146 © 2019 The Hong Kong Institution of Engineers HKIE Transactions | Volume 26, Number 4, pp.146–156 • https://doi.org/10.33430/V26N4THIE-2019-0023 * The first author who was at the age of 35 or below on the closing date of submission for the HKIE Outstanding Paper Award for Young Engineers/Researchers 2019. Automated assessment of reinforced concrete slabs using a pseudo-lower bound method: Case studies ABSTRACT Despite the widespread usage of reinforced concrete slabs in construction industry, nowadays disasters due to sudden failure of such structures still occur. Assessment analyses of reinforced concrete slabs can nowadays be performed with a multitude of techniques, but many such methods are computationally too onerous, non-automated or over- conservative. This paper proposes applications of a novel pseudo-lower bound method for evaluating the load-bearing capacity of slabs and estimating the most critical collapse mechanism. Such applications compare the results of analytical solutions, experiments and other bound methods with those obtained with the proposed pseudo-lower bound method. The case studies show evidence that great accuracy is achieved in terms of both estimation of load-bearing capacity and detection of collapse mechanism. The analysed case studies include different geometries, boundary conditions, loads and reinforcement layouts. Such numerical applications are presented in order to benchmark the accuracy and usefulness of the method. KEYWORDS Computational mechanics; concrete plasticity; limit state; lower-bound method; reinforced concrete slabs; yield-line Michele De Filippo * and J S Kuang Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, People’s Republic of China. CONTACT Michele De Filippo Received 15 March 2019 michele.defi[email protected] 1. Introduction Structural assessment of reinforced concrete slabs is required for designing new structures and estimating the load-bearing capacity of existing structures. In design phase, structural analyses require “safe” designs, so that newly-constructed structures at least satisfy the required design load-bearing capacity by a certain margin. If safety factors are not implemented in the analysis, designers cannot be confident that the design admits a safety margin and hence the analysis cannot be considered as “safe”. “Unsafe” designs can lead to premature collapses, which may subsequently cause financial losses, damages or more importantly casualties. The same concept applies for assessments of existing structures. In such cases, unsafe analyses are obtained through overestimations of collapse loads, leading to inaccurate evaluations of structural adequacy and missed structural rehabilitations. A study of the Federal Highway Administration (FHA) estimated that every day in the US there are approximately 185 M crossings underneath 56,000 structurally deficient bridges (Cook, 2014; Federal Highway Administration, 2019). In the UK, it was found that 20% of collapsed bridges failed after erroneous structural assessment (Brinckeroff, 2003). The above statistics suggest that there is still room for improvement in the accuracy of methods of analysis for assessing reinforced concrete slabs and the necessity for further research on the topic is evident. It is a common practice to perform assessments of reinforced concrete slabs with limit state analyses, specifically through upper or lower bound methods (Chen and Han, 1988). Upper bound methods seek a kinematically admissible collapse mechanism minimising the load-bearing capacity, whilst lower bound methods seek a plastically admissible stress field, maximising the load- bearing capacity, hence these methods tend to respectively over- and underestimate capacities of slabs (Johnson, 2006). The two methods are complementary, and an exact solution for a given problem is reached when upper and lower bounded solutions match. Lower bound methods, due to their conservativeness, always guarantee a “safe” assessment, whilst upper bound methods tend to produce “unsafe” estimations. The literature on proposed limit state analyses for assessing slabs is wide ranging (Denton and Burgoyne, 1996; Nielsen and Hoang, 2016), with hand upper and lower bound techniques being the pioneer limit state methods (Hillerborg, 1996; Braestrup, 2009). Over the past decades, the constant desire for improvement in this field and the mature understanding of the structural performance of slabs have led to significant progresses in terms of computer aided upper and lower bound analyses of reinforced concrete slabs (Johnson, 1996; Thavalingam et al., 1999; Krabbenhoft and Damkilde, 2003). The most up-to-date automated upper bound method is provided by Gilbert et al. (2014) and He et al. (2017), who developed a technique based on a discontinuity layout optimisation algorithm for finding an upper bound solution on mesh-less geometries. Such technique showed evidence of providing accurate results. However, it is upper-bounded, thus the outputs may tend to “unsafely” overestimate the load-
Automated assessment of reinforced concrete slabs using a pseudo-lower bound method: Case studies
Jun 24, 2023
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