GB2197356A - Prestressed beam - Google Patents

Abstract

A prestressed concrete elongate article 10, e.g. a beam, incorporates a crank along its length and longitudinal, prestressed, reinforcement strands 22 deflected at the crank, the crank being defined by at least two adjacent changes in direction 18,20 both in the overall article and in the reinforcement at the crank. Mouldage for such an article is also described and includes longitudinally extending members e.g. I- beams 62 for resisting this prestress in the strands which are pivoted or hinged at two points 68 and 70 along the length of the cranked beam or other article with the pivot centre axis lying substantially on the centroid of prestress of the longitudinal prestressing strands of the beam or other article. The longitudinally extending members may be interconnected by large diameter machined rollers engaging part-cylindrical bearing surfaces on the ends of the members. The rollers permit angular adjustment of the mould sections. The centre line of the pins forming the axes of the rollers are substantially coincident with the centroid of prestress in the strands 22. The strands are supported at the deflection points by deflector castings (100, Fig. 18) which are lost in the moulded product. <IMAGE>

Description

SPECIFICATION Beams and other prestressed concrete articles of cranked form This invention relates to cranked prestressed concrete beams and other prestressed concrete articles of cranked form. The invention also relates to mouldage for the production of such articles.

It has already been proposed to incorporate in prestressed concrete beams longitudinal reinforcing strands which are deflected at some point, generally the centre of the length thereof. When manufacturing such beams and other articles the associated mouldage must and has hitherto always incorporated very substantial structure to resist the sideways forces which arise at the point of deflection of the reinforcment strands. Clearly the need for such structure which is essentially permanently sited not only substantially increases the cost of the mouldage but renders it totally immobile primarily on weight considerations.

For conventional rectilinear beams and other members the deflected reinforcement has some advantage inasfar as the number of items in the reinforcement stirrup can be reduced to some extent without weakening the beam or other article.

For certain structures such for example as multi-storey carparks the use of cranked beams and in particular cranked beams in which the change of direction is not symmetrical lengthwise have appreciable advantages at the construction stage inasfar as it is frequently possible to reduce the number of support columns and hence the overall cost of the structure. Moreover, in use the absence of columns which are rendered unnecessary by the cranked beams has advantages from the standpoint of persons making use of the building.

The advantages of such cranked beams can be offset if the mouldage becomes particularly costly and inflexible at the workshop and this has not encouraged the development and use of cranked, pre-stressed concrete beams, at least in the United Kingdom.

According to the present invention there is provided a prestressed concrete elongate article such as a beam incorporating a crank along its length and prestressed reinforcement strands deflected at the crank, the crank being defined by at least two adjacent changes in direction both in the overall article and in the reinforcement defining the crank.

Further according to the present invention there is provided mouldage for the production of a cranked prestressed concrete beam or other article, the mouldage including longitudinally extending reinforcing members which are pivoted or hinged at two points of the cranked beam or other article with the pivot centre -axis lying on or close to the centroid of prestress of the prestressing strands of the beam or other article.

Further according to the present invention there is provided a system for the manufacture of a prestressed, cranked beam or other concrete article comprising the provision of mouldage incorporating prestressing units at each end, said mouldage further including longitudinal members arranged to resist, in compression, the prestress applied to the longitudinal reinforcement strands of the beam, and means at the crank for maintaining the strands in a deflected condition, the longitudinal members of the mouldage being substantially aligned and connected together at two pivots at or adjacent the crank with the pivot axes at least substantially coincident with the centroid of prestress.

