Integrating automated machine learning and interpretability analysis in architecture, engineering and construction industry: A case of identifying failure modes of reinforced concrete shear walls Dong Liang 1 and Fan Xue 2∗ This is the peer-reviewed post-print version of the paper: Liang, D. & Xue, F. (2023). Integrating automated machine learning and interpretability analysis in architecture, engineering and construction industry: A case of identifying failure modes of reinforced concrete shear walls. Computers in Industry, 145, 103883. Doi: 10.1016/j.compind.2023.103883 The final version of this paper is available at https://doi.org/10.1016/j.compind.2023.103883 The use of this file must follow the Creative Commons Attribution Non-Commercial No Derivatives License, as required by Elsevier’s policy. Abstract Machine learning (ML) has been recognized by researchers in the architecture, engineering, and construction (AEC) industry but undermined in practice by (i) complex processes relying on data expertise and (ii) untrustworthy ‘black box’ models. As a result, ML results of complex non-linear AEC problems, such as the failure mechanism of reinforced concrete (RC) shear walls, are not comparable with empirical and mechanics-based models. This paper aims to integrate automated ML (AutoML) and interpretability analysis to study the failure mechanism of RC shear walls. In this study, we collected a dataset of 351 comprehensive samples for the failure mode identification of RC shear walls. First, the AutoML model trained using the dataset outperformed a set of conventional ML methods in terms of the F1 accuracy score. Then, three model-agnostic interpretability analysis methods confirmed the trustworthiness of the AutoML model. The contribution of this paper is three-fold. First, AutoML sheds light on the automatic identification of failure modes of RC sheer walls. Second, the interpretability analysis can validate ‘black-box’ ML models against long-established domain knowledge in solving non-linear AEC problems. Third, for AEC industrial practitioners, the whole process is automatic, accurate, less reliant on data expertise, and interpretable. Highlights • Integration of automated Machine Learning (AutoML) and interpretable analysis for accurate and trustworthy ML • A comprehensive dataset of 351 tests of failure mode identification of RC shear walls • A ‘black box’ AutoML model outperformed five conventional ML algorithms in terms of F1 accuracy score • Complementing local and global interpretability analysis methods for explaining and validating the AutoML model • Trustworthiness of the AutoML model confirmed with domain knowledge via interpretability analysis results 1 Dong LIANG, MSc, PhD student. leodong@connect.hku.hk , : https://orcid.org/0000-0002-2918-7898 Department of Real Estate and Construction, The University of Hong Kong, Pokfulam, Hong Kong SAR, China; Email: 2 Fan XUE, PhD, Assistant Professor. xuef@hku.hk , : http://orcid.org/0000-0003-2217-3693 Department of Real Estate and Construction, The University of Hong Kong, Pokfulam, Hong Kong SAR, China; Email: *: Corresponding author, Tel: +852 3917 4174, Fax: +852 2559 9457; Email: xuef@hku.hk 1 Keywords Machine learning; Interpretability analysis; RC shear walls; Failure mode identification; AutoML 5 10 15 20 25 30 35 1 Introduction Architecture, engineering, and construction (AEC) researchers have been increasingly interested in complex non-linear problems (Su et al. 2021). In civil engineering, for example, traditional physics-based models often cannot achieve accurate results and efficiency at the same time. A typical problem is the failure mode identification of reinforced concrete (RC) shear walls, a vital lateral-force-resisting component for buildings in wind-prone or earthquake-prone areas (Farrar et al. 1993). The failure mechanism, involving various geometrical configurations and material parameters, is too complex to formalize empirical or mechanics-based models. The failure mode identification of RC shear walls used to depend on computation-intensive and time-consuming continuum-based finite element models (Dashti et al. 2014); the high costs restricted its capability in many cases, such as building vulnerability analysis and regional risk assessment (Mangalathu et al. 2020). Machine learning (ML) has been used to analyze AEC problems (Singh 2021) due to the data availability (Vadyala et al. 2022). In ML, target variables are predicted using data science assumptions without references to rigorous physical theoretical inference (Montáns et al. 2019). Example applications are structural behavior prediction (Luo & Paal 2022) and structural health monitoring (Zhang et al. 2020). In fact, advanced ML techniques provide possible alternatives to physics-based models for computationally demanding numerical modeling, as well as dangerous and expensive experimental testing (Deka 2019). However, ML applications in AEC have lagged behind expectations (Deka 2019) due to two groups of possible reasons (Kuwajima et al. 2020): (i) complex data-expertise-reliant modeling process and (ii) untrustworthy ‘black box’ models (Feng et al. 2021). First, developing a high-quality ML model is time-consuming and requires multiple steps and expert knowledge (Hidalgo-Mompeán et al. 2021). Furthermore, most advanced ML methods, such as artificial neural network (ANN), support vector machines (SVM), bagging, boosting, and deep learning, are often ‘black box’ models, as opposed to ‘white box’ models based on mechanics or empirical regression (Feng et al. 2021). The missing or uninterpreted step-bystep details in these ‘black box’ ML models lead to untrustworthiness for industrial practitioners and regulators who are sensitive to risks in healthcare, legislation (e.g., European Union’s GDPR), and critical engineering (Linardatos et al. 2020). Two new emerging technologies, i.e., automated ML (AutoML) and interpretability analysis, have the potential to solve the two issues in complex AEC problems. AutoML is a technique of automating the time-consuming and expertise-reliant procedures in selecting and training ML models; it has been successfully applied in many science and engineering areas such as medical science (Tran et al. 2021; Anwar 2021), public service (Li et al. 2019), finance (Vakhrushev et al. 2021), materials (Bangaru et al. 2019), and the construction domain (Zhao 2 40 45 50 55 60 65 et al. 2023). Interpretability analysis refers to developing human-readable explanations to facilitate practitioners’ comprehension of why an ML model makes certain decisions or predictions (Molnar 2020; Guo et al. 2022). This study aims to integrate AutoML and interpretability analysis to study failure mode identification of RC shear walls. This research involves a dataset of 351 shear wall failure samples collected from the literature, 15 low-level ML algorithms in AutoML, and three interpretability analysis methods for validating the trained AutoML model. The contribution of this paper is three-fold. First, AutoML sheds light on the automatic identification of the failure modes of RC sheer walls. Second, interpretability analysis can validate the ‘black-box’ models with long-established AEC domain knowledge. Third, for AEC industrial practitioners, the whole process is automatic, accurate, less reliant on data expertise, and interpretable. 