1. Introduction
With the rapid development of China’s urbanization, transportation has become increasingly important [
1]. As a link connecting cities, transportation directly affects the lives of urban residents, and its quality is a direct reflection of the degree of urban development. Bridge structures are an important part of the transportation system and play an important role. The reinforced concrete (RC) simply supported girder bridges, with the characteristics of higher quality and lower cost, have been widely used for small-to-medium-span highway bridges in China [
2]. The bridge structure in service is subjected to the coupling effect of environmental erosion and traffic load, which will cause the aging of concrete components, further reduce the bearing capacity, and even cause the collapse of the bridge [
3,
4,
5]. The collapse of a bridge often means not only economic losses and traffic jams, but also irreparable casualties [
6]. In order to ensure the remaining service life of a bridge structure and the safety of people’s lives and property, damage detection and performance evaluation of structures have developed rapidly, providing a basis and guidance for bridge maintenance and management decisions [
7,
8,
9]. In recent years, algorithmic and mathematical methods have been widely used in structural damage detection, such as the Kalman filter and eigen perturbation theories [
10,
11]. However, there are many random factors in the performance evaluation of a bridge structure, such as resistance and load effects [
12,
13]. Therefore, probabilistic methods are often used to evaluate the performance of a bridge structure, such as reliability and fragility models [
14,
15,
16]. In this paper, reliability is selected as the probabilistic measure for structural safety assessments.
A bridge structure is a complex system with multiple components. There exist many failure modes, such as flexural failure and shearing failure [
17]. The reliability of a single component cannot reflect the overall performance of the bridge structure. For structural designers, performance is often concerned with the reliability evaluation of bridge systems. In order to evaluate the system reliability, multiple integrals are often calculated. For a complex bridge structure, it is not only difficult to obtain the joint probability density function for each random variable, but it is also difficult to calculate multiple integrals. Therefore, some approximate calculation methods to find the system reliability are proposed, such the wide bounds [
18] and narrow bounds methods [
19]. However, these approximate calculation methods only consider the linear correlations between failure modes and ignore the nonlinear correlations. The evaluation results cannot accurately reflect the actual condition of the bridge system.
In order to obtain an accurate evaluation result, the copula theory has been introduced into system reliability evaluations recently. The copula function can not only describe the nonlinear correlation between failure modes, but can also solve the joint failure probability of multiple failure modes. Liu et al. [
20] presented mixed copula models for the system reliability evaluation of a multiple-component system by considering the nonlinear correlations between failure modes, and utilized a simply supported cored slab bridge to illustrate the feasibility and application of mixed copula models. Sun et al. [
21] proposed a reliability model of failure crack propagation based on a time-varying copula and calculated the time-varying joint failure reliability by considering two failure modes in a numerical example. Fan et al. [
22] proposed a new model to evaluate a time-dependent bridge system reliability by considering nonlinear correlations between failure modes based on a Gaussian copula function and Bayesian dynamic linear models, and used an actual series system with two failure modes to illustrate the feasibility of the proposed model. Lu et al. [
23] proposed a method for the system reliability analysis of engineering structures by considering multiple dependent failure modes based on a copula and introduced copula functions into the narrow bounds theory. Song et al. [
24] applied a copula technique to describe the nonlinear dependence among typical component seismic demands of the commonly used concrete continuous girder bridge style and analyzed the influence of the real dependence among components on the seismic reliability. Fang et al. [
25] used a copula function to model the dependency between two degradation processes in a bivariate degradation model of a coherent system. Wang et al. [
26] proposed a system reliability evaluation method based on non-parametric copulas and utilized four examples to illustrate the feasibility and efficiency of the proposed method. Significant studies have been devoted to the reliability evaluation of complex systems based on copula functions. However, most studies only use the binary copula function to consider the case where two failure modes occur together. For a bridge structure composed of multiple components, the system reliability evaluation may consider the occurrence of multiple failure modes simultaneously. Therefore, a multivariate copula function needs to be constructed.
In order to evaluate the bridge system reliability, the relationship between individual components and the overall system should be identified first, such as a series system, parallel system, and series-parallel system. This relationship is also called the failure criterion of the bridge system. Liu et al. [
20] proposed a failure criterion where the failure of any two adjacent components would cause the entire system to fail and calculated the system reliability using a simple supported hollow slab beam as an example. Tu et al. [
27] assumed that the failure of any three adjacent beams would lead to the destruction of the entire structure and calculated the system reliability of a widened T-girder bridge using the proposed failure criterion. Wang et al. [
28] proposed a semi-analytical method to assess the time-dependent reliability of an aging series system subjected to non-stationary loads and considered that the failure of any single component would cause the entire system to fail. However, the selection of the failure criterion is ambiguous in the system reliability evaluation of the bridge structure. There are no rules for determining which failure criterion should be chosen. Therefore, multiple failure criteria should be comprehensively considered to make the evaluation result more reasonable. The AHP–EW method is introduced to solve this problem, which can obtain the comprehensive weight of the subjective weight and the objective weight of each evaluation index. The analytic hierarchy process entropy weight (AHP–EW) method is gradually being accepted and recognized in various research fields [
29,
30,
31].
