US3687380A

US3687380A - Prestressing apparatus

Info

Publication number: US3687380A
Application number: US49628A
Authority: US
Inventors: Leroy Magers; John H Riddle; August D Eitzen
Original assignee: Gulf General Atomic Inc
Current assignee: General Atomics Corp; Gulf General Atomic Inc
Priority date: 1970-06-25
Filing date: 1970-06-25
Publication date: 1972-08-29
Anticipated expiration: 1989-08-29
Also published as: GB1330204A; DE2131707A1; JPS5618743B1; FR2143995A5; DE2131707C2; CH556458A; ES392598A1

Abstract

A method of and apparatus for banding a wall of a structure with a tensioned tendon. The apparatus comprises a carriage movable about the wall of the structure and a tendon tensioning means for stressing the tendon while the tendon is held and moved along a substantially straight line path of movement aligned with a tangent to a point at which the tendon is banded on the structural wall.

Description

United States Patent Magers et al.

[ 1 Aug. 29, 1972 [54] PRESTRESSING APPARATUS [72] Inventors: Leroy Magers, Setauket, N.Y.; John H. Riddle, Del Mar, Calif.; August 1). Eitzen, Ville Centre, N.Y.

[73] Assignee: Gulf General Atomic Incorporated,

San Diego, Calif.

22 Filed: June 25,1970

21 App1.No.: 49,628

52 U.S.Cl. ..242/7.21, 226/172, 226/195, 242/147R 51 1m. (:1. ..B21f17/00,B21f45/00 Field of Search ..242/7 .21, 7.22, 7.23, 7.02, 242/156,156.2,195,172, 155 BN, 147 R, 75.53

[56] References Cited UNITED STATES PATENTS 2,164,596 7/1939 Simonds ..242/75.53

2,372,060 3/1945 Crom ..242/7.21 X 2,425 ,496 8/ 1947 Tyler ..242/75 .53 2,785,866 3/1957 Vogt ..242/7.21 3,010,631 11/1961 Gretter ..226/172 3,221,401 12/1965 Scott et a1 ..242/7.23 X 3,227,344 l/ 1966 Rutter ..226/195 X Primary Examiner-Stanley N. Gilreath Assistant ExaminerMilton Gerstein Attorney-Anderson, Luedeka, Fitch, Even and Tabin [57] ABSTRACT 27 Claims, 13 Drawing Figures e0 92 5:3 l-///l/ Patented Aug. 29, 1972 9 Sheets-Sheet 1 S 8 v medfim v 0Q I] W m 00 5 9 M y w M .5. 3 w 5 Am. mm mm B. 00 ri N0. H 0.. nc. 7 1 .PQ P P- 1a m2 8. 0W0 no 3. 0m I a rm Na 0 0N0 18 mm; 0 v mm F s g Patented Aug. 29, 1972 '9 sheets-sheets v llll rh Patented Aug. 29, 1-972 9 Sheets-Sheet 5 FIG?- l III I I I I I I Ir Patented Aug. 29, 1972 9 Sheets-Sheet 6 QQE PRESTRESSING APPARATUS This invention relates to an apparatus and method for prestressing by banding continuous structures circumferentially with tensioned tendons and more particularly to an apparatus and method in which a carriage moves relative to a wall of the structure while tensioning the tendon and paying out the tensioned tendon to the structural wall.

The art of prestressing is well known, well established and has proven in many applications to be successful. In one example of prestressing, high tensile strength tendons are wrapped about a continuous wall of a structure of any suitable material, such as one made from reinforced or un-reinforced concrete or steel, with the tendons stressed in tension so that the wall of the structure is placed in compression before any substantial load is developed against the wall. When an internal load is placed against the wall, the compressive stress developed in the structure must first be overcome before the wall goes into tension and added tensile stress or tension is developed in the tendon.

Many methods have been suggested heretofore for continuously prestressing structures. In addition, various kinds of apparatus have been proposed to carry out the methods. One system involves the development of a tensile load in the tendon by means of passing it through a die to actually deform the tendon. Such a system is disclosed in US. Pat. No. Re. 22,762, issued June 4, 1946. Another system requires the tendon to be wrapped around a wheel or a capstan where a series of independently operated clamps grasp the tendon and develop a stress in it as it travels about the wheel or capstan. Such a system is disclosed in US. Pat. No. 3,229,924 to Vogt, issued Jan. 18, 1966. Another capstan or wheel system in which several convolutions of a tendon are wrapped around a wheel is found in US. Pat. No. 2,711,291 Kennedy, issued June 21, 1955. The wheel or capstan apparatus and methods, like the die system, cause a non-even stress development in the tendon since the outer portions of the tendon are bent and curved about the wheel or capstan and, hence are stressed to a greater degree along the outer side of the tendon than along the inner side of the tendon. This stressing during bending results in non-even distribution of stresses across the tendon.

Also, many of these prior art methods and kinds of apparatus are of limited usefulness for large size tendons and for banding large structures for which the operational speed of the apparatus becomes important. Thus, for large structures large quantities of tendon must be tensioned and payed out at fast operational speeds for a commercially feasible method.

Accordingly, a general object of the invention is to provide an improved apparatus and method for prestressing a structure.

Another and more specific object of the invention is to tension a tendon more uniformly across its cross sectional area by stressing the tendon while the tendon is moving along a path which is aligned substantially with a line tangent to the wall of the structure.

A further object of the invention is to provide an articulated tensioning mechanism on a carriage movable about the structural wall in order that the tendon may be stressed and maintained on the tangential path for different diameter structures or during a build up of successive convolutions of tendons on the structural wall.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is an illustrative view of a structure and an apparatus for prestressing the same and capable of carrying out the method of the invention;

FIG. 2 is a partially schematic perspective view of an apparatus in accordance with the present invention shown in relation to a continuous wall of a structure;

FIG. 3 is a top plan 'view of a tensioning mechanism and support of the present invention;

FIG. 4 is a partially sectional side view of the tensioning mechanism of FIG. 3;

FIG. 5 is a schematic side view of the tensioning mechanism and vertical alignment system of the present invention;

FIG. 6 is an enlarged sectional view through the tendon and tensioning mechanism;

FIG. 7 is an enlarged sectional view showing the linkage arrangement for the restraining elements of the prestressing mechanism;

FIG. 8 is an enlarged sectional view of a means for accommodating elongation of the tendon during stressing;

FIG. 9 is a fragmentary top view of the restraining elements;

FIG. 10 is a fragmentary side view showing the interrelationship of the restraining elements;

FIG. 11 is a schematic flow diagram showing the drive arrangement for the tensioning head and track assemblies;

' FIG. 12 is a view similar to FIG. 6 but showing a flat strip tendon being prestressed; and

FIG. 13 is a view similar to FIG. 8 but for prestressing a flat strip tendon.