2 Overview of AutoML and interpretability analysis 2.1 AutoML AutoML is the process of automating the time-consuming and iterative procedures involved in building ML models (Feurer et al. 2020). Figure 1 shows the full process of developing a highquality ML model. Traditionally, appropriate ML algorithms, extensive computational resources, time, and domain expertise are essential for training or building an ML model (Gijsbers et al. 2019). Even professional data scientists may be confused in each phase by many factors, such as unbalanced class, parameter noise, and selection of performance measures. As a result, there is an increasing interest in automating and democratizing the processes in Figure 1. Figure 1 Workflow of machine learning, where the gray box indicates the scope of AutoML AutoML, as shown in the gray area in Figure 1, automatically completes up to four tasks, including feature engineering, algorithm selection, parameter tuning, and evaluation (Elshawi et al. 2019). Thus, AutoML is more efficient and productive while maintaining the ML model quality. For example, advanced AutoML algorithms outperformed human experts in five out of six construction management datasets (Zhao et al. 2023). In the software market, there exist commercial AutoML platforms, such as Google Cloud AutoML and Microsoft Azure, and open-source libraries, such as Auto-sklearn, MLBox, and TPOT (Elshawi et al. 2019). Yet, the ‘black box’ problem of ML models remains regardless of the use of AutoML; 3 70 75 80 85 90 to a certain extent, an AutoML model is even harder to explain due to the sophisticated parameter controls in AutoML algorithms. 2.2 Interpretability analysis and applications in civil engineering Interpretability analysis aims to explain the rationale behind the decisions and outcomes of a computerized model so that a human can comprehend it (Gilpin et al. 2018). There are two categories of interpretability analysis: model-specific and model-agnostic (Molnar 2020). Model-specific interpretation tools are limited to intrinsically interpretable ML models, such as a decision tree model of project-related waste generation behaviors (Yang et al. 2021). In comparison, a model-agnostic interpretation can be applied to any ML model after the training process. Furthermore, model-agnostic interpretable methods can be divided into two groups: global methods and local methods. Global model-agnostic methods describe the average behavior of an ML model (Friedman 2001), while local model-agnostic methods, such as local surrogate (LIME) (Ribeiro et al. 2016), explain a subset of prediction results. Model-agnostic interpretability analysis has been applied to ML models in the AEC industry in recent years. Table 1 summarizes the application problems, ML task, selected interpretability analysis methods, sample size, contribution, and limitations in seven related studies. In short, existing studies focused on the global model-agnostic interpretability of the regression model’s partial dependent plot (PDP) and coalitional-game-theory-based SHapley Additive exPlanations (SHAP) are popular methods. Yet, disadvantages were observed as well. For example, PDP was criticized for the feature interaction that can produce wrong interpretation results (Molnar 2020). SHAP has three limitations in unintuitive feature attributions, possibly biased and misleading interpretations, and unclear estimation thresholds. Furthermore, local model-agnostic methods received less attention. Thus, there is a need to gauge and compare global and local interpretability analysis methods. In this paper, two global model-agnostic interpretation methods: PDP and ALE, and one local method, LIME, are studied for the failure mode identification problem. Table 1 List of recent model-agnostic interpretability work of ML models in civil engineering Local / Sample Tools# Source Contribution Global* size Ultimate load of Regression G PD analysis Le & Phan 99 PD analysis of each rectangular (2020) input variable’ CFST columns influence on the ultimate load Shear Strength Regression L & G SHAP Feng et al. 434 Coalitional gamePrediction of (2021) theory based SHAP to Squat RC Walls explain predictions from both local and global aspects Targets of ML ML task Ground motion Regression L & G SHAP characteristics (PGV and PGA) Somala et al. (2021) 4 4,000 SHAP methods to explain the influence factors for estimating PGA&PGV Limitations 1) PD analysis ignores feature dependence. 1) TreeSHAP (a modelagnostic subclass of SHAP method) leading to unintuitive attributions, 2) Possibly problematic explanations by SHAP, 3) Unanswered thresholds by the SHAP. Ditto. 95 100 105 110 115 Auxetic behavior of cementitious cellular composites Regression G Magnitude of moment capacity at failure; and the failure mode/causes Regression and Classificati on G Structural Regression performance of GFRP elastic gridshells Seismic Regression response of R/C buildings G * # G SHAP Lyngdoh et al. (2022) 850 SHAP to analyze the 1) No local influence of each input interpretability analysis variable on the Pois- involved, son’s ratios 2) Lack validation of interpretation with expert knowledge. PDP, Naser 103 An explainability case 1) Many interpretable SHAP, et al. study on reinforced methods without crosssurrogate, & (2021) concrete (RC) beams references, feature strengthened with fiber- 2) Feature interaction not interaction reinforced polymer considered in other (FRP) composite methods such as PDP, laminates 3) No local interpretability analysis involved. PDP, ALE, Kookalani 400 Three interpretable 1) No local & SHAP et al. methods to analyze the interpretability analysis (2022) influences of variables involved. on the stress prediction PDP & Demertzis 5,850 Use two methods to 1) Lacking of SHAP et al. analyze the influence of comprehensive (2023) Housner intensity on interpretability analysis the maximum interstory on all parameters, drift ratio 2) No local interpretability analysis involved. : L: Local; G: Global. : PD: Partial dependence; SHAP: SHapley Additive exPlanations; PDP: PD plot; ALE: accumulated local effects. 