US3034536A

US3034536A - Prestressed concrete pipes

Info

Publication number: US3034536A
Application number: US716135A
Authority: US
Inventors: Hugh F Kennison
Original assignee: Lock Joint Pipe Co
Current assignee: Lock Joint Pipe Co
Priority date: 1958-02-19
Filing date: 1958-02-19
Publication date: 1962-05-15
Anticipated expiration: 1979-05-15
Also published as: GB865496A; DE1159221B

Description

May 15, 1962 H. F. KENNISON PRESTRESSED CONCRETE PIPES I'll l'lll 2 Sheejzs-Sheet 1 M/l/ENTOE HUGH F: KEN/V/SO/V A TI'OR/VEY Filed Feb. 19, 1958 May 15, 1962 H. F. KENNISON 3,034,536

PRESTRESSED CONCRETE PIPES Filed Feb. 19, 1958 2 Sheets-Sheet 2 United States Patent 3,034,536 PRESTRESSED CONCRETE PIPES Hugh F. Kennison, Essex Fells, N.J., assignor to Lock Joint Pipe Company, East Orange, N.J., a corporation of New Jersey Filed Feb. 19, 1958, Ser. No. 716,135 5 Claims. (Cl. 138176) This invention relates to the manufacture of prestressed concrete, and more particularly to pipes, tubes, conduits and other tubular bodies of originally plastic and subsequently hardened materials for confining fluids and in which the fluid-confining conduit portion thereof comprises a hollow monolithic core or barrel which is preloaded in compression by a prestressed wire wrapping around the outside of the core and by a plurality of longitudinally extending prestressed wires embedded in and reacting on the core.

In prestressed tubular bodies of this character, such as concrete pipe, the force of the tensioned wire wrapping is a function of the hydrostatic head to which the pipe is to be subjected when in use. The prestressed longitudinal reinforcing inhibits cracking of the concrete which otherwise would occur because of the shear and bending stresses effected by the tensioned wire wrapping. The longitudinal reinforcing also supplies the beam strength for the bending resistance required during manufacture and for resisting live and dead loads that may be placed on the pipe when it is in use. Because of the high tensile stresses involved (upwards of 120,000 pounds per square inch, for example), the prestressing technique requires that high tensile steel be used. Since wires of this material are costly it is desirable that full use be made of their strength in order to avoid an uneconomical construction.

The compressive stress in the concrete in the lengthwise direction of a pipe should be sufiiciently high to prevent cracking of the concrete in zones which are subjected to the most severe conditions of shear and o bending stress. These zones of potential cracking are located in the general vicinity of the transverse planes in which the tensioned wire winding terminates and also inward from the open end of the bell of a bell and spigot pipe where an abrupt change in the wall thickness of the pipe occurs.

The prestressed longitudinal wires usually extend into or through the zones of potential cracking and are available to exert the amount of compression of the concrete necessary to prevent cracking, but an excessive amount of steel must be used unless the ends of the wires are adequately anchored. This is because the longitudinal compression imparted by a prestressed wire embedded in concrete is less along a limited distance from an unanchored end of the wire than the maximum compression reached further inwards along the Wire where the bond between the wire and the concrete has built up to its maximum value. Owing to this phenomenon more steel is called for to effect the degree of compression required in the ends of a pipe than is necessary for attaining the compression required along the mid-length of the pipe unless the ends of the wires are positively fixed as by attachment to steel or iron rings supported on the end faces of the pipe.

If ring anchorages located externally of the concrete are not to be employed, more longitudinal steel (heavier wires or a greater number of wires) is indicated to be necessary than should be required to provide the proper amount of compression to prevent cracking in the ends of the pipes where the shear and bending stress are severest. It is a principal purpose of the invention to avoid the use of external anchorages for the longitudinal wires and to enable more efiicient use to be made of the tensile capacity of longitudinal wires for effecting the degree of compression required in the ends of prestressed pipes and to thereby accomplish greater economy in the manufacture of such pipes. It is also a purpose to provide adequate end anchorages which are embedded in and protected by the concrete and to thereby attain equally strong pipe with less longitudinal steel, or a pipe capable of withstanding a higher working pressure with the same quantity of longitudinal steel.

