GB2356413A - Metallic tied-portal or roof frame - Google Patents

Abstract

A method of construction a metallic tied-portal (10) or roof frame has an upper structure (20) for supporting roofing material ans a lower connecting tie structure (32). The upper (20) and lower (32) structures are braced by upwardly diverging structural braces (36, 38, 40), adapted to transmit loads between the upper (20) and lower (32) structures. The braces (36, 38, 40) are designed so that in response to unsymmetrical loading of the roof frame an axial load is generated in at least one of the bracing members (36, 38, 40). The roof frame is arranged so that a portion of any loading of the structure is carried by a shear load in and bending of the upper supporting structure.

Description

2356413 METHOD AND APPARATUS FOR CONSTRUCTING BUILDINGS USING BRACED TIED

PORTALS This invention relates to a method and apparatus for constructing buildings using braced tied portals.

The principal embodiments of the present invention relate to buildings and their associated roof frames which are constructed as metal fabrications, usually of mild steel or the like. The present invention is concerned with technical and commercial aspects of constructing roof frames for buildings of this kind, taking account of the time and cost factors associated with conventionally-available techniques for manufacturing such fabrications, whether by a bolted(or other fasteners) technique or by a welding (or the like) technique. The advantages of the invention may be applicable to certain other materials which may compete with metal fabrications for particular types of buildings.

It is to be understood however that the economic and technical aspects of the invention do not apply to wooden 2_ ID framed constructions, in which the technical and commercial problems of producing a structurally rigid fabrication by welding and/or bolting techniques do not seriously arise with wooden constructions.

In the case of metal fabrications, rigidity can be ensured by the adoption of a welding technique and pre fabrication, but such an approach then has serious implications for tranpport arrangements. The practicalities of on-site-assembly and achieving structurally rigid constructions from multi-piece bolt-together frameworks are also significant consideration.

The conventional method and apparatus employed for constructing roof frames for buildings provides various structures adapted to be used as one of a series of similar roof frames connecting to columns or uprights in the building walls. Typically, the frames are arranged in 2 parallel dispositions at spaced intervals dependent upon the weight and other factors relating to the roof itself. Each frame includes an upper roof -supporting structure to extend across the space to be roof ed and to support a roof ing material such as cladding sheets or a tiled structure or the like.

One prior form of roof structure used within steel buildings is a truss type of roof structure. Such a structure is characterised by the use of a number of interconnected triangulated members. The interconnections between the members are idealised to pin connections and structurally no pivotal moments are transmitted by the confections between the connected members. As such only axial loads are carried by the individual members. It will be appreciated though that in practical examples the interconnections may not be purely pin type connections, but that from structural, load analysis, and design perspectives in such structures the connections are idealised to pin connections with no significant pivotal moments transmitted ?0 between the members. In such truss structures the loads on the structure are designed to be carried by axial loads (ie tension and compression) within the individual members. As such the individual members can be relatively light weight, since only axial strength is required, which is a further characteristic of truss structures. The individual members are arranged within the truss structure such that all the horizontal and vertic4l forces at any interconnection point can be resolved into axial loads within the members meeting at that connection point or node. To transfer load along the structure the angle between each of the members at an interconnection point is practically generally limited to between 30 and 70 degrees. As a result to span a large distance a large number of individual members are required.

Furthermore these truss frames tend to have a span to depth ratio of about 10, producing a relatively deep roof 3 structure. This in some cases may. be undesirable, in particular where large spans are required for example in aircraft hangers, and certainly makes transportation of prefabricated roof trusses difficult due to their height/depth. It should also be noted that on site assembly of truss structures is also problematic and time consuming, due to the large number of individual members required to form the structure.

Conventionally in truss structures the roof loads are applied at the nodes and interconnection points to ensure that the loads within the roof frame are carried by axial loads alone. In some truss structure however some local loads are applied along the members and the individual members do carry a limited amount of local shear load and bending moments. Such moments however are very limited and do not relate to the overall structural load path and associated with the roof frame as a whole, since these local shear loads and bending moments are not transferred, resisted or carried over the interconnections between the individual members. As such the local shear loads and bending moments are limited to the individual members rather than to the roof f rame as a whole.

An alternative type of roof structure is a portal roof structure. Such roof structures comprise rafters which are rigidly connected to vertical wall members or stanchons.

There is no structural additional structure below the upper roof rafter members. 'In such portal roof structures the loads are designed to be carried by means of bending action of the members. The interconnections between members are accordingly rigid and can resist relative pivotal movement of the connected members. The rigidity of the interconnections between the members is designed and arranged to be sufficient to support and withstand such bending moments. As a result the individual members are heavier and more rigid than those of a truss structure.

4 Since the individual members of the structure are designed to carry bending moments, shear loads and axial loads, the frame as a whole resists some of the applied frame loads through bending moments and shear loads in the roof frame as a whole. As a result some of the restrictions arising with truss structures are lessened. In particular the number of individual members is reduced, although the individual members may be heavier and more substantial to carry the bending moments. Such frames are quicker and easier to fabricate due to the fewer members. In addition such individual roof members can have a higher span to depth ratio, of around 70 for example which means that the members can be more easily transported in a fabricated state.

