Tower crane safety on construction sites: A complex sociotechnical system perspective

Safety Science 109 (2018) 95–108

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Tower crane safety on construction sites: A complex sociotechnical system perspective

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Wei Zhou, Tingsheng Zhao , Wen Liu, Jingjing Tang School of Civil Engineering & Mechanics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Tower crane safety Systems theory AcciMap Questionnaire survey

Tower crane is the lifeline of the construction industry, but tower crane accidents are still too frequent. Despite the significant progress in tower crane safety research, system thinking-based approaches are lacking. The aim of this paper is to analyze tower crane safety from a complex sociotechnical system perspective through implementing both qualitative and quantitative analysis methods. Characteristics of five tower crane safety system components are summarized with a new framework comprised of five system-based levels based on Rasmussen’s risk management theory. Fifty-six contributing factors of tower crane safety were identified. The AcciMap technique was applied to qualitatively build a generic model for tower crane safety, which comprehensively presents the systems levels and casual paths of the contributing factors. A survey was conducted to quantitatively research the tower crane safety system. Nine main dimensions and 25 critical factors were found pertaining to the tower crane safety system. These results provide a new lens for tower crane safety and contribute new systems thinking applications in tower crane safety management.

1. Introduction

some aspects (e.g., equipment, worker and environment) (Lee et al., 2009; Li et al., 2012a,b; McDonald et al., 2011). More comprehensive factors are required to be systematically identified. Furthermore, tower crane safety issues can be regarded as a system problem. From a system lens, safety and indeed accidents are emergent properties of nonlinear interaction among different components of a complex sociotechnical system (Leveson, 2004; Mohaghegh et al., 2009). It is the interactions between the components that are of importance. Prior research identified tower crane safety factors through decomposing the system into component parts and analyzing these parts alone, the complex hierarchy and correlations among tower crane safety factors have been ignored. A sociotechnical system-based analysis is required to understand tower crane safety, so that systematic strategies can be developed to improve tower crane safety. Tower crane safety issues can be regarded as complex sociotechnical system problems with multiple technological, environmental and societal components. The tower crane is always in high frequency usage after structural safety checks and site assembly (McDonald et al., 2011). The equipment is composed of multiple components and devices. Stakeholders involve manufacturers, the main contractor and subcontractor (Shin, 2015). From start to finish, the main staff includes designers, supervisors, crane drivers, signalers, slingers and erection/ dismantling workers (Raviv et al., 2017a,b). Construction sites usually have crowd working faces, adjacent building and facilities, complex

As the main vertical and horizontal transportation tool in construction, the tower crane is the lifeline of the building process. However, accidents are frequent. According to MOHURD statistics, in July of 2017, 14 crane accidents occurred in China, resulting in 9 deaths and 11 injuries. For example: on July 22, 2017, a tower crane collapse accident occurred in Guangdong, Guangzhou, resulting in seven deaths and two injuries. Tower crane accidents not only threaten the safety of workers, but also cause immediate damage to machinery, equipment and buildings (Marquez et al., 2014; Swuste, 2013). Besides, tower crane construction can threaten the safety of pedestrians and facilities near the construction site due to its large size and broad coverage area even beyond the boundary of the construction site, causing a serious social impact (HSL, 2010; McDonald et al., 2011; Tam and Fung, 2011). Therefore, establishing the safety factors that can improve the tower crane safety management is essential. Much research has contributed to this area of study (Shapira and Lyachin, 2009; Shapira and Simcha, 2009a, b; Shin, 2015; Swuste, 2013). Prior research has promoted the development of targeted preventive strategies for tower crane accidents. However, these studies primarily adopted a reductionist approach to identify safety factors affecting tower crane concerning a certain phase (e.g., the operation phase, the installation and dismantling phase) (Shapira and Lyachin, 2009; Shin, 2015) or

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Corresponding author. E-mail addresses: [email protected] (W. Zhou), [email protected] (T. Zhao), [email protected] (W. Liu), [email protected] (J. Tang).

https://doi.org/10.1016/j.ssci.2018.05.001 Received 18 December 2017; Received in revised form 29 March 2018; Accepted 1 May 2018 0925-7535/ © 2018 Published by Elsevier Ltd.

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including checking site and equipment before erection of tower cranes; improving site supervision; improving qualification and experience requirements of subcontractors and workers. Accidents analysis carried out by Raviv et al. (2017a,b) presented the key causes of tower crane failures including: failure of lifting accessories, overriding of limiters, improper rigging, error of signalers, bad visibility, communication failure, inattention, strong wind, rail failure, operator fatigue, technical failure in crane, load mishap, wrong layout of crane, error of crane diver, horizontal pull of load and no signaler. Shin (2015) conducted focus group interviews to investigate factors influencing the safety of tower crane installation and dismantling, and found several majors factors including inadequate knowledge and skills of the erection and dismantling workers, insufficient instructions on safe work procedures, deterioration of crane components; insufficient supervision on construction site, time pressures, and space constraints. Prior research has promoted the development of targeted preventive strategies for tower crane accidents. However, these studies primarily adopted a reductionist approach to identify factors affecting tower crane safety, and system thinking was still lacking.

weather phenomena, and a dynamic operating environment for the tower crane (Aneziris et al., 2008; Shepherd et al., 2000). Tower crane on-site work phases include installation, operation, maintenance and disassembly (Li et al., 2012a,b; Shin, 2015). Moreover, a high degree of uncertainty and a complex environment add to the construction site dynamics (Li et al., 2012a,b). In summary, tower crane safety issues come from the coupling effects of various components across the system, and influencing factors of tower crane safety require system thinking and analysis. This paper presents a process of systematic thinking and a tower crane safety analysis, which captures the complex system of factors influencing tower crane safety. From a qualitative point of view, the hierarchy and framework of the tower crane safety system are based on Rasmussen’s (1997) risk management theory, and the characteristics of tower crane safety influencing factors were analyzed by the AcciMap method. The critical factors and main dimensions of the tower crane safety system were explored through statistical data analysis from a survey. System analysis results not only elucidate the structure and characteristics of a tower crane safety system, but also provide a system foundation for tower crane accident prevention and analysis. Furthermore, a tower crane safety evaluation and control system can be established based on these key factors and main dimensions of system safety, making the promotion of tower crane safety management a top priority. The paper is organized as follows: A literature review of tower safety is part of an introduction to the system theory-based approach, which includes Rasmussen's (1997) risk management framework and the AcciMap method. Second, the research process design and research methods are presented. Third, the structural framework, critical factors and main dimensions of the tower crane safety system are delineated. Finally, both qualitative and quantitative analysis are used to discuss the systematic thinking and implications of tower crane safety.

2.2. Rasmussen’s (1997) risk management framework and AcciMap technique At present, system thinking is advocated to understand and enhance the safety performance of complex sociotechnical systems. From a system lens, safety and indeed accidents are regarded as emergent properties of nonlinear interaction among different components of a complex sociotechnical system (Leveson, 2004; Mohaghegh et al., 2009). Safety cannot be understood by simply breaking the system into components and examining these parts individually. From the perspective of system safety based on system theory and control theory, safety is actually a system property acquired through imposing constraints on the interaction of system components (human, technology, environment, and so on), making safety issues control problems (Leveson et al., 2003; Rasmussen, 1997; Reason, 1997). To improve tower crane safety and prevent accidents, one of the essential premises is to systematically analyze its structure and components.