As shown in the drawings for purposes of illustration, the invention is embodied in an apparatus 11 for banding a wall 13 (FIG. 2) of a structure 15, such as a restraining barrier or tank, with stressed tendons 17 which are used to develop compressive stresses in the wall and to prestress it circumferentially before an internal pressure is applied within the structure. The apparatus is particularly suited for banding very large structures, such as for example 40 ft. to 200 ft. diameter'cylindrical vessels at relatively high speeds with large size tendons-although the apparatus is useable with smaller or larger sizes of both tendons and structures.

Generally, the apparatus 11 includes a carriage 19 having a tendon tensioning means or mechanism 21 thereon for banding the wall 13 by the method steps of: continuously moving the carriage 19 along a path adjacent the wall 13 of the structure 15, paying out the tendon 17 from the carriage along a straight line which is tangential to the structural wall, and tensioning and prestressing the tendon 17 while it is moving along a path substantially aligned with this straight line tangential to the wall. With the method and apparatus of this invention, the tendon is stressed without being bent or distorted while in-line with a tangent line from the tendon tensioning means to the point at which the tendon is banding on the wall or other previously laid tendons in one of a series of grooves 13a 13d. Thus, with the present invention, the tendon receives a more uniform stress distribution than is achieved with conventional tendon stressing operations such as with a die or with a relatively small radius wheel or capstan about which the tendon is bent while being stressed. Such bending while stressing causes a non-even stress distribution across the diameter or equivalent dimension of the bent tendon.

Also, in accordance with the present invention, the tendon tensioning mechanism automatically adjusts its position to remain aligned with the straight line tangential path to accommodate increasing diameters caused by overlapping a series of previously laid tendon convolutions; and also to permit its use with structures having different diameters. To these ends, the tendon tensioning mechanism 21 may be pivotally mounted on the carriage 19 to swing automatically into alignment with the tangential path under forces exerted by the tensioned tendon itself. Also, the pivoting of tendon tensioning mechanism 21 on the carriage 19 advantageously assists in isolating the tendon stressing mechanism from variations in movement of the carriage toward or from the wall 13 as the carriage 19 travels about the wall.

Also, as will be explained in detail, the preferred tendon tensioning mechanism 21 may be shifted vertically on the carriage 19 so that succeeding vertically spaced wraps may be laid adjacent one another in a groove of the structural wall or in one or more of adjacent grooves 13a 13d without shifting the carriage vertically. Means are also provided on the carriage 19 to abut the wall and hold the carriage against rotation, that is twisting or turning when the tendon tensioning mechanism 21 is shifted vertically from a centered position on the carriage 19. The carriage 19 is driven along the wall by a traction drive means 26 which preferably includes caterpillar tracks 28 which have wide surface contact with the outer face wall 13 to distribute and reduce the load per unit of surface area and which provide good tractive force to prestress relatively large tendons while propelling the carriage at operational speeds. The traction drive means 26 also may be driven in the reverse direction to remove thetendon 17 from the wall 15. That is, the tendon tensioning mechanism is driven in a reverse manner and the carriage is moved in the reverse direction to remove tendon from the structure while maintaining tension in that portion of the tendon not yet removed. On the other hand, in conventional kinds of apparatus the tendon becomes slack when the tendon is taken up.

Other and important features of the invention are a selective control of the prestressing of the tendon 17 by the tendon tensioning mechanism 21 in which tendon engaging holding or restraining elements 27 (FIG. 6) are carried on endless bands or carriers 29 to move into engagement with the tendon 17, travel therewith while restraining and prestressing the tendon, and then release the tendon to return to engage another upstream portion of the tendon. The tendon tensioning means accommodates elongation of the tendon during stressing to prevent slippage of the restraining elements along the surface of the tendon, i.e., the restraining elements 27 are mounted in a manner that the restraining elements 27 may move relative to the endless bands during elongation of the tendon. Also, the tendon engaging surfaces 30 on restraining elements 27 of the restraining mechanism may be of a softer material than the tendon material to prevent damage to the surface of the tendon. Additionally, the restraining force applied by the tendon tensioning mechanism 21 may be selectively automatically controlled to obtain the desired prestressing.

Referring now more specifically to the details of the illustrated apparatus 11, the carriage 19 (FIG. 2) is preferably formed with two units or sections, namely a supply or service section or unit 33 and a trailing prestressing unit or section 35. The position of the units may be reversed, if desired so that the supply unit follows the prestressing unit. The units 33 and 35 are connected in tandem for simultaneous traversal along the wall 13, preferably by means of upper and lower connectors in the form of bars 37 which are fixedly secured at one end to the prestressing unit 35 and pivotally connected to the supply unit by pivot pins 39. The pivot pins 39 are disposed vertically in alignment to define an axis allowing the units to pivot independently in a horizontal plane and relative to each other. The tendon prestressing mechanism 21 is carried by the prestressing unit of the carriage and a supply reel 41 carrying a coil of unstressed tendon 17 is carried on the supply unit. Thus, irregularities in movement of the supply unit 33 in the horizontal plane are not directly transmitted to prestressing unit as if the units were rigidly attached to each other.

The carriage 19 is held inwardly and against the structure wall 13 by means in the form of one or more restraining bands 43 which are endless loops passing around the structure and over and through the carriage 19. Preferably, the restraining bands 43 are held against the wall by tension and exert radial inwardly directed forces on the carriage holding the carriage against the wall 13. The upper restraining band 43 extends across the upper and outer side of the carriage and is guided by and disposed within upper pairs of grooved sheaves 45 rotatably mounted on the respective prestressing and supply units. In a similar manner, the lower restraining band 43 extends across and is guided within a pair of sheaves 47 similarly mounted on both the prestressing and service unit. The restraining bands may be suitably tensioned by means such as a take up mechanism (not shown) on the sheaves.