An inherent characteristic of portal structures a is significant deflection of the structure under applied external loads. Indeed to some degree such deflections are required in order to generate a counteracting bending moment force within the structure to resist the external applied load. With such portal structures these deflections need to be minimised resulting in the section size of the steel work having to be increased as compared to truss structures.

A development of the regular portal structure described above is a tied portal. Such structures are generally similar to regular portal structures. In addition though there is a tie member which extends between the eaves (where the rafters connect to the vertical wall members). This tie is arranged to carry 4 axial load only (typically tension) and may in some case simply comprise a cable. The tie member restrains relative lateral deflection of structure at the eaves. Whilst the rafters still carry an bending moment to resist the applied load, the tie member and axial load carried therein also contributes.

Such a tied portal structure is relatively stiff under symmetrical vertical external loads with only slight deflections arising. However under horizontal loading, or unsymmetrical external loading the horizontal loads and/or unsymmetrical loads are still only resisted by the bending moments generated within the structure and significant deflections result unless large section sizes for the individual members are used.

As described above, and readily appreciated by those skilled in the art, there are considerable and significant differences between these different types of roof/building structures (truss, portal, tied portal). These are primarily due to the ways in which the different configurations are adapted, designed and arranged to carry and resist the applied loads. Each different type has its own considerably different design considerations and factors depending upon the way in which the loads are designed to be carried, although on first inspection the basic structure of the roof structure of the different types may appear initially similar. The way in which the structure is adapted to carry the anticipated loads has to be considered when analysing different roof structures.

2n It should also be noted that the design of roof structures requires, and is dictated by, a detailed structural analysis of the structure and its designed response to loading. Such a detailed and careful structural analysis determines the individual positioning and arrangement of members actually within the structure.

US 4,616,453 and GB 2,094,372 both show clear examples of steel truss type rqqf structures, as explicitly referred to in these patents. These structures comprise a plurality of compression and tension members which carry axial loads and are fabricated from light gauge steel. These arrangemtns are also in practice suitable for only domestic building systems with small spans. A further variation of a roof truss structuref which clearly highlights the key feature of a truss structure is shown in GB 128,901 in which multiple ties (cables) carrying axial loads only are used to brace 6 the structure. The structure does not exert horizontal thrusts on the supports and the individual members are clearly pin jointed together.

GB 2,142,061 is described as a portal structure to be primarily constructed from wooden material rather than steel with which the present invention is concerned. This structure is not though a tied portal structure since it is apparent to those skilled in the art that the horizontal member (shown as item 42 in the disclosure) is not a tie member carrying axial load, but the primary roof member of the portal frame carrying substantial bending moments with a roof of a truss type (as described in the disclosure) provided on top of the horizontal primary roof member of the portal frame. Furthermore this arrangement is again in practice for a domestic, house frame structure with light gauge members.

US 5,577,353 shows a steel building system. This disclosure though is concerned with simplified connecting plates between the members, which appear in practice to be simply pin type joints. There is no mention of how the loads are carried within this disclosed structure, and it provides those skilled in the art with no teaching to address the above mentioned problems and in particular those arising from unsymmetrical loading and designing structures to adequately handle such loading with reduced deflections.

The present invention seeks to provide a significant improvement in relat;'on to presently available techniques for the construction of tied-portal roof frames for buildings of the kind concerned. As such the present invention is not concerned with simple truss type roof structures, which as explained above are completely different in their design and in the way in which the loads are carried.

We have determined that the known arrangements of regular portals and tied portals, although used 7 traditionally over many decades, are subject to significant shortcomings which have been evidently accepted for corresponding very substantial periods. These shortcomings include notably an inability to resist satisfactorily asymmetric point loads applied to the structure, and unsatisfactory performance generally with respect to anything other than uniformly-applied loads. We have discovered that where substantial asymmetric point loadings are applied to these known roof structures, there results a very substantial deflection of the roof structure which leads to generally unsatisfactory performance and the need for (in comparison with the embodiments of the present invention) unsatisfactorily high specifications for the structural members of the roof structure.

As a corollary to this discovery, we have identified a significant need for improvements in relation to the construction of tied-portal roof frames and related structures whereby enhanced performance with respect to the shortcomings identified above, for example resistance to asymmetric point loadings and other non-uniform loadings, is achieved.

According to the present invention there is provided a method and apparatus as defined in the accompanying claims.

An embodiment of the present invention provides a tied portal roof frame hav'.pg improved resistance to asymmetric loads applied to the roof, this is achieved in a simple and cost-effective manner whereby substantial savings of materials and therefore cost can be achieved, which have hitherto not been available.

In an embodiment of the invention there is provided a method of constructing a fabricated metallic roof frame of tied-portal configuration for use in an at least partially metal-framed building as one of a series of similar roof 8 frames disposed across a space to be roofed.

In the embodiment, the tied-portal roof frame comprises a fabricated metallic upper pitched roof structure to define the upper profile of said portal which extends across the space to be roofed, and which supports a roofing material such as cladding sheets or a tiled roof.