2. Literature review 2.1. Tower crane safety research Lots of research has been carried out on tower crane safety, which includes accident analysis, interviews and surveys of construction sites as well as modeling analysis (Shin, 2015). Research on the operation phase of tower cranes safety is abundant. Beavers et al. (2006) analyzed crane accidents occurring in the USA between 1997 and 2003 and found that low safety performance of the crane drivers and slingers were the main cause of crane accidents. Sertyesilisik et al. (2010) investigated the lifting operation in Britain and found that experience and safety knowledge of the lifting team needed to be strengthened. Tam and Fung (2011) found the main factors influencing tower crane safety were negligence or misjudgment of participants, inadequate training, multi-level subcontracting systems and schedule pressure. Their questionnaire survey and structured interviews showed provision of safe systems and safety programs thorough inspection, effective communication and provision of safety training for workers. Shapira and Lyachin (2009) used structural interviews and a survey to research safety factors influencing tower crane operation and identified 21 factors grouped into four categories: project conditions (obstacles and congested site, power lines, blind lifts, overlapping cranes, sight distance and angle, cab ergonomics, length of work shift, multiple languages, operator aids and type of load); environment (winds, weather and visibility); human factor (operator proficiency, operator character, employment source, and superintendent character and signal person experience); safety management (site-level management, company-level management and maintenance management). Guidelines on safety of tower cranes (CIC, 2010), established by the Construction Industry Council of Hong Kong, recommended several measures for enhancing tower crane safety

2.2.1. Rasmussen’s (1997) risk management framework Rasmussen proposed a safety risk management framework for complex sociotechnical systems, and described the sociotechnical systems as a hierarchical structure comprised of six levels, as shown in Fig. 1. From top to bottom, a system can be divided into: a government level, regulators and associations level, company level, management level, staff level and work level (Rasmussen, 1997; Svedung and Rasmussen, 2002). These levels influence each other through top-down decision flows (such as laws, regulations, and policies, etc.) and downto-top information feedback (such as the actual state of the system, and changes in the external environment, etc.). Instead of being static, these levels are constantly affected by the external environment, such as technical, economic and policy circumstances. Rasmussen considers accidents as the result of out-of-control of hazardous work processes. The out-of-control scenario is not just caused by certain actions or errors (such as workers' unsafe behavior), but instead a broader sociotechnical context should be used to gain more focus. Therefore, system safety is influenced by the decisions and actions of all actors across all levels of the system (Salmon and Lenné, 2015). Maintaining the safety of the system is essentially a dynamic control process involving all levels of the whole sociotechnical system, and a vertically integrated view of system behavior is required. 2.2.2. AcciMap technique To support the use of a risk management framework for system analysis, Rasmussen developed the AcciMap approach as an appropriate methodology for modeling the sociotechnical system. The approach can be applied to describe “how the conditions, and decisions 96

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Public Opinion

Government Laws

Regulators, Associations

Regulations

Company

Tower crane stakeholders

Changing political climate and public awareness

Tower crane staff Changing market conditions and financial pressure

Tower crane safety

Tower crane equipment

Company Policy

Work environment

Management Changing competency and levels of education Plans

Staff

Action

Work

Work process

Fig. 2. The tower crane safety system.

Therefore, the process of generalization is necessary not only through a set of accident scenarios, but also through literature, expert experience and system thinking before modeling.

Fast pace of technological change

Hazardous process

2.3. Tower crane safety system

Fig. 1. Rasmussen’s risk management framework (adapted from Rasmussen, 1997).

The system is the organized whole, which is composed of interrelated and interacting elements (or parts) and has a certain structure and function (Larsson et al., 2010; Leveson, 2004). The tower crane safety on the construction site is a complex sociotechnical system problem. System components involve tower crane stakeholders, equipment, staff, work process and environment (Fig. 2). These system components are interrelated and couple with each other before finally realizing the function of the tower crane safety. This section will preliminarily analyze the system components of the tower crane.

and actions of various actors within the system interact with one another to create the incident under analysis” (Newnam and Goode, 2015). Moreover, the AcciMap is a process for generalizing the structure and function of the system, so that the casual network of contributing factors and the hierarchy of system can be described through graphic representation (Goode et al., 2017; Stevens and Salmon, 2016). Such graphic representation not only helps to create an overview of complex occurrences, but also contributes to communicating assumptions and findings when analyzing safety risks. Specifically, the AcciMap technique decomposes the system in question into six levels: (1) government policy and budgeting, (2) regulatory bodies and associations, including local area government planning and budgeting, (3) technical and operational management, (4) physical processes, (5) actor activities, and (6) equipment and surroundings (Rasmussen, 1997; Svedung and Rasmussen, 2002). Contributing factors at each level are identified and linked together according to their interrelationships. The steps to apply the approach can be described as follows (Newnam and Goode, 2015).

2.3.1. Tower crane stakeholders A tower crane is the major equipment used for lifting activities at the construction site, and its stakeholders involve the manufacturer, main contractor and subcontractor. Generally, the tower crane is designed and manufactured by the manufacturer according to relevant directives and regulations. Then, the leasing company (the subcontractor) will buy the tower crane and take responsibility for the safety of tower crane installation/dismantling and maintenance. The main contractor (the constructor) will sign a contract with the subcontractor to lease the tower crane for moving heavy objects on the construction site. To achieve tower crane safety practices, each stakeholder of the tower crane has a different task division and safety obligation (CECE and FEM, 2012; Shin, 2015; Tam and Fung, 2011). Specifically, the responsibilities of the manufacturer involve: (a) designing and manufacturing tower cranes according to design directives; (b) providing a crane instruction manual and residual risks list. The subcontractor requirements are to: (a) provide labor and tools for tower cranes, (b) carry out safety training for tower crane workers, (c) maintain the tower crane components in good condition, and (d) conduct routine inspection at regular intervals (CIC, 2010; Tam and Fung, 2011). The measures that the primary contractor should take are: (a) providing a functional working environment for the subcontractor, such as foundation structure, roads, operating space, and so on, (b) working out the safety plan and program of the tower crane with the subcontractor, and (c) providing support and supervision concern for tower crane safety.

(1). Building the systems levels to reflect specific situations according to the system framework. (2). Collecting cases and information to analyze what contributing factors should be considered. (3). Identifying the level location of the factors and placing contributing factors into the correct level. (4). Linking the factors paths to form a causal diagram according to the cause-effect relationship of these factors. The AcciMap can be used as an independent post-analysis method to analyze accidents in various fields (such as road safety (Newnam and Goode, 2015; Stevens and Salmon, 2016), outdoor activity (Salmon et al., 2014a,b), public health (Vicente and Christoffersen, 2006), and aerospace accidents (Branford, 2011)). However, an AcciMap assessment of one particular accident may very likely be ad hoc (Rasmussen and Svedung, 2000). A generic AcciMap framework is required to present an overview of the contributing factors and casual flows of the system, so that the proactive risk management process can be built to systematically devise safety risks prevention and mitigation strategies (Rasmussen and Svedung, 2000). The prerequisite for the generic AcciMap is to comprehensively make clear the contributing factors, the level location of the factors, and the casual paths of the systems.

2.3.2. Tower crane equipment features Tower crane equipment consists of a variety of components, such as metal structures, mechanisms, electrical systems, connecting components, ancillary equipment and safety devices, etc. Although different types of tower cranes exist, their main components are the same. Any 97

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Table 1 Responsibilities/requirements of tower crane staff. Work phase

Staff type

The whole process

Supervisor

Operation phase

Crane diver

Operation phase

Slinger

Operation phase

Signaler

Erection/dis-mantling and climb phase

Erection/dismantling worker

Responsibilities/requirements a. To supervise the whole process of tower crane (installing, operating, disassembling) on the construction site b. To establish safety system, and carry out safety risk assessment, safety training and directing, etc. a. To correctly operate the tower crane according to the guidelines of the manufacturer and the safety system b. To effectively collaborate and communicate with the slinger and signaler, and understand and follow the correct instructions a. Familiar with lifting and rigging assembly methods b. Able to establish weights and judge distances, heights and clearances c. Capable of directing the movement of the crane and load in the safety manner a. Good eyesight, hearing and reflexes and familiar with the signal code b. Able to convey instructions and provide vision information to assist the crane driver in safe operation c. Able to transmit the instructions of the slinger to the crane driver in a clear and precise manner a. Familiar with the components, installation tools and equipment of the tower crane b. Familiar with the crane manual, and able to follow the safety program of installing, climbing and dismantling the tower crane

structure and function system. A tower crane is assembled on the construction site by connecting its main components: the mast, slewing unit, driver cabin, jib, and counterweight. It is worth noting that the mast should be attached to the base and building, which give the tower crane structural support. The climbing phase refers to the process of changing the height of the tower crane by means of a telescopic job as the height of the building rises. Activities of hoisting and moving heavy materials are carried out in the operation phase of the tower crane. The dismantling phase can be roughly equivalent to the inverse process of erection. Characteristics of different phases may lead to different safety factors and types of accidents (Shin, 2015). For example, a lot of lifting activities occur during the operation phase. These lifting activities call for effective coordination and communication among the crane driver, signaler and slinger. Safety risk assessment and lifting line plans are necessary for preventing collisions (Sertyesilisik et al., 2010). In addition to the operation phase, during the climbing and erection/dismantling phase, the structure system of the tower crane is in a timevarying state due to adding and removing of sections of the tower. In this case, reliable tower crane accessories and correct procedures carried out by the erection /dismantling workers are critical for the tower crane to maintain its structure stability. Besides, the PPE (personal protective equipment) of the erection /dismantling workers is a key factor because of the aloft work.