3 The problem of failure mode identification of RC shear walls RC shear walls are the primary lateral load-carrying structure elements in many high-rise buildings due to superior high lateral strength and stiffness (Paulay 1975). For the desired ductile failure modes, RC walls are designed to have a ‘strong shear and weak flexure,’ which is commonly accomplished by restricting the axial compressive force ratio and adding boundary components to the structure (Lefas et al. 1990). According to the aspect ratio (shear span length divided by wall length), shear walls are typically classified as either squat (lowrise) or slender (tall, high-rise) (Birely 2012). A ductile failure mechanism characterized by flexural yielding toward the base is more likely to occur in slender walls. Squat walls are more prone to a nonductile shear-controlled failure mechanism, which is best defined as an abrupt reduction of stiffness and strength under seismic loads. According to ASCE/SEI 41-17 (ASCE 2017), walls with an aspect ratio larger than 3.0 (often controlled by flexure) are regarded as slender. In contrast, those with an aspect ratio less than 1.5 (mostly controlled by shear) are considered short and squat. The behavior of walls having an aspect ratio between 1.5 and 3.0 is influenced by both shear and flexure. It is also suggested in FEMA 306 (ATC 1998) that well-designed slender shear walls (with aspect ratios of more than 3.0) are more likely to experience ductile flexural failure. Ductile flexural failure is caused by concrete crushing or a longitudinal reinforcement fracture at the plastic hinge zone, as shown in Figure 2(Ⅰ). Generally, flexural failure is uncommon in squat shear walls, particularly those with aspect ratios below 1.0. However, 5 120 125 130 135 140 depending on the reinforcing characteristics, flexural failure is possible in such walls, and shear failure may also be seen. The nonductile shear failure mechanism of squat shear walls was divided into three categories by Paulay and Priestley (1992) sliding shear failure, diagonal tension failure, and diagonal compression failure, as shown in Figures 2(Ⅱ) and 2(Ⅲ). The presence of one or more corner-to-corner diagonal cracks, as seen in Figure 2(Ⅱ), is diagnostic of diagonal tension failure (T), which may happen when a wall does not have enough horizontal shear reinforcement. The concrete crushes under this compression develop widespread crack patterns in diagonal compression failure mode (C), as illustrated in Figure 2(Ⅲ). Sliding shear failure (S) may be caused by two different things: (1) several major cracks at the wall base and (2) the buckling of rebars and the crushing of concrete in a narrow band along the base of the wall, as illustrated in Figure 2(Ⅳ), following considerable flexural reinforcement yielding. In RC design and retrofitting, designers and engineers are more concerned about the ductile or non-ductile failure mode of shear walls than detailed classification. Therefore, this study divides the classification work into two phases: the first stage, classifying ductile and nonductile failure modes, and the second stage, classifying three detailed non-ductile failure modes; the first stage often satisfies most engineering needs. Figure 2 Typical failure modes of RC shear walls, where dashed boxes indicate the first-stage classification 4 Research methods Figure 3 briefs the three-step flowchart: dataset preparation, AutoML using Auto-sklearn, and interpretability analysis in failure mode identification of the RC shear wall in this study. Figure 3 The proposed workflow for failure mode identification of RC shear wall 6 145 150 155 4.1 Feature settings Since there are few studies on the failure mode identification of RC shear walls using datadriven methods, this study mainly refers to national regulations about design criteria of structural concrete, including JGJ 3-2010 (China code) (MOHURD 2010), ACI 318-19 (U.S. code) (ACI 2019), EC-8 (Europe code) (CEN TC-250 2004), mechanics theories (Vecchio & Collins 1986), and an existing study by (Mangalathu et al. 2020) to set up features. Among these existing references, a total of four types of features, material property (fy,vw, fy,hw, fy,cb, fc), reinforcement details (ρvw, ρhw, ρvb, ρhb), geographic details (lw/tw, M/Vlw, Ab/Ag, CS), and load (P/fcAg), were considered to influence the seismic behavior of RC shear wall designs. Specifically, 13 features were selected for classifying the failure mode of RC shear walls, and a description of each feature is given in Table 2. The study by Salonikios et al. (1999) described more details on the sensors and methods for measuring these features in laboratory tests. These 13 features are also considered essential to influence the seismic behavior of other load-bearing structures in many studies related to data-driven structure behavior prediction (Sujith Mangalathu 2018; Hosein Naderpour 2021). Table 2 Feature setting (excluding classification target) for the problem Group Material property Reinforcement details Geometric characteristics Load Symbol fy,vw fy,hw fy,cb fc ρvw ρhw ρvb ρhb lw/tw M/Vlw Ab/Ag CS P/fcAg Unit MPa MPa MPa MPa 103 103 103 103 103 103 Feature description Yield stresses of vertical bars in web Yield stresses of horizontal reinforcement in web Yield stress of confinement reinforcement in boundaryelement Concrete compressive strength Web vertical reinforcement ratio Web horizontal reinforcement ratio Boundary region vertical reinforcement ratio Boundary region horizontal reinforcement ratio Proportion of length and thickness Aspect ratio Proportion of the area of the boundary element and the gross sectional area Cross-section type, i.e., Rectangular (R), Barbell (B), and Flanged (F) Axial load ratio Figure 4 Schematic diagram of RC shear wall tests with three types of cross-sections 7 160 165 170 175 180 185 190 4.2 Data collection In the process of collecting data, the data needs to first meet the requirement of containing all 13 features listed in Table 2. Some extra rules are followed to reduce the impact of irregular data for the main purpose of this study. Specifically, the cross sections of all selected shear walls are one-story, one-bay, symmetric, and without openings. In addition, all selected walls have continuous longitudinal reinforcement without lap splice, as well