The various aspects of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a side elevation of an upwardly convex, double T prestressed beam in accordance with the invention; Figure 2 is a plan view of the beam of Figure Figures 3A to 3F are respective half sections on the lines A-A; C-C; D-D;E-E; and F-F of Figure 1; Figure 4 is a scrap view illustrating the location of the reinforcement stirrups and end portions of the longitudinal prestressing strands of the beam of Figure 1; Figure 5 is a fragmentary view illustrating a detail of the cranked portion of the beam of Figure 1, to an enlarged scale; Figure 6 is a side elevation of an upwardly concave cranked double T beam in accordance with the invention; Figure 7 is a plan view of the beam of Figure 6; Figures 8A to 8D are respectively half sections on the lines A-A; B-B; C-C;D-D; of Figure 6; Figure 9 is a highly diagrammatic side elevation illustrating the essential components of mouldage in accordance with the invention for the manufacture of a beam according to Figures 1 to 5 or Figures 6 to 8; Figure 10 is a section on the line 10-10 of Figure 9; Figure 11 is a fragmentary elevation of a detail of a part illustrated in Figure 9; Figure 12 is a fragmentary side elevation of a further detail at the crank of the mouldage as illustrated in Figure 9; Figure 13 shows further details of the fragment illustrated in Figure 12; Figure 14 is a section on the line 14-14 of Figure 13; Figure 15 illustrates to an enlarged scale and highly diagrammatically the possible range of movement of contiguous parts of the mouldage at the crank;; Figure 16 illustrates diagrammatically and to an enlarged scale an arrangement for resisting forces generated by the prestressing strands during the manufacture of an upwardly convex beam; Figure 17 is a view similar to Figure 16 but for an upwardly concave beam; Figure 18 is an elevation of a metal casting used for maintaining the strands in their correct location at one point of deflection; Figure 19 is a section on the line 19-19 of Figure 18; Figure 20 is a plan view of the casting of Figure 18; Figure 21 is a diagrammatic view illustrating means for retaining the casting of Figures 18, 19 and 20 in location during the manufacture of an upwardly convex double T beam; Figure 22 is a fragmentary view illustrating a detail of the means illustrated in Figure 21;; Figure 23 is a side elevation illustrating means for locating the casting during manufacture of an upwardly concave double T beam; Figure 24 is a side elevation of mouldage in accordance with the invention for the construction of beams illustrated in Figures 1 to 5 or Figures 6 to 8; Figure 25 is a fragmentary plan view of an end portion of the mouldage illustrated in Figure 24; Figure 26 is a fragmentary view of a detail illustrated in Figure 24; Figure 27 is a section on the line 27-27 of Figure 26; Figure 28 is a section on the line 28-28 of Figure 24; Figure 29 is a section on the line 29-29 of Figure 28; Figure 30 is a section on the line 30-30 of Figure 24; Figure 31 is a side elevation of a detail shown in Figure 29;; Figure 32 is a section on the line 32-32 of Figure 31, and Figure 33 is a detail illustrating the manner of support of the longitudinal elements of the mouldage of Figure 24.

Referring now to the drawings, two forms of cranked double T beams are disclosed in detail and mouldage therefor, but it will be apparent that the invention is also applicable to other forms of cranked, prestressed concrete articles such for example as component parts of bridges and roofs. The primary interest of the invention is, however, in cranked double T beams since they are particularly useful in the construction of multi-storey carparks where changes in inclination are required to accommodate ramps from one storey to the next.

In Figures 1 to 5 an upwardly convex double T beam 10 is illustrated including a relatively long, horizontal portion 12 (as illustrated); a relatively short inclined portion 14; and an intermediate portion 16 joined to the longer portion and the shorter portion at deflection planes indicated generally at 18 and 20.

Reinforcement strands, six in number and referenced 22, are shown in chain lines and these pass through castings 24 at each of the deflection points. Reinforcement stirrups 26 are spaced along the length of the beam in accordance with standard practice for prestressed beams although the numbers required are less than would be necessary in the absence of the deflected longitudinal reinforcement strands. At the left hand end (as shown) a step 28 is provided and in view of the stress concentration involved at this step, the number of stirrup reinforcements incorporated in this end portion is substantially greater than along the length of the beam. At each end portion a lifting strand 30 of inverted V form is provided as shown conveniently in the detail of Figure 4.