failure of a component may lead to instability of the tower crane structural system, which can then evolve into accidents. Besides, after the repeated process of installation, operation and disassembly, the components of a tower crane is prone to aging, corrosion and wear because of its moist and harsh environment, which highlights the importance of the maintenance and repair of the tower crane components. Furthermore, under the work conditions of aloft work, heavy blocks and dynamic loads, the tower crane system has the characteristics of “poorly understood dynamics, unobservable processes and intrinsically poorly monitored and instrumented processes, and critically narrow limits of safety”(Swuste, 2013). 2.3.3. Tower crane staff The major staff of the tower crane on construction sites are supervisors and workers. Critical for tower crane safety are the supervision and guidance of supervisors from the main contractor and subcontractor, and cooperation among the tower crane workers (crane driver, signaler, slinger and erection/dismantling worker). These staff members work in different phases of the tower crane assignments, and have different responsibilities/requirements (OSHB, 2011) as shown in Table 1. 2.3.4. Work environment of the tower crane The entire process of the tower crane is exposed to the open air, so various environmental factors affect the safety of the tower crane, such as weather, temperature, and working face. Weather factors may affect the stability of the tower crane structure (i.e. excessive wind force can affect the stability of the tower crane structure) and the lifting loads (i.e. the wind load effects on the windward of the loads, leading to swinging or rotating). Visibility is also affected by weather factors (i.e. fog, dust, overcast sky, and stormy days). Therefore, appropriate safety measures should be taken under adverse weather conditions. Temperature factors can also affect the body functions of the worker (e.g., when workers need to allot part of their physical and mental energies to deal with high temperatures) (Shapira and Lyachin, 2009). The working face factors contain cross-working conditions (such as overlapping crane work, cross operation with other equipment, etc.) (Hwang, 2012; Zheng et al., 2013), obstacles (such as power line, various temporary facilities, etc.) and the crane driver’s vision (such as blind lifting zone), etc. (Shapira et al., 2008). The potential of collision and object strike accidents could be improved due to these factors of working face.

3. Research methodology 3.1. Research design In order to systematically analyzing tower crane safety system in the context of complex sociotechnical system, this paper achieved research objectives through two phases. In the first phase, the system levels framework of the tower crane safety system was built based on the system thinking and Rasmussen’s risk management framework. The contributing factors of the tower crane safety system were systematically identified through literature review, exploratory interview and a pilot study. These factors were the research basis for the next phase. In the second phase, the characteristics of the tower crane safety system were studied by a mixed methods research, which is the process of collecting, analyzing and integrating qualitative and quantitative research data in a single study or a program of inquiry (Tashakkori and Creswell, 2007). The rationale for mixing quantitative and qualitative research is that both quantitative and qualitative methods have their own advantages for researching tower crane safety systems. They should be integrated to obtain different but complementary data to best understand the research problem. The mixed methods research adopted in the second phase was a

2.3.5. Work process of the tower crane The whole process of the tower crane on the construction site involves four phases: erection, climbing, operation and dismantling. The erection phase is the assembling process, which establishes the stable 98

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Fig. 3. Research process design.

(1) Regulatory bodies: activities, decisions, actions, etc. made by regulatory bodies for construction crane safety supervision. This level mainly presents the safety supervision of regulators and safety regulation on the safety of hoisting equipment. (2) Tower crane stakeholders: activities, decisions, actions, etc. made by the manufacturer, the leasing company and the constructor of the tower crane. Factors at this level mainly describe the safety duties and attributes of each stakeholder. (3) Construction site management: activities, decisions, actions, etc. of safety management made by the tower crane stakeholders on the construction site. This level mainly shows the different task divisions and safety management responsibility of the leasing company and constructor of the tower crane on the construction site. (4) On-site tower crane staff activities: decisions, actions, etc. undertaken by the supervisor and workers of the tower crane. The factors at this level not only describe the safety supervision of officers on the workers and site crane operations, but also the tasks and behavior of tower crane workers during the erection/dismantling, climbing and operation phase of the tower crane on the construction site. These factors have distinct human-machine interaction features. (5) Environment and equipment: This level describes contributory factors associated with the environment and tower crane equipment during all tower crane activities on the construction site. These factors affect the safety of tower cranes. Therefore, this level has strong dynamic characteristics.

convergent parallel mixed methods design (Creswell and Plano Clark, 2017). Qualitative and quantitative research were implemented during the same timeframe and with equal importance (Creswell and Plano Clark, 2011). Qualitative and quantitative data were collected and analyzed in the concurrent but separate way. AcciMap analysis and semi-structured interview were applied in the qualitative research section. Questionnaire survey, factor analysis and statistical ranking analysis were adopted in the quantitative research section. Qualitative and quantitative results are integrated in the discussion section to form an in-depth understanding of the tower crane safety system. The reason for adopting this design is that the qualitative research helped to establish the framework and factor paths of the tower crane safety system. The quantitative research was used to explain the main dimensions and critical factors of the tower crane safety system. The integration of qualitative and quantitative results contributed to link the characteristic of the tower crane safety system together to form a systematic thinking and understanding. Fig. 3 presents the research process and methods of this paper. 3.2. Building the system levels framework of the tower crane safety system To enable the AcciMap technique to be used in the analysis of the tower crane safety, a system levels framework needs to be built. In general, the higher the organizational level, the more slowly the factors at each level change. Considering that change of policy and budgeting takes a long time, the relevant factors are basically static relative to the whole process of the tower crane on the construction site. Therefore, we consider the government policy and budgeting level as the external level rather than an internal level of the tower crane safety system at the construction site. Finally, based on the Rasmussen's risk management framework and analysis of the system components of the tower crane as shown in Fig. 3, we adopt five system levels to describe the safety system of the tower crane. The definition of each level is as follows:

3.3. Systematically identifying safety contributing factors First, the literature review, prior accidents analysis and system thinking are combined to develop an in-depth understanding of the factors influencing tower crane safety. A list of 61 factors with potential effects on the tower crane safety was identified. Then the factor items and descriptions were built to form the questionnaire. Second, 99

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were summarized into the contributing factors based on the category of factors in the Original Prototype. And the identified contributing factors and relationships were noted by linking the factors path in the Original Prototype. For example: “Tower crane dismantling worker A had weak safety values. He drank alcohol before the operation and did not wear the safety helmet, seat belt, antiskid shoes and other labor protection products. After removing the first pull rod (which was used to fix tower crane bollard) on the 22 floor, worker A stood above the air conditioning well on the west side of the 22-floor balcony and outside the pull-bar on the window. When he was pulling the second pull rod to the east, he did not stand firmly and fell from the height to the ground.” was summarized as that “Erection/dismantling worker: safety operation” is affected by “Worker: safety values” and “Erection/dismantling worker: safety protection equipment”. The two factor paths were identified and noted in the Original Prototype. Finally, all path results from accident analysis were aggregated into an accident causal paths model (referred to as Path Prototype A). Expert interviews were also conducted to identify the causal paths through expert experience. The expert panel was comprised of the seven experts from the step one. First, the seven experts were asked to independently identify the causal paths in the prototype according to themselves’ work experience about tower crane safety. Each expert noted the identified results in the prototype to form seven drafts. Second, one researcher reviewed the seven drafts to aggregate them into an experience causal paths model (referred to as Path Prototype B). Finally, the Path Prototype A and Path Prototype B were integrated to construct a preliminary AcciMap model.

