US3602000A

US3602000A - Reinforced steel pipe piling structure

Info

Publication number: US3602000A
Application number: US859403A
Authority: US
Inventors: Homayoun Joe Meheen
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-09-19
Filing date: 1969-09-19
Publication date: 1971-08-31
Anticipated expiration: 1988-08-31

Abstract

Reinforcement for steel pipe piles and piling structure such as that supporting an offshore oil platform, which has deteriorated and lost its strength. The pile is reinforced in situ by cutting an access opening into its interior, or cutting such an access opening through the pile to communicate with the interior of a steel bracing pipe in the structure, and introducing a partially prestressed and partially reinforced concrete column inside the steel shell.

Description

United States Patent Inventor llomayoun Joe Meheen Box 515, Rte. 3, Golden, Colo. 80401 859,403

Sept. 19, 1969 Aug. 31, 197 1 Continuation-impart of application Ser. No. 712,187, Mar. 11, 1968, now Patent No. 3,403,707

Appl. No. Filed Patented REINFORCED STEEL PIPE PILING STRUCTURE 10 Claims, 18 Drawing Figs.

US. Cl 61/46, 61/53, 6l/53.52, 61/56, 52/223 lnt.Cl E0211 5/40, E02d 5/58, E04c 3/34 Field of Search ..6l/56, 56.5,

[111 gown [56] References Cited UNITED STATES PATENTS 3,256,694 6/1966 Siedenhans 61/56 X 3,293,811 12/1966 Rice 52/223 3,382,680 5/1968 Takano 6 l/56 3,385,070 5/1968 Jackson 6l/56.5 X 3,466,879 9/1969 Justice 61/56 X FOREIGN PATENTS 152,244 7/1953 Australia 61/56 484,960 9/1953 Italy Primary Examiner.lacob Shapiro Attorney-Berman, Davidson and Berman ABSTRACT: Reinforcement for steel pipe piles and piling structure such as that supporting an offshore oil platform, which has deteriorated and lost its strength. The pile is reinforced in situ by cutting an access opening into its interior, or

cutting such an access opening through the pile to commu" nicate with the interior of a steel bracing pipe in the structure, and introducing a partially prestressed and partially reinforced concrete column inside the steel shell.

PATENTED AUB3Y um 3,602,000

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PATENTED AUG31 197i 3,6020 0 0 SHEET 3 0F 4 FIG. 10.

v; l/IIIIIIIIIllI/I ATENTEU AUGSI 1911 SHEET [1F 4 FIG. .16.

REINFORCED STEEL PIPE PILIN G STRUCTURE This application is a Continuation-in-Part of application Ser. No. 712,187, filed Mar. I1, 1968, and allowed July 16, 1969, now [1.3. Pat. No. 3,403,707.

This invention relates to a method for reinforcing steel pipe piling in situ and the resultant pile construction.

The method of this invention is particularly applicable to steel pipe piles in situ, such as those supporting offshore oil platforms. Because of wave and wind action, the steel piling is subject to substantial lateral stresses and high bending moments, continuously. Furthermore, after a period of time, the corrosive effect of the salt water surrounding the piles causes them to lose strength. The combination of these factors often results in buckling of the piles.

In order to restore the piling to its original strength, I introduce a partially prestressed and partially reinforced concrete pile inside each steel pipe pile, in situ. The resultant pile will have a much higher capacity to resist both vertical and lateral stresses, while resisting corrosion and deterioratron.

Accordingly, it is an object of this invention to provide a method for reinforcing steel pipe piling, in situ, by introducing a partially prestressed and partially reinforced concrete pile inside of each steel pipe.

A further object of this invention is to provide an improved pile consisting of an outer steel shell, reinforced by an inner concrete pile which is free from cracking, is watertight, and which can withstand high lateral stresses and bending momcnts.

