Durability design of reinforced concrete structures: a comparison of the use of durability indexes in the deemed-to-satisfy approach and the full-probabilistic approach

Abstract: To show the application of the chloride conductivity index test in service life prediction (SLP) using both the deemed-to-satisfy and probabilistic approaches to performance-based durability design. It is desirable to adopt a performance-based approach with respect to durability design of reinforced concrete (RC) structures. This is based on the perception that the durability of RC is achieved when the limiting value from an established test method is met. In South Africa, the durability index (DI) approach has been developed, which permits performance-based specifications for durability of RC. This approach involves the application of a test method together with a SLP model. This integrated approach links material properties directly with the expected service life of RC structures and environmental conditions. Two DIs are relevant to degradation processes in RC: the chloride conductivity index which is related to chloride ingress, and the oxygen permeability index related to carbonation. The study presented here focuses on the application of the chloride conductivity index as the main input parameter of a SLP model concerned with chloride-induced reinforcement corrosion. The methodology and output of the SLP model as applied in the deemed-to-satisfy approach are compared with those of the probabilistic approach. Both approaches are exemplified using a concrete pier cast in situ in a marine environment. The performance-based durability specifications from the deemed-to-satisfy approach are found to be more conservative compared to those of the probabilistic approach

Neural field models with threshold noise

The original neural field model of Wilson and Cowan is often interpreted as the averaged behaviour of a network of switch like neural elements with a distribution of switch thresholds, giving rise to the classic sigmoidal population firing-rate function so prevalent in large scale neuronal modelling. In this paper we explore the effects of such threshold noise without recourse to averaging and show that spatial correlations can have a strong effect on the behaviour of waves and patterns in continuum models. Moreover, for a prescribed spatial covariance function we explore the differences in behaviour that can emerge when the underlying stationary distribution is changed from Gaussian to non-Gaussian. For travelling front solutions, in a system with exponentially decaying spatial interactions, we make use of an interface approach to calculate the instantaneous wave speed analytically as a series expansion in the noise strength. From this we find that, for weak noise, the spatially averaged speed depends only on the choice of covariance function and not on the shape of the stationary distribution. For a system with a Mexican-hat spatial connectivity we further find that noise can induce localised bump solutions, and using an interface stability argument show that there can be multiple stable solution branches

Analysis of uncertainties in analytical pile design methods in South Africa.

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Buckling uncertainty analysis for steel pipelines buried in elastic soil using FOSM and MCS methods

Generally, buried steel pipes are designed for good transverse behavior by neglecting soil–structure interaction effect. Steel pipelines are also usually designed to prevent from the important failure mode of buckling. However, the design of this type of structures does not normally consider the uncertainties in soil and structural properties. To address the above issues, the paper estimates the uncertainties in terms of the coefficient of variation of critical buckling displacement, CVw using subgrade reaction theory (Winkler model) and first-order second-moment (FOSM) method. Two cases of boundary conditions have been considered in this study. In the first case, CVw is calculated within an infinitely thick soil as a function of uncertainty of subgrade reaction modulus (Ks). In the second case, CVw is calculated in a thick soil cylinder as a function of the uncertainty of the effective subgrade reaction modulus (K′S). Furthermore, the uncertainty of pipe flexibility (Sf) is also taken into account in the two cases. Uncertainty calculations by the FOSM method are then validated with those obtained from traditional Monte Carlo simulations

Geotechnical uncertainty, modeling, and decision making

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