Figure 1: Construction of arch unit using pre-cast individual voussoir concrete blocks (Taylor et al, 2007)

Transcription

1 CONSTRUCTION OF TIEVENAMEENA BRIDGE USING A FLEXI-ARCH SYSTEM A.GUPTA 2, S.E.TAYLOR 1, J KIRKPATRICK 2, AE LONG 1 and I HOGG 2, 1 School of Planning, Architecture and Civil Engineering, Queen s University Belfast, 2 Macrete Ireland Ltd, Toomebridge, Co Antrim, Northern Ireland Abstract This paper presents the further development and the construction of the flexi-arch bridge system and it follows a previous paper presented at the 2006 Bridge Engineering Research in Ireland conference. The Tievenameena Bridge was the first flexi-arch bridge for the Department of Regional Development, Northern Ireland and follows Approval in Principle of the arch bridge system for Tievenameena. The design philosophy is in line with advice given by the UK Highways Agency (BD 57/01 & BA 57/01 Design for Durability). The arch ring uses plain structural elements and eliminates the use of corrodible reinforcement as recommended by this Department Standard. The arch is made of un-reinforced pre-cast concrete voussoirs and is transported in flat-pack form. The construction phase makes use of polymer grid reinforcement to support the self weight during lifting. Once in the arch form it behaves as a masonry arch bridge. The flexi-arch system has been developed under a Knowledge Transfer Partnership between Queen s University and Macrete Ltd and it should be noted that it is patent protected and there is a licence agreement between the Partners. Keywords: masonry arches, polymer reinforcement, flexible pre-cast concrete 1. Introduction It is no longer economically viable to construct masonry arch bridges in the traditional method due to the high cost of the skilled labour required to build the accurate centring and to cut and place the masonry blocks. This flexi-arch system is constructed and transported in the form of a flat pack by utilising a polymeric reinforcement to carry the self weight during the lifting phase but it behaves as a masonry arch once in place. The voussoirs are pre-cast individually, laid contiguously horizontally with a layer of polymeric grid material placed on top. An in-situ layer of concrete is cast on top and when hardened forms an interconnection between the voussoirs (Figure 1). The arch unit can be cast in convenient widths to suit the design requirement, site restrictions and available lifting capacity. When lifted, the wedge shaped gaps close, concrete hinges form in the outer most layer of concrete and an arch is formed. The arch ring is then placed on a pre-cast anchor block or seating units and the selfweight is then transferred from tension in the polymer to compression in the arch ring. This paper presents an overview of the most recent developments of this system and describes the most recent bridge construction projects. Insitu screed cast over Individually cast voussoirs Polymeric reinforcement Figure 1: Construction of arch unit using pre-cast individual voussoir concrete blocks (Taylor et al, 2007) A. Gupta, S.E. Taylor, J. Kirkpatrick, AE Long & I. Hogg - 1 -

