Auckland Harbour Bridge SkyPath Cycle/Walking project: Important information on the structural integrity of the Auckland Harbour Bridge from NZTA

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Auckland Harbour Bridge Pathway Structural Feasibility StudySummaryThe Pathway Working Group (PWG) has proposed a shared footway/cycleway on theAuckland Harbour Bridge to be supported from the eastern side of the southbound extensionbridge. NZTA commissioned Beca to assess the structural implications of the proposedPathway on the bridge seeking to find a feasible solution that may be taken forward fordesign development. PWG members, Airey Consultants and Copeland Associates Architectsdesigned the Pathway. Beca has assessed the load effects on the box girder extensionbridge.This summary outlines the scope of the structural feasibility study and the key findings fromthe load assessment identifying further steps that will be necessary to develop the Pathwayproposal.Section 1.0 describes the structural load assessment carried out in J une 2011 using NZTAstandards. The findings of the assessment were that unrestricted loading on the Pathwaywould exceed the load-carrying capacity of the bridge over a proposed 20 year operatingperiod.Section 2.0 describes a pedestrian load management proposal by the Pathway WorkingGroup that would allow use by limited numbers of pedestrians if NZTA permitted thisdeparture from loading standards. Traffic loading from a September 2011 study and historicload data was used to predict future traffic load growth. The estimates of allowablepedestrian numbers were based upon future load growth predictions.Section 3.0 describes the sensitivity of the assessment to possible variations in loading andoutlines further design development that will be required to take the project forward topreliminary design stage.A cross section of the proposed footway/cycleway is shown in Figure 1 below.

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1.0Scope of Structural AssessmentThe purpose of the assessment was to determine whether the box girder structures werecapable of carrying local and global Pathway loads together with southbound traffic loadingand to feedback findings to the Pathway Working Group so that a feasible solution satisfyingNZTA's requirements could be identified.The structural load assessment was carried out to NZTA standards as covered by the NZTABridge Manual.Pathway dead loads were provided by Airey Consultants based on the concept design.Traffic loading for the assessment was taken from a bridge specific assessment live load forsouthbound traffic from 2005 increased by 10% to account for changes in heavy vehiclecontent and loading since then. The Assessment Loading was applied for the purposes ofthis study before the updated load study was available. Pedestrian live loads were derivedfrom standardBD37/01 in accordance with the Bridge Manual as applicable to steel boxgirder bridges.Local cross girders supporting the Pathway were assessed for shear and bending effectsusing previously calculated girder load-carrying capacities.Global load effects were calculated and compared with the capacity of the bridge which wasstrengthened in2010. The most critical areas of the bridge were typically those that hadpreviously been strengthened as much as possible, and these were checked for anyincrease in load effects from the Pathway loads.NZTA requires at least 20 years of forward traffic load growth margin for the southboundextension bridge to allow unrestricted traffic movement. To investigate potential future trafficloading in 2031 a prediction of future traffic load growth was made by extrapolating forwardfrom historical southbound traffic load data and reported in the Southbound Extension BridgeTraffic Load Study 2011. Itwas found that the 2031 loading corresponded to approximately10% traffic load growth above the Assessment Loading. The full scope and findings of theassessment was reported in the AHB Pathway Concept Structural Assessment TechnicalReport. A summary is given below.1.1Structural Assessment FindingsA range of footway/cycleway options were assessed starting with the proposed 4m wideshared use footway!cycleway, followed by a similar facility reduced to 3.2m wide over thenavigation span and adjacent spans, and a 2.5m wide option.The cross girders were assessed to have sufficient local capacity to support all options.Pier brackets which support the box girder at piers 4, 5 and 6 were found to requirestrengthening to cater for increased loadings from traffic and Pathway combined,The box girder bridge structure was found to be most critical in the mid-span region of thenavigation span and also to have limited capacity at piers 1 and 2. Piers 3, 4, 5 and 6 werealso found to have load effects that in some areas were anticipated to exceed capacities

