CA1270355A

CA1270355A - Truss, especially for bridge construction

Info

Publication number: CA1270355A
Application number: CA000499543A
Authority: CA
Inventors: Francois Conversy
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-01-16
Filing date: 1986-01-14
Publication date: 1990-06-19
Also published as: EP0188395A2; JPS61225406A; FR2576053B1; EP0188395A3; DE188395T1; FR2576053A1

Abstract

ABSTRACT OF THE DISCLOSURE

A prestressed truss for constructing bridges is supported at a number of points (1, 2) in order to form two series of truss elements (4a, 4b, 4c, ... 5a) in respect of each span. Each truss element comprises a top chord member (6a) of concrete and a bottom chord member (8a) of concrete, said members being connected together by means of bars (10a, 11a, 12a) forming an N-panel. The vertical bars (10a, 11a) are placed near the ends of the chord members and essentially resist tensile forces. The third bar (12a) resists compressive forces and is upward-ly inclined in the case of the first series of truss elements (4a, 4b, ...) and downwardly inclined in the case of the second series of truss elements (5a, ...) located beyond a point M at which the shearing forces under continuous load are substantially zero.

Description

~L~'7~355 A TRUSS, ESPECIALLY FOR BRIDGE CONSTRUCTION

BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to a prestressed truss consistin~ at least partly of prefabricated elements, especially for the construction of a bridge.
The truss is intended to he supported at two or more points in spaced relation so as to form one or a number of spans.

Description of the Prior Art Trusses are already known in which each span . i9 made up of two series of truss elements placed on each side of a point of the span at which the shearing forces under continuous load are substantially zero, these truss elements being assembled together by means of prestressed concrete reinforcements or so-called tendons, each tendon being so arranged as to extend over more than one element.
Each truss element consists of a top concrete chord member and a bottom concrete chord member which extend in the longitudinal direction of the span. These elements are assembled together in end-to-end relation so that the top and bottom members of the different truss elements are located in the line of extension of each other.
The top and bottom chord members of each truss ' ' , ~L27~3~

element are connected together by means of at least one group of three substantially coplanar bars forming an N-shaped assembly designated as an N-panel.
Under an applied load, the truss is subjected to shearing forces which are transmitted by the connecting bars between the top and bottom chord members of the truss elements. Certain bars are subjected to tensile forces and others are subjected to compressive forces~
By carrying out a very extensive analysis on the one hand of the forces applied to the bars arranged in an N-shaped panel assembly and on khe other hand of the forces which traverse the joint surfaces of the chord members in elements constituting trusses of known types, the present Applicant has found that it was possible to optimize the transmission of these forces in such a manner as to increase the resistance of the structure and even to reduce the welght of the structure if necessary.

SUMMARY OF THE IN~ENTION
The aim of the present invention is to propose a truss which is particularly well-suited to construction from prefabricated elements and offers enhanced resistance to shearing stress.
The distinctive feature of the truss in accordance with the invention accordingly lies ln the fact that two bars of the N-panel connect the two chord memoers to each other and extend transversely to the 3~5 longitudinal direction of the span at a point near the ends of the chord members and are anchored to these latter so as to ensure that they essentially resist tensile forces whilst the third bar of the N-panel extends between the two ~ars aforesaid. Furthermore, in the first series of truss elements, said third bars are upwardly inclined when considered by a viewer moving in a given direc~ion along the span from one support to the next whereas said third bars are downwardly inclined in the second series of truss elements considered in the same direction and located beyond a point at which the shearing forces under a continuous load are substantially zero so that said third bars are essentially subjected to compressive forces.
Thus the truss in accordance with the invention comprises at least one group of two bars on each side of the coextensive ends of two adjacent truss elements.
Said bars are essentially subjected to tensile forces and are anchored to the concrete chord members in such a manner as to resist such forces.
On the other hand, the arrangement of the so-called "third bars" is such that they are essentially subjected to compressive forces and the opposite ends of said third bars are applied against the top and bottom chord members.