The detail illustrated in Figure 5 is of particular importance since the provision of two deflector assemblies is critical to successful carrying out of the invention and these deflector assemblies are indicated by the reference 32. Details of these assemblies are disclosed in relation to other Figures and will be described in greater detail hereinafter. In relation to Figure 5 it is particularly to be noted that the "crank" in the beam is defined by two changes in direction and not one, thus introducing the intermediate portion 16 of the beam referred to in relation to Figure 1. By so doing the reaction forces at each change in direction are halved in comparison with the reaction forces which would arise if a single directional change were to be selected.It will be noted that two reinforcement stirrups 34,36,38,40 are disposed immediately adjacent each deflector 32 to accommodate the stress concentrations immediately adjacent the deflection planes.

Figure 5 also illustrates the paths of the reinforcement strands 22 and it will be noted that the upper strands 42,44 illustrated in chain lines remain parallel to one another throughout the length of the beam whereas the lower strand 46 undergoes a greater deflection at each deflection point and does not remain parallel to the strands 42,44 except within the intermediate portion 1 6.

Figures 6 to 8 disclose an upwardly concave beam 48 and broadly speaking the construction corresponds to the upwardly convex beam of Figures 1 to 5. It follows that the same reference numerals have been given with the addition of a prime and it is only necessary to note certain changes in relation to the upwardly convex beam. As will be apparent in Figure 6, the reinforcement strands at the deflection planes are supplemented by additional strands 50 which extend only a short distance beyond each deflection zone. The additional strands 50 are welded to a series of stirrups 52 which do not extend over the whole depth but provide interconnection between the main strands 42',44',46' and the additional strands.The reason for this additional reinforcement is, of course, the existence of an area of high tensile stress at the lower edge portion of the beam 48 where the change in direction occurs.

It will be noted that both in this embodiment and in the first described embodiment of an upwardly convex beam, the cross member 49 of the double T is reinforced by a standard mesh fabric 51 as shown as a breakaway fragment in Figure 7.

Figures 8A to 8D illustrate the stirrups and reinforcement details at various sections along the length of the upwardly concave beam. As is apparent in Figure 8A, the centroid of prestress substantially coincides with the intermediate strands 44', the remaining four strands 42',46' being closely adjacent to the centroid.

As will be supported in these Figures the stirrups extend upwardly beyond the level of the cross-member 49 and are bent to lie parallel to the upper surface of the member 49. This arrangement assists keying to any further layer of concrete applied to the beam when in situ.

In order to manufacture a prestressed concrete beam or similar article which is cranked along its length, it is essential properly to accommodate the lateral forces which are generated in the reinforcing strands 42,44,46 at the crank. Hitherto, it has been proposed at least in the United States to accommodate these lateral forces by applying substantial weights to the strands at the plane of deflection but this arrangement has the obvious disadvantage that it is inflexible both because the installation has to be fixed and the changes in the angle of deflection cannot be accommodated because of the widely differing lateral counterforces which need to be applied.At each plane of deflection it is necessary not only to withstand the lateral forces during the manufacturing stage but to provide for a continuing distribution of the lateral forces generated in the reinforcing strands throughout the life of the corresponding beam. Furthermore, during the moulding stage when the concrete provides no strength the mould itself has to withstand longitudinal and lateral forces at the point or points of deflection because, as is apparent hereinafter, the prestressing units form essential parts of the mouldage.

Figure 9 illustrates in outline a mould con struction 60 including longitudinally extending members of I-section 62,64,66, the latter be ing interconnected at the hinge points 68,70 interconnecting the left hand and the right hand portions through the intermediary of the intermediate section 64. As will be readily ap parent two deflection planes are provided for the purpose explained hereinbefore.

Figure 10 illustrates a cross-section of the mould 60 including four longitudinally extending I-sections 62 and also outlines an A-frame 72 described hereinafter in greater detail in relation to Figure 16 and other Figures. It will be apparent that similarly there are four I-section members 64 and 66.

Figure 11 illustrates a detail of one end of one of the longitudinally-extending members 62 and in outline the prestressing grillage 74 which is conventional and forms parts of the mouldage as a whole. Since it is basically conventional it will not be further described.

