Structural trusses or girders, normally manufactured from steel, have conflicting requirements of shallow depth to avoid unnecessary and expensive building height and a larger depth to allow a more efficient structure with less steel used.

Additional roof and/or floor depth may also be required for services such as air conditioning ducts and distribution of electricity, water and gas. An attractive compromise can on occasion be achieved by providing openings in trusses and running services through these openings. However, with a typical truss comprising upper and lower chords and bracing between the chords, providing large gaps between braces for service openings is normally structurally inefficient. For example, a vierendeel girder having upright braces and joints with chords which can transmit moments provides good service access but tends to be an inherently less efficient structure and require a substantial amount of steel. An alternative triangulated truss, for example the Warren truss, is more efficient structural but has poorer service access.

In simply supported triangulated structures such as the Warren truss the main forces to be resisted by the members are axial tensions and compressions. The chord forces are maximum at midspan and reduce towards the supports. The bracing member forces are maximum near the supports and reduce towards midspan. In practice to simplify fabrication the chords are generally continuous with the same section provided over the entire span, or possibly two sections spliced together for larger spans.

This means that the section is fully utilised only near midspan. Elsewhere

there is spare capacity.

In contrast the members of a simply supported vierendeel girder have to resist various combinations of axial force, shearing force and bending moment to support the applied load. As with the triangulated structure the axial force in the chords is maximum at midspan and reduces towards the support. However, in addition there is a shear force and associated bending moment in the chords which is maximum at the support and reduces towards midspan. The chords therefore tend to be more fully stressed than those in the Warren Girder. However the critical location for the chord design in practically designed vierendeel girders tends to be at approximately the third points of the span where significant axial forces coexist with significant bending moments, leading to heavier chords than an equivalent Warren truss. The vertical members too are heavier and fabrication more complex.

Whilst the chord bending moments can be reduced by introducing more frequent vertical members or by introducing haunching to achieve smaller chord sizes, the savings in chord weight tend to be offset by increased vertical member weights, significantly increased fabrication and reduced space for service access.

An object of the invention is to provide a structural truss with bracing which provides an efficient structure and large openings.

In accordance with the present invention there is provided a structural truss such as a roof or floor truss comprising an upper chord, a lower chord and bracing between the chords, characterised in that the bracing comprises braces arranged in pairs with a first brace of each pair on one diagonal of the truss and the second brace of the pair on an opposed diagonal and overlapping the first brace, the braces of each pair crossing

at a point intermediate of the upper and lower chords, a pair of braces being spaced along the truss from the next pair of braces.

A pair of overlapping diagonal braces can serve broadly the same function as a vertical vierendeel brace but can be lighter because the individual braces do not carry substantial bending or shear loads and for the same reason can have simple pin jointed connections to the chords, ideally suited to bolted connections.

With this structural arrangement the bending moment in the chords can also be altered by varying the spacing between adjacent pairs of braces.

Indeed the spacing can, if desired, be adjusted so that the bending moment present in the chord at each location exactly matches the available spare capacity in the chord. The resulting gap can be turned to good use by providing improved service access, but with a structural efficiency equivalent to that of a Warren truss or similar triangulated structure. Of course, larger service openings can alternatively be provided with some loss in efficiency but retaining the same simplifie construction details.

In a simple comparison, the space for services between pairs of braces is slightly less than between uprights of vierendeel girders but full width is available at the overlaps or intersections between two braces of a pair and the spaces between adjacent pairs of braces could possibly be enlarged due to the improved efficiency of the bracing. Also, full depth openings between chords are available between adjacent bracing pairs unlike the reduced depth in triangulated trusses.

Preferably, the two braces in a pair have equal but opposite inclinations and cross at their centre points. However, degrees of asymmetry may be acceptable or even desirable in special circumstances. For example, the

inclinations of the two braces could be unequal. Similarly, the braces could be arranged to cross near their upper or lower ends. The braces are preferably connected at their points of intersection.

Preferably, the chords are straight and parallel to each other but the invention may also be applied to arch trusses or to trusses of tapering depth or other configurations.

With the conventional and convenient constant chord cross-section throughout the length of each chord, the bracing requirement to achieve satisfactory truss strength may vary along the length of the truss.

