Bridges – Truss – Details
Reexamination Certificate
2000-07-06
2003-04-01
Will, Thomas B. (Department: 3671)
Bridges
Truss
Details
C014S004000, C014S013000, C052S693000
Reexamination Certificate
active
06539571
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system for constructing lattice panel bridges, and is of particular relevance to such bridges of the “Bailey” type.
BACKGROUND TO THE INVENTION
In a typical “Bailey” bridge construction there is provided a main girder at each side of the bridge, transoms extending between the girders and a deck supported on the transoms. The girders are formed from prefabricated lattice panels of steel which are joined together longitudinally. Two or more sets of the lattice panels may be secured together in the vertical direction so as to provide the required depth of girder, and to this end the panels are generally of a rectangular configuration. A common type of lattice panel consists of upper and lower longitudinally extending chords which are joined together by a lattice of web members. These will generally include vertically extending web members and angled web members which may for example extend at an angle of about 45° to the chords. Various panel configurations are disclosed in British Patent Application 2 251 018 and U.S. Pat. No. 5,065,047, for example.
To join the panels together in end to end relationship to provide the required length of girder, pin and aperture joints are usually employed to ensure speed and simplicity of assembly on site. One end of each chord will be formed with a female portion and one with a male portion. These are mated when the panels are to be joined together and then a transverse pin is inserted through apertures in the portions.
Bridges of the above type are often used as temporary structures, for example to replace bridges destroyed by floods, earthquakes or acts of war. They are supplied as prefabricated components which are assembled on site. In one common method of construction, the bridge girders are assembled on one side of the gap to be bridged, such as a river or ravine, and pushed out and over to the other side. Other methods are possible but in general important factors are speed and ease of assembly. Although the bridges are intended primarily for temporary use, they are often capable of long term use and if properly treated against corrosion, such as by galvanising may be capable of lasting for fifty years or more. Given the expense of dealing with other problems arising from natural disasters or acts of war it is desirable if the bridges can be left in place for a significant period to obtain maximum value and to delay as long as possible the expense of a conventional permanent type of structure.
One problem with conventional types of Bailey bridge is that the pin joints use may be subject to misalignment developing. Whilst the joints are stable under tension, under compression there may be a degree of lateral misalignment. The upper chord of a lattice panel will normally be in compression in use, and to compensate for the problem, lateral supporting members may be provided to resist lateral defection of the upper chords.
This problem is addressed in United Kingdom Patent Application 2 251 018, which proposes that the pin joints for the upper chords of a panel should be replaced by plates which are clamped together by longitudinally extending bolts. The specification states that by using abutting plates, lateral flexion at the joints is greatly inhibited. The plate joints may be between reinforcing chords which are attached to conventional upper chords with pin joints, or alternatively a lattice panel may have an upper chord which is provided with the plates instead of the pin joint portions.
There is a tendency for a bridge of the Bailey type to sag, this being caused by a number of factors including vertical flexion of the joints between both upper chords and lower chords. The degree of sag will depend upon many other factors including the span, the depth of the lattice panels and other structural elements used, the physical properties of the various structural elements, the weight of a deck on the bridge, the load to be carried and so forth. Whilst in engineering terms it is acceptable to have a degree of sag, the appearance of a sagging bridge may have a negative effect on users and this may reduce their willingness to tolerate a Bailey bridge as a long term construction. In any event, reduction of excessive sagging may prolong the life of a bridge or reduce the frequency or expense of maintenance work.
The system shown in GB-A-2 251 018 will inevitably reduce vertical flexion in much the same way as it reduces lateral flexion, because the upper chords are joined together by abutting plates clamped together by longitudinal bolts passing through apertures. However, this may not be sufficient to eliminate sag, or reduce it to an acceptable level, due to the influence of the other factors.
SUMMARY OF THE INVENTION
In accordance with the present invention, sagging can be reduced or eliminated by introducing spacers between the plates where the upper chords are joined together so as to impart a tendency towards an upwardly directed camber. The net effect of this and the various sagging effects will be to reduce sagging to a degree that is perceived by users to be acceptable, or to eliminate it, or even to provide a positive upwardly directed camber in the finished bridge.
Joining of the upper chords will be more time consuming than in the case of a conventional pin joint, and in a typical arrangement in accordance with the invention a number of threaded fasteners and the spacer will be required in place of a single pin. This detracts somewhat from one of the aims of a “Bailey” type of bridge construction, namely simplicity of construction. However, the additional time spent is worthwhile since it enables sagging to be reduced or eliminated. Furthermore, the use of plates to form a joint, a spacer and threaded fasteners, for example, reduces costs. Although pin joints are simple to assemble, they require the use of expensive forgings to constitute the male and female portions.
In accordance with the present invention, pin joints are retained for joining together the lower chords of the lattice panels. These will permit two joined panels to pivot with respect to each other in a vertical plane, to account for the spacer positioned between the upper chords, and of course preserve the simplicity of joining together the lower chords.
The upper chords in a “Bailey” type of construction are not always in compression. In some arrangements intermediate supports may be used, upon which the girders will rest. Above these supports the chords will be in tension and pin joints are more appropriate. Accordingly, there may be provided in this region panels whose upper chords have at one end a plate joint and at the other a pin joint. One type of panel could be formed with a plate joint at one end and a male pin joint at the other, and another type could be provided with a plate joint at one end and a female pin joint at the other. These two types could be joined together directly by means of the pin joints, although in preferred arrangements there would be one or more intermediate panels of a conventional type with pin joints at either end of the upper chord.
It will be appreciated that for use in “Bailey” type bridges a system in accordance with the present invention will include a number of panels supplied as prefabricated units together with the spacers and usually component to join the chords together in both the pin type arrangement and the plate type arrangement in accordance with the invention. The panels will normally be substantially rectangular, enabling them to be stacked on top of each other and secured by suitable means to provide in a simple manner a girder of increased depth. Within any bridge, panels of different depths may be combined. The panels will generally be of steel, with the individual elements welded together, although other materials and methods of construction would be possible.
The invention may be viewed from various different aspects, dealing with the system in broad terms, a bridge constructed using the system, a method of constructing such a bridge, and novel lattice panels
Bacon & Thomas PLLC
Mabey & Johnson Limited
Pechhold Alexandra
Will Thomas B.
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