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within the future 20 year period, however, it is recognised that these areas can bestrengthened further to increase load-carrying capacity.For all options most areas of the bridge were found to have sufficient traffic load growthmargins. However, the structural capacity of centre of the navigation span was the limitingfactor and assessment findings described in this summary relate only to that critical area ofthe bridge.The 4.0m and 3.2m wide options were found not to satisfy assessment standards using fullunrestricted pedestrian loading. The 2.5m wide option was assessed to have a traffic loadgrowth margin representing approximately 10 years of estimated traffic load growth. Thisoption, however, was deemed unacceptable by the Pathway Group on aesthetics andoperational safety grounds.To summarise the assessment findings for an unrestricted access pathway:

4m wide pathway was found to have a shortfall in bridge capacity of approximately8% of Assessment Loading and a shortfall of 18% for the anticipated traffic loads in2031

3.2m wide pathway over spans 1,2 and 3 was found to have a shortfall in bridgecapacity of approximately 1% of Assessment Loading and a shortfall of 11% for theanticipated traffic loads in 2031

2.5m wide pathway was found to comply with standards for the Assessment Loadingbut to have a shortfall in capacity of approximately 4% for the anticipated traffic loadsin 2031

In conclusion the box girder structure was found to have insufficient capacity to carry theAssessment Loadings for unrestricted pedestrian use. In order to find an acceptable feasiblesolution that provided the required margin for NZTA's unrestricted future operation of theAHB a reduction in Pathway loading was found to be necessary.

2.0PathwayGroup Proposal for Pedestrian Load ManagementThe Pathway Working Group proposed a 4m wide Pathway with control of pedestrianloading on the bridge by managing the total number of people that can access thewalkway/cycleway facility at any given time.The Pathway is proposed to have toll gates and security staff controlling entry and exit fromthe bridge. The toll gates would count the number of people on the Pathway and restrictaccess if the maximum number were reached. It is proposed that the facility would haverules, signage and closed circuit TV in place to ensure that users do not congregate inlocations other than on proposed observation decks at piers.The Pathway would have tailsafe systems enabling it to be closed off completely at bothends in extreme weather events or if other issues arose.

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Special events would be specifically catered for, for example, by arranging traffic laneclosures on the bridge.An appropriate operation and management plan will be prepared by the Pathway WorkingGroup to identify measures to enforce the proposed load limiting strategy.If a reduction in standard code-derived pedestrian loading were permitted by NZTA a rangeof people numbers allowed on the Pathway at anyone time was estimated, depending onthe weight of the facility and other load effects ye t to be assessed fully. Further pathwaydesign development and detailed bridge assessment is necessary before the full extent ofload effects can be accurately determined and so the numbers of allowable pedestrians onthe bridge are an approximation atthis stage. The estimated maximum number of peoplethat could be allowed on a 4m wide Pathway at anyone time are based upon anticipatedtraffic loading predicted from future growth in heavy vehicle percentages. Due to theuncertainties in estimating these people numbers they are not a reliable basis for anyeconomic feasibility of the Pathway.The estimated number of people that could be allowed to use the Pathway at anyone timebased upon March 2011 traffic loading for the critical 3-4pm weekday period was found to beapproximately 600.Based on anticipated 3-4pm weekday period traffic loading predicted from future growth inheavy vehicle percentages, the estimated number of people that could be allowed to use thePathway at anyone time in 15 years time was found to be approximately 350.Care must be taken when using historic data to predict future traffic growth as changes inlane use, heavy vehicle regulations, economic growth and other extemal factors can causean increase in loading in a short period of time.The maximum pedestrian numbers would need to be reduced inthe future if traffic loadingincreased. Traffic loading would need to be monitored at regular intervals to make sure thebridge capacity was never exceeded. Measured traffic loading would be used to determinethe reductions in numbers of pedestrians that may be necessary to allow unrestrictedvehicular use of the bridge.Inthe following stages ofthe project optimisation of these numbers will be sought byinvestigating the potential for:

A lighter or narrower Pathway Control limits on pedestrians or segregation of cyclists in the critical spans Adjustment of traffic loading in the future.



