Design Working Life Research Articles

Ultimate behaviour of hybrid stainless steel cross-sections

Hybrid steel plate girders are used worldwide as primary structural members in steel and composite bridges, when there is a need for deeper sections with greater stiffness and bending resistance than rolled sections to carry heavy loads. With the increasing importance of sustainability and lifecycle (cost) analysis, design for maintenance has become an important consideration for infrastructure projects like bridges which have a design working life exceeding 100 years over which they need to be regularly inspected and maintained. Stainless steels are known for their excellent corrosion resistance and low maintenance costs and thus the application of hybrid plate girders in bridge designs could be explored. This paper reports a numerical study on stainless steel hybrid plate girders subjected to compression and to bending and assesses relevant recommendations for their design. Previously developed FE models validated against stub column and four-point bending tests are employed and a parametric study is conducted on hybrid I-sections over a wide range of cross-section slenderness and aspect ratios. Based on the obtained results, the EN 1993-1-4 design predictions for stainless steel cross-sections in compression and in bending are assessed and the accuracy of the codified slenderness limits for both homogeneous and hybrid stainless steel girders is discussed. Furthermore, the numerically obtained deformation capacity at ultimate load is plotted against the CSM base curve originally derived for homogeneous sections and the accuracy of the CSM predictions of the cross-section resistance is also assessed, demonstrating that the CSM can be employed to predict the cross-section resistance of hybrid girders.

On the use of climate models for estimating the non-stationary characteristic values of climatic actions in civil engineering practice

The characteristic values of climatic actions in current structural design codes are based on a specified probability of exceedance during the design working life of a structure. These values are traditionally determined from the past observation data under a stationary climate assumption. However, this assumption becomes invalid in the context of climate change, where the frequency and intensity of climatic extremes varies with respect to time. This paper presents a methodology to calculate the non-stationary characteristic values using state of the art climate model projections. The non-stationary characteristic values are calculated in compliance with the requirements of structural design codes by forming quasi-stationary windows of the entire bias-corrected climate model data. Three approaches for the calculation of non-stationary characteristic values considering the design working life of a structure are compared and their consequences on exceedance probability are discussed.

Comparison study of time-varying seismic fragility of precast segmental and cast-in-place bridge columns in high-speed railway bridges

In recent years, precast segmental bridge columns (PSBCs) have been widely used in high-speed railway bridges, but the lack of long-term seismic performance limits PSBCs application in harsh environments and high-intensity areas. However, the selection of high-speed rail lines in high-intensity offshore areas is inevitable. Therefore, considering the impact of environmental erosion and earthquake, it is necessary to study the seismic performance of PSBC. In order to study the application of PSBCs in offshore high-intensity areas, this paper comprehensively compares the time-varying seismic fragility of PSBCs and cast-in-place bridge columns (CPBCs) in high-speed railway bridges. Based on offshore high-speed railway columns, the finite element models of representative PSBCs and CPBCs are established and verified by experiments. Then, the verified finite models are utilized to implement the time-varying fragility analysis by considering chloride ion erosion under four damage states. The main conclusions are as follows: the exceeding probabilities of PSBCs and CPBCs are close in intact bridges under different damage states and seismic intensities. With the prolongation of bridge service time, the steel bars corrosion rate of PSBCs is faster than that of CPBCs due to the discontinuity of PSBCs segments, the exceeding probabilities of PSBCs increase more rapidly than those of CPBCs. When the columns reach the design working life, the exceeding probability and PGA (Peak Ground Acceleration) median of PSBCs are higher than those of CPBCs under four damage states. Taking the moderate damage state as an example, the maximum exceeding probability of PSBCs and CPBCs were 97.2% and 89.1%, respectively, and the PGA median of PSBCs were 24% lower than that of CPBCs at 100 years. This work provides a theoretical foundation for the better application of PSBCs in offshore high-intensity areas.

