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REHABCON IPS-2000-00063 Annex L Strengthening of concrete structures using external post-tensioning EC DG ENTR -C-2 Innovation and SME Programme IPS-2000-0063 ________________________________ REHABCON Strategy for maintenance and rehabilitation in concrete structures ________________________________ Annex L Strengthening of concrete structures using external post-tensioning by A F Daly and R J Woodward, TRL Summary: These design guidelines deal with the strengthening of concrete structures
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  Annex L Strengthening of concrete structures using external post-tensioning EC DG ENTR -C-2 Innovation and SME Programme IPS-2000-0063  ________________________________ REHABCON Strategy for maintenance and rehabilitation in concrete structures  ________________________________    REHABCON IPS-2000-00063  Annex L Strengthening of concrete structures using external post-tensioning  by A F Daly and R J Woodward, TRL Summary: These design guidelines deal with the strengthening of concrete structures using external post-tensioning and have been developed on the basis of current knowledge and  practice. The technique has been found to provide an efficient and economic solution for a wide range of structures types and conditions. In addition to increasing strength, external  post-tensioning can be used to improve the serviceability behaviour of existing structures and to delay or prevent the onset of cracking. TRL PROJECT REPORT PR/CSS/10/04 Prepared for: Project Ref: REHABCON IPS-2000-00063 Client: European Commission  Annex L: Strengthening of concrete structures using external post-tensioning CONTENTS  page 1Description........................................................................................................................1   1.1   Introduction..............................................................................................................1   1.2   Scope........................................................................................................................1   1.3   Definition.................................................................................................................1   1.4   Case studies..............................................................................................................1   1.5   Preliminary investigation.........................................................................................2   1.6   Structure types..........................................................................................................2   1.6.1   Beams..................................................................................................................2   1.6.2   Slabs.....................................................................................................................3   1.6.3   Trusses.................................................................................................................3   1.7   Transverse post-tensioning......................................................................................5   1.8   Vertical post-tensioning...........................................................................................5   2Design criteria...................................................................................................................5   2.1   Limit states...............................................................................................................6   2.2   Partial safety factors.................................................................................................6   2.3   Flexure.....................................................................................................................6   2.4   Shear........................................................................................................................6   2.5   Lateral bending........................................................................................................7   2.6   Vibration..................................................................................................................7   2.7   Prestress losses.........................................................................................................7   2.8   Design of anchorages and deviators.........................................................................7   3Execution...........................................................................................................................8   3.1   General.....................................................................................................................8   3.2   Tendon lay-out.........................................................................................................9   3.3   Anchorages..............................................................................................................9   3.3.1   End anchor blocks..............................................................................................10   3.3.2   Bonded anchor blocks........................................................................................10   3.3.3   Diaphragm anchoring........................................................................................14   3.4   Deviators................................................................................................................14   3.5   Replaceability.........................................................................................................17   3.6   Health and safety....................................................................................................17   3.7   Equipment requirements........................................................................................17   3.7.1   Tendons.............................................................................................................17   3.7.2   Corrosion protection..........................................................................................18   3.7.3   Anchorage systems............................................................................................18   3.7.4   Stressing equipment...........................................................................................18   4Quality assurance.............................................................................................................19   4.1   Acceptance.............................................................................................................19   4.2   In-service maintenance and inspection..................................................................19   5References.......................................................................................................................19   Appendix A: Case studies A.1 Clifton Bridge A.2 Kingston Bridge A.3 Friarton bridge  Annex L: Strengthening of concrete structures using external post-tensioning page 1 1DESCRIPTION 1.1   Introduction The design guidelines presented here deal with the strengthening of concrete structures by external post-tensioning and are based on current practice and experience. The method has  been used in many countries and has been found to provide an efficient and economic solution for a wide range of structures types and conditions. It has been applied mainly to  bridges and the technique is growing in popularity because of the speed of construction and the minimal disruption to traffic flow. In addition to increasing strength, external post-tensioning can be used to improve the serviceability behaviour of existing structures and to delay or prevent the onset of cracking. It can also be used to reduce or close pre-existing cracks. The increased stiffness provided by external post-tensioning can reduce in-service deflections and vibrations. In addition, the stress range can be reduced and the fatigue performance thus improved. The presence of a deformation or sag can be reduced or removed by using an appropriate tendon lay-out. The method is generally applicable to a wide range of structure types although its use has  been primarily aimed at strengthening bridges and bridge supports. There are many examples of its application around the world including steel truss bridges, steel girder bridges, steel box girders, prestressed and reinforced concrete bridges. Many case studies have been reported in the UK and elsewhere and some examples are provided in Appendix A. The use of external  post-tensioning as a means of strengthening is particularly useful for short span bridges where under-strength is the result of changes in the specified design highway loading. External post-tensioning is a convenient technique for raising capacity by up to 30%. For application of the technique, these guidelines should be used in conjunction with a recognised code of practice for the design of concrete structures and structures with external and unbonded prestressing, eg, EC 2, BS 5400: Part 4 (British Standards Institution 1990), BS 8110 (British Standards institution 1985), BD 58 (Highways Agency 1994b). 1.2   Scope The technique involves applying an axial load combined with a hogging bending moment to increase the flexural capacity of a beam and improve the cracking performance. It can also have a beneficial effect on shear capacity. Recommendations are given on the types of structure that can be strengthened using external  post-tensioned tendons and the factors that need to be taken into account in the design, installation and maintenance of external tendons. 1.3   Definition The term external post-tensioning is applied to that class of prestressed structure where  prestressing bars or tendons are placed outside the member cross-section and the prestress force is transferred to the member through end-anchorages and deviators. Where the technique is applied retrospectively to strengthen an existing structure, it is recommended that the installation system be devised to ensure inspectability of the post-tensioning components. Consideration should also be given to future re-stressing and tendon replacement. 1.4   Case studies Anumber of case studies of strengthening using external post-tensioning are included in Appendix A. These are useful in providing general ideas on the use of the technique and to identify criteria that might be used in the selection of systems and the detailing of anchorages
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