GB2039566A - Underpinning - Google Patents

Abstract

A sacrificial prop for temporarily supporting a wall during building in of an in situ cast beam comprises three splayed legs (7, 8) which extend from a part (5) and have feet (10) which engage the wall above. A plate (17) rests on the wall below and has a socket (18) receiving the end of a screw threaded spindle (14) which screws through the part (5). The prop is tightened up by unscrewing the spindle (14), by means of a fixed nut (16). <IMAGE>

Description

SPECIFICATION Stool for in situ cast beam The invention is concerned with stools for use in the construction of in situ cast beams in the walls of buildings. Such beams are used in the underpinning and repair of buildings, for example when underpinning a cracked wall, the foundations of which have been subject to subsidence. The beam is inserted in the wall of the building parallel to the plane of the wall and the load will then be redistributed by the beam onto the foundations, which may be renewed at the same time. Such beams are also used as lintel beams which are inserted into the wall priorto cutting away the wall beneath the central portion of the beam.

Such a beam is conventionally constructed by cutting away the wall and inserting stools at horizontally spaced positions. The stools are built in and pinned up to support the wall above. With the intervening wall between the stools cut away, reinforcing rods are inserted, formwork is erected, and an in situ concrete beam is cast incorporating the reinforcement and the stools. A beam constructed in this way is hereinafter referred to as of the kind described.

Since a comparatively large number of stools have to be handled and sacrificed in the construction of a beam of the kind described, it is desirable that they should be of the cheapest and simplest possible construction and as small and as light in weight as possible commensurate with the load which they have to carry.

Until recently the pinning up of the stools has been a laborious business and time has had to be allowed for the mortar, which was used for the pinning up, to set before the wall between the stools could be cut away and the beam cast. Proposals have been made to use, in association with the stools, a screw jack in order to tighten and pin up the stools quickly. This has enabled the stools to be inserted successively along the wall with each stool being inserted into an aperture cut simultaneously with the removal of the intervening wall between that stool and the immediately preceding stool.However, the screw jacks have involved the use of screws which were short by comparison with the height of the stool and, although they were sufficientto enable the stool to be pinned up, the stroke is generally insufficient to lift the wall above, if cracked, sufficiently to close the cracks to any significant extent. The stitching up and filling of cracks then becomes a further laborious operation.

More recently we have proposed the use of a stool comprising upper and lower body portions, the lower of which is arranged to be supported on the wall below and the upper of which is arranged to be used in load bearing engagement with the wall above. One of the body portions includes a screw threaded socket having a vertical axis and a screw threaded spindle screws through the screw threaded socket and includes means for rotating the spindle relatively to the socket. The other of the body Dortions is couDled to the end of the spindle, such as by a spigot and socket connection, so that the spindle provides an axial reaction on the other of the body portions to force the body portions apart when the spindle is screwed out of the screw threaded socket. Such a stool is hereinafter referred to as of the kind described.

We have previously made the one body portion of a stool of the kind described as a precast concrete member having a vertical passageway in alignment with the screw threaded socket to receive the other end of the spindle. Such precast concrete bodies have been shaped as cylinders, or as saddle-shaped blocks. Although these have been satisfactory, they suffer from the disadvantage that a significant volume of concrete is necessary to provide the necessary support and they have therefore taken up a significant proportion of the thickness of the wall in which the beam is to be cast. This has resulted in the longitudinal reinforcing rods for the beam, which have to be fitted passing along the beam outside the stools, being so close to the plane of the wall face that they are inadequately covered by concrete when the beam is subsequently cast.Attempts have been made to mitigate this problem by providing notches in the wall of the precast body portion of the stool to accommodate the longitudinal rods but this has significantly increased the cost of casting the body portion and reduced the load bearing capacity of the stool.

Further practical difficulties arise from the large weight of the precast concrete body portion. The weight has been so great that the one body portion has had to be used as the lower body portion, standing on the wall below, rather than as the upper body portion which has to be held balanced on top of the spindle while the spindle is rotated to bring the upper body portion into firm contact with the wall above, after which the upper body portion is stabilised between the spindle and wall above as the final tightening up of the stool is carried out. This use of the one body portion with the screw threaded socket as the lower body portion has two disadvantages. In the first place, the screw threaded socket is most simply a conventional right hand screw threaded part which is cast or otherwise fabricated into the one body portion.It will be appreciated that to screw the spindle out through this socket when the one body portion is the lower body portion requires the application to the spindle of an anticlockwise torque as seen from above. This is more difficult for a right handed operator applying a spannerto a non-circular portion of the spindle than rotation of the spindle in a clockwise direction from above would be. Secondly, if the spindle is rotated to screw it out of the socket by the application of a spanner to a non-circular portion of the spindle, that non-circular portion of the spindle will become increasingly spaced from the screw threaded socket as the spindle is screwed out of the socket.When the screw threaded socket is part of the lower body portion of the stool, there will then be no opportunity for an operator to apply a spanner to the non-circular portion of the spindle whilst resting the spanner on an upper edge of the screw threaded socket or other part of the lower body portiqn.

