US3162709A - Method of forming prestressed tubular structures - Google Patents

Description

Dec. 22, 1964 A. DAVIDSON 6 ,70

METHOD OF FORMING PRESTRESSED TUBULAR STRUCTURES Filed July 31, 1961 3 Sheets-Sheet l A. DAVIDSON Dec. 22, 1964 METHOD OF FORMING PRESTRESSED TUBULAR STRUCTURES 3 Sheets-Sheet 2 Filed July 31, 1961 A. DAVIDSON ULAR STRUCTURES Dec. 22, 1964 METHOD OF FORMING PRESTRESSED TUB Filed July 31, 1961 3 Sheets-Sheet 3 United States Patent 3,162,709 METHOD 0F FORD/ENG PRESTRESSED TUBULAR STRUCTURES Alfred Davidson, American Form & Equipment C0., 30 W. Washington St., Chicago, Ill. Filed July 31, 1961, Ser. No. 127,911 Claims. (Cl. 264-228) This invention relates to molded structures and in particular to prestressed concrete structures and the like. More specifically, the invention relates to prestressed concrete or plastic tubular structures such as pipe and conduit.

While concrete and similar materials have exceedingly high compressive strength, such materials notoriously are quite low in tensile strength. Until relatively recently, the low tensile strength characteristic of such materials has seriously limited the utilization thereof as it has been difiicult to design structural configurations wherein tensile forces are effectively eliminated. One solution to this problem has been to prestress the concrete or similarmolded material, so that when the forces tending to place the material in tension are developed therein, they merely act against a preinduced compressive force, thereby effectively precluding the development of tensile stresses in the material. The present invention is concerned with such prestressing in tubular structural configurations and comprehends a new and improved method of forming a prestressed tubular structure.

Thus, a principal feature of the invention is the provision of a new and improved prestressing method.

Another feature of the invention is the provision of a new and improved method of forming a prestressed tubular structure.

A further feature of the invention is the provision of such a method including the steps of forming a tubular lattice-cage of high creep material, helically disposing elongated members formed of a low creep material coaxially on the lattice-cage, applying a longitudinal tension stress to the elongated members, molding a tubular body with the lattice-cage and members disposed therein, maintaining the stress while the body sets, discontinuing application of the stress to cause the members to apply a compressive stress in the set body, and maintaining the structure to permit the compressive stress to cause the lattice-cage to creep and thereby distribute the compressive stress substantially uniformly throughout the tubular structure.

Still another feature of the invention is the provision of such a method wherein the support is formed of a material having a high creep, or low unit stress retaining, characteristic such as mild steel, and the elongated member is formed of a material having low creep, or higher unit stress retaining, characteristics such as high tensile steel.

Yet another feature of the invention is the provision of such a method wherein the step of forming the latticecage comprises, disposing a plurality of rings formed of a high creep material in coaxially spaced relationship.

Still another feature of the invention is the provision of such a method wherein the step of forming the latticecage comprises, axially movably disposing a plurality of rings formed of a high creep material in coaxial spaced relationship concentrically about a plurality of circumferentially space longitudinal members extending parallel to the axis of said rings.

Another feature of the invention is the provision of such a method wherein the step of forming the latticecage comprises, axially movably disposing a plurality of rings formed of a high creep material in coaxial spaced relationship.

Yet another feature of the invention is the provision 3,162,709 Patented Dec. 22, 1964 of such a method wherein the step of forming the latticecage comprises, axially movably disposing a plurality of rings formed of a high creep material in coaxial spaced relationship, each of the rings having a small hole therethrough extending parallel to the axis of the ring, and the step of disposing the elongated member comprises, extending the member seriatim through the holes of the rings.

A further feature of the invention is the provision of such a method wherein the step of forming the latticecage comprises, fixedly securing a plurality of rings formed of a high creep material in coaxial spaced relationship concentrically about a plurality of circumferentially spaced longitudinal members extending parallel to the axis of the rings.

A still further feature of the invention is the provision of such a method wherein the tension stress is applied concurrently with the disposing of the members on the lattice-cage.

