US 3183011 A
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Description (OCR text may contain errors)
May 11, 1965 b. E.-OLIVIER 3,183,011
PRESTRESSED CONCRETE PIPE STRUCTURES Filed March 22. 1961 I 2 Sheets-Sheet 1 TOR.
F E. OLIVIER ATTORNEY y 1, 1965 'D. E. OLIVIER 3,183,011
PRESTRESSED CONCRETE PIPE STRUCTURES Filed March 22, 1961 v 2 Sheets-Sheet 2 r r i H 5 n g}: g 2; LL] 5 E -O' OO 0 IO 0 0 CD q- (I) LONGITUDINAT. BENDING TENSILE LONGITUDINAL BENDING TENSILE STRESS AS A PERCENT OF STRESS AS A PERCENT OF ULTIMATE STRENGTH ULTIMATE STRENGTH a s as E888S8 PERCENT OF CIRCUMFERENTIAL PERCENT 0 IRCUMFERENTIAL G COMPRESSION DESIGN COMPRESSION DESIG United States Patent 3,183,011 PRESTRESSED CONCRETE PIPE STRUCTURES Daniel E. Olivier, liedminster, NJ., assignor to International Pipe and Ceramics Corporation, a corporation of Delaware Filed Mar. 22, 1961, Ser. No. 97,534 4 Claims. (Cl. 277-237) This invention relates to prestressed concrete pipe structures.
Reinforced concrete pipes which are classified in the art as prestressed concrete pipes are characterized by a winding of tensioned wire around a concrete conduit or core of a pipe section. It is well known that as a tensioned wire is helically wrapped about a concrete conduit, bending and shearing stresses develop in the conduit. By mathematical analysis it can be shown that the magnitude of the longitudinal bending moments in the concrete vary at different points along the conduit as the winding progresses from an initial wrap or coil, and that the Varying patterns of the longitudinal bending moments are different for conduits with the initial wraps located at different distances from end edges or end faces of the several conduits. If a helical winding is started at the edge of a conduit and continued along the conduit, a temporary bending moment will cause a longitudinal bending stress of about 35% of the induced circumferential compression in the conduit. This bending stress is temporary and is entirely dissipated as the Winding continues along the length of the conduit. On the other hand, if the winding is started at a considerable distance in from the spigot edge, that is, in the order of about ten times the wall thickness of the conduit, a permanent bending stress of the order of magnitude of about 28% of the induced circumferential compression in the conduit will remain in the conduit near the position of the initial wrap, and the bending stress at the spigot edge will be practically nil. When the initial wrap is located near a joint-sealing element and therefore nearer to the end of a concrete conduit, permanent bending stresses remain in the vicinity of the spigot edge.
Serious problems due to bending and shearing stresses in a prestressed concrete conduit have been avoided in the past by embedding a steel cylinder within the conduit. The steel cylinder precludes leaks even though the concrete conduit is cracked as the result of temporary bending stresses which exceed the flexural strength of the concrete. In addition to the steel cylinder, the ends of the conduit were provided with steel joint rings which were welded to the cylinder and helped to stiffen the ends of the conduit. This type of construction is expensive. The present invention permits these costly elements of steel to be discarded without increasing the thickness of the wall of the conduit while still maintaining the high pressure capabilities of prestressed concrete pipes of the steel cylinder type.
For practical purposes, it has been common to locate the initial wrap of a wire winding at the shoulder of a spigot adapted to seat an O-ring sealing element between the initial wrap and the free end of the spigot. In such a case the initial wrap is beyond the mouth of a socket member when a joint is closed. With the initial wrap placed several inches from the end edge of a concrete conduit, bending stress due to the wire winding and the location of its initial wrap causes a permanent bending deformation which is manifested by a small outward flaring of the spigot and the appearance of cracks crosswise of the inner elements of the conduit if the bending stress exceeds the flexural strength of the concrete. These tension cracks can appear in the spigot and the immediately connecting portion of the barrel of the conduit while the main barrel or more central portion of the conduit is still capable of resisting additional circumferential prestress. Another consequence of positioning the initial wrap removed from the spigot edge is a reduction of circumferential compression in the barrel. The latter condition is manifested by premature longitudinal cracks in the conduit near the spigot when internal fluid pressure is applied. In effect, placing the initial wrap several inches in from the spigot edge of a conduit results in the establishment of a relatively weak portion of the barrel at and near the spigot portion both from the standpoint of circumferential cracks due to the bending and longitudinal cracks due to internal fluid pressure.
