FR2576053A1 - LATTICE BEAM, IN PARTICULAR FOR REALIZING A BRIDGE - Google Patents

Abstract

THE LATTICE BEAM IS SUPPORTED AT SEVERAL POINTS 1, 2 IN A WAY TO FORM FOR EACH SPAN, TWO SERIES OF ELEMENTS 4A, 4B, 4C, ... 5A. EACH ELEMENT CONSISTS OF AN UPPER MEMBER 6A AND A LOWER MEMBER 8A IN CONCRETE, CONNECTED TO ONE ANOTHER BY BARS 10A, 11A, 12A FORMING AN N. THE VERTICAL BARS 10A, 11A ARE ARRANGED NEAR THE ENDS OF THE MEMBERS AND RESIST ESSENTIALLY FOR TRACTION EFFORT. THE THIRD BAR 12A RESISTS TO COMPRESSION EFFORTS AND IT IS RISING IN THE CASE OF THE FIRST SERIES OF ELEMENTS 4A, 4B, ... AND DOWN IN THE CASE OF THE SECOND SERIES OF ELEMENTS 5A, ... LOCATED AT- BEYOND THE M-POINT WHERE SHARPENING EFFORTS UNDER PERMANENT LOAD ARE ZERO. USE IN PARTICULAR FOR BUILDING BRIDGES.

Description

The present invention relates to a beam

  trellis constraint, constituted at least partially of prefabricated elements, in particular for the realization of a bridge. This beam is intended to be supported in two or more spaced points so as to form a

or several bays.

  Trellis beams are known comprising

  for each span two sets of beam elements

  on both sides of the span of the bay where the efforts

  sharp under permanent load are void.

  Each element of this beam comprises an upper chord and a lower concrete chord,

  extending in the longitudinal direction of the span.

  These elements are assembled end to end so that the upper and lower chords of the different elements

  are in the extension of each other.

  The upper and lower chords 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 cutting forces which are transmitted by the bars which connect the bars. upper and lower members of the elements. Some bars suffer

  traction efforts and other efforts to reduce

if we.

  By carrying out a very thorough 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 frames, 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 strength of the structure

  and possibly lighten it.

  The object of the present invention is to propose a lattice girder which is particularly well adapted to

  a construction from prefabricated elements and pre-

  feeling improved resistance to shear.

  According to the invention, this beam is characterized in that two bars of the N connect the two members together and extend transversely to the longitudinal direction of the span, near the ends of these

  frames and are anchored to the latter so as to re-

  essentially to traction forces, while the third bar N extends between the two aforementioned bars, its opposite ends being in abutment against the lower and upper chords and in that for the first series of elements, when one moves on the span from one support to the other, said third bars are rising, while for the second series of elements located beyond the point where the shear forces under permanent load are zero, said third bars are down, so that these third bars

  suffer essentially compression efforts.

  Thus, the beam according to the invention comprises at least one group of two bars located on each side of the ends located in the extension of one another

  two neighboring elements. These bars, which are essen-

  In fact, traction efforts are firmly anchored

  concrete bricks to withstand such efforts.

  On the other hand, the disposition of the other bars called "third bars" is such that they undergo

  essentially compression efforts.

  Other features and advantages of the invention

  will still appear in the description below.

  In the accompanying drawings, given by way of nonlimiting example: FIG. 1 is an elevational view of a lattice girder in accordance with the invention, FIG. 2 is a perspective view, on an enlarged scale, of a element of this lattice beam, which is made entirely of concrete '; - Figure 3 is a diagrammatic elevational view showing the lower chords of two adjacent elements partially shown and the distributions of forces therein, - figure 4 is an enlarged perspective view of a bar adapted to withstand essentially tensile stresses; FIG. 5 is a view similar to FIG. 2, in which the bars are made of steel; FIG. perspective, of a steel bar, - Figure 7 is a perspective view of two parallel elements connected by a slab and forming a voussoir, - Figure 8 is an enlarged view of one of the elements of the realizationaccording to FIG. 7, - FIG. 9 is a view similar to FIG. 7, relating to an alternative embodiment, FIG. 10 is a view similar to FIGS. 7 and 8, relating to another variant, FIG. in perspective of a voussoir comprising several elements arranged obliquely with respect to each other, - Figure 12 is a perspective view cutaway of a bridge composed of elements according to the invention. In the embodiment of FIG. 1, the lattice beam is supported at two points 1, 2, the-part

  between these two points constituting a span 3.

