Bridges – Truss – Arrangement
Reexamination Certificate
2000-10-12
2002-12-17
Hartmann, Gary S. (Department: 3671)
Bridges
Truss
Arrangement
C014S003000, C014S013000, C014S074500
Reexamination Certificate
active
06493895
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to bridges. In particular, this invention relates to a truss for redistributing and reducing the bending moment of a girder, and furthermore, reducing the deflection of the girder.
BACKGROUND OF THE INVENTION
Bridge design has developed into three basic categories in an effort to decrease the size and cost of the bridge and its supporting “bridgeworks” for long bridge spans. The three basic categories are trussed spans and arches, suspension spans, and beam, box and T girders. Trussed span and arches are generally used for supporting two types of structures, bridges and roof frames. The different types of bridge trusses include Warren bridge trusses, Howe bridge trusses, and Pratt bridge trusses. The different types of roof frame trusses include Belgian trusses, Fink trusses, Howe trusses, Pratt trusses, Crecent trusses, Fan trusses, and Scissor trusses. These conventional trussed span and arch designs employ pin-jointed lattice frameworks composed of tension and compression members. The different trussed span frameworks, although complex, obtain their strength from the simple geometric rigidity of the triangle. These conventional trussed span framework designs are composed of straight tension and compression members which extend the length of the bridge span as a uniform assembly of chords resolving loads and moments at each framework joint. Since the rigidity of the trussed span and arch framework is secured by triangles which cannot deform without changing the length of the sides, it is generally assumed that loads applied at the panel points or joint will only produce direct stress. Thus, trusses with large vertical height or depth can be designed to resist vertical loads more efficiently using trussed span and arches than beam, box or T girders.
Due to the complexity of the trussed span and arch frame work, trussed span and arches are used in bridge design only when long spans are required. The Warren bridge truss is generally thought to be the most economical of the trussed span and arch designs. A typical Warren bridge truss
100
is shown in FIG.
1
. The Warren bridge truss
100
is comprised of a top chord
105
, a bottom chord
110
, vertical web members
115
, and diagonal web members
120
. Web members
115
and
120
form the basic triangular geometry
125
common to all trussed span and arch bridge designs. The joint
130
rigidity of each triangular section resists the load applied to the bottom chord
110
of the Warren bridge truss
100
. In conventional applications, the depth of the Warren bridge truss
100
to the length of the bridge span is usually between 1:5 and 1:10. Thus, for a bridge span of 60 feet, the height of the top chord
105
of the Warren bridge truss
100
structure above the bottom chord
110
is from 6 to 12 feet. When a load is applied to a bottom chord
110
between the joints
130
, the bottom chord
110
does not directly interact with the primary truss diagonal and vertical lacing of the Warren bridge truss
100
. Instead, the load is distributed by beam action of the bottom chord
110
to the adjacent joints
130
.
Roof trusses are generally different from bridge trusses because roofs are often pitched, meaning that the top chord of the truss is set at an angle to the horizontal. Roof trusses are designed to support loads which are applied to the top chord of the roof and to accommodate the functionality of the roof as a surface which drains or sheds water, snow or other fluid loads. The bottom chord of the roof truss is considered to be axially loaded, not subjected to beam action where the member bends. A typical Belgian roof truss
200
is shown in FIG.
2
. This shows the top chord
205
pitched to the horizontal, a horizontal bottom chord
210
, parallel vertical members
215
and diagonal members
220
. The parallel vertical members
215
and the diagonal members
220
comprise the web members of the Belgian roof truss
200
.
A typical variation of the Belgian roof truss
200
is shown in
FIG. 3
where it is used as a bridge truss. This variation shown in
FIG. 3
eliminates all diagonal members
220
, and may eliminate all vertical members
215
shown in
FIG. 2
, except the vertical member
315
at the bridge midpoint
320
. The variation shown in
FIG. 3
offers support to the bottom chord
310
by creating an upwards reaction in member
315
due to the compressive loads in the diagonal members
305
. This upwards reaction at member
315
modifies the downwards load which the bottom chord
310
experiences, and consequentially modifies the strain and stress of the beam action in the bottom chord
310
. According to trussed framed theory, the load applied to the bottom chord
310
between joints
325
is distributed to the joints
325
by beam action for the beam length between the bottom chord
310
end points and midpoint
320
. However, using the theory of work, strain energy in the bottom chord
310
is modified by the reaction at the joint
325
located at the bottom chord
310
midpoint
320
and the length of the beam between end points
330
and
335
.
The second type of bridge design is a suspension span. Suspension spans utilize cable networks suspended from arches or towers to connect to and support a bridge roadway. The suspension cables serve as multiple support points for the roadway span and effectively reduce the size of the overall bridge structure. The arch or towers serve as the main support for the bridge span. The roadway can either be a beam girder or trussed structure.
The third type of bridge design is a beam, box and T girder. Beam, box and T girder bridge spans involve a structural shape, or combination of shapes, which has a section modulus and moment of inertia that supports the design load between the unsupported length of the span. Beam girder bridges rely upon the bending of the beam, or “beam action” to support the bridge load. When a beam is subjected to a load, it bends in the plane of the load. This bending action creates fields of stresses which resist the bending and create an equilibrium condition. For example, a simple beam supported at each end which bends down under a load is experiencing a shortening of the top (or concave surface), and a lengthening of the bottom (or convex surface). These changes in the beam's shape create horizontal tensile and compressive stresses at the beam's surfaces. In order for these beam's two surfaces to work together, vertical shear is developed in the beam web, which is the section located between the top and bottom of the beam. The internal moment developed in the beam section by the horizontal and vertical stresses, generally called “beam action”, resists the external bending moment of the applied load. The external bending moment calculated by summing the moments of the external forces acting at either end of the beam.
Beam girders for bridge spans are preferred over trussed span and arches or suspension spans because of their simplicity. A compact beam girder is an efficient system which transfers shear and load between the extreme upper and lower elements, in most cases flanges, of the beam. This is especially true for a rolled beam section, such as an I beam. The compact beam section of an I beam functions as a complete system requiring little or no modification in order to support its calculated load. However, for a beam, box or T girder design having a uniformly applied load per foot, the bending moment increases by the square of the span. This can cause very large increases in girder beam size with relatively small increases in span. Thus, when designing a bridge using a beam, box or T girder, the structural requirements of the girder are determined by merely adjusting the size of the girder to fit the design constraints (stress or deflection) until the size of the girder becomes so large and expensive that a shift to the more complex trussed span and arch or suspended bridge designs becomes practicable.
In the large majority of cases, bridge girder size
Gray Cary Ware & Freidenrich
Hartmann Gary S.
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