Figures 12, 13 and 14 illustrate detail 2 indicated in Figure 9 and it will be apparent that each of the longitudinally-extending members 62,64,66 at the deflection plane is interconnected by a large diameter machined roller 76 carried on a pin 78 which is also engaged in opposed pairs of flanges 80 at elongate slots 82. It is important to note that the pin 78 is not subject to shear forces, these being accommodated solely by the large diameter roller 80 which engages part-cylindrical bearing surfaces defined by blocks 81 mounted on respective ends of the I-sections. It will also be noted that the compression connection blocks 81 are bolted to end flanges 83,85 of the adjacent compression resisting I beams thus making disassembly readily possible.The rollers are capable of withstanding substantial forces and, moreover, during the setting up stage enable precise adjustment to give the three sections of the mould their correct angular relationships to provide the required beam.

Thus, the mould assembly can be used for various beams or other prestressed concrete members with a minimum of setting up time.

The centre line of the pin and of the roller is at least substantially coincident with the centroid of prestress in the prestressing strands.

Typically, if the longer portion of the beam to be manufactured has an overall length of 10500mm., the intermediate portion has a length of 1000mm. and the shorter section has a length of 5100mm. then the substantially vertical reaction at the joint between the longer section and the intermediate section will be approximately 116KN and at the joint between the shorter section and the intermediate section the reaction will be approximately 105KN. Such reaction figures can readily be accommodated by the structure outlined in Figures 16 and 17. Figure 15 illustrates in outline these typical mould dimensions and reactions.

Figures 16 and 17 illustrate in rather more detail the arrangements for an upwardly convex and an upwardly concave beam at the deflection zone, which arrangements enable the modest reactions to be accommodated.

For the upwardly convex beani illustrated in Figure 16 the upward reactions are accommo dated by the A-frame 72 which is supported on two longitudinally extending channel sec tion members 73 known as "strongbacks".

For the upwardly concave beam deflector castings which are embedded in the concrete itself the downwardly-acting resultant force is taken up by two opposed channel sections 90 (strongbacks) interconnected by bracing 92 and having brackets 94 to which the deflector castings are detachably attached as will be described with reference to Figures 18 to 23.

Turning now to details illustrated in Figures 18 to 20, a deflector casting 100, for example of spheroidal graphite cast iron grade 700/2, incorporates shaped apertures 102,104,106, the internal profiles of which accurately reflect the change in direction at the corresponding strand 42,44,46.

Each aperture is accurately formed and the edges are radiused with a view to avoiding stress concentration in the strands during tensioning and when assembled.

The casting has a downwardly tapered profile 108 at its lower end portion and this portion accommodates a tapped bore 110 illustrated in broken lines. The deflector can be used both for upwardly concave and for upwardly convex beams and other articles but the apertures will, of course, be matched to the specific requirements of the beam to be produced. In the final beam the deflector remain in situ. The manner in which the deflector is retained during the casting operation, however, differs, the arrangement for an upwardly convex beam being illustrated in Figures 21 and 22.The deflector 100 is located within the mouldage with its tapped bore 110 directed upwardly and this receives a screwthreaded rod 112 of, for example, high yield, T32 steel and as will be apparent in Figures 19 and 21 the centre of the casting is located substantially on the centroid of prestress of the article to be cast. Immediately upwardly of the deflector the substantially vertical rod 11 2 passes through a retainer collar 114 very roughly of "horse-shoe" shape and this ensures that the rod cannot move out of its design location during the operation of prestressing. Adjacent its upper end the rod 112 passes through a square plate 116 to which the upper end portion of the rod is screwthreaded.A connector bush 118 is also screw-threaded onto the final end portion of the rod and the connector bush also receives a screw-threaded lower end portion of a further steel rod 120 and this is similarly screwthreaded and is engaged in an upper cross bar 122 of the A-frame 72. In use, the deflector casting 100, the lower one of the screwthreaded rods 112 and the plate 116 are embedded in the final article. The forces generated in the rods 112,120 are transmitted to the rigid and massive A-frame 72 but otherwise there is no need to provide any form of restraint which serves under all conditions to counteract the substantial deflection force hitherto encountered. For simplicity Fig. 21 shows only one rod assembly 112, 120, but in practice four such assemblies are used, one for each deflection plane and one for each set of reinforcing strands.