  This span 3 comprises two series of beam elements 4a, 4b, 4c, ... and 5a, 5b, 5c, ... disposed on either side of the point M of the span 3 o shear forces

under permanent load are void.

  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 passing plane.

  point M and perpendicular to the longitudinal direction

tudinal D of the span 3.

  Each element 4a, 4b, 4c, ...; 5a, 5b, 5c, ...

  comprises an upper chord 6a, 6b, 6c, ...; 7a,

  7b, 7c, ... and a lower chord 8a, 8b, 8c, ...

  9a, 9b, 9c, ... concrete that extend in the direction

longitudinal D of span 3.

  The elements 4a, 4b, 4c, ...; 5a, 5b, 5c, ...

  are assembled end to end so that their frames

  in concrete 6a, 6b, 6c, ...; 7a, 7b, 7c, ... and 8a, 8b, 8c, ..

  9a, 9b, 9c, ... are in the extension of one of the

other.

  The upper chords 6a, ...; 7a, ... and lower 8a, ...; 9a, ... are connected to each other by a group of three bars 10a, 11a, 12a; 0lob, 11b, 12b; ... 13a, 14a, 15a, ... arranged substantially in a plane and forming an N. In the example shown, the elements 4a, 4b, 4c, ... 5a, 5b, 5c are written in a rectangle,

  their members being parallel to each other.

  According to the invention, for each element, two bars 10a, 11a; 10b, 11b; 13a, 14a of N connect the upper chords 6a, 6b; 7a, 7b and lower 8a, 8b; 9a, 9b between them and extend transversely

  (in the example shown perpendicular) to the direc-

  Longitudinal D of the span 3.

  In addition, these two bars 10a, 11a; 10b, 11b; 13a, 14a, are disposed near the ends such as 16a, 17a of the ribs. These two bars 10a, 11a; 10b, 11b; 13a, 14a are anchored in the concrete chords 6a, 8a; ... so as to withstand essentially tensile stresses, as will be explained in more detail

further.

  The other bars that will be called "third bars" of the N, such that 12a, 12b, ... 15a, ... extend between the two bars 10a, 11a; lOb, 11b;

  13a, 14a, their opposite ends being in abutment against parts such as 18a, 19a, forming a stop of the upper chords DTD: 6a, 6b and lower 8a, 8b;

  Moreover, 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 for the second series of elements 5a, 5b, 5c, ... located beyond the point M,

  the third bars 15a, ... are descending.

  These third bars 12a, 12b, ... 15a, ...

  undergo essentially compression efforts,

  as will be explained in more detail later.

  In the embodiment shown in FIG. 1, the elements 4a, 4b, 4c,.

  5a, 5b, 5c, ... are connected to each other by prestressing cables 20, .. DTD: 21, 22, extending in channels provided in the mem-

concrete cracks of these elements.

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

  lower chords at the ends of the span 3.

  The cable 21 is anchored in a boss 25 formed on the upper chord 6b of the element 4b. The other end of this cable is anchored in a similar boss

  located on the neighboring bay (to the left of Figure 1).

  The cable 22 is anchored in a boss 26 formed

  on the upper chord of the element 5b.

  The other end of this cable 22 is anchored in a similar boss on the neighboring span

(located on the right of Figure 1).

  In the example shown in FIG. 1, the ends in contact, such as 16a, 17a, of the upper and lower chords of two adjacent elements such as 4a, 4b are situated in perpendicular planes.

  in direction D of span 3 and parallel to each other.

