Showing papers in "Structure and Infrastructure Engineering in 2008"

Analytical derivation of the outcrossing rate in time-variant reliability problems

Abstract: The usual approach to time-variant reliability problems is based on the computation of the outcrossing rate through the limit state surface and its time integration. The so-called PHI2 method allows one to compute the outcrossing rate by solving a two-component parallel system reliability problem using the First Order Reliability Method (FORM). Following this approach, the present paper provides new analytical expressions of the outcrossing rate and their implementation. The corresponding improvement of the PHI2 method in terms of accuracy is shown. The method is first validated using a simple time-variant reliability problem, for which an analytical expression of the associated outcrossing rate exists. Then, it is applied to evaluate the reliability of a corroded steel beam submitted to a midspan random load.

Steel corrosion and service life of reinforced concrete structures

Abstract: This paper illustrates the mechanisms of corrosion of steel in concrete, and analyses its influence on the service life of reinforced concrete structures. Even though other types of corrosion are mentioned, attention is focused on the effects of carbonation and chloride penetration. Factors affecting the time to corrosion initiation are described with regards to both concrete properties and environmental exposure conditions. Propagation of corrosion and its consequences on the serviceability and performance of the structures are illustrated. Approaches for the design of durable reinforced concrete structures, as well as options available to increase the service life of structures exposed to aggressive environments, are described.

Probability concepts in engineering: emphasis on applications to civil and environmental engineering

Abstract: Probability concepts in engineering: emphasis on applications to civil and environmental engineering, 2nd edition, by Alfredo H-S. Ang and Wilson H. Tang, John Wiley & Sons Inc., 2007, 406 pp., US$...

Pseudo-static approach for damage identification of bridges based on coupling vibration with a moving vehicle

Abstract: This paper presents a methodology for damage identification based on a time domain approach, considering the coupling vibration between a bridge and a moving vehicle, including the effect of roadway surface roughness. The fundamental idea of the proposed method is to identify damage directly from changes in the element stiffness using a pseudo-static formulation derived from the equations of motion for coupling vibration. The element stiffness index (ESI), which indicates the ratio of a damaged flexural rigidity of the finite element of bridges to an undamaged one, is adopted as the indicator of bridge damage. A numerical example reveals that the concept and procedure suggested in this paper can provide a method for extending engineering choices in damage identification of bridge structures.

Probabilistic limit analysis and lifetime prediction of concrete structures

Abstract: This paper presents a general approach to the probabilistic prediction of the lifetime of reinforced concrete frames with respect to structural collapse. The structural system is considered to be exposed to an aggressive environment and the effects of the structural damaging process are described by the corresponding evolution in time of the axial force – bending moment resistance domains. The collapse load is computed by means of limit analysis. Monte Carlo simulations are used to account for the randomness of the main structural parameters. In this way, both the time-variant probability of failure, as well as the expected structural lifetime associated with a prescribed reliability level, are evaluated. An application to the probabilistic time-variant limit analysis and lifetime prediction of a reinforced concrete arch bridge is presented.

Comparative life-cycle analysis of steel-concrete composite bridges

Abstract: Competitiveness of steel construction and, in particular, steel and composite bridges, requires a broader view that encompasses the concepts of sustainability and life-cycle assessment in parallel with the classical structural and geotechnical issues. Also, from the economical viewpoint, the progressive transfer of operational duties from the National Road Authorities to private companies in the context of design, build and operate contracts, reinforces the need for an integrated approach.It is the purpose of the present paper: (i) to present an integrated methodology for a life-cycle and sustainability analysis (LCA and LCCA); and (ii) to apply such an integrated approach to a case study of a composite bridge. The proposed approach, besides structural considerations, contemplates environmental aspects (energy consumption, raw materials, and environmental impacts), economical aspects and a brief discussion of degradation and maintenance aspects. The case study consists of a three-span continuous bridge wi...

Ultimate strength of plates with random fields of corrosion

Abstract: This paper presents a study of the collapse strength of corroded plates with random spatial distributions of corroded thicknesses. As an alternative to the uniform reduction of plate thickness due to corrosion or a localized area of reduced thickness, the spatial distributions of the thickness of the corroded plate represented by stochastic simulations of random fields are considered. A non-linear time dependent corrosion model is used to define the probabilistic characteristics of the random fields based on corrosion data measured in plate elements at different locations of several bulk carriers. The random fields of corrosion are discretized and the collapse strength of the plate is then assessed by non-linear finite element analysis. The importance of the spatial representation of the corrosion by random fields as an alternative to the traditional approach, based on a uniform reduction of the plate thickness, is demonstrated.