The carriage 19 is propelled about the wall 13 by a traction drive mechanism which includes upper and lower caterpillar track assemblies 26 each having an endless caterpillar track or tread 28 extending between a pair of rollers 53 and 54 which are rotatably mounted on a stationary support frame 55 of the prestressing unit 35 for frictional engagement with the wall 13. The caterpillar tracks 28 provide excellent traction and permit distribution of the load over a relatively wide surface of the wall which is of particular advantage to reduce the load per unit area on the wall 13 when stressing large tendons with relatively high tension forces. The caterpillar treads are driven by suitable motors such as, for example, hydraulic motors 56 attached to and in driving engagement with rollers 53 of track assemblies 26. As will be explained in greater detail hereinafter a regenerative hydraulic system is used to drive the hydraulic motors 56 and the caterpillar tracks 28 although it is to be understood that a mechanical or electrical motor drive system may be substituted for the hydraulic system. In addition to the caterpillar tracks, the carriage is provided with several roller assemblies 59 and wheel assemblies 61 for rolling about the surface of the wall 13 while the carriage 19 is being driven by the respective caterpillar assemblies 28.

To resist moments tending to twist the prestressing unit 35 and to guide the carriage in a predetermined path, in a horizontal plane in this instance, the carriage is provided with several wheels and rollers which ride on the outside surface of the wall 13 and rotate about substantially vertical axes and others which project into the grooves 13a 13d and roll along a top wall 62a or bottom wall 626 of a groove. In this illustrated embodiment of the invention, the leading portion of the prestressing unit has an upper and lower roller assembly 59 each having a vertically disposed wheel or roller 63 projecting radially inwardly into a groove in the wall 13 to roll on a top or

bottom groove wall

62a and 62b while turning about a horizontal axis. More specifically, the roller 63 of the upper wheel assembly 59 engages a lower groove wall 62b and the roller 63 of the lower wheel assembly engages an upper groove wall 62a and the rollers 63 roll along the outer edge of their respective grooves while turns of tendon are laid within another groove between these grooves. The spacing of the rollers 63 and their engagement with the horizontally disposed

groove walls

62a and 62b resists twisting of the prestressing unit 35. The roller 63 is mounted on a bracket 69 secured to the prestressing unit 35 and a pair of spaced rollers 71 and 73 are carried by the same bracket for turning about substantially vertical axes while rolling along the outer face wall 13 of the structure between a pair of grooves. The service unit 33 is provided with four wheel assemblies 61 which have spaced

wheels

74 and 75 for riding on the outer face wall 13 between grooves. The respective roller and wheel assemblies 59 and 61 are preferably mounted on adjustable mechanisms (not shown) for moving them radially inwardly or outwardly and for moving them vertically to assure that their respective wheels and rollers are properly positioned for good rolling contact with the structure which may vary in configuration or dimensions between the top or bottom thereof. Also, this allows the same carriage to be used with different kinds and sizes of structures.

After banding several of the grooves 13a, 13b etc., it is necessary to displace the carriage along the wall 13in a vertical direction to position the same for banding a next series of grooves in the wall 13. The preferred manner of supporting and positioning the carriage 19 vertically is by means of a suspension system 77 (FIG. 2) which is secured on the structure in this instance. It is to be understood that the carriage 19 may be supported and positioned for vertical displacement on a frame or suspension system which is independent of the structure 15. The illustrated suspension system 77 comprises an overhead track 78 in the form of a circular I beam secured to and supported at the top of the wall 13 by radially and inwardly directed beams 76. Mounted for rolling on and along the overhead track 78 are a series of trolley units 79 having trolley wheels 80 guided for movement within an outwardly facing groove 81 in the track 78. In this instance, front and rear trolley units. support the forward and rearward ends of the respective service and prestressing units 33 and 35. The wheels will roll on a lower wall in the groove 81 of the track while the carriage is traversing about the wall 13. Depending from the trolley units 79 are free-turning pulleys 82 about which extend support cables 83 having one end 84 anchored to the frame of the service unit 33 or the frame of the prestressing unit 35. The other end of each of the respective support cables is wound about and fastened to power drive hoists 85 which may be driven to wind or unwind the cable thereon thereby adjusting the length of cable extending from the hoist 85 to the pulley 82 and down to the anchored end 84. In this manner, the carriage 19 is positioned vertically and the trolley units 79 support the weight of the carriage while rolling in the track 78 while the carriage is propelled circumferentially about the wall 13 by the traction caterpillar tracks 28.

During such movement of the carriage 19 about the wall 13, the tendon 17 is continuously prestressed and payed out from the tendon supply reel 41 on a stand 93 carried on a floor 95 of the service unit 33. The illustrated tendon 17, as best seen in FIG. 6, is of the multistranded, circular cross section kind although the apparatus 11 is capable of use with tendons of various other sizes and cross sectional shapes such as, for example, fiat rectangular cross section as shown for a flat strip tendon 17a, as shown in FIGS. 12 and 13. While the size of the tendon may vary considerably, the apparatus l 1 is particularly useful for relatively large sizes of tendons such as the illustrated seven strand tendon 17 which, in this instance, may have a diameter of 0.6 inch. The illustrated flat strip tendon 17a may typically have a 6 inch width and a one thirty-second inch thickness. The length of

tendons

17 or 17a stored on the supply reel 41 is quite long in order that large diameter structures, e.g., 40 to 200 ft. in diameter may be banded quickly without frequent stops to change supply reels. Also, long lengths of tendon on a supply reel allow the carriage 19 to maintain relatively high speeds, e.g., the minimum operating speed for the carriage in the illustrated embodiment of the invention may be as much as ft. per minute for 20 ft. dia. structures and 300 ft. per minute for 80 ft. dia. structures. From the reel 41, the tendon 17 is fed through the tendon tensioning mechanism 21 which will now be described in detail.

Referring now to the tendon tensioning mechanism 21 and more particularly to its articulation on the tendon prestressing unit 35, this articulation assures an in-line path for the tendon 17 while being stressed, i.e., in-line with a tangent to the point at which the tendon is being banded to and about the wall 13. The tendon prestressing unit comprises a box like frame 55 (FIG. 2) having upper horizontally disposed plates 100 and 101 and a lower horizontally disposed floor plate 102 and bars 103 which are joined by upright rigid bars 105. The tendon tensioning mechanism 21 is carried on a stand 107 fastened at its lower end to a floor plate 102 of the prestressing unit.

The tendon tensioning mechanism 21 is pivotally mounted on a carrier 108 carried on the stand 107 for pivoting about a vertical axis through a pair of vertical stub shafts 109 and 110, as best seen in FIGS. 2, 3 and 4 on the carrier. The stub shafts have outer ends journaled in bearings 111 fastened to top and

bottom carrier plates

113 and 114. The stub shafts 109 and 110 have inwardly projecting ends fastened to the top and bottom of a bearing block 1 (FIG. 4) which supports in cantilever fashion a supporting frame 117 for the endless chains 29 and the restraining elements 27. More specifically, the frame 117 is cantilevered by means of a pair of vertically spaced upper and lower support bars 121 and 123 which extend between the bearing block 115 and the frame 117 with theupper support bar 121 bolted to a top frame plate 125 and the lower support bar bolted to a lower frame plate 127 of the frame 117. As will be explained in greater detail, the tendon 17 acting through the restraining elements 27 and chains 29 exerts turning moments on its supporting frame 117 which in turn acts through the support bars to exert turning moments on the bearing block and attached stub shafts for turning the latter in the bearings 111 on the carrier 108.

