Events >> Abstracts: Australasian Structural Engineering Conference - 2001
Seismic Assessment and Concept Retrofit Design of the Shell Gully Overbridge
Stuart Oliver (Holmes Consulting Group)This paper describes the procedure used for the seismic assessment and concept retrofit design of the Shell Gully Overbridge and the associated Clifton Terrace and The Terrace on and off ramps. The Shell Gully structures are located in Wellington, New Zealand in a region of relatively high seismicity. They were designed in 1970 and form an important transport link from Wellington City to the North. A preliminary seismic screening study of State Highway bridges in the Wellington Region had previously indicated that the Shell Gully Structures were potentially vulnerable should a large earthquake occur.
The main Shell Gully elevated motorway structure is 500 meters long and is comprised of a concrete 'double-tee' superstructure supported on 12 m high monolithic reinforced concrete piers. The on and off ramp structures are both approximately 200 m long and consist of concrete 'single tees' supported on monolithic reinforced concrete piers.
The purpose of the seismic assessment was to determine the probable seismic demands on the Shell Gully Structures, assess the likely performance of the structures and to provide scheme and rough order cost information for any retrofit work required.
Analysis of the structures was generally based on the recommendations given in "Seismic Design and Retrofit of Bridges" written by Priestley et. al. In accordance with the project scope a relatively simple assessment procedure was adopted for the investigation, with the goal of the analysis being to get a clear understanding of the likely performance of the structures and any potential short comings. The Institute of Geological and Nuclear Sciences Ltd were commissioned to undertake a hazard investigation of the Shell Gully site and produce a site specific response spectra for the assessment. Probable seismic displacement demands on the structures were then estimated by undertaking a series of three dimensional elastic modal analyses using the site specific response spectra generated above. The results from these analyses were used to identify the critical bents within the structures and to estimate the peak displacement demands the bents were likely to sustain under various levels of earthquake (i.e. 2/3 current code, current code and maximum credible).
Two dimensional non-linear pushover analyses were undertaken on the critical bents identified above to determine the strength and deformation demands on the critical members. The analyses provided general information on the location and extent of earthquake induced damage to the bridge, and the shear force and curvature ductility demands on the critical elements.
Results of the analyses indicated the bridge was generally well detailed when compared with current building code requirements and is expected to perform adequately in a major earthquake. Proposed retrofit works identified in the assessment to improve the performance of the structures were:
Increase the seating lengths provided to the simply supported 'link spans' to prevent premature loss of seating and 'drop-off' in a moderate to large earthquake. A retrofit scheme employing steel corbels bolted onto the existing concrete support beam was proposed.
Installing 'catch cables' to the heavy precast concrete gutter units which maybe susceptible to 'knock-off' in a moderate to large earthquake.
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The Influence of Loading History on the Seismic Performance of Concrete Structures
Darrin Liddell, Dr Jason M Ingham & Dr Barry J Davidson
(Department of Civil and Resource Engineering, University of Auckland)Currently there is considerable interest in displacement-based methods of structural design, with the expectation that these methods may lead to more rational, safer, and cost-effective designs. One of the key features of displacement-based design is the selection of a target ultimate displacement, with this decision being partly based on the performance of laboratory subassemblies.
There is considerable variability in the laboratory loading regimes used worldwide, despite the fact that it is widely appreciated that the adopted loading history influences performance. The most distinct illustration of this is to compare the response of two test units, one tested monotonically to failure, while the other is cycled to failure.
On this basis, twelve nominally identical reinforced concrete beams, which had been designed for peak loads in a fully ductile 10-storey building, were tested using different loading histories. Seven of these histories were based upon laboratory procedures used in differing parts of the world, and five were based upon analytically-derived demand associated with measured earthquake records.
It was established that the ultimate displacement of the test subassemblage was dependent on the loading history, that damage indices were a useful measure of damage, and that the loading history typically employed in New Zealand resulted in comparable response to that derived from earthquake records. Details of this testing will be reported.
Composite Down-Stand Steel Beam Behaviour with a Profiled Deep-Deck Slab
Clark Hyland and Charles Clifton (NZ Heavy Engineering Research Association)
John Butterworth (Dept of Civil and Resource Eng, Univ of Auckland)
Steve Stickland (Corus New Zealand Ltd.)Reliable composite down-stand steel beam behaviour can be developed with deep-ribbed in-situ concrete slab systems, such as with the ComFlor 210 profile. Use of long shear studs and appropriate amounts of transverse confinement reinforcement at the base of the studs results in very ductile ultimate limit state and reliable serviceability limit state system performance.
This paper proposes characteristic inter-face shear capacities and more general design expressions and detailing recommendations for down-stand steel beams made composite with ComFlor 210 slabs. A summary of the push-off testing used to derive the inter-face capacities and the design expressions for inter-face shear is included.
Cyclic Fracture Limit States in Seismic Resisting Steelwork Structures
Clark Hyland, (NZ Heavy Engineering Research Association)
W.G. Ferguson (Dept of Chemical and Materials Eng, Univ of Auckland)Cyclic fracture limit states need to be considered in the design of steelwork structures resisting earthquake loads. The Northridge and Kobe earthquakes in the mid-1990's caused low energy fractures to occur in a number of modern steelwork structures. This paper provides some guidance to structural engineers to assist them to design and specify structural steelwork in such a way as to confidently suppress these important limit states.
The paper discusses the various fracture limit states that can occur in steelwork structures that must resist earthquakes. High energy fracture and low energy fracture limit states are identified and the relevant plastic strain hardening mechanisms and stress intensity effects involved in their development described. Design features that may contribute to the suppression of low energy fracture limit states are discussed. Design approaches are reviewed or proposed for each limit state and recommendations for further research and development are made.
