Peter Tanner

Lessons from incidents attributable to the uncertainties in bridge launching illustrated by a case study

Like other competitive bridge erection techniques, incremental launching is a highly vulnerable process that entails considerable risks to persons. In steel and composite bridges, in particular, patch loading and other mechanisms that induce instability must be avoided. Considerable uncertainties are associated with both resistances and support reactions during launch operations due to the broad range of factors involved. Nonlinear finite element (FE) analysis is very useful for the explicit verification of both local and overall system stability during steel structure launching. Monitoring, in turn, may be a very powerful tool for reducing the risks associated with such operations. Further research is needed both to establish safety levels for temporary structures and on-site activities and to develop a suitable design format for nonlinear FE analysis. However, in everyday practice, engineers and builders must deal with much more basic problems, often related to the interaction between the structure under construction and the ancillary resources used, as exemplified in the case study described in this contribution. The lessons learnt from such incidents are extremely useful for improving the strategies presently in place to reduce construction-related risks.

Experimental and numerical study of bond response in structural concrete with corroded steel bars

Corrosion can affect the bond between reinforcing bars and concrete and hence the transfer of longitudinal stresses. Although a number of experimental studies on bond failure have been conducted in recent years, the findings have diverged rather widely, due primarily to differing test conditions. The present paper reports on an experimental programme consisting of eccentric pull-out tests run on corroded steel bars in specimens subjected to accelerated or natural corrosion. An axisymmetric bi-dimensional FE model with finite deformations initially developed to study bond mechanics with sound steel bars, has been enhanced to consider bond effects in corroded steel bars. The model simulation is compared to some of the experimental results for corroded and sound bars and the findings are analysed.

Bond Response in Structural Concrete with Corroded Steel Bars. Experimental Results

The growing interest in upgrading existing reinforced concrete structures or extending their service life, and in ensuring greater durability in new designs, has led to a need for resistance models that take deterioration processes into account to verify structural safety. Bond activation between reinforcing steel and concrete is of cardinal importance in this context. A number of experimental studies have been conducted in recent years on bond failure, which normally leads to brittle behaviour. The findings have diverged rather widely, however, due primarily to differing test conditions. The present paper presents an experimental programme for eccentric pull-out tests in which specimens were subjected to both accelerated and natural corrosion in an attempt to surmount these inconsistencies. It also introduces an embedded fibre-optic sensing system with corrosion-resistant fibre Bragg grating sensors and discusses some of the findings.

Roof structure for the new Zaragoza Delicias station. Concept and design

One of the main infrastructures on the new high-speed railway from Madrid to Barcelona is the Zaragoza Delicias station with its outstanding roof. The paper describes the basic ideas for the conceptual design of the roof structure, bearing in mind the complex interactions between geometry, functionality, construction materials, manufacturing, erection, structural overall-concept and detailing. The adopted solution is described and the importance of a consistent conceptual design is emphasised in order to obtaining a reliable, functional and economic structure. Some hints concerning the construction are also given. The example shows that the success of a structural design strongly depends on a good co-operation between the owner, the architect and the structural engineer. It further shows that an architectural design may be improved through structural considerations.

Salto del Carnero railway bridge, Saragossa, Spain

Structural Engineering International 3/2006 In most public works, economic constraints are a decisive factor in the adoption of a given structural design from amongst various feasible options. In the present case, economic criteria were particularly important, for it is the owner’s intention to dismantle the bridge after a relatively short service period as a result of alignment changes planned for the railway it carries. In the absence of any further information about the estimated service period at the design stage, however, it was felt that structural analysis and design should be undertaken to conservative criteria, assuming the normal notional service life for railway bridges (100 years).