In addition to the upper roof supporting structure the tied portal roof frame comprises a lower connecting structure extending between opposite ends of the upper roof supporting structure. This defines the tie of said tied portal configuration which cooperates therewith to form a load-resisting frame-like structure as part of said metal fabrication. In accordance with the teachings of the prior art, this lower connecting structure is put into tension during use by the generally downwardly directed loads applied to the upper roof-supporting structure.

The roof frame of the embodiments also includes a bracing structure comprising at least one arrangement of two or more upwardly diverging structural members which in each respective arrangement extend from a respective point on the lower connecting member to points on the upper roof supporting structure.

The bracing structures are adapted to transmit loads between said upper and lower structures through, in use, axial load within the bracing structural members. In response to asymmetrical loads applied to the roof frame the bracing structural members are arranged and provide a means for altering the axial force within, and along the length of, the lower connecting structure to resist the asymmetric forces produced by the asymmetric loading.

The roof frame, as a whole, is arranged and adapted such that the upper roof supporting structure carries shear load and resists, at least a proportion of, the applied loads by bending action of the structure. This is unlike a normal triangulated truss structure in which the bracing 9 structural members have to be arranged in a fully triangulated arrangement such that the loads are carried purely by axial loads. The upper roof supporting structure of a truss structure does not carry shear loads or bending.

The roof frame is further arranged such that, when the structure as a whole is subject to asymmetric forces, the forces applied to and generated in the bracing structure are likewise asymmetric whereby out-of-balance loads are applied to the lower connecting structure (tie or strut) of the tied-portal frame. In particular the bracing structural members, and specifically the horizontal component of the axial force within them due to their angle, alter and generate a none constant, and therefore asymmetric, axial force within and along the overall length of the lower connecting structure which resists the asymmetric load and resultant asymmetric moments applied to the frame. This thereby enables the frame as a whole to resist such asymmetric loads without undue deflection or stresses.

In embodiments of the invention described below, comparisons are made with regular or standard portal-type roof structures and regular tied-portal type frames, using the same loads for direct comparison purposes. In the embodiments of the invention the provision of the generally central, or off centre, bracing structure has a dramatic effect on the momentsand deflections experienced by the structures in use.

In further embo#1 nts of the invention the upper roof supporting structure is constructed in a different way.

Instead of comprising a pair of linear beam or roof-profile (pitched roof) defining structures, the upper roof supporting structure comprises a segmental structure comprising structural elements connected end-to-end to define a generally convex roof profile. In this embodiment, the conventional haunched roof profile structure is replaced by the segmental structure and the construction of the rafter is thereby simplified and the forces and stresses and deflections and ground loadings generated in use are further reduced.

In the embodiments, the braced tied portal configuration frames can be used with or without continuous connections to steel or other columns of the wall structure which is bridged by the roof, and with or without conventional eaves haunches or apex haunches or splices or splice haunches, and likewise with a tie that may be straight or segmental, and with rafters that may be straight or segmental or curved, in circular or parabolic or other form optimised to suit the loads applied.

Also in the embodiments, the form of bracing structure may be varied significantly. For example, the upwardly diverging or generally V-shaped format may be provided with a generally central generally vertical element, or the V shaped structure may be duplicated to produce a generally W shaped structure or there may be adopted an inverted M shaped structure. For the principal embodiments of the invention, 1, 2 or 3 such bracing structures may be provided in the f rame or span of one roof structure.

Likewise, various shapes and profiles for the upper roof-supporting structure may be adopted varying from the generally linear pitched roof profile, through the segmental structure having a generally convex form, to the use of a curved or parabolic format, according to the needs of the particular building apd i. roof in question.

The principal embodiments of the present invention provide, for use in metal-framed buildings and/or their roofs, a construction of the tied-portal general configuration but in which the significant disadvantage of low resistence to asymmetric loads is very substantially reduced. As a result, the performance of such roof frames is greatly enhanced and it then becomes possible to reduce substantially the material specification of a substantial proportion of the structure of the f rame so that material cost-savings of the order of 35% in the construction of the roof frame or 10% in relation to the overall steel consumption in a building as a whole become possible. Such an advance transforms the profitability profile of a building construction of this kind to such an extent that the failure to adopt such a construction previously, as marketplace product surveys indicate, can only indicate a failure to comprehend the available benefits which this simple structural change can provide, over a number of decades.