Research on Reliability of Structural Members Subjected to Snow or Wind Load for Design Working Life of 100 Years in China

Various national and international standards for building structures and other common structures specify a design working life of 50 years. Therefore, the statistical parameters of loads and the design expressions in current design codes are also based on a design reference period of 50 years. When the design working life is not 50 years, the variable load adjustment factor for the design working life needs to be considered. The corresponding load adjustment factors of office building live load, residential live load, snow load, and wind load are given in (GB50153-2008), (GB50009-2012), and (GB50068-2018) with different design working life (5, 10, 20, 30, 75 and 100 years), respectively. However, the recommended values presented in these documents are inconsistent and no provisions are found in those specifications for the selection of load design parameters in the design expression, which will result in designers having doubts in choosing design parameters, especially for building structures designed for a working life of 100 years. Using different design parameters in the design expression, the implied reliability level of the members with a design working life of 100 years was clarified in the paper. Furthermore, guidance for the specification in actual design is provided. For structural members designed for working life of 100 years, 14 representative structural members were selected to calculate their partial factors of resistance. Considering two simple combinations (dead load and wind load, dead load and snow load) and common load effect ratios, the reliability analysis of each member are carried out according to the load partial factors in China’s old and new codes. The study indicated that the structural importance coefficient of 1.1 needs to be taken in the design expression to increase all the load effects on the structural members designed for 100 years. The basic wind pressure and snow pressure should be taken with a return period of 100 years and the variable load adjustment factor for the design working life should not be considered.

Study on live load reduction factors of train for long span multitrack railway suspension bridges

The reduction factor of live load of train for design of long span multitrack railway bridge is investigated as a parallel research program of design of the Wufengshan Yangtze River suspension bridge in China. Firstly, to simplify the calculation of cable force of the suspension bridge with higher geometry non-linearity, the force history curves of typical suspenders are developed in the simulation of random trains running on the railway on bridge. Secondly, the probabilities of the trains running on the multitrack railway crossing on the bridge are analyzed and the probability models of the force of key suspender is established by Monte-Carlo simulation based on the established suspender force history curves. Finally, the suspender force reduction factors for bridge with multitrack railway on it are determined in the sense of the same exceeding probability. The results indicate that the probability is small for the trains on different tracks crossing on bridge at the same time. The probability distribution of the maximum force of the key suspender within the design working life can be described by a combination of a weighted lognormal and a weighted normal distribution. For the two-, three- and four-tracks railway suspension bridge, the derived reduction factors of the suspender force or the live load of the trains are 0.90, 0.75 and 0.65, respectively. Comparing these factors with those specified in the China railway bridge design code reveals that the reduction factor for two-tracks railway bridge is the same, and somewhat smaller for the three- and four-tracks railway bridge.

Calculation Method and Influence Factor Analysis for the Normal Working Life of Reinforced Concrete Highway Bridges

Aimed at the working life of bridges under the reiteration function of vehicle load, which is not mentioned in the current bridge design codes, this paper introduced the concept of normal working life. The authors regarded reinforced concrete bridges as the research object and proposed a fast method to calculate the working life to judge whether the working life can fit the requirement of design working life. On the basis of the equivalent constant amplitude stress amplitude method of Miner's criterion, the cumulative damages of the bridges were calculated, the average annual damage degrees were obtained by determining the relationship between cumulative damages and times, and the working life of the bridges was estimated. This method was applied to the calculations of T-beam bridges, and the influences of spans, transverse diaphragms, reinforcement ratios, vehicle loads, and S-N curve equations were analyzed. The calculation results showed that vehicle loads and S-N curve equations determine the accuracy of working life. The calculated life in accordance with the fatigue vehicle model in the specification is much longer than the design life. The load varies greatly from region to region. Compared with Guizhou Province, Jiangsu and Liaoning provinces have a higher traffic flow of heavy vehicles over 35 t, which results in years unfilled with the design life. With the increase in spans, the working life increases accordingly because the reinforcement ratios increase with the spans. The diaphragm plates and reinforcement ratios have a great influence on the working life. The working life of bridges with diaphragm plates is 18%-63% lower than that of bridges without diaphragm plates. The reinforcement ratios are lower, and the reduction rates of the working life are greater.

