IE20190110A2 - Precast building element connector - Google Patents

Abstract

connector (4) for connecting a reinforced concrete building element such as a flight of steps (1) to another concrete element such as a landing (2), the connector comprising a rigid bar (12) with first and second ends, and defining a horizontal connection near the first end to connect to a reinforcing rod (26), and defines adjacent to the second end a horizontal through-hole to accommodate a restraining rod (14); a rigid faceplate (6) being attached to the rigid bar (12) at an intermediate position. The rigid bar (12) may be a square or rectangular section steel tube or bar, and it may also incorporate a vertical threaded through-hole, and a threaded bolt (18) that extends through the vertical threaded through- hole to act as a set screw. In use the end of the rigid bar (12) locates in an open-sided recess (20) in the other concrete element (2), the recess (20) incorporating a steel base plate (22) and two upward-projecting steel rods (24), so the rigid bar (12) extends between the steel rods (24), and so the restraining rod (14) is between the steel rods (24) and the closed face of the recess (20).

Description

Precast Building Element Connector The invention relates to a connector for connecting concrete elements together, such as a precast flight of steps to a concrete floor element such as a staircase landing, during building construction. In one aspect the invention relates to a connecting kit consisting of a connector for incorporation in a first concrete element in combination with a female element for incorporation into a second concrete element. in the construction of buildings that have a plurality of storeys it is known to create a concrete stairwell, to mount concrete landings or floor elements at desired heights within the stainivell, and then to connect precast flights of steps between successive landings. The connections between the flights of steps and the landings must clearly be able to withstand the forces imposed during normal use, but should also be able to withstand additional forces that may be experienced in the event of an accident, for example a gas explosion. The connections are Customarily made using metal brackets, with bolts, or using metal rods inserted into drilled holes into a concrete component. A simpler, stronger and adjustable connection would be desirable.

According to the present invention there is provided a connector for connecting two reinforced concrete building elements, the connector comprising a rigid bar with first and second ends, and that defines adjacent to the first end a connection to a generally horizontal transverse reinforcing rod, and defines adjacent to the second end a through hole which is transverse and in use is also generally horizontal and which is adapted to accommodate a restraining rod; a rigid faceplate being attached to the rigid bar at an intermediate position, and the rigid bar also incorporating a threaded through-hole which in use is vertical, between the faceplate and the second end, and a threaded bolt that extends through the threaded through- hole to act as a set screw.

This connector may be referred to as a male connector The rigid bar may be of square or rectangular cross-section, and may be a square section tube or a solid bar. The rigid bar and the faceplate may be of metal, such as steel.

The transverse thr0ugh-hole adjacent to the second end and which in use is generally horizontal, may be a threaded hole, and the restraining rod may also be threaded so that its position in the transverse through-hole can be adjusted without any risk of it falling out during construction.

The connector may also incorporate a saddle-shaped reinforcing rod fixed to the face of the faceplate closer to the first end, the saddle-shaped rod consisting of a U-shaped central portion whose arms project downwards on either side ofthe rigid bar, and end portions that extend parallel to the longitudinal axis of the rigid bar, away from the face of the faceplate to which the U-shaped portion of the reinforcing rod is fixed.

For example the connector may be used to connect a flight of steps to a concrete floor element. in use the connector would be incorporated in a mould in which the flight of steps is to be cast, such that the faceplate is at a surface of the mould (and so subsequently of the cast structure) and such that the second end of the rigid bar projects Out from the m0uld (and so subsequently from the cast structure). The flight of steps would include reinforcing rods (which may be called rebars). The connection adjacent to the first end of the rigid bar may be a horizontal through hole, through which one such reinforcing rod or rebar may be inserted.

Alternatively the connection adjacent to the first end of the rigid bar may be a permanent join, such as a weld, a reinforcing bar being welded to the rigid bar, so that the reinforcing bar and the rigid bar become unitary; in this case the reinforcing bar welded to the rigid bar may be a shorter length than other reinforcing rods or rebars used in forming the structure.

Preferably two such connectors would be provided at each end of a flight of steps.

There may be more than two connectors at an end of a flight of steps.