It will be appreciated, that whether the embodiments of the invention utilise a welding-type technique or a bolting (or other fasteners) technique there is an economic limit to the complexity of the framework which can be adopted while achieving the commercial advantages indicated above. We have determined that the use of much more than 3 bracing structures within the framework reduces the economic advantage available to a level at which it no longer offers sufficient of the benefits discussed above.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig 1 shows an end elevation view of a prior art construction comprising rafters providing a roof supporting structure bridging bet.yeen a pair of columns and arranged in accordance with the regular portal configuration; Fig 2 shows a similar end elevation view of a prior art construction comprising a regular tied portal; Fig 3 shows a first embodiment of the present invention comprising linear rafters providing an upper roof supporting structure and -a linear lower connecting structure and provided with a bracing structure connected therebetween, the bracing structure being of generally twin V-format; 12 Fig 4 shows a second embodiment of the present invention in which the linear rafters structure is replaced by a segmental structure and without the haunches seen in Fig 3; Fig 5 shows a third embodiment of the present invention in which the bracing structure of Fig 4 is replaced by an inverted "M"-format structure and in which the upper roof supporting structure comprises additional segments; Figs 5A and 5B show modifications of Figs 4 with more segments (5A) and a "W"-format bracing structure (5B); Fig 6 shows a sixth embodiment of the invention in which the segmental upper roof supporting structure of Fig is replaced by a parabolic structure and the bracing structure is of generally "W"-format; Fig 7 shows a seventh embodiment of the invention comprising a segmental roof supporting structure and three twin "W'-format bracing structures; Figs 8 and 9 show deflection diagrams for the regular portal structure of Fig 1 under, respectively, a linear load of 20 kilonewtons per linear metre and 1000 kilonewtons applied as a spot load at the location indicated in Fig 9; Figs 10 and 11 show similar deflection diagrams for the known regular tied portal structure of Fig 2; Figs 12 and 13 show similar deflection diagrams for the first embodiment of the invention seen in Fig 3; Figs 14 and 15 show similar deflection diagrams for the second embodiment of t,,e invention seen in Fig 4; Figs 16 and 17 show corresponding deflection diagrams for the third embodiment of the invention seen in Fig SA; and Fig 18 is a tabulation of the deflection data shown in Figs 8 to 17, the five pairs of figures (Figs 8 and 9, 10 and 11 etc) being identified as examples 1 to 5.

Fig 19 shows an eighth embodiment of the invention comprising a segmental upper roof supporting structure and 13 a pair of twin "V"-format bracing structures; Fig 20 shows a ninth embodiment of the invention comprising a segmental upper roof supporting structure, anon linear lower connecting structure and a twin "W-format bracing structure; In the specific description below we are comparing and contrasting the structure and performance of the prior art roofing structures of Figs 1 and 2, which have corresponding deflection diagrams at Figs 8 to 11, with the embodiments of the invention shown in Figs 3, 4 and 5A which have corresponding deflection diagrams in Figs 12 to 17.

Numerical data from these comparisons is tabulated in Fig 18.

Considering first, therefore, the prior art constructions of Figs 1 and 2, these comprise, as shown in Fig 1, a building 10 having columns 12, 14 defining a space 16 to be roofed. Mounted across space 16 is a roof 18 comprising a roof supporting structure 20 consisting of a series of pairs of laterally-extending rafters 22, 24 with haunches 26 connecting to side walls 12, 14 and haunches 28 at apex 30 of the roof. This roof structure may be designated a regular portal roof construction. Light hangers 33 may be provided to simply hold the tie in position.

In the prior art regular tied portal roof construction shown in Fig 2, the general structure is otherwise similar to that of Fig 1, but,j 'there is provided a lower connecting structure 32 extending between opposite ends of the rafters 22, 24 and providing a tied frame construction.

Turning now to the first embodiment of the present invention shown in Fig 3, this is otherwise constructed as in the prior art regular tied portal of Fig 2, but is provided in addition with a central bracing structure 34 which interconnects the upper roof supporting structure 20 of rafters 22, 24 with the lower connecting structure or tie 14 32.

As shown in Fig 3, bracing structure 34 comprises upwardly diverging structural members 36, 38, 40 which are adapted to transmit loads between rafters 22, 24 of the upper roof-supporting structure and the lower connecting structure or tie 32.

The upwardly diverging structural members 36, 38 and 40 are disposed, in Fig 3, as a bisected "V"-form structure in which the two outer structural members 36 and 40 respectively each form one side of a generally triangular structure (comprising in each case part of the relevant rafter 22 or 24 and half of the lower tie 32). In addition, the bracing structure 34 serves to define two further generally triangular structures between the diverging outer arms of the bracing structure 34 and the upper ends of rafters 22, 24 with the central member 38 common to each IV' brace element. Thus, it can be seen that the structural disposition of the members or elements making up the trusses 42 formed by the rafters 22, 24 and the tie 32 and the bracing structure 34, is generally triangulated to a a limited degree, and in this embodiment defines four generally triangular structures designated in Fig 3 at 44, 46, 48 and 50. It should be noted though that arrangement comprises significantly fewer bracing members than would be 2S f ound in a truss type arrangement which is, and has to be, fully triangulated.

It will be noted,.!'t,hat in Fig 3 the haunches 26 and 28 are retained.

Turning to the embodiment of Fig 4, this differs from that of Fig 3 by the use of rafters 52, 54 comprising end to-end rafter segments 56, 58 and 60, 62. The haunches of the embodiment of Fig 3 have been removed. The bracing structure of Fig 3 is otherwise substantially unchanged and is identified in Fig 4 by the same reference numeral 3S accordingly. The tie 32 can be attached to the column 12 or is to the underside of the rafter 62.

Turning to the embodiment of Fig 5, the chief difference from that of Fig 4 is twofold. Firstly, the roof supporting structure comprises six segments 64 defining a convex format approximating to a curve. Secondly, the bracing structure 66 comprises structural members 68, 70, 72 and 74 defining twin individually unsymmetrical %1Vf1 structures arranged to form an inverted I'M" structure.