Prediction Models for Serviceability Deterioration of Stormwater Pipes

Abstract: Reducing the number of annual blockages and the consequential flooding events is one of the most important tasks for stormwater pipe infrastructure managers in Australia. Blockages are more likely to occur with pipes experiencing serviceability deterioration, resulting in a reduction of hydraulic capacity. When changing from a problem-based approach to a proactive maintenance and rehabilitation (M&R) approach, the asset managers need predictive information on the serviceability condition of pipes in order to firstly prepare the necessary resources from limited annual budgets and, secondly, to allocate these resources for the maintenance of the deteriorated pipes as precisely as possible. This paper investigates the application of a Markov model and an ordinal regression model for predictions of serviceability deterioration of stormwater pipes. The first model provides the prediction at a network level, which satisfies the first requirement, and the second model predicts serviceability condition for individual pipes, given the attributes of the pipes, in order to satisfy the second requirement. Both models are calibrated using Bayesian inference and Markov Chain Monte Carlo (MCMC) simulation techniques on a dataset supplied from the City of Greater Dandenong, Australia.

Systemic approach for the maintenance of complex structural systems

Abstract: This paper deals with the general framework for the development and the maintenance of complex structural systems. In the first part, starting with a semantic analysis of the term ‘structure’, the traditional approach to structural problem solving has been reconsidered. Consequently, a systemic approach for the formulation of the different kinds of direct and inverse problems has been framed, particularly with regards to structural design and maintenance. The overall design phase is defined with the aid of the performance-based design (PBD) philosophy, emphasizing the concepts of dependability and enlightening the role of structural identification. The second part of the present work analyses structural health monitoring (SHM) in the systemic way previously introduced. Finally, the techniques related to the implementation of the monitoring process are introduced and a synoptic overview of methods and instruments for structural health monitoring is presented, with particular attention to the ones necessary...

Performance of a BOTDR optical fibre sensing technique for crack detection in concrete structures

Abstract: Strain distribution measurements and problematic crack detection are important issues in the damage detection and performance evaluation of concrete, or reinforced concrete, structures. In recent years, the Brillouin Optical Time Domain Reflectometry (BOTDR) based optical fibre sensing technique has attracted great attention as a distributed monitoring method. Current BOTDR instruments are suitable for strain measurements over a certain distance (termed spatial resolution), but damage such as cracks in concrete structures are local. It is crucial to find an effective method to detect local damage in concrete. In this study, two basic optical fibre installation methods, overall bonding (OB) installation and point fixation (PF) installation, are proposed. Then, several unique installation methods (one-round, one-round superposition and two-round superposition) are proposed and investigated experimentally for a reinforced concrete bending beam. The efficiency of the proposed installation methods and the effe...

Rehabilitation of RC frame joints using local steel bracing

Abstract: Reinforced concrete (RC) structural frames that were built prior to the 1970s generally do not meet current design code requirements and may behave in a non-ductile manner. The lateral load carrying capacity of these structures is often insufficient due to non-ductile reinforcement detailing, which includes insufficient or no beam-column joint transverse reinforcement and inadequate anchorage for the beam's bottom reinforcement. Experience shows that such frames are prone to earthquake damage and often suffer shear and bond slip non-ductile modes of failure. Recent earthquakes have demonstrated that the beam-column joint safety is an important factor in keeping the integrity of the entire structure. Concentric steel bracing systems have been used for the rehabilitation of non-ductile RC buildings in a number of countries. However, the application of eccentric (local) steel bracing in the rehabilitation of RC structures still lags behind due to the limited research and information on the design, modelling ...

The effect of chloride-induced steel corrosion on service life of reinforced concrete structures

Abstract: Chloride-induced corrosion is one of the most important deterioration mechanisms in reinforced concrete structures. This study presents detailed theoretical models for predicting service life of reinforced concrete structures exposed to chloride attack. Three stages are considered in the corrosion process: the diffusion period, the rust accumulation period and the crack propagation period. For the diffusion period, the fully coupled moisture diffusion and chloride penetration in concrete are considered as the main driving forces of the deterioration process. Two-way coupled diffusion equations are used to characterize the length of this period. For the rust accumulation period, the chemo-mechanical coupling between the formation of the rust and the development of the interface pressure is considered as the driving force. For the crack propagation period, the interface pressure induced by the rust expansion and the fracture resistance of the cracked concrete determine the rate of crack propagation. The the...