  Moreover, each bar such as 11a adapted to withstand essentially the tensile stresses, is symmetrical with the bar such as 10b-of the adjacent element with respect to the plane in which are located the contacting ends such as 16a, 17a, two neighboring elements 4a, 4b. Thus, there are on both sides contact planes between the elements two bars perpendicular to the direction D. On the other hand, the third bars, such as 12b (see Figures 2 and 3) extend along an axis To which the axis B of each member such as 8b intersects at a point situated at equal distances from the axes a and b of

bars 11la and 0lob.

  In Figure 2, there are the channels 27 formed in the lower chord 8b for the passage of prestressing cables such as 20 and the channels 28 formed in the upper chord 6b for the passage

  prestressing cables such as 21.

  In FIGS. 2 and 3, it can also be seen that the abutment portions 18b and 19b molded in one piece with the upper and lower chords have bearing surfaces 29, 30 which are perpendicular

  to the axis A of the third bar 12b.

  In the example shown in FIG.

  4b is entirely made of concrete.

  We will now explain with reference to Figure 3, the technical effects of the lattice girder

  and the elements that compose it, which we have just described.

  This figure 3 shows that the lower ribs

  8a, 8b of two adjacent elements 4a, 4b are in contact along a surface 17a perpendicular to the axis B of these frames. Bars 11a and 10b are arranged

  parallel and on both sides of this surface 17a.

  The axis A-of the third bar 12b 'of the element 4b intersects the axis B common to the lower ribs 8a and 8b at a point 17b equidistant from the axes a and

b bars 11a and 0lob.

  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

  in a force F1 which forces in traction the vertical bar

  10b and a force F2 which is oriented towards the point of intersection between the axis a of the vertical bar ila of the neighboring element 4a and the axis B of the chord 8a of

this element.

  At this point of intersection, this force F2 decomposes into a force F3 which forces in tension the vertical bar 11a of the element 4a and a force F4 which

  compresses the chord 8a of this element.

  Taking into account the precise geometrical

  the forces F1 and F3 which respectively apply to the vertical bars 10b and 11a are equal and the force F2 which passes through the contact surface 17a compresses the ribs 8a and 8b, one against the other and forms with the B axis an angle "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 F2 and the axis B is the maximum angle that can form with this axis the total force which passes through the contact surface 17a, this total force being the sum of the force F2 and the compression force F5 of the frame 8b which, for a structure

  properly prestressed must always be positive.

  In order for the vertical bars 11a and 10b to withstand the tensile forces F1 and F3, these must be firmly anchored in the concrete of the lower chords.

8a, 8b and higher 6a, 6b.

  For this purpose, it is preferable that these vertical bars lla, lob, are anchored in the concrete of these

  chords beyond the axis of these.

  The boss such as 19b formed on the frame 8b makes it possible to cash the compression force F exerted

by the third bar 12b.

  FIG. 4 represents, by way of example, a lob bar made of concrete and capable of withstanding

  pulling forces. This bar 10b has a rectangular section

  at its two ends and steel plates 31, 32 perpendicular to the axis of the bar and which overflow on either side of the concrete body of

bar.

  These plates 31, 32 are connected to each other by steel son 33 under tension, embedded in the concrete and whose ends have bulges 33a,

in contact with the plates 31, 32.

  The preloading of the bar 10b is obtained by exerting, before pouring the concrete, equal and opposite forces on the steel plates 31, 32 in a direction which tends to move the latter away from one another in order to tension the 33. These forces are maintained after the pouring of the concrete until it has acquired a

sufficient resistance.

  The concrete of the chords 6b and 8b is then cast on the ends of the bar lob so that

  the plates 31, 32 are embedded in the concrete of the members

  brures. To improve the anchoring of the bar lob in the concrete members, it is preferable to provide the ends of this bar lob steel rods 34, 35 protruding from the concrete perpendicular to the axis

  of the bar, as shown in Figure 4.

  After the casting of the frames, these rods 34, 35 are embedded in the concrete thereof and extend

  parallel to the axis of these chords.