Accordingly, the spacing between adjacent pairs of braces may vary along the length of the truss. As a further alternative, the inclinations of the individual braces may vary along the length of the truss.

Other asymmetric features may arise with trusses of tapering depth or with curved trusses.

The latticed trusses of the present invention may be fully welded structures. Alternatively the bracing modules may be bolted or rivetted to the chords.

Embodiments of the invention are described with reference to the accompanying drawings in which:- Figures 1 a and 1 b are diagrammatic representations of simple truss structures in accordance with the invention; Figures 2a and 2b show possible variations in bracing pattern; Figures 3a and 3b illustrate alternative types of truss to which the

invention may be applied; Figures 4a and 4b illustrate further variations for pairs of braces; Figure 5 illustrates in part section a latticed truss in accordance with the present invention, illustrating a method of construction; Figure 6 is an enlarged detail of the truss illustrated in Figure 5; Figure 7 is a section along the line VII-VII of Figure 6; Figure 8 is a view similar to figure 6 showing an alternative method of construction; Figure 9 is a section along the line IX-IX of Figure 8; and Figure 10 is a partial side elevation of a cambered truss formed by the method illustrated in figure 5.

Figure 1 a shows a truss 11 simply supported on columns 12 and 13. The truss comprises an upper chord 14 and a lower chord 15 with the simple vertical support loads applied to the ends of the lower chords.

Each chord is typically a rolled steel member but other forms of structural member may be employed. For convenience of bracing between the chords which is to be described, they may be of T-section. However, in order to maximise the available opening size with minimum overall weight it is desirable to select sections which have good bending strength, for example I sections.

Bracing between the upper and lower chords is provided by braces which

are arranged in pairs and four complete pairs of braces are shown in Figure 1 a. Each pair of braces comprises a diagonal member such as 16 joined at points 17 and 18 to the upper and lower chords respectively.

Points 17 and 18 are offset from each other along the truss to provide the inclination. Brace 19 is the other brace of the same pair and it extends between joints 21 and 22 connecting it respectively to the upper and lower chords. The braces may for example be of L-section (conveniently referred to as angle section) to provide some stiffness against compressive loads and to provide convenience for connecting to the chords. The two braces of the pair intersect at point 23 and connecting them to each other at point 23 is optional, but has the beneficial effect of reducing the unbraced length of the compression brace. Theoretically, connections at points 17,18,21,22 and 23 could be pin joints with simple bolted connections. Alternatively the two braces may be welded together at the point of intersection 23. Instead of connecting two crossing braces together at point 23, one brace could be in two parts, both connected to the other brace by welding or bolting.

Each end of the truss requires special bracing including a vertical brace 24 or 25 which could be an extension of the columns 12,13 and what are in effect pairs of diagonal half braces 26 repeating the pattern of the four pairs of braces.

The bracing arrangement shown in Figure 1 a is entirely symmetrical. In particular, the two braces of the same pair have equal and opposite inclinations and their positioning along the length of the truss is such that the intersection point 23 is at their centres. The individual pairs of braces are all equally spaced from each other by a common distance X which in this example is approximately equivalent to the longitudinal offset between one end such as 17 and the other end such as 18 of a brace.

This is equivalent to the distance between points 18 and 22.

Such an arrangement provides bracing between the chords at regular frequent intervals but also provides large full depth hexagonal openings within the truss between one pair of braces and the next. Parts of the width of the opening are greater than the distance X although the bracing is effected at intervals no greater than distance X. These large openings can be used for building services such as heating or air conditioning ducts.

A well braced truss is provided despite the large service openings. The dominant loading on the individual braces is purely compression or tension so their bending strength and stiffness needs to be no greater than may be required to prevent buckling and the joints to the chords can be simple bolted or welded connections. The openings are much larger than can be provided with conventional triangulated bracing between chords and the chords can be much lighter than those that would be required with a vierendeel girder construction.

Between one pair of braces and the next pair, the upper and lower chords carry equal and opposite compressive and tensile forces. Further longitudinal forces are generated within the pairs by the diagonal loading of the braces. In addition shearing forces and bending moments are present in the chords, the magnitude of the bending moment depending upon the spacing between adjacent pairs of braces.