Design and maintenance programme optimization for large infrastructure systems

Abstract: The design, construction and operation of civil infrastructure systems in modern society has become a critical issue worldwide These processes cannot be restricted to solving just the mechanical problem On the contrary, they should be designed within a framework that considers their life cycle, taking into account all costs in which the owner (or operator) will incur during the lifetime of the structure, and the benefits derived from its existence This paper presents a strategy for optimizing the maintenance of technical facilities based on their life cycle within which deterioration and sudden failure due to extreme events may occur A renewal model for the sequence of structural failures is used to define the objective function The proposed model is useful for defining both an optimal maintenance policy (number of interventions and the time between them) and the design parameters It can also be used for investment, planning and operation of new and existing facilities The results have shown that a

Reuse system of building steel structures

Abstract: This paper proposes a reuse system of building steel structures to reduce environmental burden: specific steps to establish this system are also described. The reuse system mentioned in this paper is an overall business system for realizing a cyclic reuse flow through the processes of design, fabrication, construction, maintenance, demolition and storage. In order for structural members to circulate as reusable members in the distribution system, it is necessary to establish a venous industry; as opposed to an arterial industry which would be responsible for production and supply of newly manufactured members. This venous industry is to facilitate the disposal, recycling and reusing of the products from the arterial industry.

A framework for reliability-based system assessment based on structural health monitoring

Abstract: The safety and usability of transport infrastructures is of great importance for the entire society, because disturbances of traffic networks can have significant consequences for the economy and the environment. Especially today, when bad news about deteriorated structures and shortened public budgets are omnipresent, this must not be forgotten. Structural health monitoring (SHM) can help to ensure the safety of deteriorated structures. SHM is expensive and therefore the investigation of optimized inspection and monitoring strategies is an active field of research. The Collaborative Research Centre (CRC) 477 at the Braunschweig University of Technology investigates innovative methods for SHM. In project field A1, a framework for the probabilistic safety assessment of structures has been developed using data from SHM. The current paper describes and explains the methodology of the framework and shows its application using a substitute structure, which was built for the CRC 477.

Bridge rating using in-service data in the presence of strength deterioration and correlation in load processes

Abstract: This paper presents a probability-based methodology for load rating bridges that can accommodate detailed site-specific in-service structural deterioration and response data in a load and resistance factor rating (LRFR) format. The use of site-specific structural response allows the elimination of a substantial portion of modelling uncertainty in live load characterization. Inclusion of structural ageing allows the bridge owner the choice to rate for longer intervals than, say, the usual two-year inspection cycle. This methodology allows the live load-effect sequence on bridges to be statistically stationary with a weakened mixing-type dependence that asymptotically decreases to zero with increasing separation in time, instead of making the common assumption of independent and identically distributed sequences of live loads. In addition, uncertainties in field measurement, modelling uncertainties and Bayesian updating of the empirical distribution function are considered to obtain an extreme value distrib...

Performance evaluation of monumental bridges: testing and monitoring ‘Ponte delle Torri’ in Spoleto

Abstract: This paper reports the main results of the experimental approach used for identifying the structural parameters and monitoring the dynamic response to the environmental loading of Ponte delle Torri (Towers Bridge) in Spoleto, Italy. In particular, the masonry wall drillings and laboratory tests on the cores obtained are described, together with the endoscopic survey and the flat-jack tests aimed to characterize the masonry mechanical characteristics. Then, the main focus is on the monitoring system, which is constituted of displacement transducers placed on the major cracks in the piers, seismic accelerometers placed on the bridge elevation next to the bridge deck, and a system of two ultrasonic anemometers which are used to record the wind speed fluctuations simultaneously with the bridge lateral accelerations. The results obtained give useful information to assess the actual state of the bridge and can be used for future decision making in planning maintenance strategies and restoration/consolidation ca...

A simplified methodology for the evaluation of the residual life of corroded elements

Abstract: Steel corrosion is one of the most common causes of deterioration of reinforced concrete structures. In fact, the chemical attack causes cracking in the concrete and a number of related effects that weaken the capacity of the composite, as well as reducing the cross-section of the rebars. It is therefore extremely valuable to have methods targeted to the evaluation of the structural damage induced by corrosion for estimating the residual load bearing capacity of a structure, and then for planning the maintenance interventions. The aim of this paper is the development of a simplified methodology capable of providing conservative estimates of the residual life of corroded structures. Because of the great amount of results obtained by the authors, general relationships between the corrosion level and the residual strength and ductility of beams are derived, and their evolution is predicted using conventional deterioration models. The methodology proposed is validated using experimental results from literature.