In accordance with the invention, in a stool of the kind described, the one body portion comprises a skeletal structure of a plurality of legs which at their one ends carry one or more of the screw threaded sockets, and which at their other ends are provided with spaced feet and are interconnected with one another.

The essentially skeletal structure of the legs provides a body portion having an open framework which may be cheaper both in material and construction costs compared with the previous precast concrete body portions, and which is much lighter in weight so that the one body portion may be used as the upper body portion. This enables the stool to be pinned up by screwing the spindle out of the screw threaded socket of the upper body portion by application to the spindle of a torque in a clockwise direction as seen from above. Also, if a non-circular section is provided adjacent to the end of the spindle, the torque can be applied to the spindle by applying a spanner to the non-circular spindle section whilst resting the spanner on the end of the screw threaded socket or other part of the lower body portion.

The open framework of the body portion also provides greater flexibility for fitting reinforcing rods to the body portion, or passing through the body portion.

The legs and other load bearing parts of the body portion may be made of metal such as aluminium or galvanised iron but preferably steel, or of reinforced plastics or other material. The choice will be a matter of production economy subject to compatability with concrete.

The legs of the body portion may be made of rod, tube, strip, angle or other structural section. The feet may be formed integrally with the legs or may be separate parts such as plates or cast parts which are secured to or fit with the legs. In the latter case the feet may be formed with sockets in which the ends of the legs, made for example of rod or tube, are received. Similarly the screw threaded socket or sockets may be formed in a part which is secured to the legs, or which is formed with other sockets to receive the ends of the legs remote from the feet.

The interconnection of the leg ends may be for example by means of rods or other elongate elements of structural section which are secured or otherwise connected to the feet, or to the ends adjacent to the feet of the legs. The securing of the various components together will depend upon the material from which the components are made but pressing, adhesive bonding, or welding, are possibilities.

The one body portion may be associated with two, three, four or more of the spindles depending upon the load to be carried and structural parameters. For example if there is a danger of the wall above the beam toppling over while temporarily supported on a row of stools each with a single spindle, it may be necessary to provide each stool with two spindles spaced from one another across the thickness of the wall to provide a more stable support. However in most cases it will be sufficient for each stool to be associated with a single central spindle. In any case, irrespective of the number of spindles, the legs of the one body portion are preferably inclined so that at their one ends they are adjacent to the, or a respective, screw threaded socket whilst their feet are splayed apart.The inclination of the legs provides lateral restraint for the one body portion and causes the concentrated load at the screw threaded socket to be distributed to the spaced feet and hence more evenly into the wall above or below. This is particularly important when the stool is used in a cavity wall and the axis of a single spindle is in alignment with the cavity.

The feet of the one body portion, provided that they are adequately interconnected may rest in engagement with the wall above or below. Preferably, however, an interposed steel or concrete plate is used, the plate effectively forming a part of the one body portion and either being fixed or separate from the feet. When the plate is a metal plate such as a steel plate, the feet will normally be permanently welded or otherwise fixed to it. Such a steel plate need only be some 3/8 inch thick, possible with a 1/2 inch thick steel strip extending across its centre when used across a cavity wall. This is to be compared favourably with a comparable concrete plate of some 21/2 inch thickness.Bearing in mind that this saving can be made at both upper and lower body portions of the stool, the use of a metal plate rather than a concrete plate can lead to a reduction in the number of courses of brickwork which have to be removed, or can lead to a thicker and therefore stronger beam for the same number of courses removed. However, the use of a metal plate introduces the possibility of corrosion particularly when used as part of an upper body portion extending across a cavity. This can be mitigated to some extent by laying a piece of brick in mortar on top of the metal plate in the cavity after the stool has been tightened up and while access still exists to the cavity above the stool.

On the other hand the use of a concrete casting or plate as part of the one body portion of the new stool may involve less complexity and cost than concrete plates used with our previous precast concrete body portions. This is because the previous concrete plates had to incorporate complex reinforcement to spread the essentially centrally applied load outwards towards the edges of the concrete plate, particularly to distribute the load onto the separate leaves of a cavity wall. The legs of the new body portion particularly when splayed, already transmit the load to the concrete plate via the spaced feet away from its centre so that any reinforcement of the plate can be much simpler.

