Ghosn M.,City College of New York |
Frangopol D.M.,Lehigh University |
McAllister T.P.,U.S. National Institute of Standards and Technology |
Shah M.,Shah Asso. |
And 7 more authors.
Journal of Structural Engineering (United States) | Year: 2016
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. © 2016 American Society of Civil Engineers.
Kandra H.S.,Monash Univ. |
Callaghan J.,Monash Micro Imaging |
Deletic A.,Monash Univ. |
McCarthy D.T.,Monash Univ.
Journal of Environmental Engineering (United States) | Year: 2015
Biological clogging of filtration and infiltration systems has been acknowledged as a significant problem in the case of wastewater systems. However, scant research has been conducted on biological clogging in storm water filtration and infiltration systems, with the main hypothesis being that biological clogging is insignificant due to the low level of organics present in storm water. This article tested that hypothesis, using a laboratory-based approach. Five replicates of each zeolite-based filter design were dosed with storm water manifesting the following forms: (1) typical storm water (base case); (2) storm water containing very high nutrient concentrations to accelerate biological clogging; (3) typical storm water that was sterilized to suppress biological clogging; and (4) typical storm water with the addition of chlorine to suppress biological activity. The hydraulic performances of these four configurations were monitored over time until the systems were fully clogged. Loss on ignition (LoI) and microscopic analysis of accumulated material within the top layers of the filters were undertaken in an attempt to assess the level of organic matter present in clogging layers of the filters. It was found that all configurations performed differently in comparison to the base case, which represents the most likely set of operational conditions in the field. For instance, the chlorinated filters treated about 30% more storm water compared with the base case. Columns dosed with sterilized storm water treated almost the same volume of storm water but removed a greater quantity of sediment with higher treatment efficiency. Columns dosed with a high level of nutrients clogged more quickly than in the base case. Results of loss on ignition partially confirmed these findings. However, results of microbial cell counts provided tenuous evidence of biological clogging being present. Although the evidence was not overpowering, the variations observed in this study suggest that biological clogging in storm water filters warrants greater attention, which is mostly ignored at present. © 2014 American Society of Civil Engineers.
Amarasiri A.L.,Monash Univ. |
Kodikara J.K.,Monash Univ.
International Journal of Geomechanics | Year: 2015
Approximately uniformly spaced cracking occurs in many different materials and processes during thermal or drying shrinkage. Such cracks may occur when a slab of material restrained at the bottom is subjected to a sudden reduction in temperature at the top. Previous research indicates that the crack patterns obtained depend on the Griffith crack length for a shrinking solid. This paper shows that for materials exhibiting elastic-plastic fracture behavior, the fracture energy, Young's modulus, and tensile strength in combination form a second characteristic length of fracture of the material that has a significant impact on the cracks formed. © 2014 American Society of Civil Engineers.
Chaduvula U.,Indian Institute of Technology Bombay |
Viswanadham B.V.S.,Indian Institute of Technology Bombay |
Kodikara J.,Monash Univ.
Geotechnical Special Publication | Year: 2016
The objective of the present study is to evaluate the effect of geofibers in restraining desiccation cracking of expansive clay, through controlled laboratory tests. In the study, the soil was reinforced with polyester fibers of lengths 15 mm, 30 mm, and 50 mm, at fiber contents of 0.25%, 0.5%, and 0.75%, by dry weight. Saturated soil specimens were allowed to dry at a constant temperature and relative humidity. The loss of moisture, crack initiation, crack propagation, crack morphology, and surficial crack measurements were studied. Digital image analysis was employed to measure the surficial cracks. The results indicate that the desiccation cracking of soil is significantly restrained by geofiber reinforcement due to the bridging effect. The crack area increased with an increase in the length of the fibers. The surficial crack formation of the soil is reduced by 66% when reinforced with 15 mm fibers at a fiber content of 0.5%. © ASCE.