Further objects and advantages of the invention will become apparent from the following description and claims, and from the accompanying drawings, wherein:

FIG. 1 is a side view in elevation of a steel pipe pile, partly in section, prior to being reinforced in accordance with the method of this invention;

FIG. 2 is a side view in elevation similar to FIG. 1, partly in section, and illustrating an initial step in reinforcing the pile;

FIG. 3 is a cross-sectional view taken substantially along the plane indicated byline 3-3 of FIG. 2;

FIG. 4 is a cross-sectional view taken substantially along the plane indicated by line 44 of FIG. 3, and illustrating the finished, reinforced pile;

FIG. 5 is a cross-sectional view taken substantially along the plane indicated by line 5-5 of FIG. 3;

FIG. 6 is a view similar to FIG. 3, but illustrating an alternative method of reinforcing the pile;

FIG. 7 is a cross-sectional view taken substantially along the plane indicated by line 7-7 of FIG. 6;

FIG. 8 is a cross-sectional view taken substantially along the plane indicated by line 8-8 of FIG. 6;

FIG. 9 is an end view in elevation of a representative offshore oil platform and support structure, and further illustrating the manner of practicing the invention;

FIG. 10 is a fragmentary side view in elevation of a steel pipe pile similar to that of FIG. 9, and showing an inclined steel pipe brace between the piles as well as horizontal braces;

FIG. 11 is an enlarged sectional view taken on line llll of FIG. 10, and looking in the direction of the arrows;

FIG. 12 is a sectional view taken on line ll2-12 of FIG. 11, showing the upper portion of the diagonal brace in an inter mediate step during the process of reinforcing the same;

FIG. 12a is a sectional view similar to FIG. 12, but showing the lower portion of the brace with the spiral case about the stress tendons omitted;

FIG. I3 is a sectional view taken along line 13-I3 in FIG. 12, and looking in the direction of the arrows;

FIG. 14 is a fragmentary, enlarged, sectional view showing a portion of the bearing plate and a gripper for securing one of the prestressing tendons thereto;

FIG. 15 is a sectional view similar to FIG. 14, but showing a modified bearing plate and gripper assembly;

FIG. 16 is a fragmentary, side elevation, partly in section, of a multibrace structure; and

FIG. 17 is a sectional view taken on line l71l7 of FIG. 16, and looking in the direction of the arrows.

Referring now to the drawings in detail, and particularly to FIGS. l to 5, and 9, a representative offshore oil platform construction is generally designated by the numeral 10. Platform is supportedabove waterline 12, by a number of interconnected steel pipe piles 114. Piles 14 are anchored in the sea floor l6, and telescopically receive on their upper ends, sleeves 18. The platform It) is supported on the top of sleeves 18 by beams 19. Each sleeve 18 serves as a cap for a pile l4, and is interconnected by bracing to space the piles.

In order to prepare each pile M for reinforcement in situ, a work platform 20 is suspended from platform 10. A substantially rectangular access opening 22 is cut through sleeve l8 and pile 141 to expose the interior of the pile. The opening 22 is cut as close to the top of the pile as possible, but no closer than 1 foot. The exterior of the pile M is reinforced adjacent opening 22 with stiffeners comprising

angle irons

24 and 26, welded to pile M on opposite sides of opening 22.

All mud, water, and other impurities are then removed from the interior of the pile to a depth which varies depending on the condition of pile 114i and the foundation material in sea floor 16. This can be accomplished by jetting and pumping. Once the pile is pumped to the desired depth, it should be plugged with mud or cement grout 28 so that no foreign matter will rise above this point. The pile I4 is now ready to be reinforced.

