GB2334743A

GB2334743A - System for prestressing structures

Info

Publication number: GB2334743A
Application number: GB9817296A
Authority: GB
Inventors: John Derby; David Bretton
Original assignee: Mouchel Consulting Ltd
Current assignee: Mouchel Consulting Ltd
Priority date: 1998-08-07
Filing date: 1998-08-07
Publication date: 1999-09-01
Also published as: GB9817296D0

Abstract

A flat elongate (10) is anchored in place at one end to structure (13) then stressed using a jack (18) until the second end can be anchored. The anchoring consists of housing (12) secured to structure (13), the housing having a first keying means (14) to engage a second keying means (11a) on the tendon end piece (11). The jack (18) engages the end tab (11) and reaction bar (17) joined to jacking frame (15) which itself is removably attached to the housing (12) by connection (16).

Description

SYSTEM FOR PRESTRESSING STRUCTURES This invention relates to a method for prestressing structures and a system for prestressing structural members comprising a tendon, an anchorage and a jack.

It is well known that the load capacity and/or durability of structures may benefit from the application of certain compressive forces that alter their state of stress. Systems whereby these forces are applied by means of tendons that are stretched and subsequently anchored to the structure are known as prestressing systems. Such systems may be applied to concrete, metal, masonry or timber materials within structures such as bridges, power stations, sports stadia, retaining walls, factories, and other building and civil engineering structures.

The most common prestressing systems take the form of hydraulic jacks which stretch tendons of steel strand, which are anchored to the structure by steel wedges after stressing. Steel strand has the disadvantage that it can corrode and thereby cause premature failure of the structure. More durable materials such as advanced composite materials have therefore been proposed for use as tendons to overcome this problem. More recently there have been isolated trials in which this tendon has taken the form of a flat plate made from advanced composite materials comprising carbon fibres within a resin matrix. These tendons have simply been bolted or glued to the structure to be stressed.

According to this invention there is provided: a method of fixing a tendon, which comprises an elongate web and an end tab at a first end having a first keying means, to a structural member, the method comprising the steps of: fixing an anchorage to said structural member, said anchorage having second keying means engageable with said first keying means; stretching said tendon using a jack to push said end tab into said anchorage; and engaging said first and second keying means so as to bear the load carried by said tendon.

The invention also provides an anchorage for attachment to a structural member and to receive and engage a tendon comprising an elongate web and an end tab having first keying means, said anchorage comprising; second keying means adapted to engage said first keying means and transmit a tension load in said tendon to said anchorage; and means for fixing said anchorage to said structural member and adapted to transmit said tension load to said structural member.

The invention further provides a tendon comprising an elongate web of generally longitudinal fibres embedded in a resin matrix and having attached thereto an end tab, said end tab having a first keying means adapted to engage with a second keying means in an anchorage and means to receive a pushing force exerted by a jack to extend said web.

Still further, the invention provides a system for prestressing a structural member, the system comprising: a tendon adapted to impart the prestressing load and comprising an elongate web of generally longitudinal carbon fibres embedded in a resin matrix and an end tab having first keying means; an anchorage adapted to be fixed to said structural member and to receive said end tab, said anchorage having second keying means adapted to engage said first keying means and transfer said prestressing load to said anchorage; a jacking frame detachable connectable to said anchorage and adapted to transmit loads thereto; and a jack adapted to engage said jacking frame and said end tab to push said end tab into said anchorage whereby said elongate web is extended to exert said prestressing load and said first and second keying means engage.

The present invention may be used to apply prestresses to structural members such as beams, joists, slabs, columns or trusses. The longitudinal fibres in the tendon are preferably carbon fibre but may also be of other composite materials or a combination of composite materials.

At one end of the tendon, or tendon, an end tab, which may be made of metal or an advanced composite material, is connected to the structure by connection to an anchorage. A force is then applied to stretch the tendon until the end tab at the opposite end of the tendon is also connected to the beam by a second anchorage.

The force transfer between end tabs and anchorages may be by means of bolts, pins, bearing or shear keys. The force at each end of the stretched tendon is thus transferred to the structure by way of the connections between tendon and end tabs, hence by connection of end tabs to anchorages, and hence by connection of anchorages to the structure. The tendons may achieve additional connection to be beam by adhesive applied before the stressing operation.