When a concrete plate is used, the feet may be fixed to reinforcement in or projecting from the concrete plate. Alternatively the feet may be separate from the plate and, if necessary, located relatively to the plate by a recessed or raised portion or portions of the concrete plate, to provide lateral restraint whilst the stool is being tightened into the wall. As the concrete plate is normally approximately the same width as the wall being supported and is placed centrally in the wall then the location of the stool to the concrete plate will also automatically locate the stool in relation to the wall.

The construction involving a separate framework with splayed legs has the advantage that a number of the frameworks can be nested together for storage or transport. Also with this construction the feet need only be interconnected in the sense to prevent the legs from spreading further apart and this can be achieved by providing a stirrup which surrounds the feet or adjacent ends of the legs. Thus when there are three legs, the stirrup will conveniently be triangular, and when there are four legs rectangular.

The stirrup may be separate from the legs and feet and be located in recesses in the outwardly facing sides of the legs or feet.

It is sufficient if the body portion has three legs as this provides lateral restraintforthe screw threaded socket and the three feet will adequately distribute the load onto a wall. If the triangle, at the apices of which the feet are positioned, is substantially an isosceles triangle with equal base angles of greater than 60 , the stool will have a longer horizontal dimension and the stool can be oriented with this longer dimension either transverse to a cavity wall with the legs straddling the cavity, or along a solid wall.

For certain uses, two inclined legs may be sufficient, although they are preferably made of strip metal or light metal sections with the plane of the strip extending perpendicularly to the plane containing the two legs thereby providing a beam effect against bending of the legs out of their plane. The feet at the splayed ends of the legs are then preferably welded or otherwise secured to a base plate. An advantage of this construction is that if the stool is used with the plane containing the legs aligned with the central longitudinal plane of the wall, a minimum part of the width of the wall is obscured to reinforcing rods extending outside the body portion along the beam and a concrete covering up to a third of the beam thickness or more can be provided over the reinforcement.

The base plate for this stool may have to be reinforced by a strip extending centrally of the base plate but perpendicularly to the plane containing the legs, if the stool is to be used in a cavity wall. This could be avoided however if the stool were used with the plane containing its two legs extending transversely of the cavity wall. There would still be room for the reinforcing rods with a very significant concrete cover. In this construction of stool, the screw threaded socket may be secured to the end of a vertical tube the other end of which is secured to the base plate. The inclined legs then form effectively buttresses for the tube.

This use of the tube, which is also applicable to body portions with three or more legs, will house the portion of the screw threaded spindle which has screwed through the screw threaded socket. The tube will provide some additional structural strength although at some extra cost. The tube will not usually be appropriate when a skeletal body portion framework is to be nested with othersimilarframe- works.

When the spindle is screwed outwardly through the screw threaded socket to extend the stool, there is a danger that the spindle will be screwed so far through the socket that insufficient of the trailing end of the spindle remains in engagement with the socket, leading to the possibility ofthe stool collapsing. It is unreliable to apply to the trailing part of the spindle a brightly coloured paint to warn the operator when a painted portion of the spindle appears out of the socket, as the paint is liable to be rubbed off or become obscured by dirt of grease. When the above mentioned tube is used to house the spindle, this may also screen the end of the spindle from the operator's view.

In order to overcome this difficulty, in accordance with an independent feature of the invention, in a stool of the kind described, a portion of the spindle, which trails in the direction in which the spindle is screwed out through the screw threaded socket to extend the stool, is unthreaded and unable to pass through the screw threaded socket.

With this arrangement screwing out of the spindle to extend the stool is positively limited whilst there is still a predetermined length of the spindle to the side of the screw threaded socket remote from the other body portion.

The other body portion may be a metal or concrete plate having a part complementary to the end of the spindle, such as a socket to receive the end of the spindle, or a stub axle for engagement with a socket carried by the end of the spindle. However, the other body portion may be of a more complex construction and may be similarly constructed to the one body portion. The use of a simpler or more complex other body portion provides versitility when beams of different thickness may be required.When the other body portion is constructed similarly to the one body portion, the screw threaded socket will be replaced by a part complementary to the end of the spindle, for example a cup which may be fitted to the body portion in place of the screw threaded socket, or a screw threaded stub axle which screws into the screw threaded socket of the other body portion and has a projecting end for engagement with a socket carried at the end of the spindle.

The means for rotating the spindle may be a standard nut screwed and welded onto the spindle.

When the one body portion is to be the upper body portion, the nut will then be welded onto the spindle adjacent to its projecting end for the reasons previously described. Alternatively, the spindle may be of overall non-circular, e.g. hexagonal, section, with a length screw threaded for engagement with the screw threaded socket.

