6.b Performance of Bridges
Analyses of State Highway Bridges Damaged in the Darfield and Christchurch Earthquakes
J.H. Wood
Twelve State Highway (SH) bridges subjected to strong shaking in the 4 September 2010, Mw 7.1, Darfield and the 22 February 2011, Mw 6.2, Christchurch Earthquakes received significant structural damage. The damage varied from light to moderate but did not require closing the bridges to traffic. The type of damage experienced by several bridges indicated that longer duration shaking than occurred in the Christchurch Earthquake would have caused more serious damage and major traffic disruption. The paper describes the structural damage, and compares the results from detailed structural analyses of one of the damaged bridges with its observed performance.
Seismic Improvement of the SH 5 Mohaka River Bridge
D.K. Kirkcaldie, P. Brabhaharan, C. Wang, E. Blaikie & G. Gregg
The Mohaka River Bridge, constructed in 1962, is a 216 m long steel truss bridge located on the Napier to Taupo section of State Highway 5, 56 km from Napier. The bridge comprises 3 spans of continuous steel trusses plus short land spans at each abutment supported on tall piers up to 39.6 m in height.
Following a detailed seismic assessment, completed in 2004, which identified a range of vulnerabilities to seismic response and recommended strengthening of the bridge, detailed design and construction of strengthening works was undertaken through 2008-09. The adopted performance objectives were for the bridge to:
- Withstand a 1000 year return period earthquake event as the design earthquake event, and
- To avoid collapse of the bridge in a 2500 year return period event.
Structural elements strengthened have included:
- The main spans southern abutment, which provides longitudinal restraint to the bridge as a whole
- The truss transverse bracing system, particularly in the vicinity of the supports
- The connection of the concrete deck to the trusses, and
- The shear keys providing transverse restraint of the superstructure at the main supports
The slope between the southern main abutment and the landspan abutment was also modified to avoid loading the main abutment and stabilised as well the slope below the southern pier being strengthened.
Maintaining the stability and security of the bridge during the truss strengthening works was also a significant consideration.
Post Earthquake Transportation Network Performance: Transportation of Injured to Medical Facilities
S. Rahimian & S. McNeil
Understanding and modelling both the supply of and demand for transportation services after an earthquake event are vitally important for emergency managers, and government agencies to mitigate, prepare for, respond to, and recover from the potential impacts effectively. The changes in the supply side of transportation networks includes failure of and capacity reduction for bridges and roadways, as discussed in several studies; while the demand side, capturing changes in travel patterns, has drawn less attention. Models to estimate travel demand after an earthquake are necessary to estimate the performance of the whole system. The purposes of trips after an earthquake are completely different from the normal situation. This paper analyzes the performance of the transportation system for the purpose of transferring injured to medical facilities.
Using a simple demand model the number of trips are estimated and then assigned to the disrupted network with reduced capacity. Performance measures are computed including the network total delay for medical trips, and potential human loss due to delays. Moreover, to be able to use this study for mitigation planning, probabilistic performance measures based on earthquake scenarios can be developed.
Critically damaged bridges & concepts for earthquake recovery
J. Waldin, J. Jennings & P. Routledge
This paper outlines some of the more significant damage observed to road and foot bridges following the recent series of earthquakes in Canterbury during 2010-11 and offers an explanation of the likely underlying causes of the damage. In particular, the paper considers the effects of liquefaction and lateral spreading, differential settlement and seismic inertia loads on a variety of bridge forms including simply supported, continuous and integral bridges and also bridges on slender piles. The paper highlights some issues addressed when developing temporary and long term remedial options, including those for heritage structures. The main conclusions are that the designer needs to understand how different structural forms behave in earthquakes and detail the structure to account for this. In particular, areas susceptible to liquefaction should have ground improvement to prevent approach damage and severe loading to abutments, or the abutments should be detailed to resist the loads from lateral spreading. Geotechnical investigation needs to be more extensive and accurate for continuous bridges than for simply supported construction and simply supported construction may be more appropriate where differential settlement is likely.
Finally, care should be taken not to introduce unintentional non-symmetric behaviour in the design of new or retrofit bridges.
Damage assessment, analysis and modelling of bridgesin non-liquefiable soil during Canterbury earthquakes
A. Palermo, M. Brando & E. Camnasio
On February 22, 2011 a moment magnitude Mw 6.2 earthquake occurred with an epicentre near the town of Lyttelton, 10 km South of the Christchurch Central Business District (CBD). Though the majority of the observed damage was due to liquefaction and lateral spreading of the river banks, examples of significant damage occurred to bridges on non-liquefiable sites as well. A brief summary of field observations is presented herein, highlighting the main damage typologies shown by the bridges with respect to each structural component. The paper focuses on the seismic performance of two concrete bridges: Moorhouse Avenue Overpass and Port Hills Overbridges. The assessment involved site investigations and numerical modelling, including both quasi-static and dynamic analyses. The models include features such as shear and axial bending interaction which were essential factors to properly capture the seismic performance of the bridges during Canterbury earthquakes. The results of the analyses are consistent with the observed damage.
An improved method for conveying earthquake loss data utilising Monte Carlo simulations
M.R. Cutfield & Q.T. Ma
A simple and intuitive method for estimating the probability distribution of cumulative direct earthquake losses is presented. The method is a Monte Carlo simulation which treats the occurrence of earthquake events as a random Poisson process. Examples carried out using this method show that the probabilistic skew in a cumulative loss distribution is greatest for short considered time periods and for regions with lower overall seismic hazard. Disaggregation and sample loss functions in time are used to examine which intensity level events contributed most to the overall cumulative loss estimates and how often these events occurred. Errors and limitations of the model are briefly discussed, with the conclusion that the presented method is potentially a valuable means of conveying earthquake loss information, as an addition to an expected loss term and/or loss hazard curve.