This invention contemplates reinforcing the piling by the introduction of concrete into the interior thereof through access opening 22. Concrete is moldable and readily flowable, and is an ideal structural support member for use as a support column in massive constructions, where the majority of the applied stresses are compressive. However, whenever a material is subjected to compression in one direction, there will be an expansion in a direction perpendicular to the compression axis. The tensile and transverse strength of plain concrete is very low and unreliable, compared to its compressive strength, and, in order to make concrete available for use in columns which are subjected to bending moments and some tension, it is necessary to embed steel reinforcement in the concrete member. The purpose of the steel is to carry the flexural and tensile stresses, and the union between the steel and concrete should be sufficient to make the two materials act as one. t

In particular, the columns formed in this invention will have to withstand the application of direct horizontal forces because of wind and wave motion of the sea. An eccentric force of this nature will produce an even higher bending moment on the column than normal, as setup high flexural stresses. The maximum stress will occur on the side of the column and rapidly decrease toward its center. Therefore, the column formed herein should not only be reinforced, but the reinforcement should be as close to the perimeter of the column as possible, and symmetrically spaced.

Concrete expands as the temperature of its environment is raised and contracts as the temperature is lowered. Concrete will also expand in volume if kept wet or immersed in water and contract if exposed to air. This latter property is not confined to freshly placed concrete, but is characteristic of concrete of many years service. This tendency to change in volume with different moisture conditions and changes in temperature does, of course, setup stresses of both tension and compression in a restrained reinforced concrete structure. The tensile stresses often exceed the amount that the concrete can sustain, and cracks result. Also, when reinforced concrete structures are subjected to high flexures, the concrete in the tensile zone will crack to allow the reinforcing steel to carry the stresses. In order to overcome these stresses, the concrete should be placed under an additional, axial, compressive load, which will close the cracks after the lateral loads are removed.

I achieve these objectives by providing flexible-steel tendons housed in flexible tubing within the concrete introduced into the steel pile. The tendons are supported between plates and are spread symmetrically as close to the perimeter of the steel pile as possible and stressed partially, just enough to introduce sufiicient compression in the concrete to render the concrete crack-free and watertight. The residual area of the stressed tendons will then act as normal reinforcing steel when the pile is subjected to high lateral forces, and consequently, high bending moments. If the tendons are stressed to their limit they will create excessive compression in the concrete, and the concrete would become incapable of resisting any further bending. The same tendons are used to establish an additional axial load in the concrete and for reinforcement.

A further advantage is obtained by using flexible prestressed tendons. Because of their flexibility, they can be placed easily through the small access hole and their high ultimate strength gives the piles their strength. As illustrated in FIGS. 2 to 5, eight symmetrically spaced,flexible steel tendons 30 housed in flexible tubing 49 are secured by conventional means to annular bearing plate 32 and lowered into the interior of steel pile 14 through access opening 22, and positioned close to the periphery of steel pile 14. The tendons are lowered to a point approximately 1 foot from mud or grout plug 28 and are spaced approximately 9 inches from the bottom of access opening 22.

At their upper ends, each strand 31 of tendons 30, passes through conical guides 33 secured to an upper annular bearing plate 34, seated on brackets 40 welded to the interior of pile 14. The strands 31 of each tenon 30 pass through an opening in bearing plate 34, and are secured to a prestressing washer 39. Washers 39 are commercially available, and are manufactured by The Prescon Corporation of Corpus Christi, Texas.

Lateral ties 36, spaced along the tendons, hold them in circumferential alignment during the construction, and also provide lateral restraint .for the reinforcing for resisting diagonal tension in the concrete.

A high strength, nonshrinking, concrete or cement grout is then pumped through a central opening in plate 34, into the interior of steel pipe 14, until it reaches the underside of plate 34. The grout should be pumped into the pile in suitable lifts so as not to exceed the crushing strength of the tubing 49 and the bursting strength of the pile 14. After the grout has reached adequate strength to be stressed, washers 39 are each pulled and tendons 30 stressed to the prescribed partial stress, by conventional machinery suitable for this purpose. Shims 41 are then placed under washers 39, between the washers and plate 34 to maintain the stress in each tendon. The flexible tubing 49 surrounding each tendon 30 is then pumped full of grout through central openings in washers 39, in order to create a bond between the tendons and grout and allow the remainder of the strength of the steel tendons to be used as reinforcement.