The force to extend the tendon is applied by means of a hydraulic or mechanical jack which pushes the end tab and reacts against a jacking frame. The jacking frame provides this reaction principally because of a tensile connection to the end anchorage by means of bolts, pins, bearing or shear keys. The jack reacts between the jacking frame and one of the tendon end tabs, and pushes the end tab until the required extension of the tendon is obtained, after which the end tab is connected to the anchorage. The jack may then be released, transferring the reactions at each end of the stretched tendon to the structure through the method of connection of end tabs and the anchorages.

The present invention thereby provides a simpler and more reliable method of attaching a tendon to a structure. It has particular advantages when used on site, to add an additional prestress to an existing structure or during initial construction, but may also be used off-site in prefabrication of structural members. A particular advantage of the invention is that where the tendon is to be adhered to the structure, the tension in the tendon is held by the anchorage and it is not necessary to maintain the force with the jack whilst the adhesive sets. This means that another tendon may be stressed whilst the adhesive on the first is curing. In this case the cured adhesive provides an additional means of load transfer between the tendons and the structure, but the anchorages remain to prevent the tendons pealing off.

The manner in which the anchorage is attached to the structural member will depend on the form of the structural member. In the case of a steel or iron beam it may be desirable to avoid drilling into the beam as far as possible and the anchorage may be clamped and/or adhered to the beam. In the case of a concrete beam the anchorage may be simply bolted to the beam. Embodiments of the invention in which the jacking force is applied using a jacking frame attached to the anchorage have the advantage of minimising connections to the structure whilst avoiding the need to provide a separate brace.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which : - Figure 1 shows the end tab and jack in position to stress the tendon; Figure 2 shows a pair of anchorages with end tabs and tendon in the final stressed positions; and Figure 3 shows a side elevation of the jacking frame and transverse reaction bar.

Referring to Figure 1, the tendon 10 comprising a flat strip of advanced composite material has an end tab 11 bonded to it at each end. Both of the end tabs are housed within anchorages 12, which are fixed to the structure 13 to be prestressed. The end tabs 11 are initially free to slide within the anchorages 12, but may be locked in position by shear pin 14. The shear pin 14 locks the end tab in place by passing through a slot in the anchorage to engage in a shear key 11a within the end tab.

A jacking frame comprising two side frames 15 is reieasably connected, e.g. by bolts, to one anchorage at location 16. Adhesive is applied to the tendon if required before placing against the structure surface, and one end of the tendon is locked into position at the anchorage remote to the jacking frame by shear pin 14. If necessary the tendon may be urged against the structure 13 along its length by planks or any suitable means to ensure complete bonding. A reaction bar 17 is housed within slots in the jacking side frames 15. A hydraulic jack 18 fitted between the reaction bar 17 and the end tab 11 is able to push the end tab into the anchorage by reacting against the reaction bar. The jack has a shoe 1 8a to engage the shoulder 11b on the end tab and may additionally be bolted to the tab during the stretching operation. The structural connections between the reaction bar 17, side frames 15, anchorages 14, and structure 13 result in the tendon 10 effectively applying prestress to the structure by virtue of its connection to the pushed end tab 11.

The jack 18 pushes the end tab 11 until the required prestress and tendon extension are achieved. The tendon length is arranged such that when the required extension is achieved, the shear pin 14 may be located within a shear key within the end tab. The shear pin 14 may be moved into position by hand, or automatically when the extension has been achieved, by a spring steel device 19 or spring activated mechanism. Once the shear pin has engaged the shear key lia it may be fixed in place, e.g. with bolts. With both end tabs fixed within their anchorage the jack mechanism 18 may then be withdrawn.

The anchorages may be positioned relative to each other using a jig set up relative to the known length of the tendons when unstressed. Account needs to be taken in positioning the anchorages of ambient temperatures both at the time the anchorages are set and at the time the tendons will be installed.

Further tendons with their end tabs located within the same anchorages may then be stressed. The jack 18 may be moved laterally to react against a different section of the reaction bar 17. Structures may require many tendons to be stressed adjacent to one another. This may be achieved by extending the anchorage laterally as required. When tendons placed between the side frames have been stressed, one side frame may be moved to the other side of the second frame, and the reaction bar slid along to cover a new area.