Lifecycle operational resilience assessment of urban water distribution networks

This paper proposes a lifecycle operational resilience assessment framework of urban water distribution networks (WDNs), taking accidents, pipe deterioration, and restoration into account. First, the accident occurrence rates for three common types of pipes, namely, cast iron pipes (CIPs), ductile iron pipes (DIPs), and steel pipes (SPs), are fitted using the maintenance data provided by a water administrative sector. Second, for the two most common accidents, i.e., leakages and bursts, the accident pipes in the former case are simulated by reduced flow, while the accident pipes as well as the influenced ones in the latter case are isolated completely by closing valves, which are determined by a depth-first search method. Third, two restoration strategies are considered, including plugging and replacement, and the corresponding recovery time by days are given based on the maintenance data. Meanwhile, the aging induced pipe deterioration is modeled in the operation process. Finally, the operational resilience of the WDN in Mianzhu city is evaluated by above framework and six resilience strategies are investigated and compared. Results show that replacing accident pipes with new DIPs once burst occurs improve the operational resilience of WDNs most obviously. In addition, replacing CIPs with DIPs ahead of the design working life or adding valves also help to improve the resilience level. Based on the findings, some feasible and practicable resilience enhancement suggestions are concluded to provide guidance for local decision makers.

Changing perceptions: Durability of concrete structures

Studies by PWD show a significantly high percentage of concrete structures are in distress prematurely. Coupled with additional number of new public assets created yearly, the high increase in the amount of public funding required to ensure these assets are in good condition has generated demands from the policy makers for new, innovative and more cost effective solutions to be adopted. In Eurocode 2, performance-based concepts are implemented whereby concrete structures are designed to meet “design working life” (or “service-life” or “repair-free period”) expectancy of 50 and 100 years, rather than the usual design life. Thus, concrete structure designed to Eurocode 2 requires more stringent compliance to the specified cover thickness and quality at site. Compliance to the present MS EN 1992 in itself does not guarantee compliance to 50 or 100 years design working life. The cover requirements in MS EN 1992 are essentially a full adoption of BS EN 1992. This paper discusses the implication of structural design to MS EN 1992 to meet design working life expectancy of the Code. It highlights some priorities that the industry should undertake as a way forward.

Durability for concrete structures in marine environments of HZM project: Design, assessment and beyond

This paper reviews the fundamentals of the durability of structural concrete, and then applies the concepts to the durability design, quality control, performance assessment and maintenance planning of concrete structures in Hong Kong-Zhuhai-Macau (HZM) sea link project with design working life of 120 years. The durability design adopts a multi-level philosophy for different durability risks and uses model-based approach to determine design parameters. The long-term exposure data of 30 years were used to calibrate the chloride ingress model and quantify the modelling uncertainty. The durability quality control was realized through converting the design values to laboratory characterization values, especially for the chloride diffusivity. With the data collected in construction phase, a full-probabilistic assessment is performed on the achieved performance against the chloride ingress, serving as basis for maintenance planning. The open issues related to the durability of concrete infrastructures with long service life are elaborated in the end.

Target reliability of civil engineering structures

The structural reliability recommended in Eurocodes and other international documents vary within a broad range, while the reference to the failure consequences and design working life is mentioned only very vaguely. In some cases the target reliability indexes are indicated for one or two reference periods (in Eurocodes for 1 year and 50 years), however no explicit link to the design working life is usually provided. This article attempts to clarify the relationship between the target reliability levels, failure consequences, the design working life and the discount rate. The theoretical study based on probabilistic optimization is supplemented by recommendations useful for code makers and required by practicing engineers. It appears that the optimum reliability indices depend primarily on the ratio of the cost of structural failure to the cost per unit of structural parameter, and less significantly on the design working life and on the discount rate.

