GB1573307A

GB1573307A - Reinforced structures

Info

Publication number: GB1573307A
Application number: GB8146/76A
Authority: GB
Original assignee: STRESSED CONCRETE DESIGN Ltd
Current assignee: STRESSED CONCRETE DESIGN Ltd
Priority date: 1977-02-18
Filing date: 1977-02-18
Publication date: 1980-08-20

Description

(54) IMPROVEMENTS IN OR RELATING TO REINFORCED STRUCTURES (71) We, STRESSED CONCRETE DESIGN LIMITED, a British company of Lynton House, 54, South Side, Clapham Common, London E.C.4., do hereby declare the invention, for which we pray that a patent may be granted to us, and by the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to reinforced structures such as reinforced precast concrete structures, and in particular to pre-stressed hollow structures which are stressed by cables or other flexible, elongate reinforcing elements which extend around the structure, and are post-tensioned.

From one aspect, the present invention consists in a method of post-tensioning a hollow structure, which comprises positioning a group of flexible, elongate reinforcing elements around the exterior of the external periphery of the structure to form a plurality of loops surrounding the structure, locating first portions of the loops, from which portions the loops extend in a common direction around the structure, in individual first reinforcing element-receiving passages in a common anchor member located externally of the structure and displaceable peripherally thereof, locating second portions of the loops, from which portions the loops extend in a common direction around the structure in a direction opposite the first common direction, generally symmetrically with respect to the first portions, in individual second reinforcing element-receiving passages in the anchor member, attaching an individual tensioning device to each of the second portions with the tensioning device bearing directly on the anchor member, tensioning the loops by applying to the second portions by means of tensioning devices a tensioning force acting between the second portions and the anchor member, whilst permitting displacement of the anchor member and tensioning devices relative to the structure to tend to equalise the tension in the portions of the loops on opposite sides of the anchor member, anchoring the second portions of the tensioned loops to the anchor member, and removing the tensioning devices A single anchor member may be provided, in which event each of the two or more reinforcing elements of the group may extend continuously completely around the structure, to form an associated loop, and may be tensioned with respect to, and anchored by its opposite ends, i.e. by its opposed first and second terminal portions, to, the anchor member. However, due to friction between the reinforcing elements and the external surface of the structure, it will usually be preferred to tension the reinforcing elements at two or more peripherally spaced positions, and this may be achieved by forming each loop from two or more peripherally successive reinforcing elements, anchored at their adjacent ends to, and tensioned with respect to, a corresponding number of peripherally spaced anchor members.

To this end, the invention also consists in a method of reinforcing a hollow structure, such as a pre-cast concrete structure, which comprises positioning two or more groups of flexible, elongate reinforcing elements in succession around the periphery of the structure so as jointly to encircle the structure, with the adjacent terminal portions of the reinforcing elements of successive groups being generally symmetrically arranged with respect to each other, and being disposed in individual reinforcing element-receiving passages in, and interconnected by, common anchor members located externally of the structure and displaceable peripherally thereof, with the terminal portions of the reinforcing elements of one group associated with each anchor member anchored with respect to their passages in that anchor member, and with the adjacent terminal portions of the reinforcing elements of the successive other group displacably located relative to their passages in that anchor member, attaching an individual tensioning device to each of the displaceable terminal portions with the tensioning device bearing directly on that anchor member, tensioning the groups by applying to the displaceable terminal portions by means of the tensioning devices a tensioning force acting between the latter portions and the associated anchor members, whilst permitting peripheral displacement of the anchor members and tensioning devices so as to tend to equalise the tension in the groups extending in opposite peripheral directions from the anchor memberms, anchoring the displaceable terminal portions of the tensioned groups to their associated anchor members, and removing the tensioning devices.In one embodiment, the anchor members are equi-angularly spaced around, and rest on, the external surface of the structure, for example a cylindrical or other generally circular crosssection, hollow, pre-cast concrete structure, and the reinforcing elements extending between the anchor members also directly engage the external surface of the structure.

Thus, the individual reinforcing elements of the peripherally successive groups are effectively connected in series, so as to form a number of tensioned loops encircling the structure, compressively stressing and reinforcing the structure. The reinforcing elements and anchor members, after tensioning and final anchorage of the elements, may be coated with or embedded in a suitable material, such as "gunite", applied, for example by spraying, to the external surface of the structure.