In use the restraining rod is inserted through the through hole at the second end of the connector that projects from a face of one concrete element, and the connector is arranged to locate into an open-sided recess in the other concrete element, the recess having a horizontal base from which two rigid rods project upwardly with a gap between them, such that the rigid bar can fit through the gap between the two rigid rods, the two rigid rods being opposite each other on either side of the longitudinal axis of the rigid bar, so that the restraining rod locates in the recess with the two rigid rods projecting up between the restraining rod and the faceplate, that is to say between the restraining rod and the open face of the recess. The rigid rods may also be metal, such as steel.

The open—sided recess may also be open-topped, so that the building element from which the connectors project, for example a flight of steps, can be lowered vertically into position with the connectors being lowered vertically into the open-sided recess.

Once in position, the rigid bar is supported by the base of the recess, so the weight of the flight of steps is carried by the base of the recess. The recess may incorporate a bearing plate, for example a steel plate, forming the base of the recess, to spread the load across the recess. The threaded bolt that extends through the threaded through-hole to act as a set screw may be used to adjust the exact height of the rigid bar, to ensure that the top or bottom of the flight of steps is accurately horizontal.

The projecting rigid rods in the open-sided recess may be the ends of reinforcing rods that extend through the concrete floor element. For example these may be L—shaped below the bearing plate that forms the base of the recess.

It will be appreciated that the open~sided recess and the projecting rigid rods may be formed using conventional casting techniques. Alternatively the male connector may be supplied in combination with a recess—forming insert, for example a plastic-walled box with a steel base plate with two reinforcing rods each of which has one end portion that projects through the base plate into the box, and which has another end portion outside the box; this recess-forming insert may be referred to as a female connector. The end portion outside the box may be L-shaped. This recess-forming insert would be incorporated into the mould from which the concrete floor element is to be cast. After the floor element has been cast, the t0p and side walls of the box would be removed to expose the recess.

Alternatively the projecting rigid rods may be short rods that may extend through the bearing plate, and which are welded to the bearing plate that forms the base of the recess, and may be thicker than conventional reinforcing bars; the strength of the projecting rigid rods may be further increased by welding together two or more such rods alongside each other.

Engagement of the bearing plate and the projecting rigid rods with the surrounding concrete may be enhanced by welding one or more rigid bars transversely to the underside ofthe bearing plate, these transverse rigid bars projecting beyond the sides of the bearing plate; and indeed by welding one or more horizontal rigid bars transversely to the projecting rigid rods at a position above the bearing plate, extending away from the gap between the rigid rods and so into the concrete. In addition one or more rigid rods may extend in a direction at least partly parallel to the longitudinal axis of the rigid bar (i.e. orthogonal to a line between the centres of the projecting rods), being welded to the underside of the bearing plate, so in use they are embedded in the concrete. These may comprise a single rigid bar that follows a generally U- shaped path, or a path that forms three sides of a rectangle, the ends of the U being welded to the underside of the bearing plate, or alternatively they may be separate rigid bars, and they may extend in straight lines, or they may follow cranked paths, for example being shaped so they do not interfere with the positioning of other elements that are embedded in the concrete.

It will therefore be appreciated that an alternative recess-forming insert may for example comprise a plastic-walled box with a steel base plate with the rigid rods welded to it and extending inside the box, and with the additional rigid bars extending transversely and axially and welded to the steel base plate so they are outside the box. This recess-forming insert would be incorporated into the mould from which the concrete floor element is to be cast. After the floor element has been cast, the top and front walls of the box would be removed to expose the recess, with the projecting rigid rods exposed.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a schematic perspective view of a flight of steps linking two landings, the surrounding walls not being shown; Figure 2 shows a perSpective view of one end of a flight of steps that incorporates connectors of the invention, during its connection to a landing; Figure 3 shows the end of the flight of steps of figure 2, and the corresponding recesses of the landing of figure 2, the flight of steps and the landing being shown transparent; Figure 4 shows a perspective view of one end of a flight of steps that incorporate alternative connectors of the invention, which may be referred to as male connectors, and the landing to which the flight of steps is to be connected, the landing also incorporating female connectors of the invention, and both the flight of steps and the landing being shown transparent; and Figure 5 shows a perspective view of the features of figure 4 from a different orientation, in which both the flight of steps and the landing are again shown transparent..

