Abstract:
A system for use in constructing a lattice panel bridge, includes a plurality of substantially rectangular prefabricated steel lattice panels joined in end to end relationship to form a main girder for the bridge. Each panel comprises an upper chord, a lower chord of substantially equal length, a plurality of web members interconnecting the upper and lower chords, and joint forming devices at each end of each chord. The joint forming devices at either end of the lower chord mate and receive a transverse pin for connecting the lower chords of adjacent panels together. The joint forming devices at each end of the upper chord comprises a longitudinally facing plate provided with apertures for receiving fasteners for connecting the plates of adjacent panels together. A spacer is inserted between the plates and this reduces the tendency of the bridge to sag. Optionally the spacers can be used to give the resultant bridge an upwardly facing longitudinal camber.

Description:
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 which may be used optionally in the system if desired. 
     Viewed from a first aspect the present invention provides a system comprising a plurality of prefabricated components for use in constructing a lattice panel bridge, including a plurality of substantially rectangular prefabricated lattice panels each adapted to be joined in end to end relationship with a plurality of like panels, at least some of said panels comprising an upper chord, a lower chord, a plurality of web members interconnecting the upper and lower chords, and joint forming means at each end of each chord, the joint forming means at either end of the lower chord being adapted to receive a transverse pin for connecting the lower chord to the lower chord of another panel, and the joint forming means at each end of the upper chord comprising a longitudinally facing plate which can be engaged with a corresponding plate of the upper chord of another panel in such a way as to resist vertical displacement between the two panels, characterised in that the system further comprises a plurality of spacers for positioning between the facing plates of the upper chords of two adjacent panels in use, so that the adjacent panels can be angled with respect to each other. 
     The panels in such a system will be joined in end to end relationship by means of conventional pin joints at the ends of their lower chords and by means of the plates and spacers at the ends of the upper chords. The join between two plates and a spacer must be such as to resist vertical displacement between the two upper chords. This could be provided by a number of arrangements. For example, one plate could be provided with a horizontal protrusion across its width, the other with a matching recess, and the spacer with a recess on one side and a protrusion on the other so that it can fit between the two plates. This form of joint would not resist lateral displacement to a significant degree, other than by virtue of friction between the elements when the joint is under compression. Significant lateral stability could be provided by additional means such as lateral support braces between the upper chord and another bridge component when in use. Preferably, however, the join between two plates, and the spacer, is such as to resist both lateral and vertical displacement. Thus, instead of a horizontal protrusion across its width, one plate could be provided with a rectangular protrusion to engage in a corresponding recess, two sides resisting vertical displacement and two lateral displacement. Another possibility would be to use a circular protrusion. 
     Preferably however, the join between the plates is formed by means of one or more longitudinally extending fasters passing through apertures in the plates. The fasteners could be pins provided with e.g. split pins or circlips to resist removal, or even rivets. Preferably, they are bolts provided with nuts. The longitudinal fastener should preferably be a reasonably tight fit within the apertures, to resist movement in both the lateral and vertical directions. The joint is not normally required to resist longitudinal tension forces, but an advantage of using bolts and nuts is that the plates and spacer will be clamped together to resist tensile forces if that is necessary. 
     The number and positions of apertures to take the bolts, or other longitudinal fasteners, can be chosen to give the required degree of stability. In some cases a single central fastener may be sufficient, but preferably there are additionally or alternatively two or more fasteners nearer the edges of the plates to provide enhanced resistance to lateral deflection, and preferably there are vertically spaced fasteners to provide enhanced resistance to vertical deflection. In a typical preferred arrangement, there are two vertically spaced fasteners on one lateral side of the joint, and two on the other, and optionally there may be a central fastener also. 
     A longitudinal fastener may pass through an aperture in the spacer, or through a recess in the periphery of the spacer, to assist in holding it in place. However, the spacer may be located by other means. 
     Whilst a system in accordance with the present invention includes panels and spacers which are fastened together, the fasteners themselves whether in the form of bolts and nuts or otherwise do not have to be part of the system in its broadest sense. The system can be supplied to a user without the fasteners, the user obtaining the fasteners separately. Preferably, though, the system is provided to the user with the correct fasteners to avoid problems with inappropriate fasteners being used and compromising the integrity of the structure. 