Figs 5A and 5B show modifications of Fig 4 with more segments (5A) and a "W"-format bracing structure (5B).

In the embodiment of Fig 6, the roof supporting structure is further modified so as to adopt a two-piece curved or parabolic form comprising two curved members 76, 78. Bracing structure 80 comprises twin symmetrical Vs disposed to define a "W" structure.

In the embodiment of Fig 7 three bracing structures 82, 84 and 86 are adopted in spaced-apart positions and cooperating with a multi-segment roof supporting structure 98.

Figure 19 shows a further embodiment. This is similar to that of figure 4 and 5b with rafters 90 comprising end to-end segments and a tie 32 extending between the columns 12,14 and attached either to the columns 12,14 or rafters 90. A pair of bracing structure 92,94, each individually similar to those of figures 3 and 4 however are disposed towards either lateral side of the roof structure. Each of the bracing structur( 92,94 comprises upwardly diverging structural members 102- 112 adapted to transmit loads between the rafters 90 and tie 32. The upwardly diverging structural members 102-112 of each bracing structure 92,94 are disposed as a bisected '%V" -form structure in which the two outer structural members 102,106 and 108,112 respectively each form one side of a generally triangular structure. Each of the bracing structures 92.94 of this embodiment are similar to the individual bracing structures of the embodiments 16 shown in f igure 3 and 4. Finally the embodiment shown in figure 20 is again generally similar to

that of figure 3 and 4. In this case though the lower connecting member or tie 98 comprises two parts 98a, 98b arranged end to end at an angle to each other and the horizontal providing an overall bent, segmented, none horizontal tie 98 between the columns 12,14.

It will be appreciated that all of the embodiments incorporate a bracing structure which whilst the detailed form of the bracing structure varies, comprises upwardly diverging structural bracing members extending between the lower connecting member or tie 32 and the upper roof structure. Consequently, for convenience in this description all of the bracing structures of the various embodiments can be referred to, as group, as V bracing comprising arrangements arrangement of individual structural members or V braces.

The above-described embodiments are constructed as steel fabrications using conventional welding techniques and mild steel stock sections. Other materials may be employed for the purposes of the present invention.

The connections between the various members forming the roof frame and vertical columns 12,14 are arranged to exceed or be of comparable strength to the strength of the members.

Furthermore the connection between the columns 12,14 and the upper-supporting structure, as well as the end to end connection between t1j -te segmental members of a segmented upper supporting structure are rigid so as to maintain the relative angular positions of the connected members at the connections. In other words the connections are able, and arranged, to transmit bending moments between connected members such that the connected members support shear loads, and can support bending moments about the connections and resist applied loads through bending action of the connected members. Suitable connections and techniques are known to 17 those skilled in the art and are common to portal frame roof structures designs.

Turning now to the performance of the embodiments of the invention in Figs 3 to 7 as against the prior art of

Figs 1 and 2, the calculated performance data is covered by Figs 8 to 18 of the drawings.

In Figs 8 to 17, there are shown deflection diagrams for each of the five structures shown in Figs 1 to 4 and 5A, with the data on deflections under load tabulated in Fig 8.

In each case, the test applied to the structure is twofold.

Firstly, in Figs 8, 10, 12, 14 and 16 there is shown the effect of a uniform load on the roof structures at 20 kilonewtons per linear metre. Then, in the even number figures 9 to 17, there is shown the effect of a point load of 1000 kilonewtons applied off centre at one side of the roof structure (at the 1/4 point at 1/4 of the span of the roof structure) as indicated by the downward arrows in these figures.

As shown in Figs 8 and 9 and example 1 of Fig 18, the prior art regular portal deflects substantially under uniform downward load as shown in Fig 8, and even more under the asymmetric point load of Fig 9, corresponding to wind loads for example. As stated in Fig 9, huge vertical and horizontal deflections are obtained as indicated by the arrow "D" in Figs 8 and 9.

In the case of the prior art "regular tied portal" of

Fig 2, the result in 4Iation to uniform loading as shown in Fig 10 is reasonably good, but in the case of asymmetric loads as shown in Fig 11, huge vertical and horizontal deflections are obtained as clearly shown in Fig 11. Similar deflections also occur when a horizontal load is applied to the structure.

Turning to the first embodiment of the present invention as shown in Fig 3, the results shown in Figs 12 and 13 show very substantial improvements as against those 18 of Figs 8 to 11. In the case of Fig 12, the def lections under uniform downward load are too small to be shown in Fig 12. With respect to the asymmetric load of Fig 13, the deflection can be seen in that figure, but there is a dramatic reduction in both the vertical and horizontal deflections as compared with the prior art position of Figs

9 and 11.

In the case of the embodiment of Fig 4 shown in Figs 14 and 15, again the deflection under uniform load is too small to be seen in Fig 14 and the asymmetric load results shown in Fig 15 is comparable to that of Fig 13.