exploratory interviews were carried out to select and supplement the questionnaire items and descriptions. The expert panel that served the exploratory interviews were comprised of 12 experts who were experienced engineers representing the top ten leasing companies of the tower crane or construction companies in China. After the exploratory interviews, we simplified, rewrote, merged and eliminated some factors items and descriptions. Third, we conducted a pilot study to ensure the validity, reliability, and significance of the questionnaire items with 113 construction managers who participated in professional qualification exams in Huazhong University of Science and Technology in China. Item analysis was conducted to test whether each item could separate one participant from the others. Factor items with a significant value above 0.05 and Pearson correlation coefficient below 0.4 were excluded from the questionnaire. Finally, after the exploratory interviews and the pilot study, we obtained a list of 56 factors influencing the safety of the tower crane, which formed the main basis for our AcciMap analysis. 3.4. Qualitative research section The key problem for the AcciMap analysis was to consider the contributing factors, the level location of the factors, and the casual paths of the systems (Goode et al., 2017). Because the AcciMap is a qualitative method, the whole process of the AcciMap was designed to eliminate the interference of subjective factors and to reach a consensus. The whole process consisted of four steps as follows. 3.4.1. Step one: Data collection: Accident reports and expert panel Prior to commencing the study, our team has reached a cooperative research agreement with China Construction Third Engineering Bureau Co., Ltd, which is one of the leading construction companies in China. The company's safety management department has established a safety database platform. By the end of 2017, the platform has collected more than 300 accident investigation reports in China. These reports met the requirements of the Report on production safety accident and regulations of investigation and treatment (State Council of the People's Republic of China, 2007), and covered the project situations, and the process, causes and loss of the accident. Reports were selected for analysis if the accident involved a tower crane. Finally, 46 tower crane accident investigation reports were collected from the safety database. Besides, we set up the expert panel, comprised of five senior engineers with more than 10 years working experience from the tower crane manufacturer, the main contractor or the leasing company, and two professors engaged in construction safety research, to ensure the rationality of the AcciMap analysis.

3.4.4. Step four: Building the generic AcciMap model Semi-structured interviews were conducted with seven experts individually to revise the preliminary AcciMap model. The reasons for each expert's corrections in the process were recorded. Then, the results of the corrections were compared and the differences were eliminated through a new round of discussion among the expert panel members. Finally, after combining theoretical knowledge and expert experience, the generic AcciMap model for the safety of the tower crane was built. 3.5. Quantitative research section A survey was adopted to collect data for researching the main dimensions and key factors of the tower crane safety system. The survey contains two sections. The first section collected the respondents’ background information. The second section was designed to provide a study on the influence of the 56 factors affecting tower crane safety. The Likert scale was applied to measure the extent to which each factor affects tower crane safety. A scoring range of 1, 2, 3, 4 and 5 represents the effect with a score of (1) representing almost no impact and (5) representing the strongest effect on tower crane safety. This study’s research team established a close cooperative relationship with two general companies and one tower crane manufacturer in China, the Shanghai Construction Group, China Construction Third Engineering Bureau Co., Ltd, and Fushun Yongmao Construction Machinery Co., Ltd., respectively. The construction projects of these companies are distributed throughout China. Five different cities, namely Shanghai, Nanjing, Wuhan, Zhengzhou and Shenyang, were selected for this investigative study. They are distributed in southern, central and northern China. Two-hundred-thirty-one construction site managers and 159 tower crane workers from 42 construction projects in five cities participated in the survey. In addition, 35 government officials, from safety and quality regulatory bodies in Shanghai and Wuhan also participated. During the process of investigation, trained researchers distributed the questionnaire to the respondents. All respondents were informed that: (1) the survey was completely voluntary and anonymous, (2) all items in the questionnaire must be completed, and if a question or request for information was not clear, the respondents could ask the researchers for help, and (3) discussion with

3.4.2. Step two: Identifying the level location of the contributing factors Before this step, our researchers explained the description of each factor to seven experts. Then, each expert was asked to classify the 56 factors mentioned in the previous section to determine the level location of these factors according to the system level framework of tower crane safety. Two researchers reviewed the classification results of seven experts to find any inconsistencies, which were then fed back to the group of experts to reach a consensus. The system levels framework of the tower crane safety, the contributing factors and the level location of the factors can provide the prototype scheme (referred to as Original Prototype) for identifying the causal paths. 3.4.3. Step three: Defining the causal paths across the system The causal relationships among the influencing factors were identified based on the Original Prototype, accident reports and expert interviews. Accident reports analysis was adopted to identify the factor paths in the Original Prototype. Two researchers identified the stated accident factors, and the relationships among them, which had to be explicitly described within each accident report. Researchers were not allowed to draw any inferences from the reports. The identified accident factors 100

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AcciMap.

colleagues was not allowed. Notably, each factor item and description was explained to the workers orally due to some respondents’ lower level of knowledge and understanding. Finally, 322 valid questionnaires were collected. The Statistic Package for the Social Sciences (SPSS) version 23.0 was used to make statistical analysis. Statistical ranking analysis and factor analysis methods were adopted to determine the main dimensions and key factors of the tower crane safety system.

4.2.1. Regulatory bodies Regulatory body levels contains four factors, showing the supervision of government agencies on the safety of tower crane equipment under their jurisdiction through safety regulations. The model illustrates that that “regulatory bodies” have a direct impact on the “tower crane stakeholders” level and the “construction site management” level, which presents the government supervisory organizations’ important role in supervising the safety management of tower cranes. First of all, the tower crane is categorized as special equipment. To effectively prevent a tower crane accident, a series of management regulations for the manufacture and operation of the tower crane are formulated by the government safety regulators to clarify the safety responsibility and obligations of the tower crane stakeholders. In addition, tower crane equipment and workers are also supervised by government safety regulators. Tower crane registration is conducted by government safety regulators to check and record the manufacture and operation of the tower crane. Tower crane stakeholders should acquire relevant safety and quality licenses, such as a manufacturing license, product certificate license and equipment record certificate, to achieve the permission of the government safety regulators. Moreover, the tower crane workers from the leasing company also need to pass the administrative qualification system check conducted by government safety regulators. In addition, routine supervision and inspection are

4. Result 4.1. Safety contributing factors of the tower crane Based on the results of system analysis, literature reviews, exploratory interviews, and a pilot study, a list of 56 factors were obtained. These factors provide the main basis for analyzing tower crane safety systems. All factors with detail descriptions are listed in Appendix A. 4.2. The generic AcciMap model The AcciMap analyzes the causes of accidents occurring in complex sociotechnical systems. Fig. 4 illustrates the generic AcciMap model for the tower crane safety. This section describes the contributing factors as well as the level location and casual paths of these factors on the

Regulatory bodies

Tower crane stakeholders

X36. Government safety regulators: safety regulation

X46.Main contractor: safety management system

X38 Government safety regulators: safety supervision

X37 Government safety regulators: operator certification

X35.Government safety regulators : crane registration X39. Manufacturers: qualifications

X50. Subcontractor: tower crane safety input

X41 Main contractor: safety attitude

X51 Main contractor: safety input

X48 Subcontractor: safety inspection

X47.Main contractor: site supervision

Construction site management

X45. Subcontractor: safety briefing

X44. Main contractor: safety briefing X31.Construction sites organization rationality

X30.Warning region setting

X13.Workers: safety values

X42.Main contractor: safety plans X43. Subcontractor: safety plans

On-site tower crane staff

X8.Operation communication X5.Crane driver: physical quality X2.Crane driver: character X26.Overlapping crane work

