according to one of claims 1 to 3, characterized in that the two bars (10a, 11a) of an element (4a) resistant to tensile forces are anchored in the concrete of the members (6a, 8a ) beyond the axis of these. 5. Beam according to one of claims 1 to 4, characterized in that the parts (18a, 19a) forming stops for the third bar (12a) are constituted by concrete bosses molded in one piece with the members (6a, 8a), these bosses comprising a bearing face (30) perpendicular to the axis of the third bar. 6. Beam according to one of claims 1 to 5, characterized in that the bars (10a, 11a, 12a) are made of concrete. 7. Beam according to one of claims 1 to 5, characterized in that at least a portion of the bars are sections or steel tubes (10 "b, 12'b). 8. Beam according to claim 6, characterized in that the concrete bars (10b) resistant to tensile forces, have at their opposite ends, steel plates (31, 32) substantially perpendicular to the axis of the bar and overflowing on either side thereof, these plates being interconnected by steel wires - (33) under tension which pass through the concrete of the bar. 9. Beam according to claim 7, characterized in that the bars resistant to tensile forces, consist of a steel tube (10 "b) having at its ends a steel plate (38, 39) filled with concrete - (50), substantially perpendicular to the axis of the tube, projecting on either side of it and having a central hole. 10. Beam according to one of claims 6 to 9, characterized in that the bars (10b, 10 "b) resistant to tensile forces, have near their ends, holes (36, 37; 40, 41) substantially perpendicular to their axis, intended for the passage of prestressing reinforcements. 11. Beam according to claim 6, characterized in that the bars (12'b) undergoing compressive forces, consist of a steel section or tube having at its ends a steel plate substantially perpendicular to the axis of the profile or tube and projecting on either side thereof, this plate bearing on the part (18b, 19b) forming a stop for the frame (6b, 8b). 12. Beam according to one of claims 1 to 11, characterized in that the members (6a, 6b; 8a, ...) are traversed longitudinally by channels intended for the passage of prestressing reinforcements (20, 21, 22). 13. Beam according to one of claims 1 to 12, comprising at least two parallel rows of elements - (4 ', 4 ") assembled end to end, characterized in that the upper and / or lower members of the elements make part of a slab (42) which connects the two elements. 14. Beam according to claim 13, characterized in that the lower members are connected together by concrete sleepers (43, 44). 15. Beam according to one of claims 13 or 1 ₄ , characterized in that the elements (4A, 4B, 4C, 4D) of each row are located in oblique planes relative to the slab (46) or (47 ).

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