"', "' ~ 2~ 5~i BRIEF DESCRIPTION OF THE DRAWINGS
Other features of the invention will be more apparent to those skilled in the art upon consideration of the Eollowing description and accompanying drawings, wherein :
- Fig. 1 is a view in elevation showing a truss in accordance with the invention ;
- Fig. 2 is a view in perspective to a large~
scale showing one truss element which is entirely formed of concrete ;
- Fig. 3 is a schematic view in elevation showing the bottom chord members of two fragmentary adjacent truss elements and the distributions of forces in these latter ;
- Fig. 4 is a view in perspective to a larger scale showing a bar adapted to offer resistance essentially to tensile forces ;
- Fig. 5 is a view which is similar to Fig. 2 and in which the bars are of steel ;
- Fig. 6 is a view ln perspective showing a bar of steel ;
- Fig. 7 is a view in perspective showing two parallel truss elements joined together by a slab and forming a flat-arch segment ;
Fig. 8 is a view to a larger scale showing one of the truss elements of the embodiment ln accordance . ::

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355i with Fig. 7 ;
- Fig. 9 is a view which is similar to Fig. 7 and relates to an alternative embodiment ;
- Fig. 10 is a view which is similar to Figs. 7 and 8 and relates to another alternative embodiment ;
- Fig. 11 is a ~iew in perspective showing a flat-arch segment comprising a plurality of truss elements disposed obliquely with respect to each other ;
Fig. 12 is a fragmentary view in perspective showing a bridge composed of truss elements in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the embodiment of Fig. l, the truss is supported at two points 1, 2 and the portion located between these two points constitutes a span 3. Said span 3 is made up of two~series of truss elements 4a, 4b, 4G, ... and 5a, 5b, Sc, ... placed on each side of a point M
of the span 3 at which the shearing stresses under continuous loading are substantially zero.
: 20 In the example of Fig. 1, the truss elements 4a, 4b, 4c, ... are disposed symmetrlcall~y with respect to the elements 5al 5b, 5c, ... with respect to a plane which passes through the point M and is perpendicular to the longitudinal direction D of the span 3.
25Each truss element 4a, 4b, 4c, ..... ; 5a, Sb~
5c, ... comprises a top chord member 6aj 6b, 6c, ... ;

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7a, 7b, 7c, ... and a bottom chord member 8a, 8b, 8c, ... ;
9a, 9b, 9c, ... . Said members are of concrete and extend in the longitudinal direction D of the span 3.
The truss elements 4a, 4b, 4c, O~ ; 5a, 5b, 5c, ... are assembled in end-to-end relation in such a manner as to ensure that their concrete chord members 6a, 6b, 6c, ... ; 7a, 7b, 7c, ... and 8a, 8b, 8c, . . ; 9a, 9b, 9C, ... are located in the line of extension of each other.
10 The top chord members 6a, .... ; 7a, ... and bottom chord members 8a, ... ; 9a, ... are connected to each other by means of a set of three bars lOa, lla, 12a ; lOb, llb, 12b ; 13a, l~a, 15a ..., which are located substantially in one plane and form an N-panel when assembled together.
In the example shown in the figure, the elements 4a, 4b, 4c, ... ; Sa, 5b, 5c, ... are inscribed within a rectangle and their chord members are parallel to each other~
In accordance with the invention, each truss element is so designed that two bars of the N-panel assembly, such as the bars lOa, lla,;; lOb, llb ; 13a, 14a;
... , connect the top chord members 6a, 6b ; 7a, 7b and bottom chord members 8a, 8b 9a, 9b ..., to each other and extend ~ransversely (perpendicularly in the example shown in the figure~ with:re~pect to the longit.udinal .. , . ~ ~ . :. .

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direction D of the span 3.
Moreover, said two bars lOa, lla ; lOb, llb ;
13a, 14a ; ... are placed near the ends of the chord members such as, for example, the ends 16a, 17a of said members. These two bars lOa, lla ; lOb, llb ; 13at 14a are anchored in the concrete chord members 6a, 8a, ..~ , essentially in order to resist tensile forces as will hereinafter be explained in greater detail.
The other bars which will be designated as "third bars" of the N-panel assembly such as those designated by the references 12a, 12b, ... , 15a, ...
extend between the two bars lOa, lla ; lOb, llb ; ...
13a, 14a, ... , their opposite ends being appliecl against thrust-bearing abutment portions such as those designated by the references 18a, l9a in -the case of the top chord members 6a, 6b, ... and bottom chord members 8a, 8b, ... .
It is further apparent that, when the span 3 shown in Fig. 1 is viewed from left to right in the direction of the arrow D, the third bars 12a, 12b, ... are upwardly inclined in the first series of truss elements 4a, 4b, 4c, ... whereas the third bars 15a, ... are downwardly inclined in the second series of truss elements 5a, 5b, 5c, ... , said second series being located beyond the point M.
Said third bars 12a, 12b, .... 15a, .... are essentially subjected to compressive forces as will \