For the purpose of measuring and controlling the force of restraint applied by the prestressing mechanism, the support bars 121 and 123 are preferably slidably mounted for longitudinal movement in slide bearings 126 and 128 carried in the bearing block 115. More particularly, load measuring cells 137, as best seen in FIG. 3, are acted upon by and measure the force of restraint tending to pull and slide the support bars 121 and 123 to the right as viewed in FIGS. 2, 3 and 4. The load cells 137 are disposed between the non-slideable, bearing block 115 and a vertical, rear plate 129 which'is attached to the right hand ends of the support bars 121 and 123 to move therewith. As

best seen in FIG. 3, the bearing block 115 has a pair of horizontally extending wings 141 to which are fastened the load cells 137 with plungers 143 of the load cells extending to and abutting opposing facing surfaces'145 on the vertical plate 129. Thus, the restraint applied to the tendon 17 will cause the vertical plate 129 to apply forces to the plungers 143 of the load cells 137 and hence measure the restraint applied thereat.

The bars 121 and 123 support the tendon restraining mechanism 21 in a cantilever manner; and the latter may be provided with a counterweight system 147, as best seen in FIG. 5, for providing greater stability and for applying a lifting force to the outer unsupported (right) end of the tendon tensioning mechanism 21. Fixed to the frame 117 of the tendon tensioning mechanism in a position towards the unsupported end thereof is an upstanding bracket 149 to which is connected a chain 151 which is passed around a pair of freely rotatable sprockets 153 mounted on the frame member 101 of the prestressing unit. The chain is then passed through an opening in the carrier 108 and about another sprocket 157 rotatably mounted on the floor plate 102. The chain extends upwardly from the sprocket 147 and is secured at 156 to the under portion of the carrier. Since chain 151 is of a fixed length, it will maintain the supporting frame 117 of the tendon tensioning mechanism in substantially the same attitude with respect to the structure 15 when the carrier is shifted vertically, as will be explained. The sprockets 153 and 157 may be suitably mounted for swiveling to prevent interference with pivotal movement of the tendon tensioning mechanism 21.

In many instances, it is desired to lay tendons 17 closely adjacent to one another in the grooves 13a, 13b, 13c and 13d in the structural wall; and this may be accomplished without vertically displacing the carriage 19 as the carrier 108 for the tendon tensioning mechanism 21 is mounted for sliding vertically on the stand 107 of the carriage. Also, several successive convolutions of tendons may be wrapped. in the same groove by shifting the carrier 108 without displacing the entire carriage l9 vertically along the wall 13'. The vertical stand 107 is comprised of a pair of upstanding standards 158 each having an outer tubular channel 159 (FIG. 3). The standards 158 are bolted at the upper and lower ends thereof to the top and

bottom frame plates

101 and 102 as best seen in FIG. 4. As best seen in FIG. 3, the carrier108 has four slide pads 162- engaging each of the four sides of the standards 158 for sliding therealong and guiding the carrier for straight line vertical movement.

The prestressing carrier 108 is shifted vertically on the carriage 19 by means in the form of a screw or jack drive mechanism including a pair of vertically extending screw jacks 165 (FIGS. 3 and 4) spanning the top plate 101 and bottom plate 102 and journaled for turning in bearings 167 secured to the respective top and bottom plates. The screw jacks are threaded in a pair of nuts in the form of threaded blocks 169 fixed to the carrier 108 and the threaded blocks travel vertically on the screw jacks as they are turned and thereby the carrier 108 and prestressing mechanism 21 are shifted vertically along the standards 158.

To turn the screw jacks 165, a screw drive 171 (FIG. 4) is provided and it comprises a gear box 172 driven by a belt 173 which is connected to a take-off hub 175 of a variable speed reducer 177 which is driven by a motor 179 through a belt 180. The illustrated motor 179 is an electric one although it may be of the hydraulic type if desired.

As the prestressing carriage 19 travels about the wall 13, the screw jacks 165 may be rotated so that the carrier 108 is gradually lifted upwardly (when the tendon is being wrapped from the bottom upwardly.) In order to insure that the rate of rise of the carriage 19 is controlled, means to sense the position of the carriage are provided in the form of a feeler arm 181 (FIG. 3) which will ride on the bottom horizontal surface of a groove until the first convolution of the tendon is completed and wrapped about the wall. Thereafter the feeler arm 181 rides on the last convolution of the tendon which has been laid. The feeler arm 181 is an articulated arm which is attached to the free end of the stressing mechanism 21. Also the feeler arm 181 is operatively connected to a suitable motor control device such as a micro switch 183 (FIG. 4) for an electric motor 185 which is driven in a manner to adjust the ratio of the variable speed reducer 177 to cause the rate of vertical climb of the carrier 108 to be increased or decreased as necessary.

After leaving the supply reel 41, the tendon 17 is guided through the carrier 108 to the tendon tensioning mechanism 21 by a series of guide rollers including a first pair of vertically oriented, groove rollers 186 (FIGS. 3 and 4) rotatably mounted for turning on a bracket 187 attached to the carrier and a second set of horizontally oriented rollers 188 mounted for turning about spaced vertical axes on the bracket 187 which is fixed to the carrier. The first set of rollers 188 are mounted for rotation about horizontal axes and the second set of grooved rollers 186 are mounted for rotation about vertical axes with a pathway being defined between the grooved rollers 186 being substantially in alignment with an opening 189 in the bearing block 115. After passing through the bearing block; the tendon travels between another set of horizontally oriented, grooved rollers 190 which guide the incoming tendon to its proper position between the chains 29 and restraining holders 27.

In the tensioning mechanism 21, the tendon 17 is stressed while continuously moving therethrough by means of the traveling restraining elements 27 defining a pathway for the tendon. The amount of restraint applied by the holding elements 27 to the tendon 17 determines the degree to which the tendon is prestressed and also its degree of elongation.