The upper ends of the tendons 30 and plate 34 are then embedded in grout to protect them from corrosion. The portions of pile 14 and sleeve 18, removed to form an access opening 22, are welded back in place to complete the construction.

FIGS. 6 to 8 illustrate an alternative manner of reinforcing s steel pipe pile. Elements corresponding to those shown in FIGS. 1 to and 9 are designated by primed numerals.

The method used forreinforcing 14 is substantially identical to that usedto'r'einfdrce pil 14, except that four symmetrically spaced tendons 30', which act as pure prestressing tendons only and are sheathed with flexible tubing 49', are secured to plate 32, and lowered into the interior of pile 14'. A number of smaller size flexible strands 44, which act as pure reinforcing and induce no prestressing in the concrete, are also secured to plate 32' adjacent to its periphery. The strands 44 pass through openings in a bearing plate 34 seated on brackets 40', while tendons 30' pass through conical adapters 33' on plate 34'.

Six to eight feet of grout is poured into the bottom of the pile 14' to anchor plate 32'. Alternatively, brackets 46 can be welded to the interior of pile 14' above plate 32', to prevent plate 32' from being raised.

Strands 44 are then stretched nominally with a force of about 1,000 pounds by means of a small hydraulic ram, to

keep them taut, and anchored to plate 34'. The anchorage comprises a split cone 50, having a gripping surface 52. Cone 50 is placed about strand 44, as shown in FIG. 8. Split cone 50 is in sliding engagement with a conical bore 54 in an outer sleeve 56. A coil spring 58 is seated on cone 50 and is adapted to be compressed by a cap 60 threaded into the upper end of sleeve 56. As spring 58 is compressed, it will force cone 50 downwardly in bore 54. This, in turn, will force the gripping surfaces 52 of the cone inwardly, to tightly clamp about the strand 44.

The tendons 30 are not stressed at this time. After tensioning of the strands, grout is pumped into the interior of pile 14, as described in connection with FIGS. 1 to 5, up to the underside of plate 34'. When the grout has reached proper strength, the four tendons 30' are stressed fully, in the same manner as in FIGS. 1 to 5, and anchored to plate 34'. The flexible tubing 49' surrounding tendons 30' is then grouted full. The brackets 40', plate 34', and the top of tendons 30 and strands 44 are then embedded in grout.

An offshore platform reinforced as described, has considerably more strength than it had prior to reinforcement. Since the partially prestressed and reinforced concrete pile, induced inside of the in situ steel pile is capable of resisting all the forces alone, the residual strength of steel piles will constitute added strength. Furthermore, the concrete inside of the steel pile will prevent local buckling of steel, and by this stiffening action will assist the steel pile to carry considerably more stress without failure. Therefore, any amount of strength left in the steel pile will be added as an additional safety factor.

Because of weakening and corrosion, it has been found necessary to repair the pipe braces of offshore platforms, as well as their piles. The method and structure of the invention as described above are substantially adaptable for use in repairing the piling braces as will be explained in connection with FIGS. 10-17. In these parts identical parts are identically numbered, different reference numerals being assigned to parts which differ, however slightly.

Assuming that the inclined pipe brace 62 shown in FIG. 10, as connecting a pair of vertical piles 14, is corroded and requires repair, first a man would be lowered into each of the two vertical piles 14 to locate from their interiors the positions of the ends of the brace. An access hole 64 would be cut in each pile to communicate with the connected end of the brace. It should be noted that these access holes may be located either above or below the water surface 12. A group of tendons 30, as for example eight in number, would then be inserted in the brace pipe together with a spiral cage 65 affixed thereto by welding, or any other suitable means. The cage surrounds and holds the tendons close to the periphery of the brace. A circular bearing plate 66, curved to fit the inner surface of the pile l4 and a similar bearing plate 67 are slipped over the ends of the tendons at their tops and bottoms, the tendons passing through spaced apertured in the bearing plates to hold them in the desired locations next the periphery of the brace. The bearing