Referring now to figure 1, a precast reinforced concrete flight of steps 1 is shown linking a first landing 2 and a second landing 2. Each of the landings 2 is attached to and supported by walls {not shown), and each landing 2 is of reinforced concrete, and may either be cast in situ, or precast and then connected to the walls. The flight of steps 1 may for example provide access between successive floors ofa multi-storey building, so that both ofthe landings 2 may be well above ground level. In constructing the building, the walls may be built first, the landings 2 then being mounted onto the walls, and then the flight of steps 1 being attached to the landings 2. Since the flights of steps 1 are of precast reinforced concrete, this operation requires use of a crane to lift the component into position.

Referring now to figure 2, a vertical end face 10 of the flight of steps 1 is shown, from which project the ends of two connectors 4. Each connector 4 includes a metal plate 6 which is embedded in the concrete so it is flush with the end face 10, and a square section steel tube 12 projecting from the end face 10. Near the exposed end of the square section tube 12 is a transverse hole, in which is placed a restraining rod 14, which is a short length of steel rod.

Between the metal plate 6 and the restraining rod 14 there is a threaded vertical hole through the steel tube 12 in which a bolt 18 is fitted, to act as a set screw or screw jack. The threaded vertical hole may be formed by flow drilling, to provide an increased length of thread to engage the bolt 18.

The adjacent edge of the landing 2 defines two rectangular recesses 20 which are open at the vertical edge face of the landing 2 and are also open at the top. At the base of each recess 20 is a flat steel plate 22 (shown in figure 3) and two steel rods 24 that project upward {both are visible in figure 3).

During construction, the flight of steps 1 is suspended by a crane, and is lowered into position. At both the ends of the flight of steps 1 there are projecting connectors 4, and the landings 2 at each end of the flight of steps 1 define corresponding rectangular recesses 20.

S When the flight of steps 1 is a short distance above the landings 2 at each end, an operator inserts the restraining rods 14 and adjusts the position of each restraining rod 14 to ensure it will not catch on the side of the recess 20; the operator may also adjust the bolts 18 so they project a short distance below the steel tubes 12. The crane then lowers the flight of steps 1 so that the projecting connectors 4 are within the recesses 20, the square section tubes 12 or the bolts 18 resting on the steel base plates 22, and with the two steel rods 24 in each recess 20 projecting up between the restraining rod 14 and the metal plate 6. The bolts 18 can then be adjusted to ensure that the flight of steps 1 is resting securely on all four projecting connectors , that is to say the two projecting connectors 4 at each end of the flight of steps 1.

Once any adjustments have been completed, the recesses 20 can be filled with grout, so the connector components are no longer visible and are no longer free to move. Under normal use the engagement between the projecting connectors 4 and the steel base plates 22 ensures the stability of the flight of steps 1. In the event of a severe accident which produces a horizontal force, the restraining rod 14 interacts with the two steel rods 24 on either side of the square section tube 12 to ensure that the connector 4 can’t be pulled horizontally out of the recess 20.

Referring now to figure 3, this shows adjacent faces of the flight of steps 1 and of the landing 2, with the concrete being shown transparent so that at least some of the internal components are visible. As regards each connector 4, the square section steel tube 12 extends at least as far into the flight of steps 1 as it projects out from it, and adjacent to the end of the steel tube 12 there is a transverse hole through which a steel reinforcing rod 26 can be inserted. Attached to the back of the metal plate 6 is a saddle-shaped reinforcing rod 28, that is to say a rod that has a U-sha ped central portion whose arms project downwards on either side of the steel tube 12, and has end portions 29 that extend parallel to the longitudinal axis of the steel tube 12, away from the face of the metal plate 6 to which the reinforcing rod 28 is fixed, and so further into the concrete; thus in side view the reinforcing rod 28 has an L-shape.

It will be appreciated that since the flight of steps 1 is of reinforced concrete construction, it will incorporate a multiplicity of steel reinforcing rods which are typically interlinked by wires where they cross over each other, so as to form a reinforcing cage. Only some of the reinforcing rods in the immediate vicinity of the connectors 4 are shown. The steel reinforcing rod 26 is part of the reinforcing cage. In addition a steel reinforcing rod 30 is arranged to extend transversely just above the L-shaped bends of the saddle-shaped reinforcing rods 28. This reinforcing rod 30 also forms part of the reinforcing cage. It will be appreciated that the connection between the square section steel tubes 12 and the steel reinforcing rod 26 (which is itself connected to the reinforcing cage} ensures that the connectors 4 are held securely into the concrete against any loads along the longitudinal axis of the metal tube 12, while the metal plates 6 and the end portions 29 of the saddle-shaped reinforcing rod 28 assist the connectors 4 in resisting vertical shear forces.