The load bearing capacity of the stool depends upon the security of the screw threaded coupling between the spindle and screw threaded socket. We have proposed that the screw threaded socket shall be a standard nut which is fabricated with the one body portion. If it should be considered that the safety margin provided by these interengaging screw threads is insufficient in some circumstances, additional security can be provided by means of a lock nut on the spindle, the lock nut being tightened up against the end of tbe screw threaded socket once the stool has been tightened up. If for example the lock nut is similar to that forming the screw threaded socket, the security of the coupling will be effectively doubled.The lock nut might of course also be used as the means for rotating the spindle if some means, such as a cotter pin, is provided for temporarily locking the nut against rotation at one position on the spindle for rotating the spindle, after which the lock nut is released and screwed along the spindle into engagement with the end of the socket.

Two examples of stools constructed in accordance with the present invention are illustrated in the accompanying drawings, in which Figure 1 is a perspective view of one stool; Figure 2 is a vertical transverse section through a wall showing the stool in use; and, Figures 3 and 4 are views similar to Figures 1 and 2 but showing a second stool.

The stool of Figures 1 and 2 has a first body portion assembled from a central steel casting 5 having three sockets 6 in which three legs 7,7 and 8 are push fitted. The legs 7 are formed from 12.5 mm.

diameter steel rod and the leg 8 from 16 mm.

diameter steel rod. At their other ends the legs are fitted with feet 10 each of which is a steel casting formed with a socket 9 in which the respective leg end is push fitted. The legs 7 and 8 are splayed with the thicker leg 8 at a lesser angle to the vertical than the thinner legs 7. Consequently the feet 10 are centred at the corners of an isosceles triangle with internal angles of 66 at the ends of the legs 7 and 48 at the end of the leg 8. As will be apparent from Figure 2, when the stool is used in a cavity wall, the feet at the ends of the legs 7 are intended to be in alignment with one leaf of the wall and the foot at the end of the leg 8 in alignment with the other leaf.

Atriangular stirrup 11 made of 10 mm. diameter steel wire surrounds the feet 10 and is located in a recess 12 in the outwardly facing surface of each foot 10.

The central casting 5 is formed with a central throughgoing screw threaded socket 13 through which a screw threaded spindle 14 passes. An end 15 of the spindle 14 is unthreaded so that it cannot pass into the socket 13. A nut 16 is welded onto the spindle adjacent to its other end.

The other body portion of the stool is provided by a 121/2 mm. thick steel plate 17 to which is welded a circular steel socket 18 which is just large enough to receive the end of the spindle 14 so that the spindle can turn freely relatively to the socket 18.

Figure 2 shows the use of the stool built into a cavity wall having inner and outer leaves 19. The plate 17 is rested on a reinforced concrete plate 20 which in turn rests on the wall below. The first body portion has been coupled with the second body portion by engagement of the end of a spindle 14 in the socket 18 and with a reinforced concrete plate 21 resting on top of the feet 10. Lateral restraint is provided by three dimples 22 which are cast integrally on the surface of the plate 21 and mate with complementary recesses 23 in the feet 10.

The stool is tightened up by applying to the nut 16 a spanner and unscrewing the spindle 14 through the socket 13 so that a proportion of the load of the wall above is ultimately transmitted through the stool to the wall below. The screw thread on the spindle 14 is a conventional right handed screw thread and it will be appreciated that the operator can manipulate the spanner to tighten up the stool by resting the spanner on the top of the plate 17 with the jaws in engagement with the nut 16 and unscrew the spindle by applying to the nut 16 a clockwise torque as seen from above. It will be apparent that apart from the lateral restraint provided by the complementary dimples 22 and recesses 23, the stirrup 11 will prevent any tendancy of the legs 7 and 8 to splay further outwardly as the load is taken by the stool.

As is conventional, a number of the stools are successively inserted and tightened up, after cutting away the wall at horizontally spaced intervals. The wall then exhibits a horizontal slot containing a row of the stools which support the wall above from the wall below. Reinforcement, such as the rods 24 shown in Figure 2, together with any necessary stirrups, are located in position, for example by wiring to the stools. As is apparent from Figure 2, the essentially open tetrahedral skeletal structure of the stools enables some of the rod to pass through the stools and others of the rods to pass closer to the centre line of the stool than would be possible with a solid stool. Formwork is then erected in alignment with the external surfaces of the wall and a concrete beam 25 is cast incorporating the reinforcement and stools.Between the plates 21, the wall above may be pinned up from the top of the cast beam 25 by packing in conventional earth dry mortar in the gap 26.