     Additional panels may be attached above or below the panels in a conventional manner, and known additional reinforcing chords may be attached to the upper chords of the panels and themselves joined together with the use of plates and fasteners. Reinforcing chords may also in some circumstances be provided for the lower chords. 
     Although in most circumstances the panels are supplied completely prefabricated with both the upper and lower chords in place, it would be possible to supply the panels in a form disclosed in of the embodiments of GB-A-2 251 018, in which the upper chord for example is supplied as a separate member which has to be attached to the remainder of the panel by the user. 
     The use of the plates and spacers to join together the upper chords of the panels can reduce or eliminate sagging of a bridge using the panels in the main girders, the spacers effectively increasing the length of the upper chords compared to the lower chords. If desired the spacers can be used to give positive camber to a bridge, i.e. an upwardly curved configuration rather than a level or somewhat sagging configuration. 
     The thickness of the spacers used will depend amongst other things on the length of a span, the loads to be carried by the bridge, and the desired effect i.e. whether to provide positive camber. The engineer designing the bridge will be able to take into account these and other factors and calculate the amount of spacing required between the plates. A range of standard spacers may be available for the engineer to choose from, for example with thicknesses of say 3, 5 or 10 mm. For the thicker spacers it may be desirable to form them as wedges, to be used with the thickest parts uppermost, if the angular deflection between two adjacent panels is significant. In most applications the deflection between any two adjacent panels will be small, although the overall effect will be significant, and even thickness spacers will be adequate. Where additional reinforcing chords are used, the spacers may be configured to fit between these as well as between the standard upper chords. Preferably, however, separate spacers will be used for the reinforcing chords and the standard upper chords. 
     The system may incorporate spacers which are positioned between each joint between the upper chords of the panels, or only between selected panels depending upon where it is considered necessary to compensate for sag. The spacers supplied may all be of the same thickness, or of varying thicknesses according to their positions along the bridge. Where panels are to be stacked vertically to provide greater height, the spacers for the upper panels will generally be thicker than those for the lower panels. 
     In general, the deflection in a traditional Bailey bridge, considered typically at the centre of a span, is due to three contributory factors which are evaluated separately and summated. Firstly the bending deflection is calculated using classic elastic theory with knowledge of the self weight of the structure, the value of the loading to be applied and the relevant geometric and material characteristics of the structure. Secondly, the deflection due to strain in the shear members is calculated with knowledge of the same factors, and is a summation of the effects due to each member. Finally, the movement at the pin positions in the top compression and bottom tension chords allow s a slight rotation of each panel relative to its neighbour. The angle of rotation is a function of the difference in diameter as between the pin and its hole, and of the height of the panel. The deflection of the structure is the summation of the effects of this rotation between each of the panels forming the main girders of the bridge. These three factors are summated to calculate the total deflection. 
     In accordance with the present invention, the first two factors are calculated in the same way. However, the use of the plate joints between the upper chords substantially resists unwanted relative rotation between the panels. Instead, there is a degree of positive rotation, i.e rotation in the opposite sense to that encountered at pin joints, introduced by the spacers and the effects of this can be summated. The resulting contribution from this factor will not be added to the other two factors, as is the case when analysing conventional Bailey bridge designs, but subtracted from them. By varying the types, number and positions of the spacers the effect of the positively established rotation between the panels can be designed to reduce sagging caused by the other two factors, or to eliminate it, or to provide positive camber of a required amount. 
     Viewed from a second aspect the present invention provides a method of constructing a lattice panel bridge using a system as described above, in which the lattice panels are joined together in end to end relationship to form main longitudinally extending girders, with spacers positioned between the plates of at least some adjacent upper chords. Viewed from a third aspect the present invention extends to a bridge constructed in accordance with the above method. 
     Where the system is to be used to construct a bridge with an intermediate support, for example, there may be included different types of panels to handle the transition from compression to tension in the upper chords in the region of the support. Thus, the system may include modified panels which have upper chords which have at one end a longitudinally facing plate provided with at least one aperture for receiving a longitudinally extending threaded fastener for connecting the plate to a similar apertured plate of another panel, and at the other end means adapted to receive a transverse pin for connecting the upper chord to the upper chord of another panel. The system may also include standard panels with means for forming pin joints at the ends of both the upper and lower chords, as well as reinforcing chords for attachment to the upper chords of the panels. 