as deformed and straight reinforcement. Finally, the aspect ratio (M/Vlw or hw/lw) is considered the dominant feature for the seismic behavior of RC shear walls according to the ASCE/SEI 41-17 (ASCE 2017). According to the aspect ratio, RC shear walls are conventionally classified into slender walls (aspect ratio>3.0), moderate walls (1.5 ≤ aspect ratio ≤ 3.0), and squat walls (aspect ratio< 1.5). Therefore, the data collected in this study equally covers all three types of walls to ensure the applicability of the proposed method. A total of 351 records were collected from laboratory tests reported in the literature. For example, the first 242 records were from the ACI 445B Shear Wall Database (Usta et al. 2017), with the criterion that each data record covers all features in Table 2. The rest of the 109 records were also collected from published experiments, including Chile (Massone et al. 2009), Canada (Abdulridha & Palermo 2017; Mohamed et al. 2014; Palermo et al. 2002), China (Chiou et al. 2004; Deng et al. 2008; Hwang et al. 2004; Jiang et al. 2013; Kuang & Ho 2008; Lu et al. 2018; Ren et al. 2018; Su & Wong 2007; Wen-yuan et al. 2018; Zhou et al. 2010), the United States (Terzioglu et al. 2018), Australia and New Zealand (Dabbagh 2005; Gebreyohaness et al. 2014; Tripathi et al. 2019; Yanez et al. 1991), Japan (Kabeyasawa & Matsumoto 1992; Kimura et al. 1996; Nakamura et al. 2009; Tokunaga & Nakachi 2012), Slovenia (Tomazevic et al. 1996), and Greece (Christidis & Trezos 2017). The existing open-source library—ACI 445B Shear Wall Database was also used by Mangalathu et al. (2020) for verifying the feasibility of identifying failure modes of shear walls by ML methods. A total of eight algorithms—Naïve Bayes, K-Nearest Neighbors, Decision Tree, Random Forest, AdaBoost, XGBoost, LightGBM, CatBoost, and XGBoost—are used, and the highest accuracy was 86% from Random Forest (Mangalathu et al. 2020). The quality and reliability of existing experiment-based data were initially confirmed. Table 3 lists an excerpt of the collected dataset, while the full data table is available in the supplemental materials of this paper. Figure 5 briefs the distributions and statistical details of the dataset, respectively. The dataset has a wide range of distribution, and common engineering values are covered. Figure 5 (m) shows that three types of cross-sections accounted for 69.2% (R), 18.9% (B), and 11.9% (F), respectively. The distribution of failure modes for two-stage classification is shown in Figure 5(n)(o). 195 8 Table 3: Excerpt of the collected dataset with 351 records of RC shear walls failure tests No. fy,vw fy,hw M/Vlw 1 2 3 … 351 585 585 610 … 345 610 610 610 … 345 1.00 1.00 1.00 … 2.5 140 120 (a) fy,vw (MPa) Min: 258.2 Max: 806 Median: 446 Mean: 441 Count 100 80 60 40 … CS … … … … … … … … … … … (b) R R R … R Failure mode (1st) ND ND ND … ND Failure mode (2nd) S S S … S (c) fy,cb (MPa) Min: 0 Max: 1262 Median: 443 Mean: 456 fy,h w (MPa) Min: 216 Max: 1262 Median: 443 Mean: 456 Source (Salonikios et al. 1999) (Salonikios et al. 1999) (Salonikios et al. 1999) … (Zhou et al. 2010) (d) fc (MPa) Min: 13.7 Max: 130.8 Median: 34 Mean: 35.7 20 0 300 400 500 600 700 800 400 600 800 1000 1200 200 400 600 800 10001200 3 ρy,vb (1×10 ) Min: 1 Max: 36.7 Median: 5.4 Mean: 6.5 (g) ρy,h w (1×10 ) Min: 1.1 Max: 99.1 Median: 22.4 Mean: 27.6 (h) 60 5 10 15 20 25 30 35 40 40 80 Ab/Ag Min: 0 Max: 0.38 Median: 0 Mean: 0.1 (l) P/fcAg Min: 0 Max: 0.5 Median: 0.03 Mean: 0.07 0.6 0.1 ρy,vw (1×10 ) Min: 1 Max: 36.7 Median: 5 Mean: 7.1 (f) 5 10 15 20 25 30 35 40 20 (e) Count 320 280 240 200 160 120 80 40 0 0 250 200 lw/tw Min: 4.4 Max: 45 Median: 10.8 Mean: 13 (i) Count 150 100 3 40 80 100 Aspect ratio Min: 0.28 Max: 5 Median: 1.3 Mean: 1.6 (j) 3 (k) 20 40 60 80 100 120 140 ρy,h b (1×103) Min: 0 Max: 169.2 Median: 3.7 Mean: 10.2 120 160 50 0 0 (m) 10 20 30 40 50 Section 1 2 3 4 5 (n) Failure mode - first stage 24.5% 0.2 41.3% 0.4 0.8 0.2 0.3 0.4 0.5 (o) Failure mode - second stage 10.7% 39.8% 14% 61.5% Barbell (B) Flanged (F) Rectangular (R) 200 205 58.7% 49.5% Ductile failure mode (D) Non-ductile failure mode (ND) Diagonal tension failure (T) Diagonal compression failure (C) Sliding shear failure (S) Figure 5: The distributions of the features in the dataset. (a) – (l): Numeric features; (m) – (o) categorical features and targets Furthermore, Figure 6 shows the matrix of Pearson’s linear correlations between the features. Statistically, the circles in Figure 6 indicate one close relation among fy,hw, fy,cb, and fy,vw, and another between CS and Ab/Ag. The latter’s close relation is in line with domain knowledge—Ab/Ag—which highly depends on the cross-section type. The imbalanced crosssection type shown in Figure 5 (n) might also lead to the first relation since fy,hw, fy,cb, and fy,vw 9 are not highly linearly correlated in theory. It should be noted that the circled relations might break the independence assumptions of some interpretability analysis methods, such as PDP. 210 215 Figure 6 Pearson’s correlation matrix of the 13 features 4.3 Classification using Auto-sklearn This study applies Auto-sklearn, an open-source AutoML library (Feurer et al. 2020), to the problem of RC shear wall failure mode identification. Auto-sklearn has been tested successfully in a large study based on 209 metadata sets and 39 datasets (Feurer et al. 2020). As shown in Figure 7, Auto-sklearn comprises 15 low-level ML algorithms, 14 preprocessing methods, and four data preprocessing techniques. Auto-sklearn employs Bayesian optimization to automate the processes of feature engineering, algorithm selection, parameter tuning, and the entire modeling process shown in Figure 7. Therefore, AutoML can save human experts from heavy reliance on data expertise (Elshawi et al. 2019). More details of Auto-sklearn can be found in Feurer et al. (2015b). 10 220 Figure 7 Structural diagram and process details in Auto-sklearn The training target is set to F1 accuracy, which is an unbiased performance indicator: TP Precision = , (1) TP + FP TP Recall = , (2) TP + FN n 225 Macro= F1 (∑ 2 × i =1 230 235 240 Recall(i ) × Precision(i ) )/n, Recall(i ) + Precision(i ) (3) where TP, TN, FP, and FN indicate true positives, true negatives, false positives, and false negatives for the binary shear wall failure mode classification problem, respectively. It should be noted that F1 is able to measure performance objectively when the class balance is skewed (Li et al. 2017) and is widely adopted in the latest ML-related studies (Humphrey et al. 2022; Chen & Xue 2023). After the AutoML model is trained, five manual ML algorithms—Naive Bayes, Random Forest, SVM, Artificial Neural Network, and gradient boosting—that are widely adopted in the literature are compared with the AutoML model according to the F1 value. In addition, 10-fold cross-validation is used to validate ML models for a limited sample of data. Such a method is superior to split sample validation (Ellis & Mookim 2013; Wong & Yeh 2020; Yuan et al. 2020) because using only a single split of full samples is sensitive to the particular split sample created. 