While the tendon 17 is passing through the tendon tensioning mechanism 21, a series of the restraining elements 27 will simultaneously be engaging the tendon, traveling with the tendon, and exerting a restraining force on the tendon. Each of the respective tendon restraining elements has a vertically disposed block 191 which spans a pair of vertically spaced upper and lower, endless roller chains 29, as best 'seen in FIGS. 6 and 7. The chains carry the blocks 191 about an endless path including an inner run at which the tendon 17 is engaged adjacent an inlet sprocket 195 (FIG. 3) to outlet sprocket 197 at which the restraining elements release the tendon and return along the outside paths or runs to the inlet sprockets 195. Each of the inlet sprockets 195 is a double sprocket and each of the chains 29 is a composite chain having an upper roller chain 196 and a lower roller chain 198. Each inlet sprocket 195 is fixed to a vertically extending shaft 199 which is journaled in bearings fixed to top and

bottom frame plates

125 and 127 of the tensioning mechanism. In a similar manner, each of the outlet sprockets 197 is a double sprocket and each is attached to a common, vertically extending shaft 201 which is journaled in bearings 203 carried by stationary frame plates 205 and 207 (FIG. 4) of the tensioning mechanism. The rotational axes of the inlet and outlet support shafts and sprockets are positioned on opposite sides of the tendon 17 so that the restraining elements 27 move along a substantially straight line, parallel paths between the inlet and outlet sprockets while engaging the tendon 17.

The restraining blocks 191 are attached to the upper and

lower chains

196 and 198 by elongated pins 207 (FIG. 7) which extend from the upper chain through suitable openings in the blocks to the lower chain. These long pins which extend between the chains a1- temate with short chain pins 209 which serve with the long pins 207 to fasten the respective chain links 210 in a given chain together. Suitable chain rollers 211 are mounted for rotation on each of the respective long pins 207 and short pins 209 between chain links 210 in a familiar and conventional manner.

The restraining elements 27 are mounted on the composite chains 29 in a manner to permit the tendon surfaces 30 thereon to move relative to the chains to accommodate elongation of the tendon 17 as it is being taken from substantially zero tension to full tension but without the tendon engaging surfaces 30 slipping on the tendon during such elongation. To this end, the restraining elements 27 are mounted in a manner to shift relative to the chains and preferably to shift against the bias of a spring means 213 which permits rearward shifting of restraining elements 27 relative to the chains upon compressing the spring means 213. In this instance, the spring means is in the form of a series of curved, spring discs 215, as best seen in FIG. 8, carried in a pair of bores 217 in a central part of the blocks 191 of the restraining elements 27. The spring discs each have a circular recess through which extends a guide rod 221 on the forward end of which is a plug 223 bearing against a long chain pin 207. The other end of the guide rod 221 is inserted in a smaller diameter bore 225 in a shoulder wall 227 against which bears the rearward one of the spring discs 215. To assist the spring discs 215 in sliding during compression or expansion thereof, a sleeve liner may be employed with the spring discs 215 inserted therein and with the sleeve liner disposed within the bore 217. Preferably, a pair of such spring means 213 are provided above and below the tendon path of travel as best seen in FIG. 6.

As an alternative to the preferred embodiment of the invention, the movement of the restraining elements 27 relative to the supporting chains 29 may be limited during stress and elongation of the tendon 17 to less than that needed to accommodate the elongation of the tendon at a given tension whereby the additional displacement needed to accommodate full elongation of tendon is provided by slippage or partial yielding. For example, the spring discs 215 may be selected to bottom out at a tension load which is less than that of the given full tension load; and the tension elements may have partially yieldable surfaces 30 which will yield to provide an additional displacement needed to accommodate the total elongation of the tendon at the given tension. On the other hand, the tendon may simply slip relative to the restraining elements once the springs 215 have bottomed out and the slippage is allowed to accommodate the remainder of the elongation of the tendon. With these embodiments of the invention, however, there will be an accompanying increase in the wear rate of the tendon engaging surfaces 30.

Preferably, each restraining element 27 is interconnected with its preceding and succeeding restraining element. More specifically, the rear portion of each block 191 of the restraining element includes a centrally disposed tongue 231 (FIGS. 7 and 8) which projects rearwardly into a slot 233 forming a yoke on the front of the following block 191. Each tongue has an elongated opening 235 therein through which the long pin 207 extends to permit relative movement of the block relative to the pin. Elongated openings 237 are also formed in the yokes to receive the same elongated pin 207 to permit recovery movement, i.e., forward movement of the block when the tendon is released and the spring discs are free to expand. Thus, each block 191 is connected to its preceding and succeeding block 191 and is free to move relative thereto.

To avoid damage to the tendon 17, it is preferred that the tendon engaging surface 30 be less hard than that material of the tendon engaged thereby so that the surfaces 30 will not scratch or cut the tendon surface.

should be no substantial change in major properties of the tendon such as for example, a change in the ultimate strength, yield point or other physical characteristics due to damage to the tendon surface by the tendon engaging surfaces 30. Scratching or cutting is undesirable since such scratches or cuts have a deleterious effect on the mechanical properties of the tendon. In particular such scratches or cuts may serve as a substantial source of fatigue failure. Preferably, the tendon engaging surfaces 30 may be provided with a groove sized to receive therein a portion of the arcuate periphery of the tendon.

The restraining elements 27 are urged into tight gripping relation with the tendon 17 to prevent slipping therebetween during the restraint by a pressure means 238 (FIG. 3) which comprises a series of hydraulic jacks 239 which are secured at one end to a stationary frame member 241 and which apply force at their other ends to a movable back up plate 243. As best seen in FIG. 6, each back up plate 243 carries a series of rollers 245 which bear against a flat rear wall 247 of the blocks 191 of the restraining elements. Other alternative pressure means such as screws and/or springs or other devices may be used in lieu of the hydraulic jacks to apply pressure to the restraining elements and thereby to the tendon 17. Only one set of hydraulic jacks may be used, if another back up plate 243a is secured and held to the first back up plate by tie rods 249, as shown in FIG. 3. The rollers 245 provide reduced friction surfaces by which sufiicient force may be imparted to the movable restraining elements 27 to prevent the tendon sliding on the surfaces 30 thereof. The preferred restraining elements are sufficiently long that the blocks 191 extend across at least three sets of rollers 245 with the result that the load from the hydraulic jacks 239 is distributed in a longitudinal direction along three spaced areas on each block 191. As best seen in FIG. 6, two vertically spaced rollers 245 are journaled on a common pin 251 to provide two vertically spaced contact areas. Thus, six rollers 245 are normally in contact with the surface 247 of each block. As will be explained in detail, the hydraulic jacks may each be subjected to the same hydraulic fluid force to provide a uniformly distributed load or a non-uniform distribution of load may be achieved. For example, the hydraulic jacksmay receive progressively greater hydraulic pressures so that the tendon 17 receives a substantially linear build up in stressing from the inlet to the outlet of the tendon tensioning mechanism 21.