     Viewed from a fourth aspect, therefore, the present invention provides a substantially rectangular prefabricated lattice panel for use in constructing a main girder for a lattice panel bridge by being joined in end to end relationship with similar panels, the panel comprising an upper chord, a lower chord, a plurality of web members interconnecting the upper and lower chords, and joint forming means at each end of each chord, the joint forming means at either end of the lower chord being adapted to receive a transverse pin for connecting the lower chord to the lower chord of another panel, and the joint forming means at one end of the upper chord comprising a longitudinally facing plate for connection to a plate of the upper chord of another panel, in such a way as to resist vertical displacement between the two panels, characterised in that the joint forming means at the other end of the upper chord comprises means adapted to receive a transverse pin for connecting the upper chord to the upper chord of another panel. 
     In one form, the panel has a female pin joint forming portion at said other end of the upper chord, and in another form a male pin joint forming portion. The panel may be joined by means of the plate to a like panel, to a panel with the opposite gender of pin joint forming portion at its other end, or to a panel as described earlier with a plate at either end of the upper chord. A spacer may or may not be provided between the plates, as desired. 
     The prefabricated panels used in the various aspects of the present invention will generally be of steel, with the component parts welded or otherwise permanently joined together. The components may be galvanised or otherwise treated to resist corrosion. The pins used will generally be of high tensile steel, and the fasteners may be in the form of high tensile bolts provided with nuts and suitable ancillary items such as washers, locking spring washers and so forth. In one preferred form a female type of pin joint portion comprises a pair of spaced members with aligned transverse apertures. A male type of pin joint portion preferably comprises an apertured spigot which is inserted into the female part of the lower chord of another panel, with the apertures in alignment so that a pin can be passed through them to form the joint. Suitable means such as split pins or circlips can be used to prevent axial dislodgement of the pin. 
     It will be appreciated that expressions such as “upper” and “lower” and the like used in relation to the panels refer to the intended orientation in normal use in a simple span bridge, and not to any particular orientation during manufacture, storage or transportation. Furthermore, the expression “bridge” is to be construed broadly to cover structures which might normally be referred to as “flyovers” or the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 is a perspective view of a lattice panel for use in a system in accordance with the present invention; 
     FIG. 2 is a an exploded partial view showing the mechanism for joining together the upper chords of two panels; 
     FIG. 3 is a view corresponding to FIG. 2, showing the panels joined together; 
     FIG. 4 is an exploded partial view showing the mechanism for joining together the lower chords of two panels; 
     FIG. 5 is a view corresponding to FIG. 4, showing the panels joined together; 
     FIG. 6 is a side view of two of the panels joined together; 
     FIG. 7 is a side view of a bridge using the panels; 
     FIG. 8 is a side view of a bridge using the panels, in which there is positive camber; 
     FIG. 9 is a perspective view of part of the bridge of FIG. 7 or FIG. 8; 
     FIG. 10 is a side view of a panel having a hybrid upper chord with a male pin connector at one end; 
     FIG. 11 is a side view of a panel having a hybrid upper chord with a female pin connector at one end; 
     FIG. 12 is a side view of a conventional panel which can be used in the system with pin connections for all four joints; 
     FIG. 13 is a side view of a panel as shown in FIG. 2 with a reinforcing chord attached; 
     FIG. 14 is an exploded partial view showing the mechanism for joining together two reinforcing chords; of FIG. 11; 
     FIG. 15 is a view corresponding to FIG. 14, showing the reinforcing chords joined together; 
     FIG. 16 is a partial side view showing two panels and reinforcing chords joined together; and 
     FIG. 17 is a side view of a spacer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now in detail to the figures, in the figures there is shown a lattice panel  1  for use in a lattice panel bridge of the Bailey type, where it will be joined end to end with similar panels to form a main longitudinal girder. The panel consists of a number of steel components which have been welded together and is of the same general construction as the panel described and illustrated with reference to FIGS. 9 a  to  9   d  in GB-A-2 251 018. The panel comprises an upper chord  2  and a lower chord  3 . The panel is of rectangular shape and the upper and lower chords are of substantially the same length. Each of the chords consists of a pair of spaced apart chord members  2   a,    2   b  and  3   a,    3   b  defined by channel members (see FIGS.  2  and  4 ). Between these extend vertical web members  4 , perpendicular to the chords, and angled web members  5  at 45° to the chords, although other angles may be used such as 35° to 55°. Strengthening gussets  6  are provided. The web members are received within the gaps between the chord members  2   a,    2   b  and  3   a,    3   b.  Between the spaced apart upper chord members  2   a,    2   b,  are provided apertured portions (not shown) which enable the panel to be attached by bolts to another panel vertically above it if desired, or to enable a reinforcing chord to be attached as described later. Between the spaced apart lower chord members  3   a,    3   b,  are provided apertured portions (not shown) which enable the panel to be attached by bolts to another panel vertically below it if desired, or to a reinforcing member. 