Environment and equipment

X27.Cross operation X28.Obstacles and congested site

X53 Subcontractor: safety checking

X49.Subcontractor: tower crane maintenance

X52. Main contractor: safety acceptance

X15.Supervisor: character

X1.Worker ability

X14.Supervisor: safety inspection X16.Supervisor: safety commitment

X17.Supervisor: safety instruction

X11.Erection/dismantling worker: safety protection equipment

X7.Rigger: safe operation

X3.Crane driver: safety operation

X56. Main contractor: reward and punishment

X54.Main contractor: safety education

X40.Schedule pressure

X12.Worker: job stress

X6.Signaler: safety directing

X55.Subcontractor: safety training

X10.Erection/ dismantling worker: safety operation

X9.Erection/ dismantling worker : character

X4.Crane driver: routine checking

X18.Crane X19.Assembling X34.Winds X23. structural parts: auxiliary equipment: Operator X33.Weather safety reliability safety reliability aids phenomena(excluding winds) X22.Crane X21. Crane X20.Crane safety X32. attachment device: ergonomics device: reliability Visibility safety reliability X29. Blind lifts X25.Work height

Fig. 4. The generic AcciMap model of tower crane safety. 101

X24.Tower crane foundation : safety reliability

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executed by the government safety regulators to check the implementation of on-site safety management of the main contractor and subcontractor, and non-compliant companies will be subject to administrative penalties.

4.2.4. On-site tower crane staff The “on-site tower crane staff” level contains 17 factors. This level emphasizes on-site staff members, which are important components of the tower crane safety system. The characteristics and safety activities of on-site supervisors and tower crane workers (including the crane driver, signaler, slinger and erection/dismantling worker) are described in detail. This level works closely with the “construction site management” level and the “environment and equipment” level. In this level, the safety activities of supervisors and tower crane workers are influenced by their individual character and abilities. At the same time, tower crane worker’s safety operation is also affected by the on-site supervisors’ activities and factors in at the “environment and equipment” level. Besides, factors at the “environment and equipment” level are influenced by the safety activities of on-site supervisors and tower crane workers. First, tower crane worker’s safety operation is generally influenced by workers’ character, job stress, safety values, ability, safety protection equipment and supervisors’ safety instruction. As expected, the crane driver and the erection/dismantling worker are the key personnel in the tower crane’s operation and erection/dismantling phase, respectively. A crane driver’s safe operation is not only affected by his or her own characteristics (such as physical quality, character and ability) as well as management activities (such as job stress and supervisors’ safety instruction) and signaler’s s operation communication within this level, but the crane driver is also affected by factors at the “construction site management” level and factors at the “environment and equipment” level (such as wind, blind lifts, operator aids, etc.). The erection/dismantling worker’s safety operation is also affected by his or her own characteristics and management activities. They are also influenced by safety protection equipment and factors at the “environment and equipment” level (such as wind and safety reliability of assembling auxiliary equipment). Second, supervisors’ safety activities are affected by upper level factors (such as the main contractor’s safety briefing as well as his or her reward and punishment policies), which can also affect the tower crane workers’ safe operation. Third, safety activities of on-site supervisors and tower crane workers have a direct impact on the safety state of equipment. Specifically, the erection/dismantling worker’s safety operation can influence the safety reliability of the tower crane equipment (such as crane structural parts and attachment device) in the erection/dismantling phase. Crane driver routine checking can affect the tower crane’s safety reliability in the operation phase. Likewise, a supervisor’s safety inspection is critical at every stage of the safety checks required for the tower crane.

4.2.2. Tower crane stakeholders There are five factors at the “tower crane stakeholder” level. This level mainly describes the impact of the main contractor’s safety system on the subcontractor’s safety management activities. Besides, the model shows that the activities, decisions, actions made by the tower crane stakeholders can affect the “construction site management” level, the “on-site tower crane staff” level and the “environment and equipment” level. First, the main contractor’s safety attitude can influence the proportion of safety funds in the contract with the subcontractor, which will affect the tower crane safety input of the subcontractor. Second, the main contractor’s safety system can influence the on-site self-safety management activities. For example, the main contractor’s safety system stipulates the measures and activities that must be observed, which can affect the implementation of a safety plan and supervision on the construction site. Third, the subcontractor’s safe input plays an important role in the safety of tower crane operation and its workers. For example, the safety input may affect the maintenance of the tower crane and the PPE. Fourth, manufacturer’s qualifications can directly influence the quality of tower crane equipment, which affects tower crane safety reliability. Finally, the ergonomic level of the operator cab is also influenced by the manufacturer’s qualifications (Spasojević Brkić et al., 2015). 4.2.3. Construction site management The “construction site management” level contains 14 factors. Factor paths at this level are very complex. This level presents different safety tasks and responsibilities of the main contractor and subcontractor (the leasing company). On the one hand, the subcontractor should cooperate with the main contractor to carry out safety management activities; on the other hand, on-site safety management of subcontractor is supervised by the main contractor. Furthermore, the model shows that the “construction site management” level has an impact on the “on-site tower crane staff’s” level and the “environment and equipment” level. First, on the construction site, the main contractor and subcontractor should cooperate with each other to ensure tower crane safety. For example, the safety plans and briefing of the subcontractor should be deepened and expanded based on the main contractor’s safety plans and briefing; furthermore, the subcontractor’s tower crane maintenance and safety inspection activities are influenced by the main contractor’s site supervision. Second, the main contractor’s safety management can affect the tower crane workers’ safety operation and supervisors’ safety commitment on the construction site. For example, schedule pressure directly affects a worker’s corresponding job pressures; supervisors’ safety commitment is influenced by the main contractor’s safety education and safety briefing as well as their reward and punishment policies. Third, the main contractor’s safety management can affect the tower crane’s work environment. For example, the main contractor’s safety plans can influence warning region settings and organization rationality on construction site management, which may cause overlapping crane work, cross operation and obstacles that further congest the site. Fourth, the subcontractor’s safety management can affect tower crane workers’ safety behavior and the tower crane’s safety performance. For example, the subcontractor’s safety plans, safety briefing, and safety training can determine the tower crane workers’ acceptance and completion of safe operation procedures. The subcontractor’s safety checking and tower crane maintenance are critical for the reliability of the crane structural parts, assembling of auxiliary equipment and handling of the crane safety device.

4.2.5. Environment and equipment This level contains 15 factors. Factors concerning the tower crane work environment, tower crane and auxiliary equipment are described. Factors at this level are closely related to the upper level factors, while the paths between factors at the internal level are few. Work environment factors (such as wind, weather phenomena, overlapping crane work, and blind lifts) directly affect the crane driver’s safety operation in the operation phase. At the same time, operator aid and crane ergonomics can influence a crane driver’s safety operation. Safety reliability of a tower crane foundation is influenced by the main contractor’s safety acceptance and supervisor’s safety inspection. Safety reliability of tower crane equipment (such as crane structural parts, crane attachment device and crane safety device) is mainly influenced by the factors at the “on-site tower crane staff” level and the “construction site management” level. 4.3. Main dimensions of tower crane safety Factor analysis is a statistical tool for dimensionality reduction. By exploring potential correlation patterns among a large number of original variables, the main service factors are extracted to reflect the primary information (Hair et al., 2006; Thompson, 2004). The factor 102

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Table 2 Total variance explained and names for the critical factor groups. Groups

1 2 3 4 5 6 7 8 9

Extraction sums of squared loadings % of Variance

Cumulative%

21.890 18.897 14.450 10.655 5.820 3.711 3.082 2.893 2.589

21.890 40.787 55.237 65.892 71.712 75.423 78.505 81.398 83.987

Factor variables

Group names

X24, X18, X22, X20, X53, X52, X39 X49, X46, X47, X48, X41, X54, X50, -X51, X55, X4, X56 X43, X45, X44, X42, X40 X1, X3, X13, X6, X8, X10, X7, X5, X2, X9, X12 X34, X29, X33, X32, X25 X31, X30, X26, X27, X28 X14, X16, X17, X15 X23, X21, X19, X11 X36, X37, X38, X35