The horse-shoe collar 114 is mounted on a pin 115 passing through plates 117 on either side wall of the mouldage and the assembly is only used during the prestressing operation. It is removed prior to casting.

The rod 112 is prestressed when the prestressing steel strands in the beam are tensioned. This rod takes the out-of-balance forces from the prestressing steel strands upwards to the cross frame 72 which then takes the force down to the position of out-of-balance force at the intersection of the mould struts. The rod 112 is concreted into the beam together with the place 116 at its upper end. When the beam is cast and the prestress is released from the beam at the point where the change in compression thrust in the beam produces an out-of-balance force.

This force exactly balances the change in tensile thrusts in the prestressed, thus the rod 112 is prestressed at the time of tensioning and remains stressed through the casting operation through detensioning of the longitudinal strands and throughout the life of the structure. Thus there is no need for the rod to take up tensile strain at the time of detensioning of the beam and therefore there is no risk of horizontal cracking in the concrete at the point of change of direction of the beam.

In Figure 23 the casting is again illustrated but in this instance the forces which have to be accommodated arise in an upwardly concave beam. As before, the casting 72 is inserted in the mouldage with its centre substantially coincident with the centroid of prestress of the beam. In this construction a relatively short rod 124 which is tapped at its upper end portion (as shown) has at its lower end a pair of flats 126 which assist in applying a spanner to remove the rod at the end of the casting operation. Immediately adjacent to the deflector 72 a taper cone 126 with a screw-threaded periphery is engaged around the rod and includes a screw-driver slot in the lower surface to aid removal at the end of the casting process. The lower face of the tapered cone 126 lies flush with the material to be cast and engages against a member 128 of the mouldage of shallow, inverted, channel section. Outwardly of this channel section an annular spacer 130 is provided with a central hole which is a close tolerance with respect to the rod 124. The lower end of this annular spacer 130 is engaged against one of the brackets 94 which extend from the structure including the two opposed strongbacks 90 (Figure 17). Below the bracket a taper washer 132 adapted to the mould angle of the corresponding section cooperates with a securing nut 134 of conventional form.

It is the objective of the present invention both in relation to Figures 21 and 22 and also in relation to Figure 23 that the removable parts shall be readily accessible and likewise readily removable, any hples left in the surface of the concrete being filled with an appropriate material.

The mouldage will now be described in greater detail with reference to Figures 24 to 33.

The remaining Figures illustrate a practical embodiment of mouldage for the manufacture of cranked double T beams but it will be readily apparent that the principle can be adopted by appropriate modification to the structure for prestressed concrete articles of other cross-sections. In all cases, however, the relevant article will be cranked at two spaced planes along its length.

Although various features which also appear in Figures 24 to 33 have already been described in relation to the preceding Figures, it will be more convenient and will simplify understanding by the use of a distinct set of reference numerals.

The mouldage comprises a three section structure 200,202 and.204, the section 200 being longer than the section 204 with the remaining section 202 serving as an intermediate section bridging the two deflection planes where both the double T beam 10 and its longitudinal reinforcing strands change direction.

The longer, left-hand, section 200 includes two longitudinally extending I-section members 206 (only one shown in Figure 24-both members shown in Figure 30) and form-work 208 which provides for the actual shaping of the moulded article. The members 206 are directly supported on structure 210 and the members are also supported at intervals along the length by transverse I members 212. The form-work is supported from these I members 212 by upwardly extending channel section members 214.

As is conventional, the form-work comprises a skin 216 and spaced I-section members 218 to give the necessary stregth and rigidity to the skin 216.

The long section 200 is supported at each end and at an approximately central location by structure now to be described. At the lefthand end, as shown, a support 220 is fabricated from two opposed channel section members 222 extending vertically from a base plate 224 which is anchored to the floor of the building where the mouldage is situated.