~z~ s -B-hereinafter be explained in greater detail.
In the embodiment illustrated in Fig. 1, the truss elements 4a, 4b, 4c, ... 5a, 5b, 5c, ... are assembled together by means of prestressing cables 20, 21, 22 extending within ducts formed within the concrete chord members of said truss elements.
The opposite ends of the cable 20 which extend within the bottom chord members 8a, 8br D~ 9a, ... are anchored in bosses 23, 24 formed on the two bottom chord members located at the ends of the span 3.
The cable 21 is anchored in a boss 25 formed on the top chord member 6b of the truss element 4b. The other end of said cable is anchored in a similar boss formed on the adjacent span (located on the left in Fig. 1~.
The cable 22 is anchored in a boss 26 formed on the top chord member of the truss element 5b.
The other end of said cable 22 is anchored in a similar boss formed on the adjacent span (located on the right in Fig. 1).
In the example shown in Fig. l, the contacting ends such as the ends 16a, 17a of the top and bottom chord members of the two adjacent truss elements such as those designated by the references 4a, 4b are located in planes perpendicular to the direction D of the span 3 and are parallel to each other.
Moreover, each bar such as 11a which is adapted :, , :
~ ~ ;:' ', i: ' " """"''"'- ' 1.'.' " " ' to offer resistance essentially to tensile forces is symmetrical with the bar such as lOb of the adjacent truss element with respect to the plane in which the contacting ends such as the ends 16a, l7a of the two adjacent truss elements 4a, 4b are located. Thus two bars perpendicular to the direction D are located on each side of the contact planes between the truss elements.
Furthermore, the third bars such as the bar 12b (as shown in Figs. 2 and 3) which are essentially sub-jected to compressive forces extend along an axis Awhich intersects the axis B of each chord member such as the member 8b at a point located at equal distances from the axes a and b of the bars lla and lOb.
There are shown in Fig. 2 the ducts 27 formed in the bottom chord member 8b for laying prestressing cables such as the cable 20 and the ducts 28 -formed in the top chord member 6b for laying prestressing cables such as the cable 21.
It is also apparent from Figs. 2 and 3 that the thrust-bearing abutment portions 18b and 19b which are molded in one piece with the top and bottom chord members are provided with bearing surfaces 29, 30 which are perpendicular to the axis A of the third bar 12b.
In the example illustrated in Fig. 2j the truss element 4b is entirely formed of concrete.
The technical effects of the truss and truss .,;, : : '''"'' 3~

components described in the foregoing will now be explained.
It will be apparent from Fig 3 that the bottom chord members 8a, 8b of two adjacent truss elements 4a, 4b are in contact along a surface 17a which is perpendicular to the axis B of said members. The bars lla and lOb are placed in parallel relation on each side of said surface 17a.
The axis A of the third bar 12b of the truss element 4b intersects the axis B which is common to the bottom chord members 8a and 8b at a point 17b located at equal distance from the axes a and b of the bars lla and lOb.
The vector ~ located on the axis A of the third bar 12b o the truss element 4b represents the compressive force applied on said bar. At the point of intersection of the axis A with the axis b of the ver$ical bar lOb, the force F is resolved into a force F1 which produces tension on the vertical bar lOb and a force F2 which is direct~d towards the point of intersection of the axis a of the vertical bar lla of the adjacent truss element 4a with the axls B of the chord member 8a of said element.
At the point of intersection ~ust mentioned, said force F2 is resolved into a force F3 which produces 25 tension on the vertical bar lla of the truss element 4a and a force F~ which produces compression on the chord . .-' ' :, : , 335~