Tower crane equipment quality and reliability Tower crane safety management and maintenance Tower crane safety program Workers’ safety practice Working environment On-site working condition for tower crane Supervisors’ safety practice Auxiliary safety equipment Government safety supervision

including safety plans for tower crane assembling, operating and dismantling, and safety briefing to crane workers. Summarily, these factors are concerning tower crane safety program of main contractor and subcontractor.

analysis method was used in this paper to study the main dimensions of the tower crane safety system, and the questionnaire data was analyzed by principal component analysis. According to the analysis result, the significance level of the Bartlett test of sphericity was 0.000 (< 0.05), and the value of the KMO measure of sampling adequacy was 0.792 (> 0.50), which verifies that the sample meets the basic requirements of factor analysis. The principal component analysis generated nine critical factor groups with eigenvalues greater than 1, indicating that tower crane safety system can be expressed in nine main dimensions. The percentage of variance and the cumulative percentage of variance for the nine critical factor groups are shown in Table 2, which represents the percentage and cumulative percentage of system information explained by each factor group. Table 3 shows the rotated component matrix. The factor loading is the correlation coefficient of the variable with a factor group. The level of the factor loading represents the degree of the variable needed to explain the factor group. A factor with loading below 0.5 was excluded from the factor group. The realistic meaning of a factor group can be obtained by synthesizing those variables with a relatively high factor loading. Based on the component variables in each group, the nine factor groups can be named as follows.

4.3.4. Group 4: Workers’ safety practice Group 4 consists of 11 factors. X12 can be excluded for the low factor loading. X3, X6, X7 and X10 are concerning tower crane workers’ safety behavior, such as safety operation of crane driver, riggers and erection/dismantling worker, and signaler’s safety directing. X1, X2, X5, X8, X9 and X13 reveal some variables affecting crane workers’ safety behavior, including crane workers’ character, ability, operation communication, safety values, physical quality and so on. Therefore, these factors can be summarized as workers’ safety practice. 4.3.5. Group 5: Working environment Group 5 consists of 5 factors, that is, X25, X29, X32, X33 and X34. There are some environment factors that may influence the safety of the tower crane, such as wind, weather phenomena and work height. Good visibility and less blind lifts area are important environmental condition for ensuring crane workers’ safety operation. Therefore, these factors can be summarized as working environment.

4.3.1. Group 1: Tower crane equipment quality and reliability Group 1 consists of seven factors. The factor loading of X39 is below 0.5 means it is not enough to explain the Group 1. The combination of X18, X22 and X24 reveals the quality and reliability of tower crane structure system and functional components. X52 involves main contractor's safety acceptance procedures to ensure the quality of crane foundation and installation task. X53 involves subcontractor’s inspection and maintenance on tower crane equipment, which affects the safety state of on-site tower crane. Clearly, these focus on the quality and safety reliability of on-site tower crane equipment. Group 1 can be summarized as tower crane equipment quality and reliability.

4.3.6. Group 6: on-site working condition for the tower crane Five factors comprise elements of Group 6 regarding working condition of construction site. The combination of X30 and X31 reveals that rational organization of on-site work space and warning region setting can bring out a safety working condition for tower crane. Some elements for tower crane working condition are identified, such as the situation of multiple tower cranes (X26), cross operation (X27) and obstacles in vicinity of tower crane operation (X28). Summarily, these factors indicate on-site working condition for the tower crane.

4.3.2. Group 2: Tower crane safety management and maintenance Group 2 consists of 11 factors. X4 and X56, the factor loading of which is below 0.5, are not enough to interpret the groups. X41, X46, X47, X51 and X54 are concerning main-contractor-related tower crane safety management, such as safety management system, safety training and safety input. X48 X49, X50 and X55 are concerning subcontractorrelated tower crane safety management activity, such as maintaining the crane in good condition and training tower crane workers. Clearly, these factors represent tower crane safety management and maintenance from main-contractor and subcontractor.

4.3.7. Group 7: Supervisors’ safety practice Four factors are identified in Group 7. The combination of X14, X16 and X17 reveals the safety responsibilities of supervisors, such as safety inspection, safety commitment and safety instruction. Supervisor’s character (X15) is a critical factor affecting daily safety activities of supervisor. Clearly, the four factors represent supervisors’ safety practice. 4.3.8. Group 8: Auxiliary safety equipment Group 8 consists of 4 factors that focus primarily on auxiliary safety equipment. They include: safety protection equipment for erection/ dismantling worker (X11), assembling auxiliary equipment (X19) help to ensure safety in the process of assembling tower crane. Ergonomic level of the operator cab (X21) and operator aid equipment (X23) contribute to reduce safety risk during the operation phase of tower crane.

4.3.3. Group 3: Tower crane safety program Group 3 consists of five factors. X40 is not enough to interpret the groups for the low factor loading. X42 and X44 indicate the safety program of the main contractor, including the safety plan for construction site with tower crane, and safety briefing to the administrative staffs. X43 and X45 indicate the safety program of the subcontractor, 103

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Table 3 The rotated component matrix. Factors

Component 1

X24 X18 X22 X20 X53 X52 X39 X49 X46 X47 X48 X41 X54 X50 X51 X55 X4 X56 X43 X45 X44 X42 X40 X1 X3 X13 X6 X8 X10 X7 X5 X2 X9 X12 X34 X29 X33 X32 X25 X31 X30 X26 X27 X28 X14 X16 X17 X15 X23 X21 X19 X11 X36 X37 X38 X35

Table 4 Ranking of critical factors affecting tower crane safety.

2

3

4

5

6

7

8

Ranking

Factors

Mean

Standard deviation

1

X18. Crane structural parts: safety reliability X3.Crane driver: safety operation X24.Tower crane foundation: safety reliability X20.Crane safety device: reliability X48 Subcontractor: safety inspection X19. Assembling auxiliary equipment: safety reliability X1.Worker ability X49.Subcontractor: tower crane maintenance X52. Main contractor: safety acceptance X45. Subcontractor: safety briefing X13.Workers: safety values X42.Main contractor: safety plan X14.Supervisor: safety inspection X43.Subcontractor: safety plans X47.Main contractor: site supervision X8.Operation communication X16.Super-visor: safety commitment X4.Crane driver: routine checking X34.Winds X54.Main contractor: safety education X10.Erection/dismantling worker: safety operation X53 Subcontractor: safety checking X26.Overlapping crane work X22.Crane attachment device: safety reliability X44. Main contractor: safety briefing

4.627

0.690

4.648 4.617

0.647 0.757

4.670 4.563 4.563

0.773 0.722 0.806

4.533 4.521

0.678 0.725

4.521 4.510 4.510 4.489 4.485 4.468 4.446 4.442 4.436 4.428 4.402 4.383 4.361

0.795 0.781 0.834 0.740 0.808 0.871 0.882 0.995 0.819 0.745 0.813 0.875 0.873

4.340 4.329 4.319

0.844 0.949 0.855

4.308

0.911

9

0.853 0.842 0.823 0.786 0.642 0.623 0.491

2 3 4 5 6 0.807 0.801 0.788 0.712 0.698 0.681 0.569 0.558 0.516 0.487 0.455

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

0.793 0.702 0.689 0.594 0.482 0.854 0.835 0.820 0.815 0.813 0.802 0.773 0.689 0.650 0.634 0.489

22 23 24 25

safety. The importance level of a factor was determined based on the mean and standard deviation of each factor derived from the total sample. A factor variable with a higher mean value and smaller standard deviation was considered to have greater impact on the tower crane safety. Factors with mean values higher than the average value of all factors (4.308) were selected as critical factors. The ranking results of the critical factors can be seen in Table 4. According to Table 4, 25 factors have mean values greater than 4.308 and are, therefore, considered as critical factors affecting tower crane safety. The top-five critical factors are “X18.crane structural parts: safety reliability,” “X3.crane driver: safety operation,” “X24.tower crane foundation: safety reliability,” “X20.crane safety device: reliability” and “X48 subcontractor: safety inspection.”