Still another feature of the invention is the provision of such a method wherein the members are firstly disposed in parallel rectilinear relationship concentrically about the lattice-cage, a first selected number of the members are wrapped about the lattice-cage by moving one portion of each member of the number at one end of the lattice-cage in one circumferential direction annularly about the axis relative to an opposed portion of the respective members of the number at the other end of the lattice-cage, and a second selected number of the members are wrapped about the lattice cage by moving one portion of each member of the second number at one end of the lattice-cage in an opposite circumferential direction annularly about the axis relative to an opposed portion of the respective members of the second number at the other end of the lattice-cage.

Another feature of the invention is the provision of such a method further including a step of providing antifriction means at the points of contact of the members and the lattice-cage.

Yet another feature of the invention is the provision of such a method further including a step of providing lubricating material at the points of contact of the members and the lattice-cage.

Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing wherein:

' 1G. 1 is an isometric view illustrating one method of wrapping an elongated tension member helically on a lattice-cage;

FIG. 2 is an isometric view illustrating another method of wrapping the elongated member helically on the latticecage;

FIG. 3 is an isometric view illustrating still another method of wrapping a plurality of elongated members helically in circumferentially spaced relationship on the lattice-cage;

FIG. 4 is a perspective view illustrating another method of arranging the elongated members in a helical configuration in association with a plurality of supporting rings;

FIG. 5 is a perspective view illustrating another method of arranging the elongated members in helical configurationsin association with a lattice-cage support;

FIG. 6 is a diametric section illustrating the prestressed tubular structure with the elongated members being retained by external restraint means, and illustrating the arrangement of the forms as used in molding the concrete or plastic material about the support and tension members;

FIG. 7 is a diametric section of the resultant tubular structure;

FIG. 8 is an enlarged transverse section taken substantially along-the line 88 of FIG. 7;

FIG. 9 is a transverse section similar to that of FIG. 8 but illustrating \a modified resultant structure having a plurality of parallel longitudinally extending reinforcing rods concentrically within the prestressing members;

FIG. 10 is a perspective view illustrating still another method of forming the prestressed tubular structure including a pair of lattice-cages and two sets of helical prestressing members; and

FIG. 11 is a perspective view illustrating yet another method of arranging the tension members in association with the lattice-cage.

The present invention comprehends a method of forming a prestressed tubular structure wherein the prestressing members are arranged in an open helical configuration thereby providing preinduced stresses in the tubular structure which are directed both longitudinally and circumferentially thereof. The prestressing means is effectively fully embedded within the body of molded material such as concrete, plastic, or the like and, thus, is protected from external damaging forces, atmospheric corrosion and environmental chemical attack. The application of the tensile forces to the prestressing members may be effected concurrently with the arrangement thereof in the helical configuration or may be effected subsequent thereto; however, in each instance, the application of the tensile forces acting both longitudinally and circumferentially may be effected as a single step in the process. By maintaining the tension force in the tensioning members during the time the molded material sets, a strong bond between the tensioning members and the molded material is effected, permitting a compressive force to be transmitted to the molded material when the tension force applied to the elongated members is released subsequent to the setting of the molded material.

More specifically, the invention comprehends forming the prestressed tubular structure by firstly forming a support, such as support 10, as illustrated in FIG. 1. The support includes a plurality of rings 11 coaxially spaced to define a generally tubular lattice-cage which is relatively light and open. As shown in FIG. 1, the latticecage 10 may be provided with a plurality of longitudinally extending elements 12 to which the rings 11 may be fixedly secured. The lattice-cage serves primarily as a support for the prestressing members 13 comprising elongated rods or wires wnapped helically about the lattice-cage 10. Preferably, the prestressing members 13 are wrapped in a large pitch helix, herein having a pitch greater than the radius of the helix. In FIG. 1, one such prestressing member 13 is shown as being wrapped helically about the lattice-cage 10 by rotating a disc 14 about the axis of the helix and lattice-cage, one end 15 of the prestressing member 13 being secured to the disc 14, and the other end 16 of the prestressing member 13 being secured to a fixed support such as a disc 17. To guide the prestressing member 13 as it is wrapped about the lattice-cage 10, a guide arm 18 may be associated with the disc 14. At the same time that the disc 14 is being rotated, it is urged axially to the left as seen in FIG. 1 to provide a high tension force in the prestressing member 13. The rotating and force applying means being diagrammatically illustuated at 19. Alternatively, as shown in FIG. 2, the force applying means may comprise a drum 20 driven by a suitable drive 21 and the guide 18 may be driven by a suitable separate rotating device 22.