The improved stress conditions and savings sought by the present invention are attained by means for anchoring the ends of the longitudinal wires in a manner to establish an optimum condition of compressive stress of the concrete in the ends of a prestressed concrete pipe. In its more general form the anchorage comprises a coil or loop in a wire which is formed by curling the wire over a pin before the wire is tensioned. The wire is curled over the pin at at point along the wire which is to be 0- cated as close to the end of a completed pipe as is practical. Similar anchorages are made in each wire for both ends of a pipe. One or more wires may be curled over a single pin or all of the wires may be curled over the band of a ring, and the wires are tensioned either singly or in groups, depending upon the number of wires which are curled around a common member or pin at either end of the mould in which the concrete core is to be cast.

When a plurality of pins are employed at one end of a mould, the pins are of such length as to subtend equal arcs without overlapping one another circumferentially around the mould. Particularly when the invention is to be utilized in making pipes for withstanding very high working pressures a continuous hoop may be included as apart of the anchoring means for increasing the area of stress distribution and thereby reducing the stress concentration in the concrete.

The ends of a wire running from a coil extend tangentially from the encircled pin in opposite directionsias the wires are tensioned around the pins by'forces apaplied to their opposite ends while the wires are in their proper positions in a mould. The pins and the coils may be located outwardly from the circle of wires or inwardly from the circle of wires, as desired, and a wire may be curled more than once around an engaged pin. A coil forms an effective hook for added purchase on the concrete and it supplies an additional length of the wire in the shorter lineal distance from the beginning of the coil to the nearest end of the wire and so greatly improves the development of the bond between the wire and the concrete. The encircled pins enlarge the anchorage and distribute the pull of the wires over a greater area of the concrete.

Greatly improved results have ensued in pipes which are finished with spigots having exterior jointing surfaces and in which the wire wrapping does not overlay that portion of the pipe which has the jointing surfaces. Similar results are gained in the bells of pipes which are formed integrally with the pipe barrel and are therefore subject to very high shearing stresses due to abrupt changes in the cross-section of the pipes. The resultant maximum tension in the longitudinal wires is made effective to produce optimum conditions of compressive stress throughout the areas of the concrete adjacent the pipe ends.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself,

- as to its objects and advantages, and the manner in which it may be carried out, may be better understood by referring to the following description taken in connection with the accompanying drawing forming a part hereof, in which:

core.

FIG. 1 is illustrative of a pipe embodying the principle of the invention;

' of the pipe of FIG. 1;

FIG. 4 is a transverse section on line 4-4 of FIG. 5;

FIG. 5 is a plan of a pipe end with the concrete broken away to expose and thereby illustrate an anchoring arrangement for longitudinal wires;

FIG. 6 is a plan of the. same nat re as that of FIG. 5 but showing the coils on the wires encircling a hoop or ring;

FIG. 7 is a longitudinal sectional view of a portion of the spigot of a pipe in which the anchorage includes only a pin or a single ring;

FIGS. 8 and 9 are views at right angles to one another of a modification by which a wire is tied to a single pin;

FIGS. 10 and 11 show another modification by which a wire is tied to a single pin in another form;

FIGS. 12 and 13 show still another modification in which a wire is provided with two coils in engagement with a single pin; 5

FIG. 14 diagrammatically represents a portion of a V mould with part of its outside wall broken away;

FIG. 15 is a detail of an end of a mould.

The pipe illustrated in the accompanying drawing includes a' main conduit portion 10 consisting of a hollow monolithic concrete core which extends for the full length of the pipe and includes a plurality of prestressed wires 11 embedded therein in bonded relationship to the concrete.

The core 10 is subjected to circumferential compression by a highly stressed wire 12 of high tensile steel which is wound around the core and secured in place. In a finished pipe the wire winding is usually protected by a self-hardening covering 13 of suitable thickness, such as cement mortar, but the covering is not essential to, the use of the present invention.

The ends of the core are finished to provide appropriate jointing surfaces for making joints with other pipes. One end of the pipe illustrated in FIG. 1 is in the form of a socket or ball 14, which is complementary to the spigot 15 at the other end, and it is to be understood that the invention is applicable to pipes having both ends finished as spigots or as bells, depending on the form of the pipe desired. A steel hoop 16 (FIG. 2) engages the Wires 11 at a section from which they run into the enlarged end or bell of the pipe. As is well understood, the-bell 14 is provided with an inner cylindrical surface 17 which surrounds a spigot of another pipe in making a joint and engages a gasket by which the joint is sealed.

