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  • 1 Bridge support and protection for a major ocean outfall sewer under extreme aircraft loading; Sheasby

    Bridge support and protection for a major ocean outfall sewer under extreme aircraft loading

    Authors: P.T. Sheasby, K. ONeill, M. Alexander and J.A. Hilton

    Synopsis Sydney Airport Corporation Limited requires the inclusion of a runway end safety area (RESA) at the western end of Runway 07/25 as part of a safety upgrade to cater for the new A380 aircraft and comply with the mandatory requirements of the Civil Aviation Safety Authority. In order to include the required 90 m by 90 m extension to the end of the runway, existing infrastructure within this area requires protection or support using extensive bridge structures. An existing above ground sewer known as the South Western Suburbs Ocean Outfall Sewer (SWSOOS) requires an overhead bridge protection structure that forms the surface of the RESA and in another location requires support where the new realigned airport perimeter road crosses under the SWSOOS. The new overhead bridge structure forming the RESA also extends over the perimeter road. The SWSOOS comprises a 20 m wide three celled reinforced concrete structure, 2.5 m in height and supported on precast driven piles located at a spacing of approximately 3 m. This heritage listed rigid structure was constructed in 1938 and remains in continuous operation. The bridge structure over the SWSOOS which supports the RESA is subjected to extreme live loads exerted when aircraft under full braking conditions enter the RESA. This paper presents the make-up of these loads as well the design of the deck protection structure to account for these loads. This paper also covers the design development of the support structure for the SWSOOS which comprises a large longitudinally and transversely post-tension beam and Slab Bridge, constructed under the SWSOOS. Unlike most bridge structures this structure is designed for control of deflections as it replaces the original support piles. An assessment methodology to determine the allowable deflections the existing SWSOOS can accommodate is also presented.

  • 2 Bridge support and protection for a major ocean outfall sewer under extreme aircraft loading; Sheasby

    1. Introduction The provision of larger runway end safety areas at Australias airports is a mandatory safety requirement set by the Civil Aviation Safety Authority and is in line with international aviation safety standards. Sydney Airport Corporation Limited has begun the construction of a larger runway end safety area (RESA) at the western end of the east-west runway, Runway 07/25. This runway end safety area will include an 8,100 square metre concrete land bridge that will provide a cleared area measuring 90 by 90 metres from the end of the runway strip to assist with deceleration of an aircraft. The introduction of large aircraft like the Airbus A380 has necessitated the extension of runway end safety areas at many airports.

    Figure 1: Airbus A380

    The 07/25 Runway RESA is the last and most complex RESA to be constructed at Sydney Airport Corporation Ltd (SACL) due to its proximity to the Cooks River and various major infrastructure assets, as illustrated in Figure 2. They include Sydney Waters South West Sydney Ocean Outfall Sewer or SWSOOS, the M5 East Motorway Tunnel and the airport perimeter road.

    Figure 2: The new RESA and adjacent infrastructure

    Cooks River

    Sydney Water SWSOOS

    M5 East

    Airside Perimeter Road

    RESA

    Runway 07/25

  • 3 Bridge support and protection for a major ocean outfall sewer under extreme aircraft loading; Sheasby

    In 2006 Aurecon was engaged by SACL to review the preferred option developed to date. It became evident early on that there were major cost implications in implementing the preferred option and SACL increased Aurecons engagement to further investigate other feasible options. The major challenge was to incorporate the new infrastructure requirements within the restricted area bordered by the various significant pieces of existing infrastructure and the Cooks River. New options were considered and a solution that comprised a two lane perimeter road configuration that met with SACLs financial constraints was eventually selected for implementation. Detail design and documentation was then carried out to meet the predetermined construction commencement date of 15 October 2008, being the date for which ministerial approval had been granted for temporary closure of the 07/25 Runway. The major components of the project include:

    A two span bridge structure over the SWSOOS comprised of precast prestressed planks with a cast in-situ top slab. Plank depth varies from 240 mm to 380 mm

    Continuation of the above bridge structure also comprised of two spans of precast prestressed planks with a cast in-situ top slab spanning over ground and the perimeter road structure. Plank depth is 700 mm.