plates

66, 67, each have a precut access hole 68 for passage of grout to the interior of brace 62. The plates are then welded to the walls of the pipe pile l4; leveling shims 69, FIGS. 12 and 15, are slipped over the tendons, and then grippers 56, including half-cones 50, the springs 58 and nuts 60, illustrated in FIGS. 8 and 15, are installed. At this time, the tendons 30 are stressed to a desired tension from either end and anchored by tightening the gripper assemblies 56. Once the tendons are stressed, the brace 62 can be filled with concrete grout either as a separate filling operation, or simultaneously when the pipe pile 14 is grouted. It should be noted that the ends of the tendons protrude beyond the bearing

plates

66,67 into the piles 14 where, when grouted, they provide additional anchorage to the brace 62.

It is sometimes difficult to preassemble and maneuver the bearing plates, spiral cage 65 and tendons 30 within the brace,

and in such instances the spiral cage may be omitted so that the tendons and one of the bearing plates can more easily be assembled, the tendons inserted into the brace by lowering,

and the other bearing plate threaded over the ends of the tendons in the next pile. Such omission of the spiral cage is shown in FIG. 12a.

An alternative construction to that of FIG. for anchoring the tendons in the bearing plates is shown in FIG. 14 wherein the threading and spacing apertures 70 for receiving the ends of the tendons are conically shaped, and a pair of half-cone grippers 50 are utilized directly in these apertures to anchor the tendons. This structure, illustrated in FIG. M, differs from that of FIG. 15 in that the latter shows an inclined cylindrical aperture 71 coaxial with an threading the tendon, and a cylindrical shim 69 cut off along side 72 at an angle to the axis of the tendon to provide a leveling surface 73, at right angle to the axis of the shim and coaxial with the prestressing tendon, as a bearing surface for the gripper assembly 56.

In FIGS. 16 and I7 is shown amultibrace structure, after repair, for corroded and weakened parts. The reinforced joint comprises a vertical pile l4 surrounded by a pipe jacket 74 of slightly larger diameter and possibly 50 inches in length which forms a support for the platform, not shown, a 14-inch diameter pipe diagonal brace 62 and a 10% inchpipe horizontal brace 75 both intersecting the pile and surrounding sleeve in the same area and being joined thereto by welding. To repair and reinforce the diagonal pipe 62, a set of twelve tendons 30" is inserted therein through an access opening at the upper end of the brace similar to opening 64' cut in the pile and surrounding sleeve at the lower end. The set of tendons is held within the diagonal brace by an apertured stressing plate 76 at each end of the brace and supported on a plurality of abutment ribs 40" welded to the interior of the brace. The tendons are prestressed and gripped at the exteriors of the stressing plates by grippers 56 in the manner previously described. Each stressing plate 76 includes four larger and spaced apertures through which are inserted the high-strength rods 77, each about 1% inch in diameter and 3 feet long. The rods are provided with heads 78 at their outer ends and are secured at their inner ends in threaded sockets 79 abutting and secured to the inner surfaces of the stressing plates.

In the horizontal brace 75 is inserted a plurality (preferably eight) of high-strength steel rods 80, parallel to the axis thereof and close to the perimeter of the brace, each rod being approximately 4; inch in diameter and 2 feet, 6 inches in length. The rods are each centrally secured in a 4 to 8 inch long threaded sleeve 81 held in the brace access opening 64" by a curved plate 82 provided with eight apertures spaced near the periphery in a circular manner to pass and secure the sleeves. The circular plate 22 is welded to the interior of the pile 14 to cover the access opening. Rods 80 are headed at one end by a small head and are threaded at the other end to receive a nut after the rods have been threaded into the sleeves 811. This structural reinforcement is used for smaller braces or braces carrying very little stress. Such braces do not have any of the prestressing tendons in them, therefore, the high strength rods 80 are used in both directions to make a solid connection between the brace and the pile.