In order to reduce the size of the motor used to drive the carriage about the structural wall and to prestress the tendon, a regenerative system 260 (FIG. 11) may be used in which the work developed within the tendon tensioning mechanism 21 is used to assist in driving the respective track assemblies 28 to propel the carriage 19 about the structural wall 13.

As explained above, the upper and lower track assemblies 26 each have a roller 53 driven by a main hydraulic motor 56 which is connected by a hydraulic line 261 to a pump 263 driven by a prime mover 265 such as a diesel motor. A common hydraulic line 264 extends between the hydraulic lines 261 from the pumps 263 to provide a common system for the track drive motors 56. To assist in driving the track assemblies, a regenerative hydraulic motor 267 is attached to each of the caterpillar rollers 54 to assist the main hydraulic motors 56 in moving the carriage 19 about the structural wall 13. The regenerative hydraulic motors are driven by regenerative hydraulic pumps 269 which are connected by a drive 270 to sprocket shafts 199 of the tensioning mechanism and are driven thereby to pump hydraulic fluid through hydraulic lines 271 connected to the regenerative hydraulic motors 267. A common hydraulic line 272 interconnects the hydraulic lines 271 to assure that the regenerative motors 267 are being driven equally by a common hydraulic system. Thus, the work being done by the tendon tensioning mechanism 21 is used to drive the regenerative hydraulic pumps 269 and regenerative hydraulic motors 267 to assist in propelling the carriage 19 thereby decreasing power requirements necessary for the main pumps 263 for propelling the carriage l9.

The manner of controlling the amount of tension in the tendon 17 may also be controlled with the regenerative system 260 as by either increasing or decreasing the amount of torque allowed to be done by the regenerative hydraulic pumps 269 and motors 267. More specifically, the load cells 137 which measure the tension in the tendon 17 maybe connected to a mixing amplifier 273 which in turn is connected to servo valves 275 which regulate the angle of swash plates within the regenerative pumps 269 thus determining the torque produced by the pumps 269 for transmittal to the regenerative motors 267.

For the purpose of insuring that the regenerative hydraulic pumps 269 are actually transmitting the desired torque to the regenerative hydraulic motors 267, feedback transducers 277 may monitor the pumps 269 and provide outputs to a summing amplifier 279 which adds these signals together and compares their total to a reference signal for the desired load. If further correction is necessary the summing amplifier 279 causes the amplifier 273 to cause the servo valves 275 to be again changed. Therefore, the actual tension being developed in the tendon 17 is under constant and automatic surveillance and correction.

In the embodiments of the invention illustrated in FIGS. 12 and 13, tendon restraining elements 27a are provided and these are similar to the above described tendon restraining elements 27 except for having flat tendon engaging surfaces 30a for the flat strip tendon 17a rather than having the grooved tendon engaging surfaces-30 for the circular cross sectioned tendon 17. The remaining elements of the tendon tensioning mechanism shown in FIGS. 12 and 13 are substantially similar to elements above described in connection with FIGS. l-ll and the identical reference characters have been used for these similar elements.

In operation of the apparatus 11, the carriage 19 is propelled by the caterpillar tractive drives 28 in a clockwise direction as viewed in FIGS. 1 and 2. With the right hand end of the tendon 17 anchored to the structure, the tendon will extend in a straight line tangential path to the wall 13 of the structure from the car riage. Within the traveling carriage tendon is being uncoiled from the reel 41 and passed through the tendon tensioning mechanism 21 at which the tendon is being stressed in tension by opposed sets of traveling restraining elements 27 on the pair of endless bands or chains 29. The restraining elements are spaced apart by a distance determined by opposite sets of rollers 245 and define a line pathway for the tendon and this pathway is aligned with the tangential line to the structural wall. Hydraulic cylinders 239 act through the rollers 245 to apply forces direct normal to the direction of tendon travel against the restraining elements 27 to prevent slippage between the latter and the surface of the tendon 17. Also, the multiple sets of restraining elements 27 provide multiple areas of holding contact to prevent slippage.

Usually, several convolutions of prestressed tendon 17 are laid vertically along each groove interior wall 620 and the inner convolutions of tendon are overlaid with several outer convolutions of tendon until the desired amount of tendon is laid within a given groove. During the overlaying of convolutions within a groove, the tendon tensioning mechanism 21 may pivot slightly in the clockwise direction, as viewed in FIG. 1 from a position, diagrammatically illustrated in dotted lines in FIG. 1 to another position, diagrammatically illustrated in solid lines in FIG. 1 to accommodate a slight difference in angle due to the difference in diameters of the respective convolutions of tendon which results in different angles for the tangent line path from the carriage to the structure. If different parts of the structure have different diameters, then the pivoting of the tendon tensioning mechanism 21 will be more pronounced for the different parts of the structure.

Advantageously, all of the restraining forces applied by the tendon tensioning mechanism 21 to the prestressing unit 35 are applied at the pivot axis of the tendon tension mechanism on the carrier 108 through the stub shafts 109 and 110. This tends to isolate the tendon tensioning mechanism 21 from radially directed movements of the prestressing unit 35 and supply unit 33 and facilitates measurement of the prestressing forces by the load cells 137.

From the foregoing it will be seen that the apparatus 11 provides a more nearly uniform stress distribution and good control of stress applied to the tendon. Also, the apparatus may operate at relatively fast operating speeds particularly for large sizes of tendons and for large sizes of structures. Additionally, a regenerative system provides economies in the amount of power consumed for the prestressing operation. Also, the ability to reverse the movement of the carriage while maintaining full tension permits correction of an error in laying without unwrapping the entire length of cable.

While a preferred embodiment has been shown and described, it will be understood that there is no intent to limit the invention by such disclosure but, rather, it is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. Apparatus for prestressing a tendon and banding a circumferential wall of a structure having a longitudinally extending axis with a tensioned tendon, said apparatus comprising a carriage movable about the circumferential wall of said structure, means for moving said carriage about the circumferential wall, tendon tensioning means on said carriage having means engaging said tendon along an extended linear portion thereof for applying tensile forces thereto to stress the tendon, said means engaging said tendon and applying tensileforces thereto along a substantially straight line path of movement without substantial bending stresses being applied to the tendon, and means pivotally mounting said tendon tensioning means on'said carriage with said means engaging said tendon along an extended portion thereof for turning about an axis substantially parallel to the longitudinal axis of said structure to align said tensioned-linear portion with a tangent line extending from said tendon tensioning means to a point at which said tendon is handed on said structural wall.

