How Tower Cranes Work Tower cranes are extensively used for lifting materials in construction sites. Most construction sites are very confined and close to public. Tower crane accidents not only hazard workers in construction sites, but also pedestrians. This paper investigates tower crane safety in related to the understanding and degree of executing statutory requirements and non-statutory guidelines for the use of tower cranes in the Hong Kong construction industry. A questionnaire survey and structured interviews are conducted. It is found that human factors are attributed to tower crane safety. Indolent performance of requirements or responsibilities of practitioners in tower crane operations is found. Inadequate training and fatigue of practitioners are one of the main reasons causing unsafe practices of tower crane operations. Recommendations for improving safety performance in tower crane operations are also discussed. Research highlights This paper investigates tower crane safety in related to the understanding and degree of executing statutory requirements and non-statutory guidelines for the use of flat top tower crane in the Hong Kong construction industry. It is found that human factors are attributed to tower crane safety. Indolent performance of requirements or responsibilities of practitioners in tower crane operations is found.  Inadequate training and fatigue of practitioners are one of the main reasons causing unsafe practices of tower crane operations. Tower cranes are a common fixture at any major construction site. They're pretty hard to miss -- they often rise hundreds of feet into the air, and can reach out just as far. The construction crew uses the tower crane to lift , concrete, large tools like acetylene torches and generators, and a wide variety of other building materials. When you look at one of these cranes, what it can do seems nearly impossible: Why doesn't it tip over? How can such a long boom lift so much weigh-t? How is it able to grow taller as the building grows taller? If you have ever wondered about how tower cranes work, then this article is for you. In this article, you'll find out the answers to all of these questions and more! Weather monitoring in construction sites is important, but especially when luffing jib tower crane are used. A strong gust of wind can destabilize the load and structure, causing a collapse. Project managers should constantly check weather forecasts, and avoid lifting operations with unfavorable weather. A weather monitoring system at the project sites can warn about dangerous wind conditions that are not covered in forecasts. Tower Crane Support System One of the first questions that may be asked by someone looking at a tower crane is these structures stand upright. There are several elements that contribute to the tower crane’s stability. The concrete pad is a concrete foundation made by the construction company several weeks prior to the crane’s arrival. Typical measurements for the pad are 30x30x4 feet (10x10x1.3 meters), with a weight of around 400,000 pounds. Large anchor bolts are deeply embedded in the concrete pad, and these elements support the base of the crane. Tower cranes are delivered to construction projects in parts, which are then assembled on-site. Qualified installers assemble the jib and the machinery section, these horizontal elements are then positioned on the mast, which is only 40 feet tall initially. Once this assembly is completed, the counterweights are placed by a mobile crane. The mast rises from the concrete pad, and it remains upright thanks to its triangulated structure. To increase the crane height, the crew adds sections to the mast with a climbing frame: A weight is hung on the jib to balance the counterweight. The slewing unit is detached from the top of the mast and hydraulic rams in the top climber push the slewing unit up 20 feet. The crane operator uses the crane to lift another 20 ft mast section into the gap and then it is bolted in place. These steps are repeated continuously until the desired height is achieved. Once it is time to remove the tower crane from the construction site, the crane disassembles its own mast and smaller cranes are used to disassemble the rest. Tower crane accidents frequently occur in the construction industry, often resulting in casualties. The utilization of tower crane spare parts involves multiple phases including installation, usage, climbing, and dismantling. Moreover, the hazards associated with the use of tower cranes can change and be propagated during phase alternation. However, past studies have paid less attention to the differences and hazard propagations between phases. In this research, these hazards are investigated during different construction phases. The propagation of hazards between phases is analyzed to develop appropriate safety management protocols according to each specific phase. Finally, measures are suggested to avoid an adverse impact between the phases. A combined method is also proposed to identify hazard propagation, which serves as a reference and contributes to safety management and accident prevention during different tower crane phases in the construction process. In construction sites, tower cranes are used for the vertical and horizontal transportation of materials [1]. It is essential equipment for most construction projects, especially for high-rise buildings [2]. Typically, they need to be reinstalled on the construction site once the components of the tower crane leave the factory. As the height of a construction project increases, tower cranes are necessary, and they eventually must be climbed. Furthermore, maintenance and dismantlement must be performed. Thus, a tower crane is not only a piece of auxiliary equipment in construction but also a construction object with complicated processes [3]. This negatively impacts on-site construction safety. In this investigation, 149 accident analysis reports on a tower crane in construction sites in China were collected for the period from 2015 to 2019. The accidents resulted in a total of 216 deaths and 89 injuries and led to adverse social impacts. Therefore, it is essential to analyze the hazards associated with the deployment of tower cranes on construction sites to prevent such accidents. Tower cranes on construction sites consist of the following phases: installation, usage, climbing, and dismantling. According to the investigated accidents, the processes and constructors are not the same for the different construction phases. This results in the occurrence of different types of accidents during different phases. Moreover, hazards propagate between each phase and the propagation also affects the safety of the tower crane. Therefore, it is necessary to analyze the hazards associated with each construction phase and to explore the differences and interrelations between them. Furthermore, although different hazards may occur during different phases, previous works often focused on the usage phase [9, 10]. Some researchers have investigated dynamic structural performance, the interaction effects of multitower crane operation, the load, and the environment of the tower crane in the usage phase [11–13]. In addition, the factors that impact safety during the installation (including climbing) and dismantling phases have been analyzed [14]. However, there are few comparative studies on the multiple phases of tower crane mast section on the construction site. Equally important are the interrelationships between the hazards associated with different phases, which have not been investigated to date. In this paper, we address the aforementioned limitations in the literature. 