From another aspect, the invention consists in an anchor member dimensioned and arranged substantially as and for the purposes herein set forth for use in the methods defined, which comprises a rigid block provided with first passages and second passages extending therethrough between opposite end faces of the block, and dimensioned to slidably receive individual, flexible, elongate reinforcing elements of longitudinally successive groups of reinforcing elements, the first passages being generally symmetrically disposed with respect to the second passages, and being arranged, intended or adapted, at or adjacent their ends opening into one end face of the block, to cooperate with means to anchor the reinforcing elements of one group, when the elements extend through the first passages, against movement through the block in a direction from said one end face towards the opposite end face and the second passages being arranged, intended or adapted, at or adjacent their ends opening into the opposite end face of the block, to cooperate with means to anchor the reinforcing elements of a successive group, when the elements extend through the second passages, against movement through the block in a direction from said opposite end face towards said one end face, the block being directly cooperable with tensioning devices to apply, individually, a tensioning force between the block and the reinforcing elements of one of the groups, and being provided with skids extending generally in the axial direction of the passages, and serving, in operation, to space the block from a hollow structure to be reinforced by post-tensioning, and to facilitate displacement of the block relative to the hollow structure in a direction corresponding generally to said axial direction.

The invention also consists in a hollow structure such as a pre-cast concrete structure, reinforced by the methods defined, or by means of a post-tensioning system including anchor members as just defined.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Figure 1 is a schematic, fragmentary perspective view of a hollow, pre-cast concrete structure provided with an external, post-tensioned reinforcing system embodying the present invention; Figure 2 is a side elevation, on an enlarged scale, of an anchor member for use in the system shown in Figure 1; Figure 3 is a section on the line A-A of Figure 2; and Figures 4 and 5 are end views of the anchor member viewed, respectively, from the right and left hand sides in Figure 2.

Referring to Figure 1, the side wall 1 of a massive, pre-cast hollow concrete structure, for example a cylindrical tank or silo of 30' to 50' or more in diameter, is reinforced by a post-tensioning system indicated generally at 2. This system comprises first and second groups of flexible, elongate reinforcing elements, such as first and second pairs of multifilament steel strands or cables 3a and 3b respectively. The groups are arranged in succession in a circumferential direction, and each group extends approximately 1800 around, and in contact with, the external surface of the side wall 1. The system further includes identical diametrically opposed anchor members, in the form of plates or blocks 4 resting on the external surface of the side wall 1, and the overlapping ends or tails of the groups of strands or cables are interconnected by and anchored to these blocks. The cables 3a and 3b are tensioned, and the tension is maintained by the blocks 4, as will be described. Thus the cables form, in effect, two loops encircling the structure which pre-stress and reinforce the side wall 1 by maintaining it in circumferential compression.

Although only one post-tensioning system is shown in Figure 1, any number of such systems may be provided, spaced apart in the axial directon of the side wall 1.

The system 2 is applied to the structure by positioning the cables 3a and 3b around the side wall 1, individually anchoring the cables 3a, at one of their ends, in a first pair of passages 4a in one of the blocks 4, and similarly anchoring the cables 3b, at one of their ends, in a first pair of passages 4a in the diametrically opposed block 4. The cables 3a, at the other of their ends, extend through a second pair of passages 4b in the latter block, spaced apart by, and disposed symmetrically with respect to, the passages 4a, whilst the cables 3b, at the other of their ends similarly extend through a second pair of passages 4b in the first-mentioned block 4.The cables 3a and 3b are simultaneously tensioned by applying a tensioning force, in the direction of the arrows, to the cable ends or tails extending from the passages 4b, the force being applied between the cable ends and the associated block 4 by individual tensioning devices, for example by hydraulic jacks or rams (not shown), which grip and pull the cable ends whilst bearing directly against the blocks. When the required tension has been achieved, the cable ends are anchored in their respective passages 4b, and the rams removed.