Immediately above the base 224 tranverse 1section beam 226 extends between opposed supports 220 and is movable vertically between the channel section members 222 so as to provide adjustment for the angular relationship between the long section 200 and the intermediate section 202. The channel section members 222 each have series of apertures 228 engageable by pins (not shown) which serve to provide a coarse location of the left-hand (as shown) end portion of the long section 200.

The I beam 226 carries a roller assembly including a roller 230 rotatably mounted on two upright members 232 which may be of channel section and which form parts of the support structure 210. The uprights are tied at their upper ends by members 234. An end portion 236 of the transverse I-section member 226 has an aperture 238 which enables the member 226 to be adjusted vertically. As shown in Figures 28 and 30 staging 239 is provided for access purposes.

Fine adjustment of the location of the transverse beam 226 is provided as indicated in Figure 33 in the form of packer plates 240 which are engaged as required around bolts 242 extending between the beam 226 and a support member 244. By means of this arrangement the required degree of precision in the angular relationship of the long member 200 and the intermediate member 202 can be achieved. At the upper end of each vertical support 220 a tie 246 is provided to the opposite vertical support. Finally, each vertical support member 220 is reinforced by a buttress member 248 anchored to the floor at a plate 250 (Figure 24).

As is best apparent in Figures 24 and 25, the mouldage includes a self-prestressing arrangement 260 including a framework 262 and a conventional prestressing cylinder 264.

These parts will not be further described since they are conventional in the art. In practice the prestressing cylinder 264 will be used to apply the prestresses as the friction forces generated will be lower than when the right hand cylinder 264' is used. The frame work 262 is welded to an end portion of a corresponding one of the beams 206.

An intermediate support 270 includes a base plate 272 and two vertical channel section members 274 carrying a transverse support member 276 which can be held in position at various locations along the length of the members 274. The support member 276 carries a transverse beam 278 generally similar to the transverse beam 226 of the support assembly 220. The other end of the transverse beam 278 is, of course, similarly supported by a corresponding support assembly 270 at the other side of the mouldage. A roller assembly 280 is generally similar to the roller assembly 230 and provides for direct support of the members 206. As for the left-hand end support 220, a fine-adjustment arrangement similar to that shown in Figure 33 is provided for the roller assembly 280.A buttress member 282 secured to the floor of the workshop at 284 assists in resisting forces tending to move the support 270 from its truly vertical position. A tie 286 is provided at the upper end of the intermediate support and interlink the vertical members 274 to ensure rigidity. The precise height of each of the support assemblies 220 and 270 will depend upon the requirements of the crank angle of the beam to be manufactured.

The support assembly at the right-hand end, shorter section 204, is generally similar to the left-hand support assembly 220 and will not be further described. Like parts have been given the same reference numerals but with the addition of a prime. Similar remarks apply to the right-hand, shorter, section 204 and this will likewise not be further described.

The structure at the deflection planes will now be described with reference to Figures 24, 26, 27, 28, 29, 31 and 32. This structure includes vertical assemblies 290 (four in number) which may be of I-section and these are interconnected at their upper ends by transverse I-section members 292. Unlike the end supports 220,220' there is no requirement for vertical motion and the support assembly 290,292 carries directly transverse Ibeams 294 which, in turn, support the formwork 216, previously referred to.

Outwardly of the longitudinal I beams 206 the A-frames 296 previously referred to is supported at end portions of the beams 294.

As will be understood two A-frames are necessary, one for each of the deflection planes. Each A-frame 296 is made up of Isection members 298 (vertical) and 300 (horizontal). The horizontal members of each Aframe are themselves interconnected by two pairs of back-to-back channel section members 302 which ensure rigidity of the two Aframes and serve a further purpose to be described hereinafter. The A-frames are readily removable by releasing bolts which provide the connection between the vertical members 298 and the transverse members 294 through the intermediary of longitudinally-extending I beams 299.