Widening of the Elche de la Sierra arch bridge, Spain

This article describes the design and construction process for widening a reinforced concrete double-rib arch bridge in Spain. The deck needed widening from 6 metres to 9 metres. The conceptual design involved the construction of a new cast-in-place concrete slab, with the existing deck slab acting as formwork for the central part of the new slab and being incorporated into the structural system of the widened bridge. Structural assessment was conducted to determine whether the bridge in its new construction would be serviceable. This was based on a combined application of deterministic and probabilistic assessment methods, using information from the time of the original construction, a visual inspection and a geotechnical survey. In construction, the existing concrete cover was partially removed to leave strips which assisted in bonding of the new slab. A travelling formwork was used to cast the new deck slab cantilevers. The bridge was load-tested after paving operations were complete.

Thoughts on Construction Risk Mitigation and Acceptance

Abstract Civil construction is a highly uncertain endeavour that entails a considerable risk, as attested to by the succession of incidents and accidents reported the world over. The reasons for this situation need to be identified before strategies can be adopted with a view of reducing the construction-related risks. Taking the forensic investigations of a series of accidents as a starting point, this article explores the generally applicable lessons learnt from those case studies. Such surveys show that the problems to be dealt with in everyday practice are rather elementary and often attributable to human error. Basic principles and recommendations are therefore put forward for the design, erection and use of temporary structures in general and ancillary equipment for construction processes in particular, aimed at reducing the risks associated with such activities. Moreover, as there is no general consensus on reliability requirements for the temporary structures, this paper addresses the establishment of acceptable life safety risk and the associated reliability levels. The requirements put forward aim to ensure the same statistical fatality rate per unit of time as for the permanent structures. The effect of the established principles, recommendations and requirements on the design and assembly of the temporary structures is illustrated by means of an example.

Indicators for the performance assessment of road bridges through monitoring

Different causes may lead to the non-compliance of a particular requirement related with an existing infrastructure. Often, they may be traced back to deviations from expected actions or resistances. The quantification of parameters related with such influences may provide evidence about the degree of compliance of a given structure with a particular serviceability or safety requirement. Such parameters may therefore be called indicators and associated threshold values can be established on a risk basis, as well as admissible average frequencies for outcrossing. Indicators can be monitored comparing the measured values continuously to the previously established threshold values. Alarm systems may be installed which are activated in case of outcrossing. Safety measures can therefore be adopted depending on the consequences of the observed non-compliance. Based on such an approach and by using modern information technology, inspections of large infrastructures may be automated and optimised. A series of indicators for use in road bridge inspection are proposed hereunder, together with their respective threshold values and allowable mean frequency of outcrossing. The paper also includes a practical application in which these criteria are applied to a road bridge with an unknown reliability level.

Learning from incidents during bridge erection

Bridge building is a highly uncertain endeavour that entails considerable risk, as attested to by the succession of construction-related incidents and accidents recently reported in Spain and elsewhere. While efforts are being made to improve on-site safety, many issues are still outstanding, such as the establishment of reliability requirements for the ancillary systems used. The problems that must be dealt with in everyday practice, however, are more elementary and often attributable to human error. The overall organisation of the use of bridge construction equipment is in need of improvement. Close cooperation between the bridge engineers responsible for construction planning and ancillary element suppliers is imperative, for flawed interaction between building equipment and the bridge under construction may generate structural vulnerability. External quality assurance should likewise be mandatory

Application of risk analysis in structural engineering – gas explosions

The present contribution describes the development of methods and tools intended for the practical application of explicit risk analysis in structural design. More specifically, it deals with the risk of personal injury caused by structural damage resulting from gas explosions, regarded as a significant threat to structural safety. Risk quantification entails the use of models able to quantify the uncertainties associated with the basic variables involved. A probabilistic model for the predominant action involved in explosions, explosion pressure, was deduced from test results available in the literature. This model was subsequently used to analyse the reliability level of a representative series of reinforced concrete beams against the implicit requirements laid down in building codes. The wide scatter observed is an indication of the lack of consistent calibration of current design rules. The findings may be used in future studies to deduce a consistent level of acceptable personal risk associated with gas explosion-induced structural collapse.