Service life of reinforced structural lightweight aggregate concrete under chloride-induced corrosion

Knowledge of structural lightweight aggregate concrete (SLWAC) durability is still limited, but its potential durability is demonstrated by its good performance in existing structures. This paper focuses on chloride-induced corrosion of the reinforcement in SLWAC, which is one of the most frequent and serious causes of concrete deterioration. Based on a previous comprehensive experimental work, this study aims to predict the chloride-induced corrosion service life of SLWAC produced with different types of lightweight aggregate and types of binder. The service life prediction is based on a performance-based model suggested in the specification E465 (Concrete. Methodology for estimating the concrete performance properties allowing to comply with the design working life of the reinforced or prestressed concrete structures under the environmental exposures XC and XS. LNEC specification, Portugal 2005), considering a safety factor method. New abacuses are defined to predict the service life of uncracked SLWAC as a function of their chloride diffusion coefficient and concrete cover. Finally, limiting values of concrete composition are recommended for the most common uncracked normal weight concrete and SLWAC, for an intended service life of at least 50 years. This deemed to satisfy approach is extended for concrete with fly ash, silica fume and lime filler.

Flexible Plug Joint System with New Material

Up to now the common material used for flexible plug joints of bridges has been based on bituminous mixtures consisting of aggregates and elastomeric-modified binders. All existing national regulations and also the European Guideline for Expansion Joints ETAG 032 are restricting flexible plug joints to these traditional bituminous mixtures. All existing asphaltic mixtures are susceptible for plastic deformations at higher temperatures including rutting whereas they cause significant reaction forces at low temperatures. These properties limit the application of the traditional joints to areas with moderate climate. In practice asphaltic plug expansion joints also often fail in the intersection between adjacent pavement and joint, causing cracks and leakages. The new expansion joint system shows essential improvements in the intersection pavement/joint by mechanical means and uses a new material based on polyurethane, which does not show negative temperature affine properties and can be processed in cold condition. The new flexible plug joint system also enables new applications, both for a wider climatic range and more complicated geometries. Furthermore the new flexible plug joint allows for larger movements up to more than 100mm and addresses a longer design working life. As the new design is not covered by existing standards and verification methods in the first phase a comprehensive research had to executed after consultation with leading European experts from Germany, Switzerland and Austria. The paper covers reports on material and system tests executed during national and European approval procedures at several European testing institutes and shows examples of installation.

Target Reliability Levels in Present Standards

Abstract The target reliability levels recommended in national and international documents vary within a broad range, while the reference to relevant costs and failure consequences is mentioned only very vaguely. In some documents the target reliability index is indicated for one or two reference periods (1 year, 50 years or life-time) without providing appropriate links to the design working life. This contribution attempts to clarify the relationship between the target reliability levels, costs of safety measures, failure consequences, reference periods and the design working life

Optimisation of the target reliability for temporary structures

The target reliability levels recommended in various national and international documents vary within a broad range, while the reference to the failure consequences and design working life is made only very vaguely. In some cases, the target reliability indexes are indicated for one or two reference periods (for 1 year and 50 years); however, no explicit link to the design working life, which is important particularly for temporary structures, is usually provided. This contribution attempts to clarify the relationship between the optimum target reliability of temporary structures, failure consequences, the design working life and the discount rate. The theoretical study based on probabilistic optimisation is supplemented by recommendations useful for code-makers and required by practicing engineers. It appears that the optimum reliability indexes depend primarily on the ratio of the cost of structural failure to the cost per unit of the structural parameter and less significantly on the design working life and the discount rate.

Seismic Assessment of RC Structures in Service Affected by Reinforcement Corrosion

The safety and serviceability of RC structures are time-dependent due to degradation phenomena, therefore assessing the fulfillment of required levels of safety and serviceability up to the end of the design working life of the structure is particular important. Among the most frequent environmental attacks affecting RC structures, reinforcement corrosion is considered the most dangerous source of degradation, which may occur only after depassivation due to carbonation of concrete cover, penetration of chloride ions, or a combination of both. In this paper, the effects of rebars corrosion on seismic response of RC structures were presented. A case study was analysed using time history analyses, in sound as well as damaged conditions, assuming a moderate corrosive attack. The outcomes were discussed in terms of displacement time history curves and acceleration time history curves. In addition, the interstory drift obtained form time history analyses was compared and discussed with the limit value provided by Code for seismic design of building.