4.4 Interpretability analysis Three interpretability analysis methods are implemented to explain the trained AutoML from quantitative and qualitative aspects in this study. Two global model-agnostic interpretability analysis methods are PDP and ALE, and a local model-agnostic interpretability analysis method is LIME. Overall, the collective results of the three independent methods can jointly verify and confirm the explainability of the trained AutoML model. 4.4.1 Partial dependent plot (PDP) 11 245 PDP illustrates the marginal impact of one or two features on an ML model’s predicted output (Friedman 2001). PDP demonstrates whether a result and a feature have a linear, monotonic, or complicated relationship. The partial dependence function is defined as (Molnar 2020): ∧ ∧  = f S ( xS ) E= X C  f ( xS , X C )    250 255 260 265 270 275 ∧ ∫ f ( x , X )d ( X ) . S C C (4) The xS are features related to the partial dependency function, whereas the XC are other features utilized in the ML model f^. The features are handled as random variables. Partial dependency operates by marginalizing the output of the ML model across the distribution of features in set C, such that the function reveals the link between set S and the predicted result. By marginalizing the features, one can get a dependency function on S features, including interactions with other features. The PDP method is intuitive, clear, and easy to implement. However, the PDP presumes the marginal feature set is uncorrelated with the feature set S. If the assumption does not hold, the results of the PDP may be unreliable (Molnar 2020). Figure 8 illustrates the estimation of PDPs. With the feature ‘aspect ratio’ as an example, the estimating process is as follows: 1) Choose a feature, such as ‘Aspect ratio.’ 2) Create a grid. 3) For each grid value: a. Replace the feature with the grid value, such as setting all aspect ratios to 1.5 in Figure 8(b). b. Calculate the individual conditional expectation (light blue curves) for each single data piece). c. Average the predictions of an individual conditional expectation. 4) Complete a curve, shown as the black curve in Figure 8(a). Figure 8 Example PDP analysis of the ‘aspect ratio’ feature. (a) PDP series plot; (b) each point in (a) indicating the individual expectations with a condition of aspect ratio = 1.5 4.4.2 Accumulated local effects (ALE) ALE plots average the changes in predictions (Apley & Zhu 2020), which could block the effect of other features (Grömping 2020). Therefore, ALE plot is a less biased substitute for PDP when the features are correlated. In ALE, the features are divided into intervals. Essentially, ALE plots take the average of the changes in the predictions and aggregate them throughout the intervals to evaluate local effects: 12 ∧ ∂ f ( xS , xC ) f S , ALE ( xS )= ∫ ( ∫ d (= X C | X S zS )d )dzS − constant z0,S xC ∂xS ∧ 280 285 290 295 300 xS (5) where zs denotes the interval value of the concerned feature of ALE, and other symbols are the same as those in Eqn. (1). Figure 9 shows ALE estimation of two correlated features X1 and X2. First, the feature X1 is divided into intervals, indicated as vertical dash lines. The predicted data points are replaced by the difference in each interval. The differences are accumulated and centered, resulting in the final ALE curve. Overall, the ALE method is more complex and less intuitive than PDP; therefore, ALE is often used together with the PDP method. Figure 9 ALE results of two correlated features (X1 and X2) 4.4.3 Local interpretable model-agnostic explanations (LIME) LIME (Ribeiro et al. 2016) trains local surrogate models, as opposed to global surrogate models in PDP and ALE, to approximate the predictions of the underlying black box model. LIME aims to fully understand the rationale behind the predictions of an ML model. First, LIME creates a new dataset of samples’ perturbations and their predictions, then develops an interpretable model of a weighted new dataset according to the closeness of the samples to the instance of interest. Commonly interpretable ML models for LIME are linear regression, logistic regression, and decision tree. The predictions from the ML model should be reasonably approximated by the learned model at the local level. Figure 10 shows the LIME algorithm’s sampling and local model training. The whole process is based on a classification example using tabular data: a) Prepare the ML prediction results based on features X1 and X2, such as the predicted classes 1 (green) and 0 (orange) in Figure 10(a). b) Select an instance of interest (big dot with cross sign) and small dots sampled. c) Weigh the points that are located closer to the instance of interest. d) Learn decision boundary Locally from the weighted samples, shown as the dashed line in Figure 10(d). 13 305 310 315 320 325 Figure 10 LIME algorithm for local interpretability. (a) ML prediction results; (b) sampling; (c) local model training; (d) local interpretability results In summary, PDP and ALE are appropriate and complementary methods for explaining the general mechanisms in the trained AutoML model. PDP is intuitive but better for less correlated features, while ALE can work with correlated features. LIME is also an intuitive local method and can explain individual predictions locally. 5 Experimental results 5.1 Experimental settings All software was operated in single threading mode on a desktop computer with Intel i7-10700 2.9 GHz CPU, 32 GB memory, Python 3.7, and Ubuntu 18.04. The Auto-sklearn toolkit (ver. 0.14.7) was used, while five baseline ML algorithms were from OrangeML (ver. 3.32) (Demsar et al. 2013). Three open-source tools—PDPbox (ver. 0.2), Alibi (ver. 0.8), and LIME library (ver. 0.2.0)—were selected as efficient implementations of PDP, ALE, and LIME, respectively. 