0.823 0.801 0.757 0.668 0.522 0.797 0.755 0.667 0.613 0.557 0.904 0.876 0.788 0.652 0.781 0.726 0.656 0.501

5. Discussion 0.879 0.866 0.854 0.757

The authors applied system analysis methods to study tower crane safety system from a sociotechnical systems perspective. Fifty-six factors affecting tower crane safety were identified. Hierarchy, framework and factor paths of a tower crane safety system can be acquired through the generic AcciMap model in the qualitative research. Critical factors and the main dimensions of a tower crane safety system are also presented in the quantitative research. The generic AcciMap model cannot only contribute to understanding the tower crane safety system, but also provides a system framework for tower crane accident prevention and analysis. Critical factors and the main dimensions of tower crane safety system can help to establish the tower crane safety evaluation and control system, thereby strengthening tower crane safety management. The mixed methods research results revealed the hierarchical structure, contributing factors, interaction and key factors at each level and for the main dimensions of the tower crane safety system. The integration of qualitative and quantitative results contributed to link the

4.3.9. Group 9: Government safety supervision This group includes four factors that are related to the supervision of government agencies on the safety of tower crane equipment, such as tower crane registration management (X35), construction safety management regulations for crane (X36), operator certification (X37), supervision and inspection of safety management on the construction site (X38). Therefore, these factors can be summarized as government safety supervision.

4.4. Ranking of critical factors affecting tower crane safety This section identifies the critical factors affecting tower crane 104

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Table 5 Test of Rasmussen's predictions in tower crane safety. Predictions

Support for prediction

1. Tower crane safety is an emergent property of a complex socio-technical system. It is impacted by the decisions of all staff members, not just tower crane workers

Contributing factors and relationships between factors of the tower crane safety system were identified across five complex sociotechnical levels. These factors have uncertain characteristics. In the case of uncertainty, system performance is unpredictable due to the requirement of the system to adapt to changing demands and conditions. At the same time, single factor performance is predictable, but the combination of multiple factors is difficult to predict. Thus, the performance of the tower crane safety system was identified as an emergent property The AcciMap shows multiple contributing factors across all levels of the tower crane safety system. Many of these factors are also influenced by other causal factors. For example, crane safety operation is not only impacted by job stress, but also by visibility obstacles and work environment The model identifies multiple nonlinear interactions across the system levels of the tower crane safety system. For example, there are a lot of coordination and feedback among factors at the “construction site management” level, the “on-site tower crane staff” level and the “environment and equipment” level. The failure of any factor will threaten tower crane safety. For example, insufficient safety training may lead to unsafe worker operation, which can then lead to accidents The AcciMap identifies several examples of feedback relations across different levels of the tower crane safety system. For example, although supervisors regularly conduct safety inspections on main structures and components of the tower crane, tower crane safety parameters cannot be sufficiently acquired in real time due to the high-altitude operations The whole process of tower crane operation is in an open-air environment, and system behavior will not remain static under the dynamic environment. Financial and schedule pressure also affects the system's related factors. For example, financial pressure affects workers' safety protection equipment and tower crane assembling auxiliary equipment. Schedule pressure also affects the worker's workload, thereby affecting worker safety The AcciMap identifies several examples of degraded work practices at multiple levels, i.e., crane structural parts’ safety reliability and crane driver’s safety operation. Some factors affecting the system clearly degenerated systematically over time. As the construction process progresses, the height of the tower crane gradually increases and the number of hoisting blind areas also gradually increases, posing a challenge to the driver's safe operation. Tower crane components also gradually wear with time, and the connecting device will weaken due to time-varying loads during construction

2. Tower crane safety is influenced by multiple contributing factors, not just a single catastrophic decision or action

3. Threats to tower crane safety can result from a lack of effective communication and feedback (or ‘vertical integration’) across levels of the system, not just from deficiencies at one level alone

4. Lack of vertical integration is caused, in part, by lack of feedback across different levels of the tower crane safety system

5. Tower crane safety system behavior is not static. Behavior patterns migrate over time and under the influence of various pressures such as financial and psychological pressures

6. Behavior pattern migration occurs at multiple levels in the tower crane safety system 7. Migration of practices causes system defenses to degrade and erode gradually over time—not all at once

interaction and influence paths among the contributing factors of the tower crane safety system need attention. Safety intervention measures should be systematically designed based on these interactions and influence paths, and vertical integration principles are supposed to be applied to understand and improve communication and feedback across the tower crane safety system. For example, to improve crane drivers’ safe operation, system intervention measures must be taken based on each company’s safety management and organization on the construction site, the training and responsiveness of the crane drivers themselves, coordinated control of the work environment, and provision of safety smart equipment.

characteristic of the tower crane safety system together to form a systematic thinking and understanding. A system of sub-system perspectives was established to understand the tower crane safety system more deeply. 5.1. Predictions and implications from system thinking 5.1.1. Rasmussen’s (1997) risk management theory in tower crane safety To test the applicability of systems-based framework in a certain field, the AcciMap output should be compared to Rasmussen’s seven predictions regarding accident causation, which underpins the AcciMap analysis (Newnam and Goode, 2015; Salmon and Lenné, 2015; Stevens and Salmon, 2016). This process has been applied in many areas, such as bushfire response (Salmon et al., 2014a,b), beach driving (Stevens and Salmon, 2016), and road freight transportation (Newnam and Goode, 2015). The current paper shows that the predictions made by Rasmussen’s risk framework are all extant in the AcciMap output, which ascertains the applicability of systems-based framework for tower crane safety. Seven predictions regarding the tower crane safety system and support can be seen in Table 5.

5.2. Implications from the mixed methods research The qualitative research results present hierarchy, framework, contributing factors and impact paths of the tower crane safety system. Critical factors and main dimensions of the system were also acquired through the quantitative analysis. The combination of qualitative and quantitative results contributes to in-depth understanding of the tower crane safety system. First, the quantitative analysis can further describe the main dimensions of the tower crane safety system within the framework of the five system levels provided by qualitative analysis. Specifically, the contributing factors of tower crane safety were refined into nine dimensions: namely, (1) tower crane equipment quality and reliability, (2) tower crane safety management and maintenance, (3) a tower crane safety program, (4) workers’ safety practice, (5) the working environment, (6) on-site working conditions for a tower crane, (7) supervisors’ safety practice, (8) auxiliary safety equipment, and (9) government safety supervision. Each dimension explains the tower crane safety system from a new perspective, reflecting the specific behavior

5.1.2. Implications from system thinking The system thinking framework has some implications for tower crane accident prevention and safety improvement. The AcciMap enables understanding of factors affecting the tower crane safety under a more prescriptive lens. Tower crane safety is the result of multiple factors across system levels; it is not an individual factor. Therefore, in future tower crane safety research and practice, systems perspective should be set to consider the systems factors, rather than to solely focus on the front-end factors that lead to accidents (such as crane driver’s unsafe operation and the failures of mechanical structure). In addition, 105

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levels consisting of “regulatory bodies”, “tower crane stakeholders”, “construction site management”, “on-site tower crane staff” and “environment and equipment”. A list of 56 factors affecting tower crane safety were identified through system analyzing. The paper applied AcciMap technique to build the generic AcciMap model for tower crane safety, which comprehensively presents the systems levels and the casual paths of the contributing factors of tower crane safety system. A survey was conducted to research the impact degree of the contributing factors on tower crane safety. Through the principal component analysis, nine main dimensions of the tower crane safety system were identified. They are: (1) tower crane equipment quality and reliability, (2) tower crane safety management and maintenance, (3) the tower crane safety program, (4) workers’ safety practice, (5) working environment, (6) on-site working conditions for tower crane operation, (7) supervisors’ safety practice, (8) auxiliary safety equipment, and (9) government safety supervision. Furthermore, critical factors of tower crane safety were also identified through statistical ranking analysis. The top-five critical factors are “crane structural parts: safety reliability”, “crane driver: safety operation”, “tower crane foundation: safety reliability”, “crane safety device: reliability” and “subcontractor: safety inspection”. With the limited resources available for improving safety and preventing accident, factors with higher ranking should be given greater attention. Limited safety resources can be allocated on these critical factors to achieve best tower crane safety on a construction site. The mixed methods research results enable forming new understanding of the tower crane safety system, such as nine sub-systems of tower crane system, a system of sub-system perspective, and the key factors at a certain system level. In closing, tower crane safety issues are complex sociotechnical system problems. Tower crane safety is affected by multiple factors, which reside at all systems levels of Rasmussen’s framework. Tower crane safety management practices need to focus on the both ends of the system from the beginning to the end. Suites of countermeasures that target vertical integration of the tower crane safety system should be designed in the future.