In FIGS. 1 and 2, a single prestressing member 13 is illustnated; it is understood, of course, that a suitable plurality of such helically arranged prestressing members may be provided on the lattice-cage 10 as desired. In FIG. 3, a method of simultaneously winding a plurality of prestressing members 13 on the lattice-cage 10 is shown to comprise a method generally similar to that of FIG. 1, but wherein the left-hand end of each of the prestressing members 13 is secured to the disc 14 for concurrent rotation thereof about the axis of the lattice-cage, the righthand end of the prestressing members 13 being simultaneously affixed by the fixed disc 17 It should be noted that the lattice-cage It in FIG. 3 may be provided with rings 11 which are movable on the longitudinal members 12. Also illustrated in FIG. 3 is the arrangement of the inner core 23 and outer shell 24 defining the forms in which the molding material is poured to embed the lattice-cage 1t and prestressing members 13. As shown in FIG. 6, the discs 14 and 17 are arranged to maintain the prestressing member 13 under substantial tension while the material, such as concrete, sets in the forms.

In applying the tensile force to the prestressing members, some constriction of the lattic-cage 10 occurs. When the tensile force is discontinued, the rings will expand, thereby inducing stresses in the rings 11 tending to oppose the inwardly directed prestressing forces of the members 13. Toward the end of effectively eliminating this undesirable effect, it is preferred that the latticecage rings 11 be formed of a mild steel having a high creep characteristic whereby the rings tend to become set in the constricted configuration after a short period of time. On the other hand, it is preferred that the prestressing members 13 be formed of a high tensile steel having a low creep characteristic so that these members will continue to exert prestress forces on the concrete during the life of the structure.

As illustrated in FIG. 7, the completed structure 25 is obtained by removing the tension applying means subsequent to a setting of the concrete. By setting the concrete with the prestressing members 13 embedded therein, a bond between the concrete and the prestressing members iseffected, assuring a transmission of the prestressing forces uniformly throughout the concrete of the tubular structure 25. Means for providing a restraint at the opposite ends of the prestressing members 13 may be provided in the form of anchors 26 which may be disposed externally of the structure 25 as shown at the upper end thereof in FIG. 7 or disposed outwardly of discs 14 and 17 as shown in FIG. 6, or internally thereof as shown at the lower end of the structure 25 in FIG. 7. To provide a further improved bond between the molded material and the prestressing members 13, suitable adhesive material as shown at 27 may be provided on the prestressing members.

As seen in FIG. 8, the lattice-cage 10 and the prestressing members 13 are effectively fully embedded within the molded material C in the completed structure 25. Thus, the prestressing members are effectively protected from the atmosphere and damage from external sources.

When the tension force is released subsequent to the setting of the molded material, the prestressing members 13, by virtue of their bond to the molded material, apply a substantial compressive force thereto which, as a result of the helical configuration of the prestressing members, prestresses the molded material both longitudinally and circumferentially in the tubular structure 25. To provide additional longitudinal prestressing, additional prestressing members 2 8 may be provided concentrically within the lattice-cage 10, as shown in FIG. 9. Further, by releasing the tension force on the longitudinal members 28 prior to the release of the tension force on the prestressing members 13, an improved bond between the prestressing members 13 and the material C is obtained.

Referring now to FIGS. 4, 5, 10 and 11 the invention further comprehends a number of modified methods of forming the helical prestressing member arrangements. As shown in FIG. 4, the support 10 may comprise a plurality of rings 11 retained in coaxial spaced relationship by a suitable means such as a split mandrel 29. The prestressing members may be arranged in a helical configuration about the rings 11 which effectively define a tubular lattice-cage and may be slidingly connected to the rings 11 by suitable ties 30. Thus, the prestressing members 13 may be arranged in the helical configuration prior to the application of longitudinal tensile stresses in the members 13, permitting the split mandrel 29 to be removed subsequent to the installation of the members 13 thereon. The longitudinal tensile forces may then be applied to the opposite ends of the prestressing members 13, movement between the prestressing members 13 and the rings 11 being permitted by the ties 30 to accommodate the resultant displacement of the prestressing members.

