bridge support and protection for a major ocean outfall ... · 2 bridge support and protection for...

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. O’Neill, 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.


1. Introduction

The provision of larger runway end safety areas at Australia’s 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 Water’s 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


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 Aurecon’s 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 SACL’s 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


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


• 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 manufacturer’s 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.


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 Jumbo aircraft since they were first introduced. Given that aircraft loads are strictly controlled the live load factor of 1.5 is perhaps on the high side but as the structural adequacy of the deck structures was determined to be workable with the adoption of this high factor, it was retained at this higher value. The relevant point however, that is the pair of nose wheels impart a significant load onto the deck and support structure well in excess of that usually used in the design of road bridges.


This significantly higher load is however catered for in the design of the superstructure. In the longitudinal direction the prestress in the planks is increased and in the transverse direction the significantly increased sag bending is resisted by using closely spaced large diameter reinforcing bars. While the Nose gear loads produce more severe global effects on the short span plank superstructures, the heavier but more widely distributed loads of the Wing and Body gears produce more severe effects on the longer spans, particularly the spans comprised of Super T girder units. By comparison, a Body gear imparts a total un-factored load of 1,655 KN over an area of some 8 m2 while a single lane of SM1600 loading imparts a comparative 1,590 KN over a larger 80 m2. Although aircraft loading is significantly larger than used for roadway bridge design, the adoption of standard precast deck units was determined to be possible. Although heavy, both prestress quantities and deck slab reinforcement demands were determined to be within practical limits. 3. SWSOOS Support Structure

As the perimeter road comes out from underneath the bridge structure which comprising the RESA, it has to traverse under the SWSOOS. The existing closely spaced support piles carrying the SWSOOS have to be removed and replaced with an alternative support mechanism. The adopted solution comprises the construction of a new post-tensioned concrete support structure which spans across the perimeter road and supported on large diameter piles taken to bedrock. At this location the SWSOOS is a combination of interlinked three, two and one cell reinforced concrete box sections that span between adjacent closely spaced headstocks. Expansion joints are present at some headstocks. The existing SWSOOS with its current support structure is shown in Figure 6.

Figure 6: Existing SWSOOS structure

The new support structure comprises post-tensioned concrete edge beams with a post-tensioned transverse slab carrying the dead and imposed liquid loads from the SWSOOS. This new structure is supported on four 1200 mm diameter cast in-situ concrete piles founded on bedrock some 20 m down. At the tops of the piles flat jacks are incorporated to allow for level adjustment


against initial and future pile settlement and axial shortening during take-up of the dead loads from the existing SWSOOS. The existing headstocks remain in place and are encapsulated by the slab of the new support structure. The existing concrete piles are however removed to make way for the perimeter road. The geometry of the existing headstock arrangement makes the placement pattern of the tendons in the transverse slab unique. A three dimensional model was produced to align the tendons in both the transverse slab and the longitudinal edge beams to ensure no clashes and satisfactory fit. The transverse tendon layout was thus governed by the geometry and location of the existing headstocks and the interface with the longitudinal tendons in the edge beams. In the transverse direction an additional stiffening edge stiffening beam was also found to be necessary. Post-tensioning is applied to a level required to control deflections. Unlike most bridge structures, the SWSOOS support structure is required for the control of deflection of the SWSOOS it supports. Figures 7 and 8 present the main features of the SWSOOS support structure.

Figure 7: New SWSOOS support structure (note encapsulation of existing headstocks)

Figure 8: Cross section through SWSOOS supports structure

Considering that deflection control is the most important design criteria, the construction details adopted ensure that the original support points to the SWSOOS remain unchanged, that is, they remain at the at the existing headstocks. The four support pile positions were set by geometric constraints


and are located outside the footprint of the perimeter road structure which will run underneath the support structure. A three dimensional model finite element model of the new transfer slab with the existing SWSOOS in place was developed using the design software package LUSAS Bridge Plus. The software was chosen for this complex analysis as it has the function to: a) Model multiple prestressing cables that curve in both the vertical and

horizontal planes. b) Carry out a contact analysis between the existing headstocks that

support the SWSOOS and which are encapsulated within the new slab structure, and the SWSOOS, both in the short and long term.

c) Combine volume and thick shell elements in a single model. The model was built using a series of volumes linked together by common nodal points. Each volume was assigned with hexahedral brick elements which the analysis software automatically meshed to ensure the mesh in adjacent volumes were aligned and connected. The SWSOOS structure and existing headstocks to remain in place outside the plan area of the new transfer structure were made up of a series of plates, assigned with quadrilateral thick shell elements. The expansion joints between adjacent sections of the SWSOOS were also modelled as these provide articulation between adjacent sections. Post-tensioning cables were accurately modelled to include the drape on each individual cable in both the vertical and horizontal planes. Short term losses and long term losses due to creep, shrinkage, friction, wedge slip etc were accounted for in accordance with AS5100.5-2004.

X

Y

Z

Figure 9: The structural model


To accurately model the interaction between the new SWSOOS Support structure and the SWSOOS, a method known as “slidelines” was adopted. This contact analysis method allows structural elements that are in contact but are not physically connected to move relative to each other until such a time as equilibrium of the system is found. The stiffness of both slabs and their relative deflections are considered in this analysis including the hog due to post-tensioning. Given the rigidity of both structures, this analysis was considered the most appropriate to accurately model the interaction between the two structures. The non-linear contact analysis carried out to check the structural interaction between the existing SWSOOS and the new support structure confirmed that the deflections were acceptable and that the induced actions on the SWSOOS are within the capacity of the reinforced concrete wall elements comprising the SWOOS. From the analysis outputs, the contact analysis confirms that the SWSOOS moves relative to the support structure, but within acceptable limits. The presence of expansion joints within the SWSOOS contributes to the favourable results.

Loadcase: 2:Increment 2

Results file: RESA - SW SOOS Support StructureRev108.m ys

Entity: Displacement

Component: DY

0.663074E-3

0.473624E-3

0.284175E-3

94.7249E-6

-94.7249E-6

-0.284175E-3

-0.473624E-3

-0.663074E-3

-0.852524E-3

-1.04197E-3

-1.23142E-3

-1.42087E-3

-1.61032E-3

Maxim um 0.68984E-3 at node 11498

Minimum -1.67828E-3 at node 23581

X

Y

Z

Figure 10: Deflection contours

(maximum short-term deflection 1.6 mm downward)

4. Concluding remarks

The loads from the new generation aircraft like the A380 when entering the RESA are significantly greater than those normally associated with road bridge design. However, these heavy applied loads have not precluded the adoption of standard precast and prestressed girder planks and girder units for successfully carrying these aircraft loads. An increase in the amount of


prestress in the precast elements as well as an increase in the reinforcement within the cast in-situ topping slab ensures adequate load transfer. The SWSOOS support structure which spans some 20 m provides an altered support condition to the SWSOOS. The inclusion of post-tensioned cables limits the deflections the SWSOOS is subject to, ensuring controlled displacement. Anticipated movements and resulting actions have been determined to be within the serviceability load carrying capacity of the SWSOOS structure, thus not adversely impacting on the functionality of this major piece of infrastructure. The use of an advanced computer model and inclusion of non-linear contact analysis to model the interaction between the SWSOOS and support structure has shown that the SWSOOS can be successfully supported on the new structure.

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