     At the right hand end of the lower chord  3  is provided the female part  7  of a pin joint, and at the left hand end of the lower chord is provided the male part  8  of a pin joint which is adapted to mate with the female part of the lower chord of a like member. The construction of these will be described in more detail below with reference to FIGS. 4 and 5. At the ends of the upper chord  2 , in place of the pin connection portions there are provided matching longitudinally facing plates  9  which are illustrated in greater detail in FIGS. 2 and 3. To construct a girder for a bridge, a number of panels  1  are joined together in end to end relationship, two joined panels being shown in FIG.  6 . The lower chords  3  are joined together in a conventional manner by means of the male and female pin joint portions  7 , 8 . However, in accordance with the invention the upper chords are joined together by means of the plates  9 , with an intervening spacer  10  as shown in FIGS. 2 and 3. This imparts a degree of relative rotation of two joined panels about the axis of the pin joint between the lower chords. 
     As shown in FIGS. 2 and 3, the plates are welded to the ends of upper chord members  2   a,    2   b.  Each plate  9  and each intervening spacer  10 , which in this case are of matching rectangular shape, is provided with a central aperture ( 11 ,  12 ), two vertically spaced apertures ( 14 ,  15 ) on one side and two vertically spaced apertures ( 16 , 17 ) on the other side. A large central bolt  18  is provided, together with a matching nut  19 , together with four bolts  20  and matching nuts  21 . Ancillary items such as washers may be provided as necessary. In use, the upper chords are joined by bolting together the plates  9 , with the intervening spacer  10 , by means of the bolts and nuts as shown in FIG.  3 . This provides a secure connection which is better able than a pin joint to cope with compressive stresses during use. 
     As shown in FIGS. 4 and 5, the pin joint between the lower chords of two panels is of a conventional types. The female part  7  comprises a pair of apertured forgings  22  welded to the lower chord members  3   a,    3   b,  which are also apertured. The male part  8  comprises an apertured spigot  23  which is welded between the lower chord members  3   a,    3   b.  The spigot  23  is inserted into the female part  7 , and a transverse pin  24  is used to hold the lower chords together, the pin being retained by circlips  25 . The finished joint is as shown in FIG.  5 . 
     FIG. 6 shows two panels  1  joined together in the manner described above, with the pin joint between the lower chords indicated at ‘A’ and the plate joint, corresponding to FIG. 3, at ‘B’. In use a number of the panels are joined end to end to form a main girder for a bridge. FIGS. 7 and 9 show a simple bridge  26  of the Bailey type bridging a gap between two points  27  and  28 . The bridge comprises a pair of laterally spaced main girders  29  each of which consists of a number of lattice panels  1  which have been joined together end to end in the manner described above. A deck  30  has been laid on the girders, with transverse support members  31  being used also. In this configuration the upper chords  2  are in compression and the lower chords  3  in tension. In this embodiment the bridge the bridge has been designed to remain level, without sagging, as a result of the plate joints with spacers used between the upper chords in place of the conventional pin joints. In this example, optional transverse support struts  32  are used to provide additional lateral support. 
     The same thickness of packing plate may be used throughout, or packing plates of differing thickness. For some joints packing plates may be omitted. For typical bridges with rectangular panels of dimensions of say about 3 m×1.5 m or 3 m×2 m, packing plates of 3 to 10 mm may be used for normal spans and loads. For thicker packing plates and greater deflections between adjacent panels it may be necessary to use wedge shaped packing plates, whose faces will be angled to match the degree of rotation between the joined panels. 
     FIG. 8 shows a bridge  33  which is a modification of the bridge of FIGS. 7 and 9. Whilst its general construction is identical, thicker spacers have been employed. In this case the bridge has been designed to have a degree of positive camber, the centre of the span being raised by a distance ‘d’ relative to the base line ‘l’. 