In FIG. 5, a modified lattice-cage 31 is shown to comprise a plurality of rings 32 retained in fixed coaxial spaced relationship by longitudinal members 12. The rings 32 are provided with a plurality of circumferentially spaced holes 33 extending parallel to the axis of the rings, through which the prestressing members 13 may be threaded in a helical configuration. The holes 33 are preferably slightly larger than the diameter of the prestressing members 13 so that free movement of the prestressing members through the rings may be effected to permit the repositioning thereof upon the application of the longitudinal tension forces subsequent to the threading of the members through the rings.

In FIG. 11, the prestressing members 13 are wrapped helically about a lattice-cage 34 which is generally similar to lattice-cage 10. The rings 11 may be axially movable on the longitudinal members 12 and the end rings 35 fixedly secured to the ends of the longitudinal members 12. Thus, the rings 11 may move during the tensioning of the prestressing members 13, permitting the tension ing to be effected subsequent to the helical wrapping. Alternatively, the rings 11 may be fixedly secured to the longitudinal members 12. throughout.

In the arrangements of each of FIGS. 4, 5 and 11, the helical wrapping is preferably effected in complementary pairs of members 13 wherein the respective members are wrapped in opposite directions, i.e., one of the members may be wrapped in a right-hand helix and the other may be wrapped in a left-hand helix. Thus, by applying the tension force to the paired members 13, when the rings 11 are loose, the rings will be retained by the respective members against rotation notwithstanding their free retention.

To provide improved free movement of the members over the rings, during the application of the longitudinal tensile force, anti-friction means may be provided; herein suitable lubricant 36 is provided at the point of contact between the prestressing members and the rings of the support.

Referring now to FIG. 10, still another arrangement of the prestress means is shown to comprise a pair of concentrically related support-prestressing member assemblies 3'7 and 33 each of which is generally similar to the arrangement illustrated in FIG. 3. Assembly 38 is slightly smaller than assembly 37 and is coaxially disposed within assembly 37. As shown in FIG. 10, the assemblies may be formed by concurrent wrapping of the prestressing members 13 of each assembly as disclosed relative to FIG. 3.

In illustrating the invention, the molded material C has been described as formed in place about the assembled support and helically arranged prestressing members as by pouring the plastic molding material into suitable forms placed therearound subsequent to the assembly thereof. It is to be understood that other methods of embedding the prestressing assemblies in the molding material may be utilized in conjunction with the invention. Illustratively, the molding material may be extruded so as to embed the assembly therein, or it may be centrifugally spun to embed the assembly therein. Where the material is extruded, both the inner form 40 and outer form 41 are completely omitted and where the material is spun, the inner form 40 may be omitted.

Each of the prestressing members defines an open relatively large pitch helix. Longitudinal stability of the helix is provided by the light structural support which permits relatively free longitudinal movement of the prestressing members and limited reduction in the helix diameter. The individual prestressing members may be selectively wrapped in the same direction on the support as where the support is rigidly assembled, or may be wrapped in opposing pairs of opposite-hand helices as where the support rings are loosely retained. Further, all of the members may be concurrently twisted into a helical configuration where aranged initially in axial parallel, circumferentially spaced relationship. The support and prestressing members are effectively bonded within the molding material for improved transfer of the prestressing forces to the material and for improved protection of the prestressing members in use. The tensioning forces may be applied while wrapping the helices or may be applied subsequent thereto as discussed above relative to the different embodiments of the invention. Desirable control of the pretensioning forces may be effected by suitably selecting the number of prestressing members to be utilized in the structure, the open pitch arrangement of the prestressing member helices permitting a wide variation in this selection.

As will be obvious to one skilled in the art, the invention comprehends an improved method of forming prestressed structures which may be utilized with both solid and hollow structures. Thus, the term tubular as used herein is to be taken to include both such solid as well as hollow structures. Further, it is to be understood that the lattice-cage discussed above may have other configurations as well as the spaced ring and ring and longitudinal member structural combinations; -i.e., one or more spiral elements may be substituted for the ring or ring and longitudinal member structures to provide both the circumferential and longitudinal support means.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. The method of forming a prestressed tubular structure comprising the steps of: forming a tubular latticecage of high creep material; helically disposing elongated members formed of a low creep material coaxially on the lattice-cage; applying a longitudinal tension stress to the elongated members; molding a tubular body with the lattice-cage and members disposed therein; maintaining said stress while the body sets; discontinuing application of said stress to cause said members to apply a compressive stress in the set body; and maintaining the structure to permit said compressive stress to cause the latticecage to creep and thereby distribute the compressive stress substantially uniformly throughout the tubular structure.

