A case study of performance based seismic design (PBD) for a critical wharf facility is presented and compared to conventional design methods. The Oil Wharf at the Lyttelton Port is a critical infrastructure link for New Zealand’s South Island. The wharf suffered extensive damage due to the Canterbury earthquakes and is to be replaced with a new structure. The design of a replacement wharf utilised PBD principles throughout every aspect of the project, including probabilistic seismic hazard assessment, selection of design ground motions, nonlinear site response analyses and nonlinear, dynamic deformation and soil structure interaction analyses.
The site is located on soft marine sediments only four kilometres from the epicentre of the M6.3 22 February 2011 Christchurch earthquake. A unique strong motion dataset is available at the site; a seismograph on soft soil is located 100m away and another on rock is located 900m away. Site response analysis using the recorded rock ground motions as input was able to match the recorded soft soil ground motions over a range of earthquake sizes. Site response analyses were then used to derive the design spectra for structural design.
Nonlinear deformation analyses (NDAs) were able to replicate key displacement mechanisms observed in the Canterbury earthquakes. Predicted displacements matched the surveyed slope deformation data which enabled real world validation of the analyses results. A series of NDAs were performed to design the new wharf structure. Performance based design resulted in significant construction cost savings for the design of the new wharf structure when compared to conventional design methods.
Lyttelton Port of Christchurch (LPC) has experienced many large earthquakes and thousands of aftershocks during the 2010-2011 Canterbury Earthquake sequence. Consequently, LPC and their consultants and contractors have a very clear understanding of Performance Based Design as they have observed the actual performance of their land and structures through a wide range of earthquake magnitudes.
An example of one of their damaged structures is the Oil Berth, which is a critical facility that supplies oil, petrol, LPG and bitumen to New Zealand’s South Island. The Oil Berth suffered extensive damage in the 2010-2011 Canterbury Earthquake sequence.
A replacement wharf has been designed which will replace the existing wharf. This paper describes the performance-based design of the replacement wharf, including the following key aspects:
- Utilising the extensive post-earthquake observations (including survey measurements, structural inspections, strong motion data) to benchmark geotechnical analysis and design
- Design to the performance based ASCE 61-14 code for the Seismic Design for Piers and Wharves using advanced methods for site response analysis, seismic hazard assessment and dynamic non-linear deformation and soil structure interaction analysis
A comparison between the performance based design approach and conventional design is presented.
2 LYTTELTON PORT OIL BERTH
The Oil Berth is located in Lyttelton Harbour, New Zealand. The existing wharf structure comprises a wharf supported on timber piles, pipelines, a reinforced concrete seawall, mooring bollards and thrust blocks. Fender blocks are supported on four 600mm diameter steel encased concrete piles.
A new oil wharf, 63.3m long by 15m wide is proposed. The proposed wharf has a 0.8m thick reinforced concrete deck and is supported on 39 no. 1.2m diameter steel encased concrete piles. A 36.5m long by 6m wide access jetty will link the wharf with the shore, which will incorporate a 0.8m thick reinforced concrete deck.
The site is situated on reclaimed land that overlies harbour deposits consisting of soft to stiff silts with medium dense to very dense sands from approximately 40 m depth and volcanic boulders and rock at approximately 50 m depth. The reclamation process involved construction of a breakwater by end dumping quarried basalt gravel on top of the natural marine deposits. Progressive slope failures were reported during construction. Reclamation of land behind the breakwater was then carried out with hydraulic fill dredged from the harbour bed.