2. An apparatus in accordance with claim 1 in which said tendon tensioning means includes a series of traveling, opposed restraining elements for engaging the tendon and for forming a pathway therebetween for said tendon, said last mentioned pathway being aligned with the tangent line to said structure wall.

3. An apparatus in accordance with claim 2 in which endless bands move about endless paths and in which said restraining elements are carried by said endless bands to engage said tendon and move along said straight line path therewith while the tendon is being tensioned and in which said restraining elements are carried into and from engagement with the tensioned tendon by said endless bands.

4. An apparatus in accordance with claim 1 in which said tendon tensioning means includes means for engaging said tendon and for displacing with elongation of said tendon during stressing thereof to prevent slipping of said means along the surface of said tendon.

5. An apparatus in accordance with claim 1 in which means are provided on said carriage for displacing said tendon tensioning means transversely to the path of carriage movement to position adjacent tendons side by side while said carriage remains in a given path of carriage movement.

6. An apparatus in accordance with claim 5 in which means on said carriage engage said structural wall at spaced locations and hold said carriage against twisting on said structural wall after a transverse shifting of said tensioning means and tendon.

7. An apparatus in accordance with claim 1 in which said means for moving said carriage includes caterpillar tracks for engaging said structural wall and for providing a propelling force to move said carriage about said wall.

8. An apparatus movable about a wall of a structure for tensioning a tendon and for applying the tensioned tendon to said wall, said apparatus comprising a carriage movable about the wall of the structure, said carriage having a supply unit and a tendon tensioning unit, caterpillar track means on at least one of said units having tractive engagement with the structural wall, means pivotally connecting said units for movement together as a tandem about the structural wall, a supply reel of tendon carried on said supply unit for providing tendon for tensioning, and means on said tendon tensioning unit for tensioning the tendon and paying out the tensioned tendon along a line substantially tangential to said structural wall.

9. An apparatus in accordance with claim 8 in which means on said carriage engage said wall at spaced points and hold said carriage against twisting on the structural wall.

10. An apparatus in accordance with claim 7 in which means are provided to shift said tendon tensioning means to permit a plurality of tendons to be applied at longitudinally spaced positions along said structural wall without shifting said carriage along said wall.

11. Apparatus for tensioning a tendon and for banding a wall of a structure with a tensioned tendon, said apparatus comprising a carriage movable in a forward or reverse direction about the wall of the structure, tendon tensioning means on said carriage for tensioning said tendon while said tendon is moving along a substantially straightline path which is along a tangent to the structural wall, said tendon tensioning means including a series of restraining elements for engaging and restraining said tendon and for traveling therewith while tensioning the same along an extended linear portion of said tendon, means mounting said tendon tensioning means on said carriage for alignment with a tangent line extending from said tendon tensioning means to said structure, said restraining elements being movable in one direction when said carriage is paying out said tendon and being movable in an opposite direction when said carriage moves in the reverse direction to take upsaid tendon and means for moving said carriage in the forward direction to pay out the tensioned tendon and in the reverse direction to take up the tensioned tendon while holding said tendon under a controlled, predetermined tension.

12. Apparatus in accordance with claim 11 in which said means for moving said carriage includes a motor drive and caterpillar tracks for engaging the structural wall and in which said tendon tensioning means includes a pair of endless carriers for carrying said restraining elements into engagement with said tendon to travel therewith and from said tendon for return to positions to re-engage said tendon.

13. A prestressing apparatus as defined in claim 12 wherein said restraining elements are slidably mounted and have tongue and yoke portions, said yoke portion including a pair of flanges, and the tongue portion of each element adapted to be slidably positioned within the flanges of an adjacent element.

14. A prestressing apparatus as defined in claim 12 wherein said restraining elements include a resilient member whereby the sliding movement of the restraining elements in passing through the tensioning zone is substantially equal to the elongation of the tendon in passing through said zone.

15. A prestressing apparatusfor circumferentially tensioning a tendon about a continuous structure by elongating the tendon a desired extent, said apparatus comprising a support carriage movable about said structure, a tendon tensioning mechanism carried by said carriage means mounting said tendon tensioning mechanism on said carriage for articulation about an axis to align said tendon tensioning mechanism with a line tangent to said structure, and means for shifting the mechanism along a path parallel to said axis to adjust said line of tangency relative to said carriage and to said structure.

16. A prestressing apparatus as defined in claim 15, wherein the tensioning mechanism includes a pair of opposed loops each comprised of flexible links engaging a pair of spaced, rotatable sprockets fixedly posi tioned to maintain said loops taut, means for rotating a sprocket in each pair in timed relation whereby said loops define a pathway for the tendon and create a zone for progressively increasing the tension in said tendon as it passes through said zone.

7 17. A prestressing apparatus as defined in claim 16 wherein said opposed loops include a plurality of ten- .don restraining elements for engaging the tendon throughout its travel along the pathway defined by the opposed loops. j

18. A prestressing apparatus as defined in claim 17 wherein said tendon restraining elements have a surface thereon for engaging the tendon, the hardness of said surface being less than that of the tendon.

19. A prestressing apparatus as defined in claim 16 and further including a back-up plate positioned within each loop and extending substantially the length of the pathway formed by the loops, at least one of said plates adapted to be moved whereby the pressure exerted by the restraining elements on the loops against the tendon may be varied.

20. A prestressing apparatus as defined in claim 17 wherein said restraining elements are supported by a plurality of rollers whereby the distance between the holding elements of the opposed loops is determined by the distance between the opposed rollers.

21. A prestressing apparatus as defined in claim 15, in which a movable carrier for the tendon tensioning mechanism'is provided on said carriage and said tendon tensioning mechanism is supported thereon in a cantilevered condition, and in which a counterweight arrangement comprising a flexible connector of a fixed length is attached to one side of the cantilevered portion of the tensioning mechanism and passed over a roller fixed on the support carriage and attached to the carrier on the side thereof opposite the attached side of the cantilevered portion whereby said tensioning mechanism is supported by said connector as the carrier is moved relative to the carriage.