2 2. Tievenameena Bridge 2.1 Location and structure Tievenameena bridge is located adjacent to Plumbridge near the Sperrin Mountains in County Tyrone, Northern Ireland. It carries the U1236 road over a tributary of the Glenelly River. The arch ring provides a 5m wide by 2m high clearance at high water level and the details of the bridge are given in Table 1. The design philosophy is in line with advice given by the UK Highways Agency (BD 57/01 & BA 57/01 Design for Durability). The arch ring uses plain structural elements and eliminates the use of corrodible reinforcement as recommended by this Department Standard. The arch is made of un-reinforced pre-cast concrete voussoirs and is transported in flat-pack form. The construction phase makes use of polymer grid reinforcement to support the self weight during lifting. Once in the arch form it behaves as a masonry arch bridge. Pre-cast spandrel walls act to retain the backfill and, in this bridge, are stone faced. The flexible and pre-cast elements enable an efficient construction time and provide a cost effective and aesthetic bridge form. 2.2 Arch ring detail The arch bridge system was monitored during the concrete backfilling operation and under a static test load and the results have been published in a previous papers (Taylor et al, 2006 and Taylor et al, 2007). A third scale model arch was also backfilled, using firstly granular material and then concrete, the arch was then monitored during load testing at third span in the laboratory at Queen s. Control samples of concrete from the voussoirs and screed were tested and the material properties are given in Table 1. Advancements in composite material technology and the ability of the polymer in this system to be sufficiently strong to resist the self weight bending moment generated during lifting and flexible enough to develop the arch shape when placing, have provided the key to the success of the flexi-arch. Material tests were carried out on samples of the polymeric reinforcement (Figure 2) and the average tensile strength is given in Table 2. A corbel detail has also been incorporated into the casting of the voussoirs which gives better aesthetics to the intrados of the arch. The arch rings were lifted from flat to arch form by supports at the 7 th, 13 th and 17 th voussoir positions. A total of eight arch rings were used for the bridge to give the required road width. Table 1-5m x 2m arch details Voussoir dimensions 324mm 200mm 300mm Clear span: 5.00m Effective span: 5.24m Internal height: 2.00m Depth of arch ring: 0.240m (40mm top screed) Width of arch ring: 1.00m Polymer Reinforcement 150/100 Tensile strength (2 layers over middle 17 blocks and 1 layer for outer 3 blocks) kn/m Average arch ring concrete compressive 56N/mm 2 strength* Backfill lean mix concrete to 0.4m above arch extrados * is based upon average 28 days cube compressive tests and concrete in the arch ring in excess of 28 days. A. Gupta, S.E. Taylor, J. Kirkpatrick, AE Long & I. Hogg - 2 -

3 Figure 2: Typical failed specimen of polymer reinforcement Table 2: Tensile strength of polymeric reinforcement Sample no. Load (kn) Tensile strength (kn/m width) Maximum creep at ultimate load Average Construction sequence A critical influence on the arch geometry is the dimensions of the voussoir and very rigorous quality control has been employed during their manufacture to ensure mould dimensions within 1mm tolerance by using a steel mould. Galvanised HYS stirrups cats into the voussoirs ensure a mechanical bond between the voussoirs and the screed. The precast concrete voussoirs were then laid contiguously on a flat bed and the total length measured and compared to the calculated design length of the arch ring. The required numbers of polymer layers as recommended by the designers, QUB and Amey for this bridge, were placed under the projected steel stirrups and secured firmly at the two ends to ensure the polymer lays flat and adjacent to the top surface of the voussoirs as shown in Figure 3. The top surface was then cast with a high slump concrete. After an hour, crack inducers were formed above each joint. Figure 3: Polymer reinforcement prior to casting top screed and top screed in place A. Gupta, S.E. Taylor, J. Kirkpatrick, AE Long & I. Hogg - 3 -

4 A lifting beam was used to lift the arch ring using the three lifting points at 7 th, 12 th and 17 th block. The arch ring is then easily placed on a trailer for flat-pack delivery to the site. On site the arch ring was lifted onto the concrete footing block as shown in Figure 4. The remaining arch units were placed contiguously and a seal made between the units using standard expandable foam. a. seating units in place above foundations b. lifting sequence c. positioning of the first arch ring d. positioning of the subsequent arch rings Figure 4: Construction sequence 2.4 Spandrel walls and backfill operation The first head wall unit was then placed on the inside face adjacent to the outer arch ring and push/ pull props on the inside of the spandrel wall units allowed vertical positioning. The two spandrel walls were then connected using the straps. The arch was then backfilled with a 500mm layer low strength, low slump concrete to both sides of the arch units simultaneously. A minimum concrete thickness of 1000mm was placed from the foundation level. Then backfill, in layers of 250mm simultaneously, was placed to both sides of the arch to the top level of the spandrel wall units. The spandrel walls were then faced with stone and the finished bridge is shown below in Figure 5. A. Gupta, S.E. Taylor, J. Kirkpatrick, AE Long & I. Hogg - 4 -