The spigot end has a relatively nan'ow circular surface 18 (FIG. 3) between the end face 19 and a circumferential groove 20. The groove is formed in part by the concrete of the core and in part by the covering 13, al-.,

though thegroove may be formed entirely in the concrete of the core in other forms of pipes without detracting from. the effectiveness of the present invention. In the pipe illustrated. in the drawing, the jointing surfaces on the spigot include the circular surface 18, the surfaces defining the groove 20 and a cylindrical surface 21 on the protective coating, and the wire winding 12 extends well into the spigot of the pipe.

The longitudinal wires 11 are distributed at equal intervals in tubular arrangement around the core in such number and under such stress as to exert the amount of longitudinal compression required on the concrete of the Wires of high tensile steel are used.

Referring to FIG. 3, one end of each wire 11 extends into the spigot portion of the core and is curled around a steel pin 22. disposed transversely of the wire. The pins for the several wires are preferably made from round stock. The curled end of the wire forms a coil 23 which together with the pin provides a relatively extensive area normal to the longitudinal direction of the wire. The other end of each wire is curled around a similar pin 24 (FIG. 2) and forms a coil 25 close to the end face 26 of the bell. The pins are of sufficient diameter and hardness to resist shearing by the wires as, the wires are initially tensioned in the mould before the concrete is cast. For a quarter-inch wire a pin having a diameter of fiveeighths of an inch has proven satisfactory.

In pipes designed to contain very high working pressures, steel hoops or rings 27 and 28 are located in abutting relationship to the coils at either end of the core in order to increase the area of stress distribution and reduce the stress concentration on the concrete, but these rings may be omitted in pipes designed to contain lower working pressures.

A form of anchorage including only a coil 29 and a pin 30 is shown in FIG. 7.

As more clearly shown in FIGS. 4 and 5, two of the longitudinally extending wires 11 are engaged 'with a single pin 22 at an end of the core. While the wires of each pair of wires are curled around a common pin in a manner to provide coils of opposite pitch or hand, the two coils may be formed with the same pitch. The pins are preferably of such length as not to interfere with one another when the respective pairs of wires are stretched as they are stressed in tension, but each pin is sufiiciently long to effect uniform compression of the concrete.

As an alternative arrangement, the several longitudinal wires 32 (FIG. 6) may be coiled around a single hoop or ring 33 extending entirely around the core. If the anchorages at both ends of a mould are formed in this manner all of the wires should be stretched and tensioned at one time, but they can be separately tensioned when the pulled ends of the wires are individually curled around different pin members.

In FIGS. 8 and 9 there is shown a form of anchorage for a single wire. The anchorage includes a generally L-shaped pin 34 having one leg 35 extending alongside of a portion of a wire 36 which runs longitudinally of a pipe and a leg 37 which is engaged by a coil 38 in the wire. The location and function of the leg 37 in a pipe corresponds with that of a pin 22 or 24 as shown in FIGS. 2 and 3. The leg 35 is secured to the wire by a lashing 39. The lashing prevents tipping or rotation of the coil when tension is applied to the opposite ends of the wire.

The modification illustrated in FIGS. 10 and 11 includes a generally U-shaped pin 40 with the legs of the pin extending across a wire 41 The wire passes through an opening 42 in the leg 43 and the leg 44 isengaged by a coil 45.

In the anchorage illustrated in FIGS. 12 and 13, a wire 46 is curled around two

legs

47 and 48 of a U-shaped member 49. The wire forms a coil 50 around the leg 47 and a coil 51 around the leg 48. V

The coils are placed in a wire before the wire is tensioned by curling the wire around the pin members that are to be used or around pins of'the same diameter at such a distance apart as to locate the coils close to the ends of the core when the wires have been tensioned in the amount required. The wires. with the pins extending through the coils are placed in a mould.