5.2 Classification results of AutoML Figure 11 compares the confusion matrices of AutoML against the baseline ML. In the classification of ‘ductile (D)’ and ‘non-ductile (ND)’ modes, the Auto-sklearn model was the best with an F1 = 90.6%, while the Auto-sklearn model was also the best (F1 = 84.5%) for classifying the three types of ‘non-ductile (ND)’ modes. It should be noted that the next best model in Figure 11a and the third best model in Figure 11b are Random Forest, which echoed the findings in Mangalathu et al. (2020). The comparison results confirmed that AutoML is automatic (saving manual interventions that relied on data expertise) and effective (less erroneous) for the failure mode identification of RC shear walls. 14 330 335 340 345 Figure 11: Comparison of the confusion matrices of four manual ML and AutoML models through 10-fold cross-validation of training: (a) first-stage classification; (b) second-stage classification The details in the Auto-sklearn training revealed more insights. In the model, an ensemble model was built for each fold of prediction, in which different algorithms with different hyperparameters are combined to achieve a better prediction performance. Figure 12 shows one ensemble model. In Figure 12, the ‘radom_forest’ classifier with a setting ‘max_features = 3, N_estimators = 512’ was one of the best low-level ML algorithms; the ‘gradient_boosing’ low-level classifier with a setting ‘early_stopping = true, L2_regularization = 8.06, learning_rate = 0.092, max_iter = 128’ was another of the best low-level algorithms. Human experts can, therefore, replicate the AutoML model manually using the tracked training record. 5.3 Results of interpretability analyses of AutoML 5.3.1 Partial dependent plot (PDP) Figure 13 shows the PDP curves of the trained AutoML model regarding all 11 assumingly independent features. The x-axes in Figure 13 denote the features, while the quantiles of the feature are the dash lines. The y-axes indicate the probabilities of non-ductile (ND) failure modes, whereas a low value (e.g., y = 0.3) represents the ductile (D) failure mode. The individual conditional expectation is illustrated with a light blue curve, while the PDP curve is drawn as a thick black curve. 15 350 Figure 12: Details of the ensemble models built in a fold of AutoML training Figure 13 PDP curves of 11 independent features regarding the trained AutoML model 16 355 360 365 370 375 380 Figure 13 (f) (i-k) shows that predictions of the AutoML model are significantly influenced by four features, namely the boundary region vertical reinforcement ratio (ρvb), lw/tw, aspect ratio, and axial load ratio (P/fcAg). First, the probability of non-ductile failure mode increases with the increase in ρvb. The evidence of this phenomenon is that a larger ρvb leads to higher flexural strength, while shear strength ensures stability. The shear walls are more likely to fail in a non-ductile (shear-damage-based) mode with larger flexural strength (Gulec 2009). In addition, Figure 13 (i) shows that the probability of non-ductile failure mode obviously increases when lw/tw increases, especially when exceeding 13.3. lw/tw increases the flexural strength due to the increase in vertical reinforcement moment with a stable crosssection of shear walls, but the shear strength does not change significantly as the area and reinforcement amount of shear walls do not change (Lefas et al. 1990). When the threshold of flexural strength exceeds shear strength, the probability of non-ductile failure mode may increase obviously. Figure 13 (j) illustrates that non-ductile failures are more likely to occur when the aspect ratio is < 1.5; the probability of ductile failure is high when the aspect ratio exceeds 2. This observation is generally consistent with the theory in (ASCE 2017), which stated that the aspect ratio 1.5 (< 1.5 mostly controlled by shear damage (non-ductile) and 3.0 (>3.0 often controlled by flexure damage [ductile]) are regarded as the thresholds of shear and flexure failure modes. Finally, Figure 13(k) shows that the higher the P/fcAg , the larger probability of flexure (ductile) failure mode predicted by the AutoML model. The trend can be explained by common knowledge about the RC structure component design theory. That is, a proper axial load could be helpful for the improvement of shear strength, and the flexural failure mode is more likely to happen (Moehle 2015). 5.3.2 Accumulated Local Effects (ALE) Figure 14 shows the ALE curves of the trained AutoML model, with background PDP curves that were adjusted to start at zero. Overall, the trends of the PDP and ALE curves are similar for most features. The interpretations of specified features by PDP curves could be explained by the ALE. The remainder of this section focuses on two more features—Ab/Ag and CS—than those verified by domain knowledge in Sect. 5.3.1. 17 ND 0.3 0.615 0.245 0.140 ALE Probability 0.2 0.1 0.0 -0.1 D 385 390 395 400 (l) Section_R Section_B Section Section_F Figure 14 Comparison of PDP and ALE curves of all features Figure 14(l) shows that barbell and flanged walls are more prone to fail in a non-ductile mode according to the trained AutoML model. Specifically, compared to RC walls with rectangular cross-sections, walls containing boundary components—barbells or flanges—are more vulnerable to diagonal compression failure, which is a non-ductile failure mode. This is because flanged and barbell walls may accommodate a larger amount of reinforcement at the ends of the wall, which, in turn, improve the wall’s flexural strength and raise the shear demands in the wall web (Gulec 2009). Surprisingly, Figure 14(m) shows that the probability trend does not change much when Ab/Ag increases from 0 to 0.38, which is not anticipated. According to civil engineering knowledge, larger Ab/Ag leads to a higher probability of non-ductile failure since non-ductilefailure-controlled flanged and barbell walls correspond to the larger Ab/Ag than rectangular walls in practice. 5.3.3 Local interpretable model-agnostic explanations (LIME) Figure 15 shows the interpretation of the trained AutoML model by LIME for two typical instances (Nos. 275 and 219). The No. 219 instance is a ductile failure mode, while No. 275 is a non-ductile failure mode. In contrast with the two global methods, LIME focuses on individual prediction results. Figure 15 lists the overall contributions of all features of the instance according to their importance, while the partial effects are illustrated. The tornado charts in Figure 15 illustrate the trained Auto-sklearn model’s key decision features’ partial effects and thresholds. 