     FIG. 10 shows an alternative panel  34  which is identical to panel  1  except as regards the joint at the left hand side of upper chord  35  where the plate has been replaced by the male part  36  of a pin joint, having a configuration corresponding to that shown in FIG.  4 . The right hand end  37  of the upper chord has the same configuration as shown in FIG.  2 . FIG. 11 shows an alternative panel  38  which is identical to panel  1  except as regards the joint at the right hand side of upper chord  39  where the plate has been replaced by the female part  40  of a pin joint, also having a configuration corresponding to that shown in FIG.  4 . The left hand end  41  of the upper chord has the same configuration as shown in FIG.  2 . It will thus be seen that panels  34  and  38  could be joined together with a pin joint between the upper chords rather than the plate joint, in circumstances where the joint might be under tension rather than compression, for example above a support column or the like. In practice, panels  34  and  38  would not normally be joined together directly but by means of an intermediate panel of a conventional type with pin joint portions at either end of the upper chord. Such a conventional panel is shown in FIG.  12 . This shows a panel  42  which is identical to panel  1  except as regards the joints at the left and right hand sides of upper chord  43 . This panel has the male part  44  of a pin joint at the left hand side of the upper chord  43 , and the female part  45  of a pin joint at the right had side of the upper chord  43 . In each case the pin joint parts correspond to the construction shown in FIG.  4 . 
     Thus where there is a supporting column there may be a transition from a plate joint to a pin joint for the upper chords, using panel  34 , then pin joints using panel  42 , and then a transition back to plate joints using panel  38 . 
     FIG. 13 shows a panel  1  as described above with a reinforcing chord  46 , in the form of a channel member, attached to the upper chord  2  to assist in resisting compressive stresses. The reinforcing chord  46  is attached to upper chord  2  by means of bolts passing through apertured portions as indicated diagrammatically at  47 . At each end of the reinforcing chord  46  is provided an end plate  48  welded to the channel member, which is arranged to be flush with the adjacent plate  12  of the upper chord of the panel. As shown in FIG. 14, each end plate  48  of the reinforcing chord is provided with a small central aperture  49  and two side apertures. A rectangular spacer  50  matching the end plates  48  is also provided with matching apertures  51  and  52 . Bolts  53  and  54 , with appropriate nuts  55  and  56  and ancillary items such as washers if desired, are used to bolt together the end plates  48  with the intervening spacer  50  as shown in FIG.  15 . FIG. 16 shows how both the reinforcing chords  46  and the upper chords  2  of two panels  1  are bolted together with their respective spacers  50  and  10 . This is preferred to using a single spacer to span the plates of the upper chords and the reinforcing chords, although there might be cases where that would be possible. 
     Finally, FIG. 17 shows a side view of a relatively thick spacer  57  which is of wedge configuration, the faces of the spacer being inclined so as to match the angle between two joined panels  1 . Such a spacer may be used between the plates of upper panel chords or of reinforcing chords, the dimensions and apertures being chosen as appropriate. 
     In a two storey arrangement, in which panels are joined in the vertical direction, adjacent panels in the lower storey will use a spacer of a first thickness, t between the upper chords. In the upper storey, the upper chords will be joined via a thicker spacer. If the panels are of the same height in the two storeys then this will have a thickness of 2 t. If there are different heights then the ratio between the thicknesses will vary accordingly. In the upper storey, the lower chords of the panels cannot be joined together in the conventional manner, whether they are pin joints or in some other form. Spacers could be used, or the chords not joined together directly, for example. The lower chords of the upper panels would already be joined to the upper chords of panels in the lower storey, and those chords are of course already joined together. 
     By means of the various aspects of the invention and the specific components specified above, it is possible to construct lattice panel bridges of the Bailey type with reduced or no unwanted sagging, or if desired with reverse camber so that it rises in the middle and falls away towards the end. The advantages of using rectangular prefabricated panels are maintained and the improvement achieved with a minimum of additional complexity. The lower chords are joined directly to each other by the pin joints, without the need for intermediate members, and the modifications concern the upper chord joints. 
     The invention is not limited to the specific embodiments described. It will be appreciated that many variations may be possible within the spirit and scope of the accompanying claims whilst still retaining the advantages of the broad aspects and specific components described above.