member 8a of said element.
Taking into account the geometrical arrangements mentioned abo~e, the forces F1 and F3 which are applied respectively on the vertical bars lOb and lla are of equal magnitude and the orce F2 which traverses the contact surface 17a has the effect of compressing the chord members 8a and 8b against each other and makes with the axis B an angle a which is relatively small since its tangent is one-half the tangent of the angle between the axes A and B.
The angle made with the axis B by the force F2 is the maximum angle which can be made with said axis by the total force which passes through the contact surface 17a. This total force is the sum of the force F2 and of the compressive force F5 exerted on the chord member 8b which, in the case of a suitably prestressed structure, must always be positive.
In order to ensure that the vertical bars lla and lOb resist the tensile forces F1 and F3, these bars must be securely anchored in the concrete mass of the lower chord members 8a, 8b and upper chord members 6a, 6b.
To this end, it is preferable to ensure that said vertical bars lla, lOb are anchored in the concrete mass of these chord members at a point beyond the axis of each member.
The abutment shoulder of the type designated by ' ~ ., ~.
"~'' ". ~' the reference l9b and formed on the chord member 8b has the function of withstanding ~he compressive force F
exerted by the third bar 12b.
Fig. 4 illustrates by way of example a bar lOb - 5 which is formed of concrete and is capable of resisting tensile forces. This bar lOb has a rectangular cross-sec~ion and is fitted at both ends with steel plates 31, 32 which are perpendicular to the axis of the bar and project on each side of the concrete body of said bar.
Said plates 31, 32 are connected together by means of tensioned steel wires 33 which are embedded in the concrete and the ends of which are provided with enlarged heads 33a applied in contact with the plates 31, 32.
Prestressing of the bar lOb is obtained by exerting equal and opposite forces on the steel plates 31, 32 before placing the concrete and in a direction which tends to move said plates away from each other in order to stretch the wires 33. These forces are maintained after placement of the concrete until this latter has gained sufficient strength.
The concrete of the chord members 6b and 8b is then placed on the end portions of the bar lOb in such a manner as to embed the plates 31l 32 in the concrete of said chord members.
In order to achieve more effective anchoring of .

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3~5 the bar lOb to the concrete of the chord members, it is preferable to provide the ends of said bar lOb with steel rods 34, 35 which project rom the concrete at right angles to the axis of the bar as indicated in Fig. 4.
After placement of the concrete chord members, said rods 3d, 35 are embedded in the concrete and extend in a direction parallel to the axis of said chord members.
The holes 36, 37 formed near the opposite ends of the bar lOb at right angles to the axis of said bar and extending in a direction parallel to the steel rods 34, 35 are intended to permit insertion of prestressing tendons (cables) such as the tendons 20, 21, 22 shown diagrammatically in Fig. 1.
In the embodiment of Fig. 5, the truss element comprises upper and lower chord members 6b, 8b which are constructed of concrete as in the case of the elements described earlier. On the other hand, the bars lO"b, ll"b and 12'b are formed of steel.
The end portions of the vertical bars lOI'b and ll"b are securely anchored to the concrete of the chord members 6b and 8b whilst the bar 12'b which is essentially subjected to compressive forces is applied against the abutment shoulders 18b and l9b of the chord members.
One o the vertical bars designated by the reference lO"b is illustrated in detail in Fig. 6. This bar is in fact a steel tube filled with concrete 50. The -:
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opposite ends of said bar are ezch provided with a steel ring 38, 39 which projects at each end of the tubular body of the bar. Said rings 38, 39 are intended to be embedded in the concrete of the chord members 6b, 8b and ensure high-strength anchoring of the hars lOI'b and ll"b, thereby endowing said bars with high resistance to tensile forcesO
Furthermore, the bar lO"b is traversed hy holes 40, 41 through which the prestressing tendons are passed.
Instead of being formed by a single row of elements 4a, 4b, 4c as indicated in Fig. l, the truss in accordance with the invention can comprise two or more parallel rows of elements.
In the embodiment of Fig. 7, there are shown two parallel and identical truss elements 4' and 4". In this case the bottom chord member is free but the top chord member 6', 6" forms part of a slab 42 which extends at right angles to the elements 4' and 4". ~he slab connects the two elements together and projects on each side of these latter.
This embodiment forms a so-called arch segment.
By means of a plurality of segments of this type which are assembled together in the line of extension of the chord members, it is possible to construct a bridge.
Fig. 8 is a deta1l view showing one of the two elements of the arch segment shown in Fig. 7O This . .