The anchorage 12 is arranged so that the shear pins which engage laterally adjacent tendons 10 are staggered longitudinally. The shear keys 11 a in the tendons are correspondingly staggered. This enables the anchorage to be made compact and the tendons to be close together whilst allowing room for the shear pins 14 to be firmly fixed in place.

Once all tendons have been connected to an anchorage, the whole anchorage may boxed in and encased in grout or galvanised or greased or wax coated or painted to protect it from corrosion or other damage.

In an embodiment of the invention suitable for reinforcing a road bridge, four tendons may be applied to the underside of a cast iron beam and prestressed to 22.10 Tonnes each. In this embodiment the tendons are made from T 700 Carbon Fibre Reinforced Polymer (CFRP) plates supplied by Fibreforce Ltd and having an ultimate tensile strength of 2500 N/mm2. The tendons are 76mm wide and have a thickness of 4mm. The end tabs are of mild steel and are 300mm long by 82mm wide and 25mm thick. They are bonded to the CFRP plate using 3M 9323 adhesive by 3M. The anchorages may be bonded to the beams using 3M 9323 adhesive and the tendons may be bonded to the beams by SikaDur 31 PBA epoxy adhesive by Sika Ltd..

The apparatus shown in Figures 1, 2 and 3 may be varied according to the application. For example, the mechanical interlock between the end tab and the anchorage may take the form of dowels or bolts according to the loads and geometry and materials used in the application. The means by which the mechanical interlock is applied may take the form of manual, mechanical, electro-mechanical or hydraulic powered mechanisms. The anchorages may be recessed within the structure, or cast within the structure, or attached to the structure by fixings which are cast into the structure. Also, the jack may be modified to allow two or more tendons to be stressed at the same end at once.

If desired, it is possible to provide multiple layers of tendons on a single structure with the tendons applied later being shorter than the earlier tendons and having anchorages positioned inside the earlier anchorages and bridging the earlier tendons.

In the embodiment described above both anchorages and both ends of the tendon are the same. This can facilitate manufacture and allow two tendons to be stressed at once by working at opposite ends. Alternatively, the "dead" end, which is fixed when loose, may be attached to the structure using a simplified anchorage.

In a further modification, for use where it is desired to apply a prestress to a wide slab, the anchorage may comprise a single box section fixed down and spaced from the slab soffit across its entire width. In this case multiple fixing points for the jacking frame may be provided spaced apart on the anchorage and the side frames may be leap-frogged across the width of the slab to stress the tendons.