Comparably good results are shown in Figs 16 and 17 for the embodiment of Fig 5A. Once again, the deflections under uniform load are too small to be clearly seen in Fig 16 and the asymmetric load result shown in Fig 17 once again shows relatively small deflections.

The results are shown in numerical tabular form in Fig 18 and in general terms it can be seen that the deflections under asymmetric load reduce by a factor of about 8 horizontally (delta H reduces from 3500 and 3200 to between 409 and 490) There is a reduction by a factor of approximately five in the vertical direction (delta V reduces from 5760 and 4963 to between 730 and 1012).

Similar results are also obtained with the other embodiments of the invention shown in figure 19 and 20.

The reason for this dramatic difference in the way in which the structures (.-of the embodiments of the invention respond and deflect under the different loading as compared to the prior art portal designs is due to the way in which the structures resist and carry the loads applied.

It will be appreciated that any load applied to a framework causes bending moments and shears along and over the span of the portal structure which vary between the support points. These moments and shears must be resisted by the internal framework of the structure to achieve a stable 19 equilibrium. It is the way in which the structure resists these moments that determines how the structures behaves and deflects. In designing a structure these moments and shears can and are calculated and accordingly the performance of the structure analysed which reveals the following fundamentally different ways in which the structures carry and resist the loads.

For purely symmetrical or uniform loading the portal frame structure, as shown in figure 1 resists the resultant moments from the applied load by bending action of the individual members, particularly the upper roof supporting members 22,24, and the loads are carried by shear loads within the members 12,14,22,24. This can be clearly seen by the resultant deflections shown in figure 8. The rafters 22,24 and columns are accordingly fabricated from I section beams which can support and resist shear loads and bending.

The connections between the members 12,14,22,24 are also substantial and are arranged to resist relative angular pivoting of the connected members 12,14,22,24 such that they can support shear loading and bending moments.

In the case of unsymmetrical loading a portal frame still resists the load and resultant moments by bending action of the individual members 12,14,22,24. However since the frame is of a symmetrical geometry, and the asymmetric loading cannot be resisted well, the structure is unbalanced by the load with the result that parts away from the unsymmetric load deflq qt in the opposite sense to compensate for the asymmetry of the loading with the result that significant bending and deflection occurs. In addition due to the asymmetric nature of the loading a significant horizontal deflection of the whole structure occurs as a result of vertical initial loading. The resultant deflections shown in figure 9 again clearly indicate the above described way in which the unsymmetrical loads are carried in this type of situation.

In a tied portal structure as shown in f igure 2, under symmetrical/uniform loading the frame moments are predominantly resisted by a combination of axial load within the bottom tie member 32 and axial load and bending in the rafters 22,24. A significant proportion is resisted by the stiffness of the rafter members 22,24 which carry and resist around S% of the moment by bending action (the tie is not designed, configured or arranged to carry bending moments).

Again this is clear from the resultant analysed deflections shown in figure 10.

Unsymmetrical loading of a tied portal generates unsymmetrical moments. Such unsymmetrical moments can, by virtue of the symmetrical geometry of the tied portal, only be resisted and carried by shear loads and bending moments within the rafter section 22,24. The tie member 32 has no effect and makes no contribution to carrying the unsymmetrical load because the axial force within the tie member 32 is constant along its length and the resistance moment produced by the tie 32 is symmetrical due to the symmetrical geometry of the structure. Consequently under unsymmetrical loading the structure behaves and carries the load in a similar way to that of a conventional portal frame structure and the tie member 32 has no beneficial effect. As a result the structure exhibits similar disproportionate 2S deflections in response to unsymmetrical loading, as shown in figure 11 which shows almost the same deflection to that of a conventional portil frame structure.

Symmetrical loads applied to the braced tied portal designs of the embodiments of the invention (shown in figures 3 to 7 and 19 and 20) are carried in similar ways to that of symmetrical loads in a tied portal, as described above. The loads in each of the embodiments are carried in similar ways and consequently in the following description references will only be made to those parts of the embodiment identified in figure 3 for convenience. It will 21 be appreciated that the corresponding. items and references of the other embodiments will act and function in similar ways to those of the embodiment shown in figure 3.

Specifically under symmetrical uniform loading the frame moments are predominantly resisted by a combination of axial load in the bottom tie member(s) 32 and axial and bending in the rafter 22,24 with a significant proportion (typically 5%) carried by bending moments in the rafters 22,24 which can resist and support bending loads and shear loads due to their designed stiffness. It should be noted that the various bracing structures 34 of the various embodiments of the braced tied portal designs of the invention are not designed or intended to carry shear or bending loads resulting from symmetrical / uniform loading.

Under uniform loading such loads are designed to be carried and resisted by shear and bending within the rafters 22,24 and axial loads in the rafters 22,24 and tie members 32, as in a tied portal configuration.

The braced tied portal of the invention, shown in the various embodiments of figure 3 to 7 and 19 and 20, however carries and resists unsymmetrical loads differently to the conventional tied portal design. This is due to the addition of the V bracing structure 34 of the various embodiments.

