Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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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.
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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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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
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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
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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
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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
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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
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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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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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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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