Protection of our bridge infrastructure against man-made and natural hazards

Abstract: Blast mitigation research on a series of bridge components to date has shown both similarities and differences between seismic events and explosive loads in terms of structural response. Similarities on the load side include the fact that both blast and earthquake loads are largely unknown in terms of location, magnitude, intensity, type, etc., and both have characteristics of rapid attenuation with distance from the source mechanism. In terms of consequences, both actions can result in progressive structural collapse, requiring redundant structural systems for mitigation. Significant differences in these two extreme event scenarios also exist; these include load duration and associated strain rate effects and the extent of initial damage or excitation. The similarities and differences between blast and seismic hazards and their responses are explored in a series of blast mitigation field tests, laboratory tests, and computational analyses to form the basis for multi-hazard mitigation in bridge structures.

Curling of concrete slabs on grade: experimental versus theoretical analysis

Abstract: Based on a mathematical approach, Westergaard (1926, 1927, 1943, 1948) derived formulae that predicted the state of stresses and strains in concrete slabs on grade subjected to temperature variations. Although Westergaard made various assumptions, his approach remained a basic tool for the design of concrete pavements. This paper presents a close experimental examination of the deformation of concrete pavement slabs subjected to temperature variations. Full-scale instrumented slabs provided data from key-performance parameters within the scope of a long-term monitoring program where concrete pavement slabs equipped with over 200 sensors are subjected to weather conditions in the absence of traffic loading. A quantitative validation of Westergaard's predictions is presented and a modification to Westergaard's curling strain formula is suggested based on temperature profiles recorded across the slab cross section. Time histories of temperature profiles showed a temperature change at the mid-slab thickness a...

The LQI standard of practice: optimizing engineered safety with the Life Quality Index

Abstract: This paper describes a standard of practice for optimizing engineered safety. The objective in managing risk is to ensure that significant risks are identified and appropriate actions taken to minimize these risks to a reasonably low level. Engineered safety is always determined on the basis of a balance between the cost effectiveness of risk control and the benefits arising from the mitigation of risk. For the net benefit to be positive, whether it accrues to the organization or to society at large, the management of risk entails a process of priority setting, as there are limits on available resources. The net benefit is measured by the consequent increment to the Life Quality Index (LQI). With this approach, enterprises, both public and private, have a more confident and rigorous basis for decision-making and planning engineering projects when life safety is a concern. The evaluation process, the requirements for data to support the analysis, and the major components of the LQI model are illustrated by...

Issues in societal optimal engineering decision making

Abstract: Optimal decision making in the context of engineering is addressed from the perspective of society, with the aim of optimal decision making being understood as to provide an informed basis for the identification of sustainable societal developments. The paper begins with a discussion of the issues that are presently of main concern in engineering decision making from a societal perspective. Following this, a suggestion is outlined for the hierarchical representation of the typical societal organizational instruments for ensuring such optimal decision making. This representation defines the boundary conditions for the optimization of engineering decision making. Thereafter, based on Bayesian decision theory, the main constituents of decision making are highlighted, and the various problems in the representation and treatment of these in the context of decision making are discussed in light of the most recent developments of research in these areas. This includes the representation of society in decision ma...

Health assessment of beams–experimental verification

Abstract: A novel nondestructive structural health assessment procedure now under development at the University of Arizona and presented in a companion paper, is experimentally verified. The experimental verifications of the procedure to identify defect-free and defective fixed ended and simply supported beams are presented in this paper. In this approach, acceleration and rotational time histories are measured at pre-selected node points. They are then post-processed to remove several sources of error including noise, high frequency content, slope, and DC bias. The post-processed response information is then successively integrated to obtain the corresponding velocity and displacement time histories. Even when these sources of error were removed from the response information, the proposed method failed to identify the beams. Several factors including noise, data latency, scale factor and cross coupling error were investigated. Amplitude and phase errors in the accelerometer's measurements were found to be the root...

Numerical study of corrosion crack opening

Abstract: Determining the reliability of reinforced concrete structures, based on visual inspections of corroding cracks on the surfaces of structures, is of great interest. In the present study, models for the deterioration of reinforced concrete structures are presented with a special emphasis on a model for corrosion crack opening. Experiments and theoretical analysis conducted by a numerical finite element method (FEM), support that the reduction of the reinforcement bar diameter due to corrosion is proportional to the corresponding increase in the crack width, measured on the surface of a concrete specimen, for a given time interval. More recently, the constant of proportionality, the so-called crack-corrosion index, has been studied further with respect to its dependence on the diameter of the reinforcement and the concrete cover. In the present paper, the above work is presented and extended with more realistic three-dimensional models of the cracked concrete beam. The crack-corrosion index is evaluated for ...