After the tendons 30" in the diagonal brace are stressed, grout may be simultaneously applied to fill the diagonal brace, the horizontal brace 75, and the pile 14, which has meanwhile been provided with a set of prestressed tendons 30, a previously described for FIGS. 2-5. In this way, the joint, filled with grout, is reinforced by the

rods

77, and 80; while the horizontal brace is reinforced by the rods 80; the diagonal brace is reinforced by the stressed tendons 30" and anchored by rods 77, yielding an excellent repair. In the diagonal brace each tendon may be stressed 5000 pounds prior to the grouting.

Although certain specific embodiments of the invention have been shown and described, it is obvious that many modifications thereof are possible. The invention, therefore, is not intended to be restricted to the exact showing of the drawings and description thereof, but is considered to include reasonable and obvious equivalents.

What is claimed is:

l. A reinforced in situ pile structure formed by strengthening a weakened hollow pile and connected bracing, comprising a substantially vertical outer steel shell, a plurality of flexible steel tendons secured to upper and lower bearing plates and disposed within said shell, an access opening cut in the wall of said shell near the top thereof for lowering said tendons into the shell and for pouring concrete thereinto, each of said bearing plates having an opening therethrough for passage of concrete, abutment means fixed to the inner surface of said shell below and near said access opening and serving to support said upper bearing plate, means for securing said lower bearing plate to the inner wall surface of the shell near its bottom, spaced apertures in said bearing plates through which the tendons pass holding the tendons longitudinally in symmetrical array closely adjacent the periphery of said shell, adjustable means gripping said tendons at the level of said upper bearing plate and accessible through said access opening for adjustment of forces in said tendons to draw the upper and lower bearing plates toward one another against the resistance of said abutment means and said means for securing said lower bearing plate, and concrete substantially filling the interior of the shell to the level of the access opening and hardened about said tendons at least some of which are at least partially stressed to exert an axial compressive force on said concrete.

2. A reinforced in situ pile structure according to claim ll, wherein said opening is closed by a cover plate secured to said shell.

3. A reinforced in situ pile structure in accordance with claim ll, wherein said tendons are each in a flexible tube and concrete fills the interiors of said tubes.

4. A reinforced in situ pile structure in accordance with claim 1, wherein said bearing plates are annular and said opening of each said bearing plates is disposed centrally thereof.

5. A reinforced in situ pile structure according to claim 1, wherein a pair of said substantially vertical steel shells are connected to a brace extending at an angle therebetween, said brace being a third steel shell, a second set of flexible steel tendons extending within and longitudinally of the brace and secured near their ends to annular bearing plates, a second and third access opening being cut in the walls of said pair of shells leading to the interior of the brace for lowering said second set of tendons into the brace andpouring concrete thereinto, said annular bearing plates being secured to said pair of shells across said second and third access openings, adjustable tensioning means gripping the tendons of said second set at said annular bearing plates, and concrete substantially filling the interior of said brace and hardened about said tendons of the second set at least some of which are at least partially stressed.

6. A reinforced in situ pile structure according to claim 5, wherein the end portions of said tendons extend beyond said annular bearing plates into said pair of shells and are embedded in concrete within the pair of vertical shells.

7. A reinforced in situ pile structure according to claim 5, wherein said second set of tendons is secured to a spiral cage of reinforcing wire.

8. A reinforced in situ pile structure according to claim 5, wherein a plurality of high strength rigid rods are secured to each of said annular bearing plates and extend into said pair of vertical shells.

9. A reinforced in situ pile structure according to claim 0, wherein said brace is diagonal to the pair of vertical shells and said pile structure includes a horizontal brace connecting the pair of substantially vertical shells, said horizontal brace being a fourth steel shell, a fourth access opening cut in the wall of one of said pair of substantially vertical shells and communicating with the interior of said horizontal brace, a plurality of high strength steel rods held in spaced circular arrangement within said horizontal brace near one end and extending through said fourth access opening into the adjacent vertical shell, and concrete surrounding said last-named rods in said horizontal brace and vertical shell.