  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 the cables 20, 21, 22 As shown in FIG. 1. In the embodiment of FIG. 5, the beam element comprises upper and lower chords 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 compression forces bears against the parts

  forming abutments 18b and 19b of the chords.

  1o''b vertical bars is shown in detail in Figure 6. This bar 10''b is a steel tube which, filled with concrete 50, has at its opposite ends a ring also steel 38, 39 making

  protrusion on both sides of the tubular body of the bar.

  These rings 38, 39 intended to be embedded in the concrete of the ribs 6b, 8b provide excellent anchoring of the bars '' b and 11 '' b and allow them to withstand the

pulling forces.

  Moreover, the bar 10''b is crossed by

  holes 40, 41 allow the passage of cable pre-

  constraint. 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

  several parallel rows of elements.

  In the embodiment of FIG. 7, there is shown

  two parallel and identical elements 4 'and 4' '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' ', connecting

  these and overflowing on both sides of these elements.

  This realization forms a voussoir. With the aid of several voussoirs of this type, assembled end to end in the extension of the frames, it is possible to make a bridge. Figure 8 shows in detail one of the 4 '

  two elements of the voussoir.represented in Figure 7.

  The element 4 'is identical to the element 4b shown in FIG. 2, except that the upper chord 6' has a smaller thickness than the chord 6b of the element 4b of FIG. 2. Moreover, the ends 10'b and 11'b of the vertical bars 10b and 11b protrude beyond the upper edge of the frame 6 'so as to be embedded in the slab 42 which is cast between the two elements

4 'and 4' '.

  Figure 9 shows an identical voussoir

  to that of Figure 7, except that the lower ribs

  are connected to each other by two cross members 43, 44 in concrete arranged in a cross, which further improves

  more the mechanical strength of the voussoir.

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

superior 42.

  In the embodiment of FIG. 11, the voussoir

  includes an upper slab 46 and a lower slab -

  47 which connect the upper and lower members of four elements 4A, 4B, 4C, 4D similar to those described above but which are arranged in oblique planes with respect to the slabs 46 and 47. The upper and central chord 6BC is common to both elements central

  4B and 4C-, the latter being connected by departing progressively

  sive of the plane of symmetry of the voussoir to two lower ribs 8AB and 8CD which are common to external zi elements 4A and 4D. The latter are each connected to a frame 6A and 6D forming part of the upper slab 46. FIG. 12 represents a bridge resting on vertical stacks 48, 49 made by means of voussoirs V1, V2, V3, V4 comprising two parallel connected elements. between them by an upper slab and a lower slab, as in the case of Figure 10, and voussoirs V5, V6, V7 also comprising two parallel elements interconnected by an upper slab, but whose lower chords are connected by crosses arranged in a cross as in the case of the voussoir represented on the

figure 9.

  The voussoirs V1. V2, V3, V4 which have a lower chord more resistant than that of V5 voussoirs,

  V6, V7, are arranged near the batteries 48, 49.

  In the case of the embodiment according to FIG. 12, the upper chords are as previously arranged along a line parallel to the direction

longitudinal of the span.

  On the other hand, the lower chords assembled

end to end define an arc.

  In such a bridge, the forces are distributed as explained with reference to Figure 3, namely that the vertical bars of the elements undergo essentially tensile stresses, while the slashes

  (called third bars) in the above description

  suffer essentially compression efforts.

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

of the invention.