The V braces 36,38,40 carry an axial load. Due to the angle of the V braces 36,38,40 there is a horizontal component to this axial force within the V brace 36,38. The attachment of the V brace 36,38,40 to the lower tie member 32 means that the V brace 36,38,40 will alter the force within the tie member 32, such that by the use of the V braces 36,38,40 and bracing structure 34 the axial force within the tie member 32 can and is varied along its length. The result is that the tie member 32 can now assist and contribute in resisting unsymmetrical loads by virtue of the V braces 36,38,40 altering axial force within the tie member 32 such that this force in the tie member 32 is no longer symmetrical when an 22 unsymmetrical load is applied to the frame. Consequently whilst some of the unsymmetrical load is carried by bending of the rafters, the V braces 36,38,40 generate and provide for varying axial loads along the length of tie member 32 so that a significant proportion of the asymmetric load can be carried and resisted by the tie member. This dramatically reduces the deflections produced, as shown in figures 13 and 15.

It should be noted though that with the braced portal structure a significant proportion of the load is still carried and resisted by bending and shear within the rafters 22,24 and the V bracing is not fully triangulated, as is the case with a truss structure which requires a large number of individual members.

The braced tied portal designs of the invention are particularly advantageous in the construction of buildings with large unsupported spans. For example such designs are particularly useful in the fabrication of aircraft hangers which may have spans of typically 20 to 100m.

The braced tied portal design of the present invention, since it is fundamentally still of a portal type, can provide a roof structure with a relatively low roof profile (i. e a high span to depth ratio). This being by virtue of the fact that the upper supporting members can support shear loads and bending moments. It will also be appreciated that the bracing structures of the invention are not required to form a fully triangul4,ted as in a truss structure and so the depth considerations imposed on truss type structures are lessened. The structure is therefore also easier to transport in an assembled condition. In addition the number of individual members forming the bracing structures and degree of bracing is considerably less in the braced tied portal designs of the invention than would be required in a truss type structure. As a result the roof frame is simpler, easier and cheaper to fabricate, particularly on site.

23 As described above by using the -bracing structure as proposed in the present invention within a tied portal roof structure the deflections are reduced and consequently the stresses in the individual members are also reduced.

Consequently for a given allowable deflection the individual members, particularly upper supporting members, do not have to be as substantial as would be required in a conventional tied portal design to limit the deflections. The weight of the structure is there reduced in this case.

In the embodiments shown in figures 7 and 19 multiple the V bracing arrangements are provided which are disposed along the span and lower connecting member 32, rather than just at the centre of the roof frame and connects to the apex. In the case of the variant of f igure 19 no V bracing is used. The use of multiple bracing arrangement and spacing them along the span and lower connecting member is particularly advantageous for large spans. Such multiple arrangements disposed along the span and lower connecting member also provide more variation and alteration of the axial force in the lower connecting member, by dividing the lower connecting member into additional portions between each V bracing arrangement. As a result the alteration in the axial force in the lower connecting member in response to asymmetric loading is more localised. This improves the way in which the braced tied portal as a whole resists the asymmetric load and reduces the deflections which arise.

It will be ap1j reciated however that the multiple bracing arrangements of figures 7 and 19 still comprise much fewer members than corresponding truss structures for similar spans and loads and that the roof f rame as a whole is far from being fully triangulated as in a truss structure. In a corresponding truss structure all of the bracing arrangements would be interconnected through common nodal points between each bracing arrangement. In the arrangements of the invention the V bracing arrangements 24 are separated and distinct along the. upper roof structure and are not directly interconnected to each other through a common nodal connection.

It will be appreciated that there are many variations and alternate embodiments of the invention following the principles described above. The features of the individual embodiments may also be combined and mixed in differnt ways in other embodiments. The detailed location and arrangement of the bracing structure will be determined by structural analysis of the anticipated loads to ensure that the loads are carried as described, in accordance with the invention.

Claims (29)

1 A method of constructing a fabricated metallic roof frame of tied-portal configuration for use in an at least partially metal-framed building as one of a series of similar roof frames disposed across a space to be roofed, the method comprising:

a) providing a fabricated metallic upper roof supporting structure to define the upper profile of said portal and to extend across said space and to support a roofing material such as cladding sheets or a tiled structure or the like; and b) providing a lower connecting structure to define the tie of said tied-portal configuration and extending between opposite ends of said upper roof supporting structure and cooperating therewith as part of said metal fabrication to form a load resisting structure; characterised by C) providing as part of said. metal fabrication a bracing structure comprising at least one bracing arrangement of upwardly-diverging structural members interconnecting said upper roof -supporting structure and said lower connecting structure; d) arranging said bracing structure such that, in response to unsymmetrical loading of said roof frame, an a';:al load is generated in at least one of said upwardly diverging structural members which is arranged to alter the axial load in a portion of said lower connecting structure; and e) arranging and adapting said roof frame such that at least a portion of any loading of said structure is carried by shear load in and bending of said upper supporting structure.

26

2 A method of constructing a tied-portal roof f rame characterised by providing a bracing structure interconnecting an upper roof supporting structure and a lower connecting structure, said bracing structure arranged such that, in response to unsymmetrical loading of said roof frame, an axial load is generated in a structural member of said bracing structure which is arranged to alter the axial load in a portion of said lower connecting structure

3 A method as claimed in claim 2 in which the bracing structure comprises at least one bracing arrangement of upwardly diverging members.