Cracking in an end portion of a prestressed conduit imposes limitations on the hydrostatic pressure to which a given conduit can be subjected. In some forms of prestressed concrete pipes, large amounts of longitudinals, tensioned to longitudinally compress the concrete, or an extraordinarily thick wall, or a combination of both, have been employed to prevent the formation of permanent cracks under the action of an amount of circumferential prestressing necessary to allow a given pipe to operate under pressures of usual magnitude found in water works operations.
An object of the invention is to compensate for the permanent bending stress described hereinabove and there by eliminate deleterious cracking in the vicinity of the spigot of a prestressed moulded concrete conduit without having to rely on an excess quantity of longitudinal internal reinforcing or on increased wall thickness and, at the same time, retain the attributes of a joint seal wherein the sealing element bears directly against the concrete of which the conduit is made. According to the invention, the varying temporary stress pattern in the conduit is modified to reduce the transient or temporary bending moments induced in the concrete as the tensioned wire of the main wire winding is wound onto the conduit by providing a supplemental tensioned wire wrapping of one or more wraps between the sealing element and the spigot edge or end face of the spigot, the supplemental winding also having the effect of offsetting the permanent bending moments which would occur but for the presence of the supplemental wrapping. As a consequence, the invention enables the production of a prestressed concrete conduit devoid of internal longitudinal reinforcement and having a relatively thin wall ranging in thickness from one-fourteenth to one-eighteenth of the inside diameter of the conduit, yet capable of withstanding hydrostatic pressures which formerly required the inclusion of internal longitudinal reinforcements, or a thicker wall.
Other objects, features and advantages of the invention will appear in the following description with refer v ence to the accompanying drawing which illustrates, by
way of example, a preferred embodiment of the invention.
In the drawing:
FIG. 1 illustrates a joint of pipe sections employing the invention, with parts broken away;
FIG. 2 is an enlarged plan showing the clamped and anchored ends of the prestressing wires of FIG. 1;
FIG. 3 is a section on line 3-3 of FIG. 2;
FIG. 4 is a graph of the temporary stresses in percent in a concrete conduit due to temporary longitudinal bending moments existing at the instant a helical winding of tensioned wire has arrived at a designated distance from an initial wrap or coil which is set back from an end edge of the conduit;
FIG. 5 is a graph of the temporary stresses in percent due to temporary longitudinal bending moments in a concrete conduit resulting from the combined effect of a supplemental prestressing wrapping and the helical wire winding of FIG. 4;
FIG. 6 is a graph of the percent of circumferential design compression in a concrete conduit caused by a helical winding of tensioned wire extending from an initial wrap set back from an end edge of the conduit and continuing to the other end of the conduit;
FIG. 7 is a graph of the percent of circumferential design compression resulting from the combined effect of a supplemental prestressin g wrapping and the helical winding of FIG. 6;
FIG. 8 is a profile of a modified spigot; and
FIG. 9 is a profile of another modified spigot.
The joint illustrated in FIG. 1 is of two similarly constructed pipe sections 10 and 11. Each section includes a concrete conduit 12 which is moulded in one piece as a hollow concrete core having complementary ends for making joints with other pipe sections.