This paper develops a novel method that can be used to evaluate the system reliability of the bridge structure by considering multiple failure criteria based on copulas and the AHP–EW method. In order to obtain accurate evaluation results, the system reliability evaluation under each failure criterion was used to adapt the copula theory. By considering the joint failure of multiple components, the multivariate copula functions were constructed. The AHP–EW method was used to comprehensively consider the subjective and objective weights of each failure criterion in the system reliability evaluation and construct the analytic hierarchy model. The specific process of the system reliability evaluation using the method proposed in this paper is shown in
Figure 1. Then, a simply supported RC hollow slab bridge composed of nine slab segments was selected as the numerical example to illustrate the applicability of the proposed method.
3. System Reliability Evaluation Method That Considers Multiple Failure Criteria
The system failure probability under different failure criteria can be calculated based on copulas. However, there is currently no standard in the system reliability evaluation of a bridge structure to decide which failure criterion to choose. In order to comprehensively consider multiple failure criteria, the AHP–EW method was applied to establish the model of bridge system reliability evaluation and calculate the comprehensive weight of each failure criterion. Peer experts were invited to score each failure criterion based on their knowledge and experience. The possible scores ranged from 1 to 5. The higher the score was, the more reasonable the failure criterion was.
3.1. AHP(Analytic Hierarchy Process) Method
AHP is a subjective decision-making method for solving the complex problems of multiple objectives [
41]. Its core idea is to determine the weight of evaluation indexes according to the experience of experts. Here, we take three failure criteria as the evaluation indexes of the system reliability.
The specific steps are as follows.
(1) Establish the hierarchical mode for system reliability evaluation:
In order to evaluate the bridge system reliability by considering multiple failure criteria, a target layer and an index layer are established. The target layer represents the system reliability by considering multiple failure criteria. Each index in the index layer represents the system reliability of each failure criterion. Then, a hierarchical structure between the target layer and the index layer is established, as shown in
Figure 5.
(2) Construct the judgment matrix .
The judgment matrix is constructed based on the scores of peer experts for each index. Equation (13) shows the definition of the element
in the judgment matrix
, in which
indicates the relative importance of the
ith index over the
jth index.
where
and
indicate the scores of the
ith index and the
jth index, respectively.
is an element of the judgment matrix.
(3) Calculate the weight vector
:
(4) Calculate the maximum eigenvalue
:
(5) Check the consistency of the judgment matrix
A:
where
CI and
CR represent the consistency index and the consistency ratio, respectively.
RI is the random consistency index shown in
Table 2.
n is the number of indexes.
3.2. Entropy Weight Method
Entropy theory was first introduced to information theory as a measure of information disorder by Shannon [
43]. The entropy weight method, an objective weighting method, utilizes entropy theory to determine the index weight according to the information provided by the observations of each index. The importance of the index is reflected by the degree of deviation between the observations of the same index. The greater the degree of deviation, the smaller the entropy value, and the corresponding index weight should be larger [
44]. The objective weight calculation procedures of the indexes are as follows.
(1) Construct the evaluation matrix M:
Let the evaluation matrix M = ()m×n be an information system according to the knowledge and experience of peer experts.
Normalize the evaluation matrix M:
In order to eliminate the impact of different index dimensions on evaluation, the evaluation matrix needs to be normalized.
where
is the standardized value of the
jth index of the
ith evaluation sample.
and
represent the number of experts and the number of indexes, respectively.
(2) Calculate the characteristic proportion (
) of the
jth index of the
ith evaluation sample:
which satisfies
.
(3) Calculate the entropy value (
) of the
jth index:
where if
or
,
.
(4) Calculate the difference coefficient (
) of the
jth index:
(5) Calculate the entropy weight (
) of the
jth index:
3.3. System Reliability Evaluation That Considers Multiple Criteria
After calculating the weight of each index using the AHP and EW methods, the subjective and objective weights of each index were multiplied and normalized to obtain the comprehensive weight
, as shown in Equation (23):
It can be seen from Equation (23) that it not only considers the knowledge and experience of peer experts to scale the indexes, but also distinguishes the quality of the index data, overcomes the shortcomings of the single-weighting method, and makes the evaluation result more reasonable.
The system reliability under each failure criterion can be obtained based on copulas. According to the analytic hierarchy model shown in
Figure 5, the system reliability obtained by considering multiple failure criteria is a weighted summation of the system failure probability under each failure criterion, as shown in Equation (24):
where
is the system failure probability obtained by considering multiple failure criteria.
is the system failure probability under the
jth failure criterion.