22. A tendon mechanism for prestressing a tendon without substantially bending the prestressed tendon before banding the tendon on a structure having a longitudinally extending axis comprising, a frame means, a support means, a pair of opposed, endless bands mounted on said support means for movement about endless paths, and a plurality of tendon restraining elements mounted on said endless bands for travel therewith and for forming a straight line pathway for the tendon to travel along, said restraining elements engaging the tendon during its travel along said straight line pathway, said tendon restraining elements exerting forces on said tendon to prestress the same while traveling along said straight line pathway and means pivotally mounting said support means on said frame means for pivoting thereon about an axis parallel to said longitudinal axis of the structure and relative to said structure being banded to align said tendon being tensioned with a tangent line to the structure.

23. A tendon tensioning mechanism in accordance with claim 22 in which said support means is slidably mounted on said frame means for movement in the direction of tendon travel and in which load sensing with claim 24 in which means are provided to allow said restraining elements to move relative to said endless bands during elongation of the tendon to prevent substantial surface slippage between the tendon and the restraining elements.

26. A tendon tensioning mechanism in accordance with claim 22 in which means are provided to allow said restraining elements to move relative to said endless bands for a predetermined displacement during tendon elongation and in which surfaces are provided on said restraining elements for partially yielding to accommodate further elongation of said tendon to its fully stressed length.

27. A method of tensioning a tendon and banding a circumferentially extending wall of a structure with the tensioned tendon by a tendon tensioning mechanism carried on a movable carriage; said method comprising the steps of: moving the carriage circumferentially about the wall, tensioning an extended linear portion of said tendon while said tendon is moving along a straight line path through said tendon tensioning mechanism without producing a substantial bending of the tendon and without drawing the tendon to produce the tension therein, aligning the-tensioned linear portion of the tendon with a straight line tangent path from the tendon tensioning mechanism to the circumferential wall of the structure, paying out said tensioned tendon along said straight line tangential path to said structure, and banding said circumferential wall with said tensioned tendon.

Claims

1. Apparatus for prestressing a tendon and banding a circumferential wall of a structure having a longitudinally extending axis with a tensioned tendon, said apparatus comprising a carriage movable about the circumferential wall of said structure, means for moving said carriage about the circumferential wall, tendon tensioning means on said carriage having means engaging said tendon along an extended linear portion thereof for applying tensile forces thereto to stress the tendon, said means engaging said tendon and applying tensile forces thereto along a substantially straight line path of movement without substantial bending stresses being applied to the tendon, and means pivotally mounting said tendon tensioning means on said carriage with said means engaging said tendon along an extended portion thereof for turning about an axis substantially parallel to the longitudinal axis of said structure to align said tensioned linear portion with a tangent line extending from said tendon tensioning means to a point at which said tendon is banded on said structural wall.

4. An apparatus in accordance with claim 1 in which said tendon tensioning means includes means for engaging said tendon and for displacing with elongation of said tendon during stressing thereof to prevent slipping of said tendon engaging means along the surface of said tendon.

11. Apparatus for tensioning a tendon and for banding a wall of a structure with a tensioned tendon, said apparatus comprising a carriage movable in a forward or reverse direction about the wall of the structure, tendon tensioning means on said carriage for tensioning said tendon while said tendon is moving along a substantially straight line path which is along a tangent to the structural wall, said tendon tensioning means including a series of restraining elements for engaging and restraining said tendon and for traveling therewith while tensioning the same along an extended linear portion of said tendon, means mounting said tendon tensioning means on said carriage for alignment with a tangent line extending from said tendon tensioning means to said structure, said restraining elements being movable in one direction when said carriage is paying out said tendon and being movable in an opposite direction when said carriage moves in the reverse direction to take up said tendon and means for moving said carriage in the forward direction to pay out the tensioned tendon and in the reverse direction to take up the tensioned tendon while holding said tendon under a controlled, predetermined tension.

15. A prestressing apparatus for circumferentially tensioning a tendon about a continuous structure by elongating the tendon a desired extent, said apparatus comprising a support carriage movable about said structure, a tendon tensioning mechanism carried by said carriage means mounting said tendon tensioning mechanism on said carriage for articulation about an axis to align said tendon tensioning mechanism with a line tangent to said structure, and means for shifting the mechanism aLong a path parallel to said axis to adjust said line of tangency relative to said carriage and to said structure.

16. A prestressing apparatus as defined in claim 15, wherein the tensioning mechanism includes a pair of opposed loops each comprised of flexible links engaging a pair of spaced, rotatable sprockets fixedly positioned to maintain said loops taut, means for rotating a sprocket in each pair in timed relation whereby said loops define a pathway for the tendon and create a zone for progressively increasing the tension in said tendon as it passes through said zone.

17. A prestressing apparatus as defined in claim 16 wherein said opposed loops include a plurality of tendon restraining elements for engaging the tendon throughout its travel along the pathway defined by the opposed loops.

21. A prestressing apparatus as defined in claim 15, in which a movable carrier for the tendon tensioning mechanism is provided on said carriage and said tendon tensioning mechanism is supported thereon in a cantilevered condition, and in which a counterweight arrangement comprising a flexible connector of a fixed length is attached to one side of the cantilevered portion of the tensioning mechanism and passed over a roller fixed on the support carriage and attached to the carrier on the side thereof opposite the attached side of the cantilevered portion whereby said tensioning mechanism is supported by said connector as the carrier is moved relative to the carriage.

23. A tendon tensioning mechanism in accordance with claim 22 in which said support means is slidably mounted on said frame means for movement in the direction of tendon travel and in which load sensing means are operated upon by said support means moving in the direction of tendon travel to provide an indication of the prestressing force being applied to the tendon.

24. A tendon tensioning mechanism in accordance with claim 22 in which means are provided to apply force to said tendon restraining elements in a direction normal to the tendon travel to hold the tendon against substantial slippage relative to said restraining elements.

25. A tendon tensioning mechanism in accordance with claim 24 in which means are proVided to allow said restraining elements to move relative to said endless bands during elongation of the tendon to prevent substantial surface slippage between the tendon and the restraining elements.

27. A method of tensioning a tendon and banding a circumferentially extending wall of a structure with the tensioned tendon by a tendon tensioning mechanism carried on a movable carriage; said method comprising the steps of: moving the carriage circumferentially about the wall, tensioning an extended linear portion of said tendon while said tendon is moving along a straight line path through said tendon tensioning mechanism without producing a substantial bending of the tendon and without drawing the tendon to produce the tension therein, aligning the tensioned linear portion of the tendon with a straight line tangent path from the tendon tensioning mechanism to the circumferential wall of the structure, paying out said tensioned tendon along said straight line tangential path to said structure, and banding said circumferential wall with said tensioned tendon.