5 a. placing of the Spandrel wall units b. backfill in place c. stone facing to the bridge d. finished bridge Figure 5: Construction sequence 3. ARCHIE analysis The arch was analysed using ARCHIE (Obvis, 2006), a numerical analysis package which takes into account the arch backfill. It is important to note that this software is also used by the DRD Road Service in Northern Ireland for load assessment analysis of their arch bridges. Therefore, assessment of the load carrying capacity of the flexible arch system using ARCHIE was a critical task in the development of the arch system. The arch unit was analysed under different wheel loading conditions. A typical case of arch unit analysis is shown in Figure 6. An arch unit of the required geometry can be created and loaded with the standard wheel loads. A line of thrust is indicated in Figure 6. Under design loading, the position of the thrust line in the arch unit gave information about the stability of the unit. Furthermore, ARCHIE was able to demonstrate the change in the thrust line by changing the height of the Backing Material (BM) at the springing level and the effect of changing the Passive Pressure (PP). Therefore, for a particular loading condition, arch ring depth and using a the appropriate for BM the passive pressure required to resist the arching thrust is given. Alternatively, the passive pressure factor can be fixed and the minimum arch ring depth established for the given loading conditions. A similar deflection response was given in the ARCHIE analysis in comparison to the test load results. However, the predicted ultimate capacity was conservative based on the actual load carrying capacity of the arch system (which was greater than the applied test loads in excess of the design ultimate loads). A. Gupta, S.E. Taylor, J. Kirkpatrick, AE Long & I. Hogg - 5 -

6 Figure 6: Typical ARCHIE analysis 4. Conclusions and future work Experience has shown that arch bridges are highly durable structures requiring little maintenance in comparison with other bridge forms. Thus, the objective of the new Highway Agency Standard (BD91/01, 2004) is welcomed especially if it encourages a renaissance in arch building using unreinforced masonry materials. The Tievenameena Bridge is the first bridge with the DRD (NI) and it is due to be monitored under service load conditions in the next few weeks. This will provide valuable data on the behaviour of the arch and will be used for validating on-going numerical modelling of the arch behaviour by QUB. Acknowledgements The authors would like to thank the Knowledge Transfer Programme at QUB and Amey for their continued support. Also the Department of Regional Development, the ICE R&D Enabling Fund, Invest NI and CBDG for their financial support. References ARCHIE-M: Masonry Arch Bridges and Viaduct Assessment Software, Version 2.0.8, OBVIS Ltd. UK, British Standards Institute, BS 8110: Part 2: Structural use of concrete: Code of practice for special circumstances Section 9: Appraisal and testing of structures and components for construction, London, British Standards Institute, BS 8110: Part 2: Structural use of concrete: Code of practice for special circumstances Section 9: Appraisal and testing of structures and components for construction, London, Highways Agency (UK), BD37/01, Departmental Standard, Loads for Highway Bridges (used with BS5400: Pt2) Design Manual for Roads and Bridges, Volume 1, Section 3, Part 14, Department of Transport, Highway and Traffic, A. Gupta, S.E. Taylor, J. Kirkpatrick, AE Long & I. Hogg - 6 -

7 Highways Agency (UK), BD44/95, Departmental Standard, The assessment of concrete highway bridges, Design Manual for Roads and Bridges, Volume 1, Section 3, Department of Transport, Highway and Traffic, Highways Agency (UK), BD 91/04, Departmental Standard, Unreinforced masonry arch bridges, Design Manual for Roads and Bridges, Volume 2, Section 2 Special Structures, Part 14, Taylor S E, Gupta A, Kirkpatrick J, Long A E, Rankin G I B and Hogg I, Development of a novel flexible concrete arch system, 11th International Conference on Structural Faults and Repairs, Edinburgh, June Taylor S E, Gupta A, Kirkpatrick J and Long A E, Testing of a flexible concrete arch system, Proceedings of the seventh New York Bridge Conference, August A. Gupta, S.E. Taylor, J. Kirkpatrick, AE Long & I. Hogg - 7 -