The blocks 4 rest on the external surface of the side wall 1, and are capable of displacement relative thereto in a circumferential direction, for example on skid bars 5. Thus, during tensioning of the cables 3a and 3b, the blocks 4 will be displaced circumferentially in a direction opposite to that in which the tensioning force is applied, so that the tension in the cables 3a and 3b on opposite sides of the blocks 4 will be equalised, i.e. the application of a tensioning force between one group of cables 3a or 3b and one block 4 serves to tension both groups of cables, at least in the proximity of that block.This is of particular advantage, since the cables 3a and 3b will frictionally grip the side wall 1, with a force which increases with the tension in the cables, and a tensioning force applied to only one of the ends of the cables would be significantly attenuated in a direction towards the other of the ends.

After completing the stressing operation, the cables are preferably protected by coating with, or being embedded in, a pneumatic mortar, such as "gunite" or the like.

Referring to Figures 2 to 5, each of the anchor members shown in Figure 1 comprises a generally rectangular metal plate or block 4, forged or otherwise formed, for example, from a high tensile steel. The block 4 is provided with opposed end faces 4c and 4d between which extend the first and second pairs of passages, 4a and 4b respectively. These passages are mutually generally parallel and equi-spaced, and the passages 4b are disposed symmetrically with respect to, and on opposite sides of, the passages 4a. The passages are dimensioned to slidably receive the ends or tails of the cables 3a and 3b which may, for example be 0.5" in diameter. The ends of the passages 4a opening into the end face 4c the ends of the passages 4b opening into the opposite end face 4d,are enlarged to form frustoconical seats 4c for split collets 6 which anchor the cables in a conventional manner.

It will be apparent from Figure 1 that the initially anchored ends of the cables 3a and 3b are located in the inner or first pair of passages 4a whilst the opposite ends of the cables, to which the tensioning force is applied, are located in the outer or second pair of passages 4b. This enables the interaxial spacing of the passages 4a and 4b to be minimised whilst providing adequate clearance between the rams, as will be apparent from Figure 2, where the heads or chucks of the rams are shown in outline at 7, butting against the end face 4d around the cables 3a and associated collets 6.

As previously mentioned, skid bars 5 are provided to facilitate sliding of the block relative to the external surface of the side wall of the structure. These skid bars are secured, for example by welding, to one surface of the block outboard of the passages 4b. The skid bars, which also provide the necessary clearance between the side wall and rams during tensioning, terminate adjacent the end face 4d so as to clear the ram heads 7, but project beyond the end face 4c, so as to be spaced away from the external surface of the side wall 1 at their extremities as shown in - Figure 3. This minimises the risk of the tips of the skid bars digging into the side wall due to tilting of the block 4 as it slides to the left as viewed in Figures 2 and 3 during tensioning of the cables 3a.

From the foregoing it will be seen that,although at each anchor block, the rams are applied to only one group of cables, since the other group is anchored to the block, and the block is displaceable, the other group will be simultaneously and automatically tensioned.

Since the passages 4a and 4b of each block are symmetrically arranged, provided that the cables of each group are tensioned equally, any tendency for the block to twist relative to the longitudinal direction of the cables will be eliminated or minimised.

The post-tensioning system may be ap plied to the construction of new structures, although it may, to advantage, be applied to existing structures, either to increase the strength thereof, or as a remedial measure, for example if existing or original, external or internal reinforcing wires, strands or cables are showing signs of failure or weakness due to corrosion or damage.

It will be understood that various modifications may be made without departing from the scope of the present invention as defined in the appended claims. For example each group may comprise more than two cables so as to form more than two loops encircling the structure, and/or the groups may have different numbers of cables.

In the specific embodiment, the axes of the passages 4a and 4b each anchor member are located in a common plane tangential to the structure. However, the passages, or additional passages, could be spaced apart in a radial direction; for example one or more additional anchor members could be stacked on and secured to the illustrated anchor member.

Instead of providing two diametrically opposed anchor members as shown in Figure 1, three or more equi-angularly spaced members, and a corresponding number of groups of cables, could be provided. For example, anchor members may be located at 45" intervals, or 1200 intervals. Alternatively, each cable may extend continuously and completely around the structure, and its opposite ends will then be anchored to a single anchor member.