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3.0 Design Development and Assessment Sensitivi tyThe AHB Pathway proposal is at concept stage and the current feasibility study has beenbased upon load assessment using 2011 load data. Assumptions have been maderegarding information that is not yet developed such as final Pathway weights, temperatureand wind effects on the Pathway and box girder strengthening loads. The load growthmargins for the Pathway were found to be small in the mid-span area of the navigation spanand so the assessment is sensitive to possible variations in applied loading.The design of the under-slung Pathway and the required strengthening of the supportingstructure will be developed as the project proceeds. More detailed assessment will also becarried out to evaluate the effects on individual structural elements within the box girders. Atthe feasibility stage a sensitivity analysis of the potential variability in loadings was carriedout to assess the risks associated with variability of loadings.The sensitivity analysis included:

Pathway dead load, surfacing, barriers and services weights increasing ordecreasing Box girder strengthening extent increasing or decreasing Differential temperature effects subject to detailed analysis increasing Wind load effects yet to be included Traffic load growth increasing or decreasing

The key variables were assessed to be traffic and temperature load effects which havedominant impacts on the bridge. The final maximum people numbers estimated abovecannot be determined until further assessment of these effects is available including theresults of wind tunnel tests.At this feasibility stage normal load factors have been applied in the assessment. In additionto its proposal to control pedestrian live load the Pathway Working Group has also proposedto more accurately determine the weight of the final pathway by weighing units andcomponents and thus apply a reduced dead load factor.The departure from the design standards for reductions in pedestrian loading would need tobe approved by NZTA.The Pathway Group has also suggested that itwould like to consider the possibility of areduction in load factors using probabilistic analysis of loading combinations applyingalternative approaches such as embodied in European standard EN 1990. This could beinvestigated as the project is progressed beyond feasibility. The further development ofdesigns and the appraisal of alternative design criteria will form part of the next stages of theproject.

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Auckland Harbour Bridge P athway (Skypath)The Auckland Harbour Bridge (AHB) "clip-ons" recently underwent significantstrengthening and the $86 million project was successfully completed in 2010.Providing that freight loadings are appropriately managed on the AHB, as the citygrows, the bridge has many decades of useful life as a strategic infrastructure assetfor Auckland and the nation.In addition to being one of the busiest stretches of State Highway 1 the AHB alsoaccommodates critical city infrastructure such as water supply, telecommunications,electricity and natural gas.The next Harbour Crossing is forecast to be required around 2030, subject to theactual level of freight loading growth. At this time freight and through traffic wouldtransfer to the additional (new) harbour crossing while the existing AHB would revertto local commuter and public transport (bus) traffic.The NZTransport Agency (NZTA) has worked collaboratively with the Pathway Trust,Auckland Transport, Architects, Engineers and the Auckland Council to investigateand agree a feasible engineering solution for a cycling and walking structure to beattached to the existing Harbour Bridge. This has been achieved (see attached) andfunded by the NZTA during 2011 and 2012. The proposed structure needs to besubjected to wind tunnel testing and this will be undertaken in 2013.The project is being lead by the Pathway Trust and is now at a point where theAuckland Council is being requested to partly fund (and partly underwrite) theproject.NZTA regards this project as a community facility, to be paid for largely by anadmission charge from the users. The pathway would be allowed to be attached tothe Harbour Bridge structure through a license to occupy at peppercorn rental.There is minor strengthening work still to complete on the AHB to support Skypath.Engineers are working to agree this detail and cost. Depending on the $amountrequired (currently in the range of $1-3 million) the NZTA may be able to contributetowards these costs.The structures required at the north and south ends of the bridge to connect fromthe ground to the Skypath are additional to the existing Skypath concept design.The NZTA has no alternative plans for a walking and cycling facility across the AHB.For more information on the Auckland Harbour Bridge:Stephen TownRegional DirectorAuckland/Northland

Steve MuttonRegional Asset ManagerAuckland/Northland

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Auckland Harbour Bridge SkyPath Cycle/Walking project: Important information on the structural integrity of the Auckland Harbour Bridge from NZTA