Identifying bridge structural condition development trends via categorical inspection condition rating with case studies

Abstract: Inspection data in a bridge management system (BMS) may provide us with comprehensive information about a structure's condition history. Identification of bridge structural condition development trends from historic inspection data for maintenance planning has drawn more and more attention, particularly from bridge agencies, who prefer ‘worst-first’ maintenance and rehabilitation strategies. In this paper, rather than correlating inspection data with fundamental structural deterioration models, a statistical inspection data-based analysis was developed. To demonstrate the inspection data-based approach in identifying bridge structural condition development trends in practical scenarios, case studies of concrete deck slabs and steel beam girders were applied. The ANalysis Of VAriance (ANOVA) technique was adopted to identify the major factors considered by a BMS to have a significant influence on the condition index. As a result, three major factors, bridge construction year, inspection year and inspector,...

Health monitoring system of a self-anchored suspension bridge (planning, design and installation/operation)

Abstract: Recent advances in sensing technologies make it feasible and practic al to install sensors on expensive civil infrastructures, such as bridges, for safety monitoring during construction and long-term assessment of the structures condition. This paper presents a case study of a structural health monitoring (SHM) and bridge management system (BMS) installed in Yeongjong Bridge in South Korea. This is a self-anchored suspension bridge on the expressway that links Seoul to Incheon International Airport. Experimental data have been collected from the bridge for two years, since its completion in 2000. This paper presents a list of examples where the structural health monitoring system can be applied and provides general guidelines and recommendations for deploying a monitoring system in terms of (1) sensing and instrumentation, (2) data collection and signal processing, and (3) information processing.

Assessing preservation needs for a bridge network: a comparison of alternative approaches

Abstract: This paper describes a methodology for bridge preservation needs assessment on the basis of two alternative approaches. The age based approach uses specified time intervals between preservation treatments, while the performance based approach uses functional and structural performance thresholds and deterioration functions. Both approaches involve the use of bridge specific parameters and preservation costs. The paper argues that the performance based approach has greater potential to yield more reliable needs estimates. Applying the methodology to a test network of highway bridges in Indiana, it is seen that 25% of the inventory will need preservation during the 2005 – 2015 analysis period at an estimated cost of $882 million. Three hundred and forty-five bridges will need replacement at a cost of $449 million, and 935 bridges will need rehabilitation at a cost of $433 million. The most frequently prescribed preservation activities in the analysis period are deck rehabilitation, deck replacement, and bri...

Health assessment of beams—theoretical formulation and analytical verification

Abstract: Theoretical aspects and analytical verification of a novel non-destructive structural health assessment procedure being developed by a research team at the University of Arizona are presented in this paper. The experimental verification of the procedure is discussed in the companion paper. The health assessments of fixed ended and simply supported beams are specifically addressed. The basic procedure is a finite element-based linear time domain system identification technique where input excitation information is not required. Beams are represented by finite elements. By tracking the changes in the moduli of rigidities of the elements, the current health of the beams can be assessed. Rayleigh damping is used in the dynamic formulation to reduce the size of the identification problem. How to establish the optimal number of finite elements is first discussed. Defects, in terms of two notches, are introduced in an element in fixed ended and simply supported beams. Analytical responses of defect-free and defe...

Evolutionary Design of Structural Systems with Time-Variant Performance

Abstract: This paper presents an evolutionary approach to the optimal design of structural systems with time-variant performance. The complexity of a multidimensional design is handled by introducing a set of quality indices, which represent the multiple targets of design, and by a systemic vision of the structural model, which can be viewed either as a whole having its own emerging properties, or as composed of elementary parts with their own specific characteristics. An overall measure of lifetime structural quality, able to also take the time-variant structural performance into account, is then obtained through a weighted integration of the quality indices over the expected service life of the structure. In addition, in order to overcome the limits of mathematical optimization methods in solving design problems of high complexity and multidimensionality, the optimal structural morphology is searched for through a two-level heuristic approach based on biologically inspired evolutionary procedures. Finally, the ap...

Serviceability behaviour of PC structures by probabilistic and fuzzy probabilistic approaches

Abstract: The evaluation of the safety level in concrete structures should be carried out considering the stochastic behaviour of the main parameters involved, not only under ultimate conditions, but also under serviceability and durability conditions. Particularly in concrete structures, the large variability of mechanical and rheological parameters may give rise to important deviations from the expected behaviour if a deterministic approach is used. On the other hand, it is well known that the probability density function and its parameters cannot be univocally defined. To overcome this problem, the fuzzy probabilistic theory may be used in the processing of stochastic parameters, taking into account their fuzzy nature. In this paper, a probabilistic analysis and a fuzzy probabilistic analysis of a bridge prestressed beam is performed in order to evaluate its behaviour under serviceability conditions, with particular attention to the crack formation limit state. Shrinkage, creep and tensile strength are considere...