Instead of forming the or each anchor member from a single block, it may be fabricated, or assembled from two or more blocks located end face-to-face and with their associated passages in alignment. The blocks may be positively secured together, or may be held in their assembled condition by the location of the cables in the passages, and by the tension in the cables which will urge the blocks together.

WHAT WE CLAIM IS: 1. A method of post-tensioning a hollow structure, which comprises positioning a group of flexible, elongate reinforcing elements around the exterior of the external periphery of the structure to form a plurality of loops surrounding the structure, locating first portions of the loops, from which portions the loops extend in a common direction around the structure, in individual first reinforcing element-receiving passages in a common anchor member located externally of the structure and displaceable peripherally thereof, locating second portions of the loops, from which portions the loops extend in a common direction around the structure in a direction opposite the first common direction, generally symmetrically with respect to the first portions, in individual second reinforcing element-receiving passages in the anchor member, attaching an individual tensioning device to each of the second portions with the tensioning device bearing directly on the anchor member, tensioning the loops by applying to the second portions by means of the tensioning devices a tensioning force acting between the second portions and the anchor member, whilst permitting displacement of the anchor member and tensioning devices relative to the structure to tend to equalise the tension in the portions of the loops on opposite sides of the anchor member, anchoring the second portions of the tensioned loops to the anchor member, and removing the tensioning devices.

2. A method of reinforcing a hollow structure which comprises positioning two or more groups of flexible, elongate reinforcing elements in succession around the periphery of the structure so as jointly to encircle the structure, with the adjacent terminal portions of the reinforcing elements of successive groups being generally symmetrically arranged with respect to each other, and being disposed in individual reinforcing element-receiving passages in, and interconnected by, common anchor members located externally of the structure and displaceable peripherally thereof, with the terminal portions of the reinforcing elements of one group associated with each anchor member anchored with respect to their passages in that anchor member, and with the adjacent terminal portions of the reinforcing elements of the successive other group displacably located relative to their passages in that anchor member, attaching an individual tensioning device to each of the displaceable terminal portions with the tensioning device bearing directly on that anchor member, tensioning the groups by applying to the displaceable terminal portions by means of the tensioning devices a tensioning force acting between the latter portions and the associated anchor members, whilst permitting peripheral displacement of the anchor members and tensioning devices so as to tend to equalise the tension in the groups extending in opposite peripheral directions from the anchor members, anchoring the displaceable terminal portions of the tensioned groups to their associated anchor members, and removing the tensioning devices.

3. A method claimed in claim 2, which includes tensioning the groups by tensioning devices comprising hydraulic jacks or rams.

4. A method as claimed in claim 2 or 3, wherein the hollow structure comprises a cylindrical or other generally circular crosssection, hollow, pre-cast concrete structure, the method including spacing the anchor members equi-angularly around, and resting

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. plied to the construction of new structures, although it may, to advantage, be applied to existing structures, either to increase the strength thereof, or as a remedial measure, for example if existing or original, external or internal reinforcing wires, strands or cables are showing signs of failure or weakness due to corrosion or damage. It will be understood that various modifications may be made without departing from the scope of the present invention as defined in the appended claims. For example each group may comprise more than two cables so as to form more than two loops encircling the structure, and/or the groups may have different numbers of cables. In the specific embodiment, the axes of the passages 4a and 4b each anchor member are located in a common plane tangential to the structure. However, the passages, or additional passages, could be spaced apart in a radial direction; for example one or more additional anchor members could be stacked on and secured to the illustrated anchor member. Instead of providing two diametrically opposed anchor members as shown in Figure 1, three or more equi-angularly spaced members, and a corresponding number of groups of cables, could be provided. For example, anchor members may be located at 45" intervals, or 1200 intervals. Alternatively, each cable may extend continuously and completely around the structure, and its opposite ends will then be anchored to a single anchor member. Instead of forming the or each anchor member from a single block, it may be fabricated, or assembled from two or more blocks located end face-to-face and with their associated passages in alignment. The blocks may be positively secured together, or may be held in their assembled condition by the location of the cables in the passages, and by the tension in the cables which will urge the blocks together. WHAT WE CLAIM IS:

1. A method of post-tensioning a hollow structure, which comprises positioning a group of flexible, elongate reinforcing elements around the exterior of the external periphery of the structure to form a plurality of loops surrounding the structure, locating first portions of the loops, from which portions the loops extend in a common direction around the structure, in individual first reinforcing element-receiving passages in a common anchor member located externally of the structure and displaceable peripherally thereof, locating second portions of the loops, from which portions the loops extend in a common direction around the structure in a direction opposite the first common direction, generally symmetrically with respect to the first portions, in individual second reinforcing element-receiving passages in the anchor member, attaching an individual tensioning device to each of the second portions with the tensioning device bearing directly on the anchor member, tensioning the loops by applying to the second portions by means of the tensioning devices a tensioning force acting between the second portions and the anchor member, whilst permitting displacement of the anchor member and tensioning devices relative to the structure to tend to equalise the tension in the portions of the loops on opposite sides of the anchor member, anchoring the second portions of the tensioned loops to the anchor member, and removing the tensioning devices.

them on, the external surface of the structure, with the reinforcing elements extending between the anchor members directly engaging the external surface of the structure.

5. A method as claimed in claim 2, 3 or 4, which includes applying a protective material to the external surface of the structure after tensioning and final anchorage of the elements, to coat or embed the reinforcing elements and anchor members.

6. A method as claimed in claim 5, wherein the external surface of the structure is sprayed with a pneumatic mortar.

7. A method as claimed in any of claims 2 to 6, wherein each group comprises a pair of reinforcing elements, the method including positioning the reinforcing elements so that the terminal portions of the pair initially anchored to an associated anchor member are directly adjacent each other, whilst the displacably located terminal portions of the other pair are spaced apart by the anchored portions.

8. A method as claimed in claim 7, which includes locating the axes of the two pairs of terminal portions in a common plane.

9. A method of post-tensioning a hollow, pre-cast concrete structure of circular or cylindrical cross-section, substantially as hereinbefore described with reference to the accompanying drawings.

10. An anchor member dimensioned and arranged substantially as and for the purposes herein set forth for use in the method claimed in any preceding claim, which comprises a rigid block provided with first passages and second passages extending therethrough between opposite end faces of the block, and dimensioned to slidably receive individual, flexible, elongate reinforcing elements of longitudinally successive groups of reinforcing elements, the first passages being generally symmetrically disposed with respect to the second passages, and being arranged, intended or adapted, at or adjacent their ends opening into one end face of the block, to cooperate with means to anchor the reinforcing elements of one group, when the elements extend through the first passages against movement through the block in a direction from said one end face towards the opposite end face, and the second passage being arranged, intended or adapted, at or adjacent their ends opening into the opposite end face of the block, to cooperate with means to anchor the reinforcing elements of a successive group, when the elements extend through the second passages, against movement through the block in a direction from said opposite end face towards said one end face, the block being directly cooperable with tensioning devices to apply, individually, a tensioning force between the block and the reinforcing elements of one of the groups, and being provided with skids extending generally in the axial direction of the passages, and serving, in operation, to space the block from a hollow structure to be reinforced by post-tensioning, and to facilitate displacement of the block relative to the hollow structure in a direction corresponding generally to said axial direction.

11. An anchor member as claimed in claim 10, wherein the skids comprise rods which project beyond the end face of the block opposite that associated with the ends of the laterally outermost passages which are cooperable with the anchor means.

12. An anchor member as claimed in any of claims 9 to 11, wherein the axes of the first and second passages are located in a common plane.

13. An anchor member as claimed in any of claims 9 to 12, wherein the first passages are located laterally adjacent each other, and the second passages are located laterally on opposite sides of the first passages. An

14. An anchor member as claimed in claim 13, including a pair of first passages and a pair of second passages.

15. An anchor member as claimed in any of claims 9 to 14, wherein the passages are provided at their ends with frust-conical seats for anchor means comprising split collets.

16. An anchor member constructed substantially as hereinbefore described with reference to Figures 2 to 5 of the accompanying drawings.

17. A hollow structure such as a pre-cast concrete structure, reinforced by the method claimed in any of the claims 1 to 8, or by means of a post-tensioning system including anchor members as claimed in any of claims 9 to 16.