In order to resist the very high forces which arise from prestressing of the strands which serve to reinforce the cranked double T beam, the members 206,206' are provided at their ends corresponding to the deflection planes with a part cylindrical bearing assembly best illustrated in Figures 26 and 27 and indicated generally by the reference numeral 310,310'.

Since these assemblies are identical, only the assembly 310 will be described in detail. A part cylindrical bearing shell 312 is supported by spiders 314 and by a transverse member 316 which provides an abutment for the central spider member 314 where a substantial proportion of the compressive load is accommodated.

The part cylindrical bearing members each receive a part cylindrical member 317 mounted on a rectangular base 318. The latter is secured to a transverse plate 320 which is itself welded to the corresponding one of the two I-section members 322 which correspond to the members 206,206' of the long and short compression-resisting beams. The plate 318 and its corresponding part cylindrical bearing member 31 7 is secured to the plate 320 by a plurality of set bolts 324 (only one shown). Because of the very heavy loads involved the plate 320 is welded at all contact surfaces with the member 322 and in addition to a central plate 326 which is itself reinforced by a plate 328 as shown in Figure 24.

It will be readily apparent that the assembly illustrated in Figures 26 and 27 is very robust but at the same time this assembly can be removed in a straightforward manner by virtue of the use of the set bolts 324. Additional security may be provided if required by the use of bolts 330 which engage in the body of the cylindrical member 316 and pass through apertures 332 in a flange member 334 extending between flanges corresponding to the flanges of the I member 206 or 206'.

The back-to-back channel section members 302 (Figures 24 and 28) serve an important purpose additional to providing rigidity between the A-frames, namely they receive an upper end portion of the rods 120 described with reference to Figures 19 and 21. The arrangement illustrated in Figures 31 and 32 provides for pivotal movement of the corresponding rod 120, each channel 302 having a reinforcing plate 303 with a central aperture 305 aligned with a corresponding aperture in the web of the channel section member, this aperture receiving a short pin 306 welded at its end remote from the plate 304 to a plate 308 forming part of a stirrup 310 which receives the end portion of the rod 1 20. The stirrup 310 also includes two plates 312 having central apertures 314 which are aligned, the upper one of the two plates 312 providing support for a washer 316 and a nut 318 threaded onto the end portion of the rod 120.

It will be readily apparent that the rod 120 can readily pivot about the pins 306, thus accommodating differences in the deflection angles.

Details of the lower end portion of the rod 120 and the deflector plates are the same as hereinbefore described.

The mouldage hereinbefore particularly described can also be used for upwardly concave double T beams by removing the A frames 296 and substituting an assembly similar to that illustrated in Figure 23.

In order to produce a double T beam in accordance with the invention as described with reference to Figures 1 to 5 or with reference to Figures 6 to 8 the mouldage as described with reference to Figures 24 to 33 is adjusted by means of the coarse adjustment and fine adjustment so that the required relationships between the three sections of the finished beam, 12, 14, 16 will be achieved. It will be readily apparent from the description given herein before that there is a substantial range in angles to produce an upwardly concave or an upwardly convex beam and even with maximum angular relationships the reac tion at the crank of the pre-stressing forces can be adequately accommodated by the mouldage itself.

After the mouldage has been set the prestressing operation is initiated by first locating the deflector castings 24 at the deflection planes and in the case of a double T beam as particularly described, four such castings will be required. If an upwardly convex beam as illustrated in Figures 1 to 5 is to be produced then the arrangement of Figures 21 and 22 supplemented by the details given in Figures 24 to 33 will be employed. If an upwardly concave beam is to be produced then the arrangement of Figure 23 will be employed.

Which ever form of beam is to be produced it is important that the intermediate strands or tendons should lie on the centroid of prestress. In this way the reaction is minimized.

Having correctly located the prestressing strands or tendons within the mouldage and engaged them in the prestressing apparatus at each end, the prestressing force is then applied by the prestressing apparatus at the end corresponding to the longer of the two mouldage sections. After the prestressing force has been applied to the correct level, the concrete is poured and vibrated in the conventional manner. After the concrete has set the means which are employed to accommodate the reaction at the deflection points are disengaged although the deflector castings and parts lying within the profile of the beam will be permanently retained.