The conduit 12 of the pipe section 10 has a socket member or bell portion 13 at one end, and a spigot at its other end which is similar to the spigot portion 14 of the pipe section 11. The spigot portion is that portion of a conduit which is inserted within and overlapped by a socket member of a joint. Each conduit 12 has a cylindrical inside surface 15. A cylindrical exterior surface 16 extends from the spigot portion, along the barrel portion of the conduit to the sloping surface of the bell.
The exterior of the spigot portion includes an annular flange 17 which is a moulded integral part of the conduit set back axially from the end face or edge of the conduit at the free end of the spigot. The flange has an outer peripheral surface 18, and a substantially cylindrical gasketsupporting surface 19 which is stepped radially inwardly from the surface 18 and radially outwardly from a substantially cylindrical surface 20 adjacent the free or outer end of the spigot. A gasket 21 seats upon the surface 19 and against the side surface 22 which extends laterally between the surfaces 18 and 19 of the flange and provides a side wall of an annular recess around the flange.
The conduit is formed in a mould which is carefully dimensioned to provide accurate surfaces for the profile of the spigot and for the interior cylindrical surface 23 of the bell. In moulding the conduit a plastic concrete mix is forcibly rammed, vibrated, or otherwise packed as by centrifugal action, to insure the production of smoothly turned surfaces.
The conduit 12 is circumferentially compressed by prestressing means which are wrapped around the conduit under tension. The prestressing means includes a tensioned wire 24 intermediate the stepped flange 17 and the free end of the spigot and another tensioned wire 25 extending from the other side of the flange to the bell end of the conduit.
The wire 24 is protected by a coating forming a collar 26 which provides a side wall 29 opposite from the side wall 22 of the annular recess in which the gasket is contained. The collar 26 is formed in place upon the end surface 20 of the conduit and against the laterally extending surface 28 of the stepped flange 17. The side of the collar which engages the surface 28 of the flange extends radially outwardly above the surface 19 on the flange to thereby provide a side wall 29 of an annular recess in which the gasket 21 is contained. The collar 26 may be formed of cement and suitable aggregates, or moulded from any other suitable plastic material which subsequently hardens into a solid member.
The joined bell and spigot portions provide an O-ring type of seal in which a gasket is forcibly confined in a groove in contact with the concrete of a prestressed conduit of one pipe section and the bell portion of another pipe section. In such a seal, a closed ring of natural or synthetic rubber or of any other suitable elastometric material having a resilience and sealing qualities of rubber may be used for the sealing element. The volume of the material in the gasket is such as to substantially fill the groove around the spigot when the gasket is forcibly deformed and contained within a joint.
By locating the gasket 21 axially inwards of the outer 4. perimeter of the flange (FIG. 1), the gasket is buttressed against extrusion from a joint by a solid integral portion of the conduit having a peripheral surface 18 which is accurately dimensioned to provide a free sliding fit within the interior cylindrical bearing surface 23 of the bell.
The tensioned wires 24 and 25 are applied under tension around the conduit after the concrete has thoroughly hardened. The wires are high tensile strength steel wires having the strength characteristics of a hard drawn, high carbon steel wire. The wires of windings in prestressed concrete pipes are customarily stressed to about 6070 percent of the ultimate strength (170,000-220,000 pounds per square inch) of the wires when applied and secured in place.
The ends of the tensioned wire wrapping 24 are tied together as in a hoop. Any suitable means may be employed for permanently connecting the ends together as, for example, a pair of sleeves 30, 31 as shown in the drawing. The sleeves are made of mild steel and are lined with sharp particles of a hard material which are frictionally seized upon the wire when the sleeves are compressed onto the wire.
The sleeves 30, 31 are initially located on the wire at points where they will be adjacent one another when the wire is tensioned and the wrapping is closed around the spigot. Tension may be applied by a hydraulic jack equipped for pulling on the two ends, or, if desired, one end may be held fast while tension is applied as the wire is wrapped around the spigot under tension. The encircling wrapping is closed when the attached sleeves are welded together, as indicated at 32.