2.2. Hazard Analysis The conventional hazard analysis methods include preliminary hazard analysis (PHA), system hazard analysis (SHA), fault tree analysis (FTA), event tree analysis (ETA), failure mode and effects analysis (FMEA), and failure mode effects and criticality analysis (FMECA) [15–17]. With the development of system thinking, system analysis methods such as AcciMap, STAMP, FRAM, and the 2–4 Model have been increasingly utilized in contemporary studies to analyze hazards [18]. According to one of the main tenets of system thinking, accidents are not caused by a series of linear events. Moreover, the relationships and interactions among the system elements should be considered [19]. A complex system of accidents may be analyzed in detail to define the relationship between several factors at different organizational levels based on the system thinking principle [20, 21]. It is an important method for the analysis of the cause of accidents and safety hazard identification. These system thinking methods have different objectives. A summary of each method is presented in Table 1. These methods have also been compared in several investigations and it was concluded that the STAMP model results in a more comprehensive set of conclusions and is more reliable than other accident system analysis methods [26–28]. The STAMP model involves various elements of a system, such as the individuals, the objects, the organizations, and the environment [29]. The most important is that the STAMP model concerns the interactions of components and systems. As the tower crane safety system is a complex system with different components and phases, the STAMP model can contribute to the safety system analysis of the tower crane during different construction phases in this study. The personnel involved in the installation, climbing, and dismantling phases of the tower crane are primarily the same. The individuals and components involved in the climbing and dismantling phases are mostly the same as those of the installation phase. The differences are the system input and the working activities. The climbing process involves repeating certain steps of the installation process, namely the installation of the mast section. The dismantling process entails the inverse of the installation process. In consideration of the similar components and interactions in installation, climbing, and dismantling, the installation phase is selected to represent others to analyze internal system hazards. In the usage phase, the lifting system that consists of the tower crane and the lifting object is considered as the controlled object. The personnel in the usage phases mainly include the operator, rigger, and signalman. This is the process in which the operator uses the tower crane to lift the objects and is relatively different from the installation phase. Hazard analysis of the usage phase is therefore performed separately. Moreover, the hazards caused by the interaction among the phases are also analyzed separately. 4.2. System Analysis of Tower Crane with STAMP The STAMP model has good performance for system modeling and safety analysis and is broadly applied to accident analysis in astronautics, fire disasters, traffic incidents, and other industries [42–44]. However, it is seldom applied to system hazard analysis in the construction industry, and the tower crane in particular. In the following, the STAMP method is adapted to model the installation and usage phases of the tower crane. Moreover, the proposed STPA method based on STAMP is applied to analyze hazards, namely, the unsafe behavior of humans and the unsafe state of the objects. Since the STAMP model is proposed in the context of system theory, the system model is considered as a hierarchical structure in which each layer imposes constraints on its lower layers. In the complete STAMP, several superstructures are involved, including Congress and Legislatures, Government Regulatory Agencies, and Companies. However, in this investigation, only hazards at the construction site are analyzed and superstructures such as government and enterprise are not considered. Thus, the core content of the STAMP model, i.e., the control loop and the process model, is utilized in this work (Figure 5). The interaction between components consists of the feedback of information and the control loops. A dynamic balance is also maintained by the system via the feedback and control of the components. The interactions between the system and the outside world include the process input, the process output, and the disturbance due to the outside world. Generally, the STAMP model is applied to system security analysis related to three aspects: component failure, component interaction failure, and external influence. There are few safety analysis methods that consider system inputs and outputs. They usually consider factors within the system. STAMP can analyze the interaction between phases via the input and output analysis. It is the main reason to choose this method in our research. The process input and output of STAMP can correspond to the IDEF0 interface. Meanwhile, the controls and mechanisms of IDEF0 can help establish the control model of STAMP. Thus, it is feasible to combine IDEF0 and STAMP in this study. This method can analyze the hazard transition between different phases. 4.2.1. Tower Crane STAMP Model for the Installation Phase Tower crane installation is a process that involves rigorous operation steps, short operation time, complicated procedures, and high professional requirements of the workers. Younes and Marzouk [13] analyzed and listed the components required for the installation of the tower crane as the foundation, basic mast, main jib, counter jib, winding gear, and operating room. All these components constitute the tower crane and form the controlled process of the system. The installation processes include sensing, controlling, and execution in the vicinity of the tower crane and its components. The supervisor acts as a sensor and collects on-site information, including the status of the tower crane and the behavior of the operators, which is then fed back to the manager. The manager acts as a controller, which involves making decisions and sending out operational commands based on the installation scheme and the information received from the construction site. Based on the directives of the supervisor, the workers install the tower crane according to the installation scheme and the operational commands from the manager. The workers consist of an installer, operator, signalman, and rigger. The latter three can be the individuals that also operate the tower crane or those who use other lifting machinery to lift the tower crane components. Moreover, the completion of the previous phase, as the process input, affects the installation process. The external disturbance affects the system components, including the construction environment and the weather conditions. Likewise, the completion of the installation phase as the process output also affects the next phase. According to the previous analysis, the system control loop and the process model for the tower crane installation process are constructed using the STAMP method, as illustrated in Figure 6.