A manner in which a pair of wires may be mounted in a mould is exemplified in FIG. 14. The mould there shown includes a mould form 53 and removable mould ends 54 and 5 5 which abut the ends of the form and are removable therefrom. At one end of the mould the wires 56 pass through openings in the mould end 55 and are secured in place by clamping wedges 57 in such a manner as to locate the coils 58 and the pin 59 close to the transverse moulding surface 60. The. other ends 61 of the wires extendthrough openings in the mould end 54 and are held in place by clampingwedges 62 after the required amount of tension has been applied thereto by a tensioning device 63. The tension in the wires efiects a very tight engagement with the

pins

59 and 64 at either end of the mould.

A section through the spigot end of a suitable pipe mould is illustrated in FIG. 15. The mould includes a split cylindrical shell 67 and a removable mould end ring 65 which axially abuts the mould shell at the circular surface 68. While a stationary mould may be used, I prefer to mould a hollow concrete core in a mould adapted for use in a roller suspension machine or in a centrifugal machine.

When rings such as the rings 16, 27 and 28 (FIGS. 2 and 3) are to be employed they are placed around the wires before the wires are tensioned. These rings are held in their respective positions during the moulding of the concrete by any suitable means such as light wire lashings.

After the wires have been tensioned and secured, the mould is rotated and filled with a rich concrete mix. The concrete becomes bonded to the wires throughout their lengths as the hardening process develops. After the concrete has thoroughly hardened, the wire-clamping devices and the mould are removed and the reaction to the tensioned wires which was previously provided by the mould is transferred to the concrete. The ends of the wires extending from the end faces of the concrete core are severed close to the end faces of the core.

As a consequence of the invention, savings in cost are effected because a less quantity of steel is required to be used in the longitudinal wires than is required for a pipe of like design and of the same strength but in which the ends of the wires are less securely anchored.

A higher pressure classification is produced for pipes having any given core-thickness and size and number of longitudinal wires as a result of the elimination of damaging cracks in the concrete.

The foregoing is illustrative of the invention and it is self-evident that various changes may be made in the details of construction without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a prestressed pipe, a monolithic concrete conduit having a spigot at one end and gasket-bearing surfaces formed from the concrete of said spigot, a plurality of longitudinally disposed pretensioned high tensile steel wires embedded in the conduit at intervals around said conduit and extending substantially the entire length of the conduit, each wire uniformly stretched between an anchor adjacent each end of the wire, the ends of said longitudinally disposed wires at the spigot end of the conduit terminating adjacent the free end of the spigot, a tensioned wrapping around said conduit subjecting said conduit to circumferential compression, said wrapping ending short of said gasket-bearing surfaces, each of said anchors on the ends of said wires adjacent the free end of the spigot comprising a pin member and a portion of one of said pretensioned Wires engaged with said pin member, said portion of said wire tensioned around said pin member in a tight coil disposed on an axis crosswise of said Wire and ended with a short straight length of the wire beyond the coil, said short length of wire extending towards the free end of said spigot in a direction substantimiy parallel to the length of the wire which continues from the other side of the coil, each coil and pin member being completely encased by the concrete of said conduit in bonding relationship therewith, whereby the tension of each of said pretensioned wires is carried into a coil and the coil together with the pin member distribute the pull of the Wire onto the immediate concrete of said spigot.

2. A prestressed pipe according to claim 1 wherein said coils of only two of said longitudinal wires engage a single one of said pin members.

3. A prestressed pipe according to claim 1 wherein said pin member is generally L-shaped and the coiled portion of a longitudinal wire is in engagement with one leg of said member, and holding means is provided to hold the other leg of said pin member substantially parallel to the wire.

4. A prestressed pipe according to claim 1 wherein said pin member is generally U-shaped and said coiled portion of a longitudinal wire is in engagement with one leg of the member and the wire is provided with another coiled portion in engagement with the other leg of said member.

5. A prestrcssed pipe according to claim 1, wherein said pin member is generally U-shaped and said coiled portion of a longitudinal Wire is in engagement with one leg of said member and said Wire extends through an opening in the other leg of said member.

References Cited in the file of this patent UNITED STATES PATENTS 468,269 Stempel Feb. 2, 1892 979,285 Gilligan Dec. 20, 1910 1,900,145 Whiting Mar. 7, 1933 2,236,167 Miller et al Mar. 25, 1941 2,662,555 Hirsh Dec. 15, 1953 2,686,951 Seaman Aug. 24, 1954 FOREIGN PATENTS 707,447 Great Britain Apr. 21, 1954