18 Feature Value Non-ductile instance (a) (b) Ductile ρvb>10 0.08 Feature Non-ductile Section=F 0.23 Aspect ratio<=1.00 0.23 lw/tw>15.71 0.21 Ductile instance 410 415 420 0.14 0.86 fy,cb>500 0.05 fy,vw>504 0.05 Value Ductile Non-ductile Section=R 0.35 405 Prediction probabilities P/fcAg>0.10 0.23 lw/tw<=8.00 0.12 10<ρvb<=20 0.12 1.33 0.50—corresponding to the non-ductile failure mode. Among the top six features, the section, aspect ratio, lw/tw, fy,cb, and fy,vw contributed to the non-ductile failure mode with scores at 0.23, 0.23, 0.21, 0.05, and 0.05, respectively. In contrast, ρvb contributed to the ductile failure mode with a score of 0.08. The flanges section contributes most to the non-ductile failure mode classification result. The evidence is that diagonal compression failure, one type of non-ductile failure mode, is more likely to occur in walls with boundary elements (such as barbells or flanges) (Gulec 2009). Meanwhile, the aspect ratio <= 1.00 contributes to the non-ductile failure mode, and it corresponds to the theory that the shear walls with an aspect ratio < 1.5 are mostly controlled by shear damage (non-ductile) (ASCE 2017). The evidence of other features, including lw/tw and ρvb, can be found in the explanation of Figure 13(i) and (f), respectively, in Sect. 5.3.1. In Figure 15(b), all top six features (i.e., section, P/fcAg, lw/tw, ρvb, aspect ratio, and fc) contribute to the ductile failure mode, of which the probability is at 0.99, and the corresponding scores are at 0.35, 0.23, 0.12, 0.12, 0.07, and 0.04, respectively. The related explanation is also consistent with existing knowledge. For instance, the rectangular section contributes most to the ductile failure mode since it has smaller flexural strength and is more prone to flexural failure mode (ductile mode). 19 425 430 435 440 445 450 455 In summary, PDP interpretability analysis indicates four uncorrelated features influencing predictions: boundary region vertical reinforcement ratio (ρvb), lw/tw, aspect ratio, and axial load ratio (P/fcAg). In detail, the probability of non-ductile failure mode increases with the increase of ρvb. Non-ductile failures are more likely to occur when the aspect ratio is < 1.5, but the probability of ductile failure is high when the aspect ratio exceeds 2. The higher P/fcAg leads to a larger probability of flexure (ductile) failure mode of shear walls. In addition, ALE curves exhibit similar trends as that in PDP analysis for most uncorrelated features. For uncorrelated features, ALE shows that barbell and flanged walls are more prone to fail in a non-ductile mode according to the trained AutoML model. Finally, the LIME provides a detailed explanation for individual prediction, including importance contributions, partial effects, and corresponding thresholds of all features. 6 Discussion 6.1 Application As the major lateral load-carrying components in structure systems, shear walls have received much attention due to the higher structural lateral resistance ability requirement for high-rise buildings. However, the existing design process or retrofitting evaluation relies on computingintensive simulation and expensive experimental testing. The fast and reliable failure mode identification of the shear walls and related explanations for prediction results can help improve the existing practices in designing or retrofitting RC shear walls. As shown in Figure 16, designers and engineers can first use AutoML tools to examine the failure mode of shear walls after acquiring a preliminary design plan based on architectural requirements and structural standards. Generally, the ductile failure mode is preferred to achieve better seismic performance. Therefore, if they get non-ductile failure results in the first step, they can use the interpretation to acquire direct information about how to reverse the failure mode by input factors modifications. Especially if the tendency, contribution ratio, and threshold of each feature are detailed and illustrated in LIME, designers could modify the feature according to the interpretation result. This modification could be an iterative process until satisfactory results are obtained, meaning the ductile failure mode of the shear wall. A similar iterative modification process based on LIME interpretation analysis results is applied in the automotive industry (Pana & Stark 2022). The iterative RC wall design modification process in Figure 16 can thus take advantage of the trained AutoML model and interpretability analysis. Compared to the conventional computing-intensive simulation and expensive experiment, the novel iterative process can improve traditional simulation-based and test-based design processes in terms of efficiency and costs of money, manpower, and simulation and experiment resources (Chiarello et al. 2021). 20 460 465 470 475 480 485 Figure 16 An iterative RC wall design process based on the AutoML model and interpretability analysis 6.2 Implications By resolving the complex data-expertise-reliant modeling process and the untrustworthy ‘black box’ of ML application in failure mode identification, the proposed integration of automated ML and interpretability analysis has the potential to solve more complex AEC problems using ‘black box’ ML techniques. Example problems are the prediction of the behaviors of structural elements and systems (Huang et al. 2022), structure design optimization (Fang et al. 2022), structure health monitoring (Yang et al. 2022), off-site modular building production (Li et al. 2021b), tower crane optimization (Huang et al. 2021), site work packaging (Li et al. 2021a), project-related solid waste generation behaviors (Yang et al. 2021), the semantic enrichment of buildings and the city (Xue et al. 2021; Li et al. 2022), and the prediction of the mechanical properties of the materials used in AEC (Nguyen et al. 2020). 6.3 Pros and Cons of this Study This study has three advantages: 1. AutoML automates and improves the ML model’s establishment, including algorithm selection, parameter tuning, and model evaluation. Auto-sklearn, the AutoML selected in this study, outperformed other 5 manual algorithms in the prediction of the ML of walls’ failure mode according to Sect. 5.2. In addition, Auto-sklearn is open source for academia and industry. 2. The global interpretation methods tested in Sects. 5.3.1 and 5.3.2 visualized the input– output correlations and sensitivities of the trained AutoML model. Civil engineers can validate (or reject) the AutoML model against the features with domain knowledge. For example, the interpretation of AutoML indicates that aspect ratios 1.5 (< 1.5 mostly controlled by shear damage (non-ductile)) and 3.0 (> 3.0 often controlled by flexure damage (ductile)) are regarded as the threshold of shear and flexure failure mode, which is consistent with existing knowledge such as code (ASCE 2017). The local interpretation could help users gain a deeper understanding of individual classification results. 