In Figure 1 b the bracing corresponds to that of Figure 1 a but a different chord support arrangement is provided. The ends of the chords are pin jointed to support columns with the result that independent tensile and compressive loads can be generated in the upper and lower chords.

However, the principles of the bracing arrangement remain the same.

Figures 2a and 2b employ broadly the same structure as Figure 1 b but with a variation in the bracing along the length of the truss to reflect the different forces in the truss along its length. Considering a typically

loaded truss as a whole, chord shear forces and bending moments tend to be zero at the centre and to increase broadly linearly towards the ends. In contrast, compressive and tensile force in the chords is at a maximum in the centre of the truss and diminishes towards the ends. In a similar way, forces within the truss bracing members between the upper and lower chords vary along the length. Braces which are near to upright are most effective at transferring vertical loads between the chords whereas more severely angled chords are more appropriate for resisting longitudinal shear loads between upper and lower chords. The requirements can of course be analysed for any given case of inherent weight, applied load and end mounting of the chords. Bending moments in particular tend to increase with increased spacing between braces. Thus greater utilisation of the capacity of constant section chords can be achieved by increasing the spacing between pairs of braces at locations where other forces in them are low.

Examples of what might be appropriate in certain circumstances are illustrated in Figures 2a and 2b. Figure 2a corresponds to Figure 1 b except that the angles of the braces vary along its length. The pair of braces 31 near the centre of the truss are more upright than for Figure 1 b.

The pairs of braces 32 and 33, approximately mid-way between the centre and the ends have inclinations nearer to horizontal than in Figure 1 b. The remaining braces are angled as in Figure 1 b. One effect of the near upright braces is to increase the available space for services. If required, pairs of braces with different points of intersection may be used along the length of the truss.

In Figure 2b, the braces retain the same inclination as in Figure 1 b but the spacing between adjacent pairs of braces is varied. In addition to matching the bracing to the required loading, such an arrangement can provide larger openings for services where they may be required.

Figure 3a is a simple illustration of the fact that the invention may be applied to a truss of varying depth, for example as may be used for a pitched roof. Figure 3b similarly shows that the invention may be applied to a curved truss. Other alternatives are possible, for example a constant depth pitched truss.

As illustrated in the above embodiments the point of intersection 23 of the braces 16 and 19 is preferably substantially at the mid-point between the upper and lower chords 14,15. As a result the shearing forces and bending moments in the upper and lower chords will be equal.

Furthermore, when the braces 16 and 19 are connected at their point of intersection 23 the unbraced length of the braces will be minimised.

However, typically the compression chord 14 of a latticed girder is of slightly heavier section than the tension chord 15, since the compression chord 14 needs to be of sufficient size to prevent buckling. In this case the bending capacity of the smaller tensile chord 15 will be lower than the bending capacity of the larger compression chord 14, and the cross over points of the braces 16,19 will ideally be offset from the mid-point towards the tension chord 15, as illustrated in Fig. 4b.

Alternatively, if a larger section tension chord 15 is used to cater, for example, for heavy local loads, it may be desirable to move the point of intersection 23 of the braces 16,19 towards the compression chord 14 as illustrated in Fig. 4a.

The trusses of the present invention may be fabricated by welding gusset plates to the chords and then welding or bolting the bracing modules to the gusset plates. Alternatively, the bracing modules may be welded directly to the chords. With welded constructions such as this full scale jigs will be required to set out the geometry of the truss accurately.

According to a preferred embodiment, as illustrated in Figs. 5 to 7, a latticed truss 110 comprises upper and lower chords 112, 114 of"I" section arranged in spaced apart relationship, parallel to one another. The chords 112, 114 are arranged such that the lower flange 116 of the upper chord 112 is opposed to the upper flange 118 of the lower chord 114.

A series of bracing members 120 are connected at one end to the upper chord 112 and at the other end to the lower chord 114, the bracing members 120 being inclined between the chords 112, 114. The bracing members 120 may be of hollow or angle section.

The ends of each bracing member 120 are cut parallel to the flanges 116, 118 of the chords 112, 114, end plates 122 being welded to the ends of the bracing members 120, so that they lie parallel and abut the surfaces of the flanges 116, 118. The end plates 122 are then bolted to the flanges 116, 118 by means of bolts 124, which engage through holes 126,128 pre-drilled in the end plates 122 and flanges 116, 118.