    Continuation of the above bridge structure but comprised of Super T precast prestressed girders with a cast in-situ top slab spanning over the M5 East Tunnel

    The perimeter road trough structure which makes possible the necessary grading of the road below sea level

    A bridge structure which supports the SWSOOS at the location where perimeter road crosses under the under the SWSOOS

    Figure 3: Section through the RESA

  • 4 Bridge support and protection for a major ocean outfall sewer under extreme aircraft loading; Sheasby

    The project has a capital value of approximately $85 million. In addition to SACL, the other asset owners who have an interest in the project are Sydney Water Corporation, owners of the SWSOOS and the Roads and Traffic Authority of New South Wales, owners of the adjacent M5 East Tunnel. While the project includes a broad range of disciplines ranging from airside pavement design, road geometrics, flood hydrology, submerged structures design, significant utility diversion, fire and safety, environmental and foundation engineering, this paper presents aspects that had a direct impact on the structural design of the bridge structures over (protecting) and supporting the SWSOOS. 2. SWSOOS protection structure At the region where the RESA passes over the SWSOOS, the SWSOOS comprises two separate structures. The one portion comprises two cells while the other comprises a single cell. Whereas the single celled structure is founded on a battery of timber piles at approximately 18 m centres and the single celled superstructure cantilevers off these support points, the twin celled superstructure is supported off headstocks spaced at approximately 3 m centres. The twin celled superstructure generally incorporates expansion joints at every fourth headstock support. Headstocks are founded on precast concrete piles driven some 12 m into the ground. The grading of the RESA which ties into existing 07/25 Runway is dictated by the runway levels. At the point at which the RESA crosses over the SWSOOS minimal vertical clearance opportunity exists dictating the use of a shallow deck section over a short span length. Site constraints thus lead to the adoption of precast prestressed concrete planks and Super T girders. The sizes adopted are:

    Span over single celled SWSOOS; Span length 5.5 m Plank depth 240 mm Approximate total width 120 m Depth of cast in-situ topping slab 175 mm

    Span over twin celled SWSOOS; Span length 9.0 m Plank depth 380 mm Approximate total width 120 m Depth of cast in-situ topping slab 200 mm

    Spans over ground and the perimeter road structure; Span length 15 m Plank depth 700 mm Approximate total width 120 m Depth of cast in-situ topping slab 200

  • 5 Bridge support and protection for a major ocean outfall sewer under extreme aircraft loading; Sheasby

    Span over the M5 East tunnel; Span length 25 m Super T depth 1800 mm Approximate total width 120 m Depth of cast in-situ topping slab 200

    Aircraft entering onto the RESA are assumed to be under full braking. Aircraft like the Airbus A380 include three types of support gears or struts, namely the Nose gear, Body gear and the Wing gear. Only the wheels on the Body and Wing gears include brakes. The application of brakes increases the vertical load on the Nose gear. Figure 3 presents typical aircraft manufacturers information on pavement loads.

    Figure 4: The A380 Airplane pavement loads

    While the vertical load on the nose gear is given under Static braking, no information is provided under the condition of Instantaneous braking which generates larger forces. The vertical un-factored load per nose wheel under instantaneous braking can however be determined and from the information in the above table was calculated at 612 KN. The Nose gear for this aircraft comprises two wheels spaced at 1.05 m centres. See Figure 5.

  • 6 Bridge support and protection for a major ocean outfall sewer under extreme aircraft loading; Sheasby

    Figure 5: The A380 Airplane landing gear footprint These two closely spaced nose wheel loads produces a very high concentration of load. Compared with normal road bridge design, the wheel load requirement to be considered in AS5100 is only a single 80 KN load. In addition, there are load factors that need to be applied to the aircraft loading. For this project the following load factors were adopted. The comparative load factors required by AS5100 are also tabulated. AS5100 RESA Project (A380

    Aircraft) Live load 1.8 1.5 Dynamic Load Allowance

    1.4 1.15

    Future increase in load

    N/A 1.18

    Load Factors for Ultimate Limit State

    The load factor adopted for future increase relates to a comparative increase in the operational load of the Jumb