Design for serviceability: evaluation and modeling methodologies
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Serviceability (structure)
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Serviceability (structure)
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One important aspect in the design of many geotechnical structures such as tunnels, retaining walls, and piles is the serviceability failure performance of the structure, i.e., when the actual displacement of the structure exceeds the maximum allowable displacement. A risk-based approach to serviceability performance "failure" is necessary to incorporate systematically the uncertainties associated with various design parameters. This paper demonstrates the use of an integrated neural network-reliability method to assess the risk of serviceability failure through calculation of the reliability index. An illustrative example for a braced retaining wall is presented using this approach.
Serviceability (structure)
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This paper compares the different reliability-based criteria adopted by different structural design and safety evaluation codes. Ongoing evolution from point-in time, member-oriented, reliability-based criteria to risk-based structural system criteria is described including recent proposals to establish quantifiable measures of structural redundancy and robustness. Issues related to the implementation of reliability-based serviceability and durability criteria are also summarized.
Serviceability (structure)
Robustness
Structural Reliability
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The implementation of reliability methods for designing new structures and assessing the safety and evaluating the performance of existing structures and infrastructure systems has gained widespread acceptance. Consequently, reliability-based design specifications in the form of load and resistance factor design (LRFD) methods have dominated the development of current codes and standards. This paper reviews the reliability-based performance criteria used to calibrate design and evaluation codes and standards for assessing the strength, serviceability, and fatigue resistance of structural components. The review shows that large differences exist in the target reliability levels adopted for evaluating the strength of various types of structural members and materials. These differences result from many factors, including (1) intended structure design and service life; (2) expected member modes of failure (e.g., ductile or brittle); (3) importance of the individual member to overall system integrity (secondary member, column, or connection); (4) experiences with previous designs; (5) material and construction costs; (6) structure type and occupancy; and (7) risk tolerance of the engineering community and the public within a code's jurisdiction. For other than seismic hazards, current specifications remain primarily focused on the evaluation of individual structural members and components, although recently proposed performance-based design (PBD) procedures apply varying target member reliability levels that depend on structure categories, modes of failure, and required levels of structural performance. The implementation of reliability-based durability criteria in design standards is still a subject of research owing to difficulties encountered in modeling material degradation mechanisms and their interactions and in the collection and mapping of long-term site-specific data on degrading agents. Because of large epistemic uncertainties, the evaluation of the fatigue safety of structural components in engineering practice still relies on conservative basic models of damage accumulation using S-N curves or basic fracture mechanics crack growth models. Overall, reliability-calibrated structural standards are producing designs that offer a good balance between safety and cost. The future implementation of risk-based methods will further enhance the ability to meet structure-specific performance requirements set by owners and users.
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In this paper, the author advances a fuzzy set theoretic approach that enables the environmental present serviceability index (PSI) loss determination to be accomplished in a better defined and repeatable manner using the same AASHTO procedures. In this approach, the designer's qualitative descriptions of the relevant attributes are transformed to well defined fuzzy sets stored in the computer. At this stage of the development, fuzzy sets mathematics principles take over the manipulation process dictated by the AASHTO guidelines, to predict the PSI loss thus ensuring repeatability and consistency. The new technique is further illustrated by predicting the PSI loss anticipated due to swelling using a computer program developed by the author.
Serviceability (structure)
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Serviceability limit state (SLS) considerations often govern the design of foundations. The objective of this paper is to review the limited literature on reliability-based design for SLS and to suggest useful directions for future work. A review of several new design codes indicates that ultimate and serviceability limit states are not verified in the same way. The former check conforms to reliability principles, but the latter is essentially verified in a deterministic way. To address SLS within the reliability framework, it is necessary to characterize the uncertainties associated with foundation displacements and tolerable displacements. The former is discussed with particular attention to spatial correlation, model uncertainties, and load-displacement nonlinearities. Uncertainties in tolerable displacements are discussed using two case histories.
Serviceability (structure)
Limit state design
Foundation (evidence)
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This paper analyzed factors that affect the serviceability of roads. Based on the analysis, a fuzzy comprehensive assessment model was established using the fuzzy mathematical method. The model was then applied to survey data of the serviceability of a freeway with reasonable results.
Serviceability (structure)
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Serviceability (structure)
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This paper describes a methodology that aims to produce a performance assessment model by applying techniques of expert and knowledge based systems, which have been used for failure analysis in other branches of engineering. Existing data from sources such as repeat CCTV surveys, event histories and hydraulic models can be used to identify changes in the internal condition of sewers over time. Pipes are categorised into either "no change" or "rapid change" groups and related to i) their physical attributes / susceptibility to surcharging / local environment and ii) to their past performance, to determine the influence deteriorating condition has had on performance. This will allow a model database to be collated of pipes in many different circumstances, with known performance histories. From this a series of cases can be built up forming a suitable archive from which to predict suitable management strategies of any pipe of known characteristics, using techniques such as case based reasoning (CBR) and classification by similarity (CBS). The paper also evaluates the current criteria for targeting maintenance activity in the UK and considers the extent to which existing "rules" are capable of identifying performance deficiencies in a sewerage network (in contrast to condition deficiencies). Suitable database structures and appropriate case retrieval functions/algorithms are discussed as a first step in the development of a tool for prioritising the asset management of sewers with a high probability of rapid deterioration and impaired future performance.
Serviceability (structure)
Sanitary sewer
Sewerage
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