21 490 495 500 505 510 515 520 525 3. Last but not least, the interpretation presented in this study triangulates the learned ML models with domain knowledge in civil engineering. Then, the mechanisms of the case AEC problem can be interpreted in more comprehensive (against all features), visual, and quantitative ways. For example, the quantified threshold of ρvb of RC shear walls between the ductile and non-ductile failure modes was detected in PDP in Sect. 5.3.1, whereas the only trend that greater ρvb leads to a greater probability of non-ductile failure was known in past civil engineering knowledge. However, there also exist four limitations in this study: 1. Compared with general AEC knowledge, the interpretation of the ML model focuses on the marginal effect (i.e., the learned discrimination “areas” in the feature space). The interpretability in this study may thus be unable to explain the whole feature space. 2. Although some interpretability methods, such as ALE, could be used to interpret the marginal effect of correlated features, the correlation among different features may influence the accuracy of the interpretability result. For example, the significance of Ab/Ag was not captured in Sect. 5.3.2. One possible reason may be that too much weight has been given to the section, which shares a similar meaning with the Ab/Ag. 3. Due to the small data samples, the time cost of different ML models is limited and, therefore, is not taken into consideration. Time cost could be a good performance indicator when meeting more complex AEC problems and larger datasets. 4. Several assumptions, such as balanced classes or continuous and average feature distribution, were required for the interpretability analyses in this study. Features of interest unable to meet the assumptions may result in unexpected curves or trends in PDP or ALE plots. For example, in the ALE analysis of the feature Ab/Ag, the result was not in line with the domain knowledge, where a possible reason was that half of the instances in the dataset had low values (0) for feature Ab/Ag. 7 Conclusion Although ML showed success in terms of error metrics in civil engineering, two fatal problems—the complex data-expertise-relied modeling process and the untrustworthy ‘black box’ nature—have undermined ML adoptions and applications due to critiques from civil engineers and legislators. This paper presents an integration of the AutoML algorithm and interpretability analysis to solve the two fatal problems with a complex AEC problem—the identification of failure modes of RC shear walls. A training dataset of 351 cyclic tests of RC shear walls was collected in this study. Based on the training dataset, 10-fold cross-validation was utilized for training and validating the AutoML model. As a result, the trained AutoML model outperformed the conventional ML algorithms. Regarding interpretability, two global model-agnostic methods—PDP and ALE—and a local model-agnostic method, LIME, were adopted to explain the trained AutoML model per feature and per instance, respectively. The interpretability results confirmed that most observed trends and correlations in the trained AutoML model were in line with domain knowledge. Overall, the proposed integration of the 22 530 535 540 545 550 555 560 AutoML algorithm and interpretability analysis is confirmed to address the two fatal issues for the case problem. The following conclusions can be drawn from this paper: 1) As a high-performance, easy-to-use, and open-source AutoML software, Auto-sklearn was confirmed to be effective and fast in the failure identification of RC shear walls. Evidence is that Auto-sklearn’s output outperformed five manual ML algorithms. 2) PDP analysis based on the partial effects confirmed a list of significant features, including boundary region vertical reinforcement ratio (ρvb), lw/tw, aspect ratio, and axial load ratio (P/fcAg). The features’ significance was consistent with long-established domain knowledge. For example, low values (<1.5) of the aspect ratio led to a high probability of non-ductile failure mode, and high values (>2) contributed to a high probability of ductile failure mode. 3) ALE found similar partial effects with PDP for uncorrelated features, of which two have different estimation mechanisms. For correlated features, such as Ab/Ag and section, interpretation results show that flanged and barbell RC shear walls are more prone to fail in a non-ductile mode. 4) LIME provides a local interpretation for an individual prediction. The partial effect of each feature and related detailed evidence is given, which could provide detailed design/retrofitting suggestions for a single building case. The contribution to knowledge from this study lies mainly in structural engineering, but also in applied computing. For the former, a theory-based, comprehensive data sampling is provided. More importantly, integration of an AutoML method and interpretability analysis provides an automatic and interpretable problem solution, which resolves the two fatal problems—meaning the complex data-expertise-relied modeling process and the untrustworthy ‘black box’ nature—of the application of ML in AEC industry. For the latter, empirical evidence is provided to the real-world performance and applicability of AutoML and three interpretability methods for a complex AEC problem. In the future, the proposed integrated workflow can be applied to extracting engineering information and knowledge from complex AEC problems. In addition, big data and other advanced interpretability methods can be adopted for interpreting complex AEC informatics with sophisticated ML methods. Acknowledgment This study was supported by Hong Kong Research Grants Council (RGC) (Nos. T22-504/21R, C7080–22GF)) and in part by Shenzhen Science Technology and Innovation Commission (SZSTI) (No. SGDX20201103093600002). We appreciate the open data sources offered in the literature listed in section 4.2. 565 References Abdulridha, A. & Palermo, D. (2017). Behaviour and modelling of hybrid SMA-steel reinforced concrete slender shear wall. Engineering Structures, 147(15), 77-89. doi:10.1016/j.engstruct.2017.04.058 23 570 575 580 585 590 595 600 605 610 615 ACI. (2019). Building Code Requirements for Structural Concrete (ACI 318-19) and Commentary. Farmington Hills, MI, USA: American Concrete Institute. doi:10.14359/51716937 Anwar, T. (2021). 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