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element is designated by the reference 4' and is identical with the truss element 4b shown in Fig. 2 except for the fact that the top chord member 6' has a smaller thickness than the chord member 6b of the truss element 4b of Fig. 2.
Moreover, the end portions lO'b and ll'b of the vertical bars lOb and llb project beyond the top edge of the chord member 6' in order to be embedded in the concrete slab 42 which is placed between the two elements 4' and 4".
Fig. 9 illustrates an arch segment which is identical with the segment of Fig. 7 except for the fact that the bottom chord members are connected together by means of two ~oncrete cross-struts 44, thus further increasing the mechanical strength of the arch segment.
In the embodiment of Fig. 10, the arch segment is also identical with those of Figs. 7 and 9 except for the fact that the bottom chord members of the truss elements 4' and 4" are connected by means of a concrete slab 45 which is parallel to the upper slab 42.
In the embodiment of Fig. ll, the arch segment comprises an upper concrete slab 46 and a lower concrete slab 47 which connect the top and bottom chord members of four truss elements 4A, 4B, 4C, 4~ which are similar to the elements described earlier but are located in oblique planes with respect to the slabs 46 and 47. The top and central chord member 6BC is common to the two central truss elements 4B and 4C which are outwardly .
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inclined in a downward direction and thus extend progress-ively away from the plane of symmetry of the arch segment.
Said central elements are joined at the lower ends thereof to two bottom chord members 8AB and 8C'D which are common to the outer truss elements 4A and 4D and these latter are each connected to a chord member 6A and 6D which forms part of the upper concrete slab 46.
Fig. 12 illustrates a bridge supported on vertical piers 48, 49 and constructed by means of arch segments V1, V2, V3, V4 each consisting of two parallel truss ele,ments connected to each other by an upper concrete slab and a lower concrete slab as in the case of Fig. 10, and also by means of arch segments V5, V6, V7 which again consist of two parallel elements connected to each other by an upper concrete slab but which in this case have bottom chord members connected to each other by diagonal cross-struts as in the case of the arch segment shown in Fig. 9.
The arch segments V1, V2, V3, V4 which have a bottom chord member of higher strength than that of the arch segments V5, V6, V7 are placed in proximity to the piers 48, 49.
In the case of the emhodiment illustrated in Fig. 12, the top chord members are placed along a stralght line parallel to the longitudinal direction of the span as in the previous embodiments.

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~3~i On the other hand, the assembled bott~m chord members placed in end-to-end relation define an arc.
In a bridge of this type, applied forces are distributed as explained with reference to Fig. 3. In S other words, the vertical bars of the truss elements are essentially subjected to tensile forces whilst the oblique bars mentioned in the foregoing description land designated as third bars) are essentially subjected to compressive forces.
It is readily apparent that the invention ls not limited to the examples described in the foregoing and that any numher of modifications may accordingly be contemplated without thereby departing either from the scope or the spirit of the invention.

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Claims

WHAT IS CLAIMED IS :

1. A truss as applicable in particular to the con-struction of a bridge, said truss being supported at a number of points (1, 2) in spaced relation so as to form one or a number of spans (3) and comprising in the case of each span two series of truss elements (4a, 4b, ... 5a) placed on each side of a point (M) of the span at which the shearing forces under continuous load are substantially zero, said truss elements being assembled together by means of prestressed concrete reinforcements or so-called tendons each adapted to extend over more than one element, each truss element (4a, 4b, ... 5a) being constituted by a top concrete chord member (6a, 6b, ... 7a) and a bottom concrete chord member (8a, 8b, ... 9a) which extend in the longitudinal direction (D) of the span, the truss elements being assembled together so that the top and bottom chord members are located in the line of extension of each other, the top and bottom members of each truss element being connected together by means of at least one set of three substantially coplanar bars (10a, 11a, 12a ;
10b, 11b, 12b, ...) forming an N-panel assembly, wherein two bars (10a, 11a ; 10b, 11b, ...) of the N-panel connect the two chord members (6a, 8a ; 6b, 8b, ...) to each other and extend transversely to the longitudinal direction (D) of the span (3) at a point near the ends of said chord members and are anchored to said members in such a manner as to ensure that they essentially resist tensile forces whilst the third bar (12a, 12b, ...) extends between the two bars aforesaid, and wherein, in the first series of truss elements (4a, 4b, ...), said third bars (12a, 12b, ...) are upwardly inclined when considered by a viewer moving in a given direction along the span from one support to the next whereas said third bars (15a, ...) are downwardly inclined in the second series of truss elements (5a, ...) considered in the same direction and located beyond said point (M), thereby ensuring that said third bars are essentially subjected to compressive forces.