Claims

Claims 1. A method of fixing a tendon, which comprises an elongate web and an end tab at a first end having a first keying means, to a structural member, the method comprising the steps of: fixing an anchorage to said structural member, said anchorage having second keying means engageable with said first keying means; stretching said tendon using a jack to push said end tab into said anchorage; and engaging said first and second keying means so as to bear the load carried by said tendon.
2. A method according to claim 1, comprising before the step of stretching the further step of fixing the second end of said tendon, which is remote from said end tab, to said structural member, the distance between the point of fixing the second end and the second keying means in the anchorage when fixed to said structural member being greater than the unstressed length of said tendon between said point of fixing and said first keying means.
3. A method according to claim 2, wherein said second end is fixed to said structural member using an anchorage similar to the anchorage used to fix said first end.
4. A method according to claim 1, 2 or 3, wherein at least a part of said web is to contact said structural member after fixing, and further comprising the steps of: providing adhesive on at least one of said web and said structural member before said step of stretching; and curing said adhesive to bond said tendon to said structural member after said step of engaging.
5. A method according to claim 4 wherein the load applied by said jack is removed before said adhesive develops a bond capable of bearing the load in said tendon.
6. A method according to any one of the preceding claims wherein said steps of stretching and engaging are repeated to fix a plurality of tendons to one anchorage.
7. A method according to any one of the preceding claims comprising the further step, after the or all steps of engaging, of fixing an additional anchoring means to said structural member overlapping the or each tendon and wherein said steps of stretching and engaging are repeated to fix one or more additional tendons to the additional anchorage.
8. A method according to any one of the preceding claims wherein said step of stretching comprises the steps of: releasably attaching a jacking frame to said anchorage; releasably attaching said jack to said jacking frame; and operating said jack to push said end tab towards the anchorage.
9. A method according to any one of the preceding claims wherein said second keying means is provided with biasing means such that, when said first and second keying means are aligned, said step of engaging occurs automatically.
10. A method according to any one of the preceding claims wherein said tendon is stretched with a force greater than about 10 Tonnes.
11. An anchorage for attachment to a structural member and to receive and engage a tendon comprising an elongate web and an end tab having first keying means, said anchorage comprising; second keying means adapted to engage said first keying means and transmit a tension load in said tendon to said anchorage; and means for fixing said anchorage to said structural member and adapted to transmit said tension load to said structural member.
12. An anchorage according to claim 11, further comprising a coupling adapted to receive a detachable jacking frame and transmit to the anchorage forces generated by a jack connected to the jacking frame and used to push said tendon into engagement with said anchorage.
13. An anchorage according to claim 12, wherein said anchorage comprises an anchorage frame adapted to be fixed to said structural member to define an open-ended cavity into which said end tab may be pushed and said jacking frame comprises two side frames detachably connectable to said anchorage frame either side of the open end of said cavity and a reaction bar releasably connectable to said side frames to span between them and adapted to receive the reaction force exerted said jack when said tendon is pushed into engagement with said anchorage.
14. An anchorage according to claim 13, wherein said anchorage frame is adapted to receive a plurality of laterally adjacent tendons and said reaction bar extends laterally to receive said jack at positions to exert force on each of said tendons.
15. An anchorage according to claim 12,13 or 14, further comprising a jack adapted to bear on said jacking frame and having a shoe adapted to engage the end tab of said tendon to push said tendon into engagement with said anchorage.
16. An anchorage according to any one of claims 11 to 15, further comprising biasing means automatically operable when said first and second keying means are aligned, to cause said second keying means to engage said first keying means.
17. An anchorage according to claim 16, wherein said biasing means comprises a spring.
18. An anchorage according to any one of claims 11 to 17, comprising a plurality of second keying means adapted to receive respective first keying means of a plurality of tendons.
19. An anchorage according to claim 18, wherein each second keying means is located at a position displaced in the longitudinal direction of the tendon, when engaged, from the or each adjacent second keying means.
20. An anchorage according to any one of claims 11 to 19, wherein the or each second fixing means comprises a shear key adapted to engage a slot in said end tab.
21. An anchorage according to any one of claims 11 to 20, wherein said fixing means is adapted to fix said anchorage to said structural member in a position bridging one or more tendons.
22. A tendon comprising an elongate web of generally longitudinal fibres embedded in a resin matrix and having attached thereto an end tab, said end tab having a first keying means adapted to engage with a second keying means in an anchorage and means to receive a pushing force exerted by a jack to extend said web.
23. A tendon according to claim 22, wherein said first keying means comprises a transverse slot adapted to receive a shear key.
24. A tendon according to claim 22 or 23, wherein said means to receive a pushing force comprises a shoulder facing toward the middle of said tendon.
25. A tendon according to claim 24, further comprising a threaded hole proximate said shoulder to which the shoe of a jack may be bolted.
26. A tendon according to any one of claims 22 to 15 wherein fibres are carbon fibres or other composite materials or a combination of composite materials.
27. A system for prestressing a structural member, the system comprising: a tendon adapted to impart the prestressing load and comprising an elongate web of generally longitudinal carbon fibres embedded in a resin matrix and an end tab having first keying means; an anchorage adapted to be fixed to said structural member and to receive said end tab, said anchorage having second keying means adapted to engage said first keying means and transfer said prestressing load to said anchorage; a jacking frame detachable connectable to said anchorage and adapted to transmit loads thereto; and a jack adapted to engage said jacking frame and said end tab to push said end tab into said anchorage whereby said elongate web is extended to exert said prestressing load and said first and second keying means engage.
28. A system according to claim 27, comprising a plurality of tendons and wherein said anchorage is adapted to receive said plurality of tendons and has a corresponding plurality of second keying means.
29. A system according to claim 27 or 28, further comprising a second anchorage and wherein the or each tendon has an end tab at each end thereof.
30. A method of fixing a tendon to a structural member substantially as hereinbefore described with reference to the accompanying drawings.
31. An anchorage for attachment to a structural member and to receive and engage a tendon constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.
32. A tendon for fixing to a structural member constructed substantially as hereinbefore described with reference to the accompanying drawings.
33. A system for prestressing a structural member constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.