2. The method of forming a prestressed tubular structure of claim 1 wherein the step of forming the latticecage comprises, disposing a plurality of rings formed of a high creep material in coaxially spaced relationship.

3. The method of forming a prestressed tubular structure of claim 1 wherein the step of forming the latticecage comprises, axially movably disposing a plurality of rings formed of a high creep material in coaxial spaced relationship concentrically about a plurality of circumferentially spaced longitudinal members extending parallel to the axis of said rings.

4. The method of forming a prestressed tubular structure of claim 1 wherein the step of forming the latticecage comprises, axially movably disposing a plurality of rings formed of a high creep material in coaxial spaced relationship.

5. The method of forming a prestressed tubular structure of claim 1 wherein the step of forming the latticecage comprises, axially movably disposing a plurality of rings formed of a high creep material in coaxial spaced relationship, each of said rings having a small hole therethrough extending parallel to the axis of the ring, and the step of disposing the elongated member comprises, ex-

tending said member seriatim through the holes of said rings.

6. The method of forming a prestressed tubular structure of claim 1 wherein the step of forming the latticecage comprises, fixedly securing a plurality of rings formed of a high creep material in coaxial spaced relationship concentrically about a plurality of circumferentially spaced longitudinal members extending parallel to the axis of said rings.

7. The method of claim 1 including the further step of providing anti-friction means at the points of contact of said members and said lattice-cage.

8. The method of claim 1 including the further step of providing lubricating material at the points of contact of said members and said lattice-cage.

9. The method of forming a prestressed tubular struc ture of claim 1 wherein the tension stress is applied concurrently with the disposing of the members on the latticecage.

10. The method of claim 1 wherein said members are firstly disposed in parallel rectilinear reiationship concentrically about said lattice-cage, a first selected number of said members are wrapped about the lattice-cage by moving one portion of each member of said number at one end of the lattice-cage in one circumferential direction annularly about the axis relative to an opposed portion of the respective members of said number at the other end of the lattice-cage; and a second selected number of said members are Wrapped about the lattice-cage by moving one portion of each member of said second number at one end of the latticecage in an opposite circumferential direction annularly about the axis relative to an opposed portion of the respective members of said second number at the other end of the lattice-cage.

References Cited in the file of this patent UNITED STATES PATENTS 1,267,835 Zwicker May 28, 1918 2,001,237 Bille May 14, 1935 2,191,025 Mitchell Feb. 20, 1940 2,303,394 Schorer Dec. 1, 1942 2,378,584 Schorer June 19, 1945 FOREIGN PATENTS 1,107,465 Germany May 25, 1961 208,958 Great Britain Jan. 3, 1924

Claims (1)

1. THE METHOD OF FORMING A PRESTRESSED TUBULAR STRUCTURE COMPRISING THE STEPS OF: FORMING A TUBULAR LATTICECAGE OF HIGH CREEP MATERIAL; HELICALLY DISPOSING ELONGATED MEMBERS FORMED OF A LOW CREEP MATERIAL COAXIALLY ON THE LATTICE-CAGE; APPLYING A LONGITUDINAL TENSION STRESS TO THE ELONGATED MEMBERS; MOLDING A TUBULAR BODY WITH THE LATTICE-CAGE AND MEMBERS DISPOSED THEREIN; MAINTAINING SAID STRESS WHILE THE BODY SETS; DISCONTINUING APPLICATION OF SAID STRESS TO CAUSE SAID MEMBERS TO APPLY A COMPRESSIVE STRESS IN THE SET BODY; AND MAINTAINING THE STRUCTURE TO PERMIT SAID COMPRESSIVE STRESS TO C AUSE THE LATTICECAGE TO CREEP AND THEREBY DISTRIBUTE THE COMPRESSIVE STRESS SUBSTANTIALLY UNIFORMLY THROUGHOUT THE TUBULAR STRUCTURE.

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