Claims (19)

1. A prestressed concrete elongate article incorporating a crank along its length and longitudinal prestressed reinforcement strands deflected at the crank, the crank being defined by at least two adjacent changes in direction both in the overall article and in the reinforce ment at the crank.

2. A prestressed concrete elongate article incorporating a crank along its length and longitudinal prestressed reinforcement strands deflected at the crank, the crank being defined by two closely adjacent changes in direction approximating to one half of the crank angle, the angular changes being apparent externally of the article and also being incorporated as angular changes in the reinforcement strands.

3. An article according to claim 1 or claim 2 comprising deflector members embedded in the concrete and each supporting the reinforcement strands at each of the changes of direction, means being provided to retain the deflector members in position throughout the life of the article, said means being preten sioned to take up the lateral components of prestress in the longitudinal reinforcement strands, the tension being mentioned through out the life of the beam.

4. An article according to any one of the preceding claims wherein the reinforcing strands lie parallel to one another intermediate the direction changes.

5. An article according to any one of the preceding claims comprising steps or shoulders at each end of the article, the article further incorporating transverse stirrups spaced along its length and stirrups at a closer spacing incorporated in each end portion of the article.

6. An article according to claim 5 wherein the stirrups extend above the upper surface of the article.

7. An article according to claim 6 wherein parts of the stirrups extending beyond the upper surface of the article incorporate portions which lie parallel to that upper surface.

8. An article according to any one of the preceding claims wherein the article is a double T beam and the cross member of the beam incorporates a reinforcement mesh.

9. An article according to any one of the preceding claims wherein at least six longitudinal reinforcing strands are incorporated and two of these strands extend non-parallel to the remaining strands except at the region of the crank.

10. Mouldage for the production of a cranked, concrete beam or other article having prestressed longitudinal reinforcing strands, the mouldage including longitudinally-extending members for resisting the prestress in the strands which are pivoted or hinged at two points along the length of the cranked beam or other article with the pivot centre axis lying substantially on the centroid of prestress of the longitudinal prestressing strands of the beam or other article.

11. Mouldage according to claim 10 wherein the beam or other article defines a crank by two adjacent changes in direction corresponding to the pivot axis, the mouldage incorporating means for containing prestress lateral component forces at each of the changes of direction.

12. Mouldage according to claim 10 or claim 11 wherein the longitudinal mouldage members are interconnected by pivot joints where the area bearing the compressive stress is a substantial proportion of the outline cross-sectional area of the members.

13. Mouldage according to claim 12 wherein the reinforcing members are additionally interconnected by a pin which serves to hold the members together during assembly operations.

14. A system for the manufacture of a cranked, beam or other concrete article incorporating prestressed longitudinal reinforcing strands, the system comprising the provision of mouldage incorporating prestressing units at each end, said mouldage further including longitudinal members arranged to resist, in compression, the prestress applied to the longitudinal reinforcement strands of the beam, means at the crank for maintaining the strands in a deflected condition during mould ing, the longitudinal members of the mouldage being substantially aligned and connected together at two pivots at or adjacent the crank with the pivot axes coincident with the centroid of prestress of the reinforcing strands.

15. A system according to claim 14 wherein the means at the crank for maintaining the strands in a deflected condition include pretensioned rods which remain in tension after the prestress has been removed from the longitudinal reinforcing strands.

16. A system according to claim 14 or claim 1 5 wherein the means at the crank for maintaining the strands in a deflected condition include a cast metal member having apertures receiving the strands which apertures are matched to the deflection angles of the strands.

17. A cranked prestressed concrete beam or other article substantially as hereinbefore described with reference to figures 1 to 8 of the accompanying drawings.

18. Mouldage for a cranked prestressed concrete beam or other article substantially as herein before described with reference to Figures 9 to 33 of the accompanying drawings.

19. A system of moulding a prestressed concrete beam or other article substantially as hereinbefore described with reference to the accompanying drawings.