As regards the recesses 20 with the steel plates 22 and the upward-projecting steel rods , in this example the steel rods 24 are the end portions of reinforcing rods 32 that are embedded in the concrete of the landing 2, the reinforcing rods 32 being bent into an L-shape so the end portions 24 extend upwards. In addition the reinforcing rods 32 are linked by transverse reinforcing rods 34 and 35 which form part of a reinforcing cage for the concrete.

Since the landing 2 is of reinforced concrete construction, it incorporates a multiplicity of steel reinforcing rods which are typically interlinked by wires where they cross over each other to form the reinforcing cage, and only some of those in the immediate vicinity of the recesses 20 are shown.

The recesses 20 and the upward-projecting steel rods 24 may be formed using conventional casting techniques. Alternatively the connector 4 may be supplied in combination with a recess-forming insert, for example a plastic-walled box which includes the steel base plate 22 and the two reinforcing rods 32, each of which has one end portion that projects through the base plate into the box, to define the steel rod 24, and the remainder of the rod 32 being outside the box and being L-shaped. This recess-forming insert would be incorporated into the mould from which the concrete floor element is to be cast. After the floor element has been cast, the top and side walls of the box would be removed to expose the recess.

It will be appreciated that the connectors 4 and the recesses 20 are shown by way of example only, and that they can be modified in various ways while remaining within the scope of the claims. For example the rigid metal bar 12 may be of rectangular cross-section, rather than square; and may be a solid bar rather than a tube. Indeed it may be a non-metallic material, for example an engineering plastic or a composite material.

Referring now to figures 4 and 5 , these show two different perspective views of an end face 10 of a flight of steps 1 and the adjacent edge of the landing 2. As described above, both the flight of steps 1 and the landing 2 are of reinforced concrete, but are shown transparent in these figures so that the components that connect them together are more readily visible. The flight of steps 1 and the landing 2 are connected by connectors 40 and corresponding recesses 60; the connectors 40 may be referred to as male connectors, while the recesses 60 may be referred to as female connectors.

The vertical end face 10 of the flight of steps 1 is shown, from which project the ends of two connectors 40. Each connector 40 includes a metal plate 36 which is embedded in the concrete so it is flush with the end face 10, and a rectangular section steel bar 42 projecting from the end face 10. To the exposed end of the steel bar is welded a long steel nut 43, so there is a threaded transverse hole, in which is placed a threaded steel restraining rod 44. Between the metal plate 36 and the restraining rod 44 there is a threaded vertical hole through the steel bar 42 in which a bolt 48 is fitted, to act as a set screw or screw jack.

The adjacent edge of the landing 2 defines two rectangular recesses 60 which are open at the vertical edge face of the landing 2 and are also open at the top. At the base of each recess 60 is a flat steel plate 62 and two pairs of steel rods 64 that project upward, the rods 64 of each pair being welded to each other and welded to the steel plate 62, and the pairs being spaced apart to leave a gap between them; the gap is sufficiently wide for the steel bar 42 to pass through, while allowing for tolerance in the exact positioning of the recess 60 relative to the connector 40.

In a similar way to that described above, during construction, the flight of steps 1 is suspended by a crane, and is lowered into position. At both the ends of the flight of steps 1 there are projecting connectors 40, and the landings 2 at each end of the flight of steps 1 have corresponding recesses 60. When the flight of steps 1 is a short distance above the landings 2 at each end, an operator inserts the threaded restraining rods 44 into the steel nuts 43, if they haven't already been inserted, and adjusts the transverse position of each restraining rod 44 to ensure it will not catch on the side of the recess 60; the operator may aiso adjust the bolts 48 so they project a short distance below the steel bars 42. The crane then lowers the flight of steps 1 so that the projecting connectors 40 are within the recesses 60, the steel bars 42 or the bolts 48 resting on the steel base plates 62, and with the pairs of steel rods 64 in each recess 60 projecting up between the restraining rod 44 and the metal plate 36. The bolts 48 can then be adjusted to ensure that the flight of steps 1 is resting securely on all four projecting connectors , that is to say the two projecting connectors 40 at each end of the flight of steps 1.