10. A reinforced in situpile structure formed .by strengthen ing a weakened hollow pile and connected bracing, comprising a pair of substantially vertical steel shells spaced from one another and connected by a brace extending at an angle therebetween, said brace being a third steel shell, a first access opening cut in the wall of one of said pair of shells near the top thereof, a set of flexible steel tendons secured near their ends to annular bearing plates and extending within and longitudinally of the brace, second and third access openings being

Claims

1. A reinforced in situ pile structure formed by strengthening a weakened hollow pile and connected bracing, comprising a substantially vertical outer steel shell, a plurality of flexible steel tendons secured to upper and lower bearing plates and disposed within said shell, an access opening cut in the wall of said shell near the top thereof for lowering said tendons into the shell and for pouring concrete thereinto, each of said bearing plates having an opening therethrough for passage of concrete, abutment means fixed to the inner surface of said shell below and near said access opening and serving to support said upper bearing plate, means for securing said lower bearing plate to the inner wall surface of the shell near its bottom, spaced apertures in said bearing plates through which the tendons pass holding the tendons longitudinally in symmetrical array closely adjacent the periphery of said shell, adjustable means gripping said tendons at the level of said upper bearing plate and accessible through said access opening for adjustment of forces in said tendons to draw the upper and lower bearing plates toward one another against the resistance of said aButment means and said means for securing said lower bearing plate, and concrete substantially filling the interior of the shell to the level of the access opening and hardened about said tendons at least some of which are at least partially stressed to exert an axial compressive force on said concrete.

2. A reinforced in situ pile structure according to claim 1, wherein said opening is closed by a cover plate secured to said shell.

3. A reinforced in situ pile structure in accordance with claim 1, wherein said tendons are each in a flexible tube and concrete fills the interiors of said tubes.

5. A reinforced in situ pile structure according to claim 1, wherein a pair of said substantially vertical steel shells are connected to a brace extending at an angle therebetween, said brace being a third steel shell, a second set of flexible steel tendons extending within and longitudinally of the brace and secured near their ends to annular bearing plates, a second and third access opening being cut in the walls of said pair of shells leading to the interior of the brace for lowering said second set of tendons into the brace and pouring concrete thereinto, said annular bearing plates being secured to said pair of shells across said second and third access openings, adjustable tensioning means gripping the tendons of said second set at said annular bearing plates, and concrete substantially filling the interior of said brace and hardened about said tendons of the second set at least some of which are at least partially stressed.

9. A reinforced in situ pile structure according to claim 8, wherein said brace is diagonal to the pair of vertical shells and said pile structure includes a horizontal brace connecting the pair of substantially vertical shells, said horizontal brace being a fourth steel shell, a fourth access opening cut in the wall of one of said pair of substantially vertical shells and communicating with the interior of said horizontal brace, a plurality of high strength steel rods held in spaced circular arrangement within said horizontal brace near one end and extending through said fourth access opening into the adjacent vertical shell, and concrete surrounding said last-named rods in said horizontal brace and vertical shell.

10. A reinforced in situ pile structure formed by strengthening a weakened hollow pile and connected bracing, comprising a pair of substantially vertical steel shells spaced from one another and connected by a brace extending at an angle therebetween, said brace being a third steel shell, a first access opening cut in the wall of one of said pair of shells near the top thereof, a set of flexible steel tendons secured near their ends to annular bearing plates and extending within and longitudinally of the brace, second and third access openings being cut in the walls of said pair of shells leading to the interior of the brace, said annular bearing plates being connected to said pair of vertical shells respectively across said second and third access openings, adjustable tensioning means gripping the tendons of said set at said annular bearing plates, and concrete substantially filling the interior of said brace and hardened about said tendons at least some of which havE been at least partially stressed.