integration of the tower crane safety system. Second, from the perspective of safety systems engineering, factors of each dimension can be considered to constitute a safety sub-system with a specific structure and function. Therefore, the tower crane safety system is comprised of nine safety subsystems. The mixed methods research results not only describe the overall structure of the crane system hierarchy, but also the subsystem content, the subsystem level locations in the system model, and the relationships between subsystems. So, the mixed methods research in this paper provide different subsystem perspectives, which can provide a theory basis for future specific system modeling, simulation and system decision making (Rae and Alexander, 2012). Third, quantitative analysis (the statistical ranking analysis) can identify the key factors at all the system levels, which should be given special attention. For example, safety reliability of crane structural parts and crane driver safety operation is respectively the key factor at the “onsite tower crane staff” level and the “environment and equipment” level. The limited resources available for improving tower crane safety should be allocated to keep them in check. 5.3. Limitation and future research As a first study to systematically analyze tower crane safety, some limitations are in this paper. First, the AcciMap analysis process may have some subjectivity due to the experience of experts. However, after accident analysis results, multiple expert discussions and literaturebased modifications, these limitations will not affect the accuracy of the final result. Notably, the survey data in this paper was mainly collected in China. Whether differences can occur in other countries, which were overlooked herein due to our restriction to one country’s experience with our chosen method of analysis has yet to be studied. This article analyzed China’s tower crane safety system, and the system framework and theory should be applied to guide tower crane accidents analysis and proactive risk management in the future. Based on the critical factors and main dimensions of system safety identified in this paper, system evaluation and control measures for tower crane safety must be established. To further research on the dynamic characteristics of the tower crane safety system, a system dynamics model needs to be built to study the dynamic characteristics of the system under the current system framework.

Acknowledgements Many thanks are given to Shanghai Construction Group, China Construction Third Engineering Bureau Co., Ltd, and Fushun Yongmao Construction Machinery Co., Ltd. for their participation in this research.

6. Conclusions

Funding

This paper presents the process of system thinking applications in tower crane safety. Specifically, the features of tower crane safety system components were analyzed from a sociotechnical perspective. The tower crane safety system framework is comprised of five system

This research was supported by National Key R&D Program of China (Grant No. 2017YFC0805500).

Appendix A Questionnaire items for tower crane safety factors Factors

Description

X1.Worker ability

The experience, knowledge, skills and qualifications that tower crane workers (crane driver, signaler, slinger and erection/dismantling worker) need X2.Crane driver: character The behavior and psychological characteristics of the tower crane driver, e.g. impulse/calm, tenacious/submissive, careful/careless X3.Crane driver: safety operation Decision and behavior that the crane driver takes to ensure the safety operation X4.Crane driver: routine checking Routine checking of tower crane components before and after working X5.Crane driver: physical quality The physical condition of the crane driver during operation X6 Signaler: safety directing Whether the signaler can identify and transmit the lift signal in a clear and precise manner X7Rigger: safety operation Whether rigger can attach and detach the load to and from the crane in a clear and precise manner X8.Operation communication Communication effects among crane operator, signaler and rigger X9.Erection/dismantling worker: character Assessing behavior and psychological characteristics of erection/dismantling workers, e.g. impulse/ calm, careful/careless

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X10.Erection/dismantling worker: safety operation X11.Erection/dismantling worker: safety protection equipment X12.Worker: job stress X13.Workers: safety values X14.Supervisor: safety inspection X15.Supervisor: character X16.Supervisor: safety commitment X17.Supervisor: safety instruction X18.Crane structural parts: safety reliability X19.Assembling auxiliary equipment: safety reliability X20.Crane safety device: reliability X21 Crane ergonomics X22.Crane attachment device: safety reliability X23.Operator aid X24.Tower crane foundation: safety reliability X25.Work height X26.Overlapping crane work X27.Cross operation X28.Obstacles and congested site X29. Blind lifts X30.Warning region setting X31.Construction sites organization rationality X32. Visibility X33.Weather phenomena (excluding winds) X34.Wind X35.Government safety regulators: crane registration X36. Government safety regulators: safety regulation X37 Government safety regulators: operator certification X38 Government safety regulators: safety supervision X39. Manufacturers: qualifications X40.Schedule pressure X41 Main contractor: safety attitude X42.Main contractor: safety plans X43. Subcontractor: safety plans X44. Main contractor: safety briefing X45. Subcontractor: safety briefing X46.Main contractor: safety management system X47.Main contractor: site supervision X48 Subcontractor: safety inspection X49.Subcontractor: tower crane maintenance X50. Subcontractor: tower crane safety input X51 Main contractor: safety input X52. Main contractor: safety acceptance X53 Subcontractor: safety checking X54.Main contractor: safety education

Whether erection/dismantling workers follow the safety procedures and instructions when working The PPEs equipped with erection/dismantling workers Construction schedule and labor strength for tower crane workers Safety awareness and attitude of tower crane workers Routine safety inspection conducted by supervisor, including patrol, monthly inspection and quarterly inspection, etc. Supervisor 's characteristics (knowledge, skills, responsibility consciousness, professional ethics, etc. Supervisors’ implementation of tower crane safety management work environment (routine safety inspections, hazard checking and risk assessment, etc.) Supervisors’ safety guidance for tower crane workers The quality and reliability of tower crane structural components and accessories The reliability of assembling auxiliary equipment (such as truck-crane, wire rope, installation tools, etc.) The reliability of tower crane safety device (brake, limiter, protection device, etc.) Ergonomic level of the operator cab for work convenience The reliability of the attachment device (welds, bolts, embedded parts, adhering bars, etc.) between the tower crane and the building Optional operation aids for increased safety, such as digital-display safe load indicators, and crane operation graphical displays The safety reliability of the tower crane foundation components (supporting structure, concrete base, tension piles, etc.) The height of the tower crane or the height for the driver operating The situation of multiple tower cranes at the construction site Conditions of other simultaneous construction activities within the radius of tower crane Obstacles in vicinity of tower crane operation, such as transmission lines, adjacent construction, etc. The blind area of the driver's visual field during the crane operation Site isolation area and warning sign settings for tower crane erection/dismantling and lifting weights Ground conditions and working space for tower crane at construction site The visibility at construction site with tower crane The weather phenomena, such as temperatures and humidity or dryness The wind conditions at the construction site Tower crane registration management conducted by the government safety regulators Construction safety management rules, procedures and regulations for crane machinery Implementation of the administrative qualification system of special workers Supervision and inspection of safety management on the construction site executed by the government safety regulators The qualifications, safety certification, performance and market reputation of crane manufacturers The schedule requirements of main contractor Main contractor’s safety attitude towards tower crane Safety plans of main contractor for construction site with tower crane Safety plans for crane assembling, operating and dismantling Safety briefing to the subcontractor, chief manager, safety officers, and supervisors on major hazards and risks at construction site Safety briefing to crane workers on major hazards and risks The rationality of main contractor’s safety management system Safety supervision of main contractor on site environment, crane operation and safety management of sub-contractor Safety inspection of subcontractor on tower crane equipment and workers To maintain the crane in good condition Sub-contractor’s safety funds for tower crane safety Main contractor’s safety funds for the construction safety Main contractor’s acceptance on crane foundation, and installation task Different levels of inspections and maintenance carried out by subcontractor Safety education for supervisors and technical personnel 107

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X55.Subcontractor: safety training X56. Main contractor: reward and punishment

The safety training for tower crane workers The reward and punishment strategy for safety management at construction sites

Appendix B. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ssci.2018.05.001.

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