The tensioned wire wrapping 24 prevents the development of a temporary bending stress in the spigot which in other circumstances would cause flaring of the spigot as the tensioned wire winding 25 is applied from the other side of the stepped flange 17.
The wire winding 25 extends from a fixed anchor 34 adjacent the flange 17, axially along the barrel of the conduit to over the outside of the hell or socket at the other end of the conduit where it is fixed by a like anchor 35. The wire is first attached to the anchor 34 and it is maintained in high tension as it is wound around the conduit and attached to the anchor 35. Any suita'ble anchoring means may be employed.
The anchor 34 includes a steel block 36 which is set into the concrete of the conduit, This block has a tapered slot containing a wedge 37 alongside a length of the wire 25. Access of the wire and wedge to the bottom of the slot is gained by a shallow ramp 39 in the concrete at each end of the block. The pull of the wire is resisted by the seizure of the wire between the wedge and a side wall of the slot.
The amount of prestressing and sizes of the wires 24 and 25, and the pitch of the winding or the number of turns of wire per unit of length of the core are governed by the amount of circumferential compression or prestress desired to be imparted to a particular conduit.
As is usual in prestressed concrete pipes, the helical wire winding 25 is protected by a coating 38 which is applied against the exterior of the conduit. A covering of mortar is generally used, but other and structurally weaker materials, such as a mastic, can be used in the embodiment shown in FIGS. 1 and 3. The coating extends to the flange 17. It need not be thicker than necessary to afford protection and it should not be thicker than the height of the flange so that it may not interfere witth the centering of coupled pipe sections or the flexibility of a joint.
The graphs show comparative conditions of the concrete of a single size of conduit with reference to distances measured from the end edge of a conduit at the end face 40 in terms of the thickness T of the wall of the conduit. The particular curves shown are based on calculations involving a concrete conduit having an inside diameter of 48 inches and a wall thickness of three inches.
In each of FIGS. 4 and 5, the circumferential prestressing winding 41 of tensioned wire extends between T and about 4.5T. The curve of FIG. 4 shows that the maximum bending tensile stress along the inside surface 42 of the conduit due to temporary longitudinal bending moments occurs at about 2.7T, which would be 8.1 inches from the end edge of a conduit having a wall thickness of three inches. Cracking would occur at this point because the magnitude of the imposed stress is equal to the ultimate bending tensile strength of the concrete which is represented as 100 percent on the graph.
In comparison, the maximum temporary bending tensile stress in the concrete in about the same transverse plane through the conduit (FIG. 5) is reduced to approximately 5 6 percent of the ultimate strength of the concrete in a conduit having the same tensioned Wire winding applied and advanced to the same position as in FIG. 4 after the spigot end had been prestressed with a tensioned supplemental wire wrapping 43. If the ultimate bending tensile strength of the concrete is taken as 780 pounds per square inch, the temporary bending tensile stress developed would be 437 pounds per square inch at the inside surface of the conduit.
The graph of FIG. 5 also shows that the combined effect of the Wire winding 41 and of the supplemental wire wrapping 43 has resulted in reversing the temporary bending stress adjacent the end face of the conduit from a condition of tension to a condition of compression.
The graph illustrated in FIG. 6 shows the percentage of final circumferential compression in one end of a concrete conduit having only a helical winding of tensioned wire 44 wtih an initial wrap located at approximately the distance T from the end edge of the spigot. The graph illustrated in FIG. 7 shows the final state of circumferential compression of a like conduit having a tensioned wire wrapping 45 adjacent its end, and a like helical winding 44. It can be seen from a comparison of these two graphs that the circumferential compression of the concrete has been increased from about 26% of the circumferential design compression to about 77% at the end face of the spigot, and that the circumferential design compression is considerably improved inwards from the spigot face towards the barrel of the conduit.