Once any adjustments have been completed, the recesses 60 can be filled with grout, so the connector components are no longer visible and are no longer free to move. Under normal use the engagement between the projecting connectors 40 and the steel base plates 62 ensures the stability of the flight of steps 1. In the event of a severe accident which produces a horizontal force, the restraining rod 44 interacts with the two pairs of steel rods 64 on either side of the steel bar 42 to ensure that the connector 40 can’t be pulled horizontally out of the recess 60.

The above description refers only to the components which are visible in use; the following description describes the components that are embedded in the concrete during production of the flight of steps 1 and of the landing 2.

As regards each connector 40, the steel bar 42 extends at least as far into the flight of steps 1 as it projects out from it, and a short length of steel reinforcing rod 56 is welded to the embedded end of the steel bar 42 so it projects transversely. Immediately behind the metal plate 36 and welded to the underside of the steel bar 42 and to the back of the plate 36 is a steel element 58 in the form of an upside-down T, so as shown in figure 5 the side arms of the T project as horizontal flanges below the steel bar 42 on each side.

It will be appreciated that since the flight of steps 1 and the landing 2 are each of reinforced concrete construction, they each will incorporate a multiplicity of steel reinforcing rods (or ”rebars") which are typically interlinked by wires where they cross over each other, so as to form a reinforcing cage. None of these reinforcing rods are shown in figures 4 or 5.

However it will be appreciated that the connectors 40 may be linked to that cage, for example by interlinking the short lengths of steel reinforcing rod 56 to the cage.

As regards the recesses 60 with the steel plates 62 and the upward-projecting pairs of steel rods 64, in this example the steel rods 64 extend a similar distance below the steel plate 62 so the bottom ends are embedded in the concrete ofthe landing 2. In addition, as regards each recess 60, short steel rods 66 are welded to the pairs of steel rods 64 above the plate 62, towards the top of the steel rods 64, and extend transversely away from the gap so their ends are embedded in the concrete. Steel rods 68 or 68a are welded to the underside of the steel plate 62 and extend away from the edge of the landing 2 (in a direction parallel to the longitudinal axis of the connector 40}; these may be in the form of a rectangle, or at least three sides of a rectangle or U-shape, as shown at 68; or may be separate steel rods as shown at 68a.

In the latter case in particular, the rods 68a may be straight, or alternatively may be shaped for example to follow a cranked path (as shown) so as to leave an unobstructed space for another item that is to be embedded in the concrete. in either case the steel rods 68 or 68a may also be connected to the reinforcing rods that form the cage within the concrete.

In addition, in this example, a transverse steel rod 70 is welded to both pairs of steel rods 64, spaced below the plate 62, on the face further from the edge of the landing 2, extending the whole width of the recess 60 and projecting out on each side; and a second transverse steel rod 72 is welded to the underside of the steel plate 62 and also welded to the pairs of steel rods 64 at the face of the rods 64 closer to the edge of the landing 2, and which also projects out on each side. It will be appreciated that when installed, both these transverse steel rods 70 and 72 are embedded in the concrete; and that they may also be interlinked to the cage of reinforcing bars within the concrete.

The male connector 40 may be supplied in combination with a female connector in the form of a recess-forming insert, for example a plastic-walled box which includes the steel base plate 62 and the pairs of steel rods 64, each of which has one end portion that projects through the base plate 62 into the box. The downward—projecting parts ofthe rods 64, and the other steel rods 66, 68, 70 and 72 would be outside the box.This recess-forming insert would be incorporated into the mould from which the concrete floor element or landing 2 is to be cast.

After the floor element has been cast, at least the top and front wall of the box would be removed to expose the recess 60.

It will be appreciated that the connectors 40 and the recesses 60 are shown by way of example only, and that they can be modified in various ways while remaining within the scope of the claims. For example the steel bar 42 may have additional transverse steel rods or bars connected or welded to it in addition to the transverse steel rods 56 at the ends of the bars 42, or for example at an intermediate position along the length of the steel bar 42 there may be a projecting element to locate one of the reinforcing bars or rebars of the cage.