As illustrated in Figure 5, the bracing members 120 are arranged in pairs which intersect each other at a point intermediate of the chords 112, 114.

In order to permit this, the bracing members 120 may be offset from one another. The bracing members 120 may furthermore be interconnected where they intersect, either by bolting, welding or similar means.

Alternatively, the pairs of bracing members 120 may be prefabricated by welding them together at the point of intersection, so that they are co- planar.

As illustrated in Figure 5, the pairs of bracing members 120 are spaced from one another.

As illustrated in Figure 10, the arrangement illustrated in Figure 5 may

easily be adapted for cambered trusses, the end plates 122 of the bracing members 120 being curved to the same radius as the chords 112, 114, if necessary. The spacing of the bolted connection of the end plates 122 to upper chord 112 may then be increased relative to the spacing of the bolted connection of the end plates 122 to the lower chord 114, to produce the camber. Similarly, undulating profiles can easily be achieved by suitable adjustments, without the need for a full sized jig.

In the embodiment illustrated in Figures 8 and 9, the end plates 140 are formed of"T"section. Angle section bracing members 142 are welded or bolted to the web portion 144 of the"T"section, while the flange portion 146 of the"T"section is bolted to the flanges 116, 118 of the chords 112, 114, in similar manner to the end plates 122. With this construction, the ends of the bracing members 142 may be cut square.

With the truss constructions disclosed with reference to Figs. 5 to 10, the heavier chords can be produced accurately and economically on an automated saw/drill production line and the lighter bracing modules, which can often be easily manhandled, can be manufactured separately on simple small jigs. The positioning of holes in the chords and end plates can thereby be accurately controlled, so that the components may be bolted together on a construction site, to create a predetermined geometry, without the need for a full sized jig and offering other benefits and cost savings.

Bolting of the bracing modules directly to the flanges 116, 118 of the chords 112, 114, will lead to a loss of capacity locally, both due to the drilling of the section and due to localise bending effects of the flanges, 116, 118, due to the transfer of the bracing force into the chord 112, 114.

The adoption of a symmetrical bolting arrangement will reduce the local bending effects, typically by half, due to the prying action of the bolts.

Also since the bracing forces reduce towards mid-span, so do the local bending effects. Thus the local bending effects due to this method of construction are greatest next to the supporting members, where spare capacity in the chord is greatest, reducing to zero at mid-span where the chord is fully utilised to resist axial forces. In practice when a constant chord section is used over the full span this will often mean that no increase in chord section size will be required to accommodate the addition local bending effects. Even when an increase in chord size is found to be necessary to overcome the loss of capacity and local bending, this is more than offset by the substantial gain in fabrication simplification due to the elimination of full size jigging and adoption of production line manufacture.

A further consequence of this method of construction, as opposed to bolted constructions used hitherto, is that the bracing bolts are stressed in both tension and shear, rather than just shear as with bolted constructions using a gusset plate. However for practical bracing inclinations, the tensile and shear forces are approximately equal, so that typically there is little loss in bolt capacity compared to a simple shear connection to a gusset. There is therefore unlikely to be any need for addition bolting and typically two or four bolt connections are likely to be sufficient for quite a wide range of structures and geometries under normal load.

The maximum benefit of the proposed bolted construction would be achieved if all fittings, for example purlins or bracings between girders, or stiffeners to enhance the flange capacity and end connections, are also bolted. However, even if some welded fittings or stiffeners are found to be necessary, they can be added, at a component level of manufacture, with greater allowable tolerances without requiring full scale jigging of the truss. The key truss geometry will still be controlled by the accurately

drilled holes of the chord flanges and the bracing members.

Various modifications may be made without departing from the invention. For example, while in the above embodiments the end plates are bolted to the flanges of the chords, the end plates may alternatively be rivetted or secured by some other similar means engaging in holes drilled in the end plates and flanges. Instead of welding, the bracing members may be brazed or bonded to the end plates and to one another to suit the materials used and the loads to which the bracing members will be subjected.

The present invention may also relate to tapered girders in which the upper and lower chords are inclined at different angles to the horizontal.