As shown in the modification of FIG. 8, an annular groove 46 for an O-ring type of joint, similar to that hereinabove described, is provided by a substantially cylindrical outer edge 47 of a flange 48, an end wall. 49 of a coating 50 and an end wall 51 of a coating 52. The coatings are made of mortar or of other hard materials. The end Walls contact the sides of the flange and protect the tensioned wires 53 and 54 by which the conduit 55 is prestressed. The outer diameters of the coatings are such as to provide a clearance inside of a gasket sealing surface of a complementary outer joint member.
In the modification of FIG. 9, the walls of the annular groove 56 are provided by a stepped flange 57 and the end 58 of the coating 59 which covers the prestressing Wire winding 60.
The application and use of the invention will be apparent to those skilled in the art in view of the foregoing disclosure. While a preferred embodiment has been described, it is the intention to reserve all modifications within the scope of the appended claims.
What is claimed is: v
1. A joint for prestressed concrete pipe sections comprising, in combination, a prestresed one-piece moulded concrete conduit of a first pipe section and a socket memher of a second pipe section, said socket member having a substantially inner peripheral cylindrical sealing surface, I
h from exterior surfaces of the parent concrete of said spigot portion at either side of the flange, so that the flange is set back axially from the end face of the conduit at the free end of said spigot portion and located a substantial axial distance inwardly from the outside edge of said cylindrical sealing surface of said socket member, the axial width of said flange being substantially less than the axial length of said cylindrical sealing surface, a gasket consisting of an O-ring of an elastorneric material confined exteriorly of said flange in an annular groove.
in said first pipe section, said gasket pressed radially inwardly against said flange by said sealing surface of said socket member, a pre-stressed wrapping of tensioned wire having at least one revolution around the one of said parent concrete exterior surfaces of said spigot portion to the side of said flange intermediate said flange and the free end of the spigot portion, said wrapping prestressing the concrete of said spigot portion in the vicinity of said flange and of the end face of the spigot portion in circumferential compression, a coating of hardenable material disposed over the one of said parent concrete exterior surfaces of said spigot portion to the side of said flange intermediate said flange and the free end of said spigot portion, said coating covering said tensioned wire wrapping and having a laterally disposed end wall contacting a side of said flange facing towards the free end of said spigot portion, a prestressed helical winding of tensioned wire around said conduit, said wire winding encircling the other of said parent concrete exterior surfaces of said spigot portion and extending from a terminus of said wire winding adjacent said flange to the end of said conduit remote from the spigot end, said wrapping and said winding prestressing said concrete conduit in circumferential compression, a second coating of hardenable material disposed over the exterior of said conduit from the barrel side of said flange to the end of said conduit remote from the spigot end, said second-mentioned coating covering said wire winding and having a laterally disposed end wall contacting the side of said flange adjacent said terminus of said wire winding, at least one of said end walls of said coatings also forming a side wall of said annular groove in said first pipe section for contacting with a side of said gasket to restrain said gasket against axial movement in respect to said flange, the outer diameters of the coatings at said end walls and along said spigot portion within said socket member being such as to provide clearance between the outside surfaces of the coatings and said cylindrical sealing surface on said socket member.
2. A joint according to claim 1 wherein said annular flange includes means providing a surface extending laterally to the axis of the conduit for contacting the side of said gasket opposite from the side thereof which is in contact with said end Wall of said coating intermediate said flange and the free end of said spigot portion.
3. A joint according to claim 1 wherein said annular flange includes means providing a surface extending laterally to the axis of the conduit for contacting the side of said gasket opposite from the side thereof which is in contact with said end wall of said second-mentioned coating.
4. A joint according to claim 1 wherein said end wall of the first-mentioned coating and said end wall of said second-mentioned coating contact opposite sides of said gasket.
References Cited by the Examiner UNITED STATES PATENTS 2,576,012 11/51 Gurck 285-288 2,707,003 4/55 Kennison 285288 3,034,537 5/62 Seaman et a1. 138-1,76
FOREIGN PATENTS 132,905 5/49 Australia.
CARL W. TOMLIN, Primary Examiner.