Claims (15)

Claims

1. A connector for connecting two reinforced concrete building elements, the connector comprising a rigid bar with first and second ends, and that defines adjacent to the first end a a connection to a generally horizontal transverse reinforcing rod, and defines adjacent to the second end a through hole which is transverse and which in use is generally horizontal and which is adapted to accommodate a restraining rod; a rigid faceplate being attached to the rigid bar at an intermediate position, and the rigid bar also incorporating a threaded through-hole which in use is vertical, between the faceplate and the second end, and a threaded bolt that extends through the threaded through-hole to act as a set screw.

2. A connector as claimed in claim 1 wherein the rigid bar is of square or rectangular cross- section.

3. A connector as claimed in claim 1 or claim 2 wherein the transverse through-hole adjacent to the second end, and which in use is generally horizontal, is a threaded hole, and the restraining rod is also be threaded.

4. A connector as claimed in any one of the preceding claims, also incorporating a saddle- shaped reinforcing rod fixed to the face of the faceplate closer to the first end, the saddle- shaped rod consisting of a U-shaped central portion whose arms project downwards on either side of the rigid bar, and end portions that extend parallel to the longitudinal axis of the rigid bar, away from the face of the faceplate to which the reinforcing rod is fixed.

5. A connector as claimed in any one of the preceding claims, wherein the rigid bar is of metal.

6. A connector as claimed in any one of the preceding claims wherein the connection adjacent to the first end of the rigid bar is a horizontal through hole, through which one such reinforcing rod or rebar may be inserted. 12

7. A connector as claimed in any one of claims 1 to 5 wherein the connection adjacent to the first end of the rigid bar is a permanent join so that the reinforcing bar and the rigid bar are unitary.

8. The use of a connector as claimed in claim 6 in the manufacture of a reinforced concrete building element, wherein the connector is mounted in a mould in which the building element is to be cast, such that the faceplate is at a surface of the mould and such that the second end of the rigid bar projects out from the mould, and also installing reinforcing rods in the mould, wherein one such reinforcing rod is inserted through the through hole adjacent to the first end of the rigid bar.

9. The use of a connector as claimed in claim 6 or claim 8 wherein at least two such connectors are provided at an end of the building element, and wherein the reinforcing rod is inserted through the through holes of each of the connectors at that end of the building element.

10. The use of a connector as claimed in any one of claims 1 to 7 to interconnect two reinforced concrete building elements, wherein one building element incorporates at least one connector with the faceplate at a surface of the building element and the second end of the rigid bar projecting out from the surface of the building element, and wherein the other building element defines at least one open-sided recess, the recess having a horizontal base from which two rigid rods project upwardly, such that the rigid bar can fit between the two metal rods, the two rigid rods being opposite each other on either side of the longitudinal axis of the rigid bar, so that the restraining rod locates in the recess with the two rigid rods projecting up between the restraining rod and the faceplate.

11. The use of a connector as claimed in claim 10 wherein the open-sided recess is open- topped, so the building element having the projecting connector can be lowered vertically into position with each connector locating into a corresponding recess.

12. A kit for enabling connections to be made between two reinforced concrete building elements, the kit comprising, in combination: - a male connector which is a connector as claimed in any one of claims 1 to 7; and 13 - a female connector comprising a rigid base plate from which two rigid rods project upwards with a gap between them such that the rigid bar of the connector can fit through the gap between the two rigid rods.

13. A kit as claimed in claim 12 wherein the two rigid rods of the female connector extend below the rigid base plate.

14. A kit as claimed in claim 12 or claim 13 wherein the female connector also comprises one or more of: - a transverse rigid bar or rod below the bearing plate, and welded to the projecting rods and/or to the rigid base plate; - a transverse rigid bar or rod that is welded to the projecting rods above the rigid base plate to extend beyond the width of the rigid base plate, and which does not extend across the gap; and — a rigid bar or rod that extends in a direction at least partly parallel to the longitudinal axis of the rigid bar, being welded to the underside of the rigid base plate, that extends along a generally U-shaped or rectangular path, or that extends in a straight line, or that follows a cranked path., for example being shaped so it does not interfere with the positioning of other elements that are embedded in the concrete.

15. A kit as claimed in claim 12 or claim 13 or claim 14 wherein the female connector also incorporates a plastic-walled recess-forming box, the box surrounding those parts of the projecting rigid rods that are above the rigid base plate. 14