In Figure 2 the plate 20 is seen to be resting on a bed of morter 27 which also fills the cavity between the leaves 19. This is possible when the beam is a low level beam in which case there is no disadvantage in filling the cavity between the lowermost few courses of brickwork. However, if the beam is to be a high level beam, for example a lintel beam, it may be necessary to leave the lower cavity open. In that case the plate 20 must be made strong enough to transmit, without breaking, the substantially centrally applied reaction from the stool to the two leaves of the cavity wall. Alternatively, if there is sufficient height available, the second body portion consisting of the plate 17 and socket 18, may be replaced buy a skeletal body portion similar to the first body portion although inverted. The only necessary modification would be to provide one of the spindles 14 with a socket similar to the socket 18 for receiving the end of the other spindle. With such an arrangement the load would be spread by the splayed feet into alignment with the leaves of the cavity wall above and below.

The second example of Figures 3 and 4 is a modification of the first example and analogous parts are given the same reference numerals with the suffix A whilst a substantially identical parts are given the same reference numerals.

The stool of Figures 3 and 4 has a first body portion, which, unlike that of the Figure 1 and 2 example, is a rigid fabrication of steel parts welded together. Thus the central steel casting 5 is replaced by a hexagonal nut 5A to which are welded legs 7A and 8A of 12.5 mm. and 16 mm. diameter steel rod respectively and 141 and 118 mm. long respectively.

The feet 10 are replaced bytriangularfeet 10A formed of 12.5 mm. thick steel plate welded to the respective feet and interconnected by three lengths of 10 mm. diameter steel rod 11A welded to the feet and legs and providing a rigid interconnection in place of the stirrup 11.

Instead of being welded on the spindle 14, the nut 16A is temporarily securable to the spindle 14A against rotation by means of a cotter pin 28 which is insertable through alignable diametrical passageways in the nut and spindle. The stool is utilised exactly as described with reference to the Figure 1 and 2 example, except that after the stool has been tightened up, additional security may be provided by removing the cotter pin 28 and screwing the nut 1 6A up into abutment with the underside of the nut 5A.

Claims (17)

1. A stool of the kind described, wherein the one body portion comprises a skeletal structure of a plurality of legs which at their one ends carry one or more of the screw threaded sockets, and which at their other ends are provided with spaced feet and are interconnected with one another.

2. A stool according to claim 1, in which the legs are inclined and are splayed outwards relatively to one another from the screw threaded socket or sockets towards the feet.

3. A stool according to claim 2, in which there are three legs.

4. A stool according to claim 3, in which the triangle, on the apices of which the feet are positioned, is substantially an isosceles triangle with equal base angles of greater than 60 .

5. A stool according to any one of claims 2 to 4, in which the ends of the legs are interconnected by a surrounding stirrup which is located in recesses in the outwardly facing surfaces of the ends of the legs or feet.

6. A stool according to any one of the preceding claims, in which the one body portion is provided with a concrete plate which is, in use, interposed between the feet and the wall.

7. A stool according to claim 6, in which the feet are located, in use, relatively to the concrete plate by means of a recessed or raised portion or portions of the plate.

8. A stool according to any one of the preceding claims, in which the legs are steel rods or tubes.

9. A stool according to any one of the preceding claims, in which the feet have sockets to receive the ends of the legs.

10. A stool according to any one of the preceding claims, in which the screw threaded socket or sockets is or are formed in a part formed with other sockets to receive the ends of the legs remote from the feet.

11. A stool according to any one of the preceding claims, in which a lock nut is provided on the spindle for tightening againstthe one body portion afterthe stool has been tightened up in use.

12. A stool accordina to claim 11. in which the lock nut is arranged to be fixed temporarily to the spindle and to act as the means for rotating the spindle.

13. A stool according to any one of the preceding claims, in which the other body portion is formed, similarly to the one body portion, as a skeletal structure with, instead of the screw threaded socket, a part complementary to the end of the spindle.

14. A stool of the kind described, wherein a portion of the spindle, which trails in the direction in which the spindle is screwed out through the screw threaded socket to extend the stool, is unthreaded and unable to pass through the screw threaded socket.

15. A stool according to claim 1, substantially as described with reference to any one of the examples illustrated in the accompanying drawings.

16. A method of constructing an in situ cast beam in the plane of a building wall, the method comprising cutting away the wall and inserting, at horizontally spaced positions, stools according to any one of the preceding claims, tightening up the stools to support the wall above and, with the intervening wall between the stools cut away, casting a reinforced concrete beam incorporating the stools.

17. A method according to claim 16, in which the stools are used with their one body portions uppermost.