4 A method as claimed in claim 1 or 3 in which the bracing structure comprises a plurality of bracing arrangements of upwardly diverging members.

A method as claimed in any one of claims 1, 3 or 4 in which the upwardly diverging members of each bracing arrangement of the bracing structure are interconnected to a common point on the lower connecting member.

6 A method as claimed in any preceding claim in which the upwardly diverging members are symmetrical about a vertical axis bisecting said upwardly diverging members.

7 A method as claimed in claim 6 in which there is provided a vertical member bisecting the upwardly diverging members of the bracing arrangement of upwardly diverging members of the bracing structure.

8 A method as claimed in any preceding claim in which the upwardly diverging members of the bracing structure are 27 disposed in a W configuration.

9 A method as claimed in any one of claims 1 to 7 in which the upwardly diverging members of the bracing 5 structure are disposed in an inverted M configuration.

A method as claimed in any one of the preceding claims in which the upper supporting structure is segmented.

11 A method as claimed in any one of claims 1 to 9 in which the upper supporting structure is curved.

12 A method as claimed in any one of claims 1 to 11 in which the lower connecting structure is segmented.

13 A method as claimed in any preceding claim in which the lower connecting structure is non linear.

14 A method of constructing a tied-portal roof frame for use in a building substantially as described herein with reference to Figs 3 to 7, 19 and 20 of the accompanying drawings.

A fabricated metallic roof frame for use in an at least partially metal-framed building as one of a series of similar roof frames disposed across a space to be roofed, comprisip!u a) a fabricated metallic upper roof supporting structure to define the upper profile of said portal and to extend across said space and to support a roofing material such as cladding sheets or a tiled structure or the like; and b) a lower connecting structure to define a tie of said tied-portal configuration and extending between opposite ends of said upper roof 28 supporting structure and cooperating therewith as part of said metal fabrication to form a load resisting structure; characterised by C) as part of said metal fabrication, a bracing structure interconnecting said upper roof supporting structure and said lower connecting structure, said bracing structure adapted within said roof frame such that, in response to unsymmetrical loading of said roof frame, an axial load is generated in a structural member of said bracing structure which is arranged to alter the axial load in a portion of said lower connecting structure; and is d) said roof frame is adapted such that said upper roof supporting structure carries, in use, at least a portion of any loading of said structure is carried by shear load in and bending of said upper supporting structure.

16 A tied-portal roof frame characterised by a bracing structure interconnecting an upper roof supporting structure and a lower connecting structure, said bracing structure arranged such that, in response to unsymmetrical loading of said roof frame, an axial load is generated in a structural member of said bracing structure which arranged to alter the axial load in a portion of said lower connecting structure

17 A tied portal structure as claimed in claim 15 or 16 in which the bracing structure comprises at least one bracing arrangement of upwardly diverging members.

18 A tied portal structure as claimed in claim 17 in which the bracing structure comprises a plurality of bracing 29 arrangements of upwardly diverging members.

19 A tied portal structure as claimed in 18 in which the upwardly diverging members of each arrangement of the bracing structure are interconnected to a common point on the lower connecting member.

A tied portal structure as claimed in any one of claims 15, or 17 to 19 in which the upwardly diverging members are symmetrical about a vertical axis bisecting said upwardly diverging members.

21 A tied portal structure as claimed in claim 20 in which there is provided a vertical member bisecting the upwardly diverging members of the arrangement of upwardly diverging members of the bracing structure.

22 A tied portal structure as claimed in any preceding claim in which the upwardly diverging members of the bracing structure are disposed in a W configuration.

23 A tied portal structure as claimed in any one of claims or 17 to 21 in which the upwardly diverging members of the bracing structure are disposed in an inverted M configuration.

24 A tied portal str cture as claimed in any one of the preceding claims in which the upper supporting structure is segmented.

A tied portal structure as claimed in any one of claims 15 to 23 in which the upper supporting structure is curved.

26 A tied portal structure as claimed in any preceding claim in which the lower connecting structure is segmented.

27 A tied portal structure as claimed in any preceding claim in which the lower connecting structure is non linear.

28 A tied portal structure comprising an upper roof supporting structure and a lower connecting structure extending between opposite ends of the upper roof supporting structure to define a tie of said tied portal structure, the upper supporting structure arranged and connected within the portal structure such that loading of the structure, in use, is resisted at least in part, by shear loads and bending moments within and by the upper supporting structure; characterised in that the tied portal structure further comprises a bracing structure comprising at least one arrangement of upwardly diverging bracing members which in the arrangement extend from a common point on the lower connecting structure and interconnect the lower connecting structure with the upper roof supporting structure, said bracing structure arranged and configured such that, in use in response to asymmetric loading of the portal structure, an axial force within a first portion and second portion of the lower connecting struc,ure on o posite sides of said common p point is different.

29 A tied-portal roof frame substantially as described herein with reference to Figs 3 to 7, 19 and 20 of the accompanying drawings the accompanying drawings.