2. A truss according to claim 1, wherein the bars which essentially resist tensile forces (11a, 10b) and are located on each side and in the vicinity of the ends (16a, 17a) in contact with the chord members (6a, 6b ; 8a, 8b) which form part of two adjacent truss elements (4a, 4b) are substantially parallel to each other, and wherein the opposite ends of the third bar (12a, 12b, ...) of each truss element which essentially resists compressive forces and extends between the two bars of said element (10a, 11a ; 10b, 11b ; ...) which essentially resist tensile forces are applied against the bottom and top chord members of said element.

3. A truss according to claim 1, wherein the axis of the third bar (12b) of a truss element (4b) intersects the axis of the bottom chord member (8b) of said element at a point located at equal distance from the axes of the bars which essentially resist tensile forces (10b and 11a) and intersects the axis of the top chord member (6b) at a point located at equal distance from the axes of the bars which essentially resist tensile forces (11b and 10c).

4. A truss according to claim 1, wherein the two bars (10a, 11a) of a truss element (4a) which resist tensile forces are anchored in the concrete of the chord members (6a, 8a) beyond the axis of said members.

5. A truss according to claim 1, wherein the portions (18a, 19a) forming abutments for the third bar (12a) are constituted by concrete bosses molded in a single piece with the chord members (6a, 8a), said bosses being provided with a bearing face (30) perpendicular to the axis of the third bar.

6. A truss according to claim 1, wherein the bars (10a, 11a, 12a) are formed of concrete.

7. A truss according to claim 1, wherein at least part of the bars are structural sections or tubes of steel (10"b, 12'b).

8. A truss according to claim 6, wherein the con-crete bar (10b) which resists tensile forces is provided at the opposite ends thereof with steel plates (31, 32) which are substantially perpendicular to the axis of the concrete bar and project on each side of said bar, said plates being connected to each other by means of tensioned steel wires (33) which extend through the con-crete of the bar.

9. A truss according to claim 7, wherein each bar which resists tensile forces consists of a steel tube (10"b) provided at the ends thereof with a steel plate (38, 39) substantially at right angles to the axis of the tube and filled with concrete (50), said plate being such as to form a projecting portion at each end of said tube and provided with a central bore.

10. A truss according to claim 6, wherein each bar (10b, 10"b) which resists tensile forces is provided near the ends thereof with holes (36, 37 ; 40, 41) which are substantially perpendicular to the axis of said bar and through which prestressing tendons are intended to pass.

11. A truss according to claim 6, wherein each bar (12'b) which is subjected to compressive forces consists of a steel structural section or tube provided at the ends thereof with a steel plate substantially perpendicular to the axis of the structural section or tube and forming a projecting portion at each end thereof, said plate being applied against the abutment portion (18b, 19b) of the chord member (6b, 8b).

12. A truss according to claim 1, wherein the chord members (6a, 6b ; 8a, ...) are traversed longitudinally by ducts through which prestressing tendons (20, 21, 22) are intended to pass.

13. A truss according to claim 1, comprising at least two parallel rows of elements (4', 4") assembled in end-to-end relation, wherein the top and/or bottom chord members of the truss elements form part of a slab (42) which connects the two elements.

14. A truss according to claim 13, wherein the bottom chord members are connected together by means of concrete cross-struts (43, 44).

15. A truss according to claim 13, wherein the truss elements (4A, 4B, 4C, 4D) of each row are located in oblique planes with respect to the slab (46) or (47).