Claims (15)

  1. A lattice girder, in particular for making a bridge, this beam being supported at several points (1, 2) spaced so as to form one or more bays (3) and comprising, for each span, two series of ele- beams (4a, 4b, ... 5a) arranged on either side of the point (M) of the span where the shear forces under   permanent load are zero, each element (4a, 4b, ...   a) having an upper chord (6a, 6b, ... 7a) and a lower chord (8a, 8b, ... 9a) of concrete, extending in the longitudinal direction (D) of the span, the members being assembled so that the upper and lower chords are in line with each other, the upper and lower chords of each member being connected to each 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, 11b, ...) of the N connect the two chords (6a, 8a, 6b, 8b , ...) between them and extend transversely to the longitudinal direction (D) of the span (3) near the ends of these members, and are anchored to the latter so as to withstand essentially tensile forces, while that the third bar (12a, 12b, ...) extends between the two aforesaid bars, its opposite ends being in ap against the lower and upper chords,   and in that for the first series of elements (4a, 4b, ...   when moving on the span of a support towards the other, said third bars (12a, 12b, ...) are rising, whereas for the second series of elements (5a, ...) located at beyond the point (M) where shear forces under permanent load are zero, said third bars (15a, ...) are descending, so that these third bars undergo essentially efforts compression.   2.A complement according to claim 1, wherein the bars, substantially resistant to tensile stresses (11a, 0lob), located on both sides and in the vicinity of the ends (16a, 17a) in contact with the ribs (6a, 6b; 8a, 8b), belonging to two elements   neighbors (4a, 4b) are substantially parallel to each other.   3. Beam according to one of the claims   1 or 2, characterized in that the third bar (12b) of an element (4b) intersects the axis of the lower chord (8b) of this element at a point equidistant from the axes of the bars that are essentially resistant to the forces of pulls (10b and 11a) and intersects the axis of the upper chord (6b) at a point equidistant from the bar axes which are essentially resistant to pull-ups (11b and 10c).   4. Beam according to one of claims 1   at 3, characterized in that the two bars (10a, 1a) of a tensile stress-resistant member (4a) are anchored in the concrete of the chords (6a, 8a) beyond the axis of these.   5. Beam according to one of claims 1   at 4, characterized in that the abutment portions (18b, 19b) for the third bar (12b) are constituted by concrete bosses molded in one piece with the chords (6b, 8b), these bosses having a face support (29, 30) perpendicular to the axis of the third bar.   6. Beam according to one of claims 1   at 5, characterized in that the bars (10a, 11a, 12a) are made of concrete.   7. Beam according to one of the claims   1 to 5, characterized in that at least a portion of the bars are profiles or steel tubes (10''b, 12'b).   8. Beam according to claim 6, characterized   characterized in that the concrete bars (lob) resistant to tensile stresses have at their ends   opposed, steel plates (31, 32) substantially   Diculaires to the axis of the bar and overflowing on either side thereof, these plates being interconnected by steel son (33) under tension which pass through the concrete of the bar.   9. Beam according to claim 7, characterized   characterized in that the tensile stress bars are constituted by a steel tube (10''b) having at its ends a steel plate (38, 39) filled with concrete (50), substantially perpendicular to the axis   tube, overflowing on both sides of the tube and feeling a central hole.   Beam according to one of claims 6   9, characterized in that the bars (10b, 10''b) resist   both at pulling efforts, present near their   ends, holes (36, 37; 40, 41) substantially perpendicular   diculaires at their axis, intended for the passage of reinforcement prestressing.   Beam according to claim 6, characterized   characterized in that the bars (12'b) undergoing compressive forces are constituted by a steel profile or tube having at its ends a steel plate substantially perpendicular to the axis of the section or tube and projecting outwards and else of it, this plate bearing on the part (18b, 19b) forming abutment of the chord (6b, 8b).   Beam according to one of claims 1   at 11, characterized in that the chords (6a, 6b, 8a, ...   are traversed longitudinally by channels intended to   at the passage of prestressing frames (20, 21, 22).   13. Beam according to one of claims 1   to 12, comprising at least two parallel rows of ele-   (4 ', 4' ') assembled end to end, characterized in   what the upper and / or lower members of the   The elements are part of a slab (42) which connects the two elements.   Beam according to claim 13, characterized   in that the lower chords are connected   together by concrete sleepers (43, 44).   Beam according to one of claims 13   or 14, characterized in that the elements (4A, 4B, 4C, 4D) of each row are located in oblique planes   relative to the slab (46) or (47).