1. Introduction
The crane has become a major symbol of building construction sites and is often the most prominent piece of equipment at a building construction site based on its size and the key role that it plays at many construction sites [
1]. The use of tower cranes (T/Cs) at construction sites has consistently increased since their introduction into the Korean construction industry in the 1980s. According to statistics on construction machinery statuses from the Ministry of Land, Infrastructure, and Transport, in 2015, Korea contained 3408 cab-control (CC) T/Cs and 272 remote-control (RC) T/Cs. Generally, CC T/Cs are used for lifting objects weighing three tons or more and RC T/Cs are used for lifting objects weighing less than three tons. In 2019, the number of CC T/Cs increased by 22% to 4385 and the number of RC T/Cs increased by over 85% to 1845 [
2].
One of the major causes of fatalities is the usage of cranes during lifting operations in the construction phase of the construction project lifecycle [
3]. As the number of T/Cs used at construction sites has increased steadily, there has been an increase in fatalities and accidents because T/Cs are relatively dangerous and various risk factors are inherent to assembly, lifting, and disassembly works [
4]. As accidents at construction sites are closely related to construction time, cost, scope, and company reputation [
5], and because construction workplace safety and health are essential elements of sustainable construction management [
6], construction accidents must be reduced for the sustainable management of construction sites. Reyes et al. [
7] stated that when quantifying the sustainable value of a construction project, health and safety indexes should be considered. Therefore, the government, academia, and practitioners in Korea have made various efforts to reduce T/C-related accidents. To reduce T/C accidents, the Korean government revised the enforcement regulations of the Construction Equipment Management Act in October of 2019, subdividing the safety training programs for construction equipment operators into 19 types and shortening the training cycle of RC T/C operators. Members of academia have also conducted research [
8,
9,
10] on various causes of accident occurrence to reduce T/C accidents.
Although many efforts have been made to prevent accidents related to CC T/Cs, research on RC T/Cs, which are becoming increasingly common at Korean construction sites, is insufficient, leading to many accidents at construction sites. Every year, the number of T/C-related accidents at Korean construction sites continues to increase. The numbers of fatalities related to T/Cs were nine in 2016, seven in 2017, six in 2018, and eight in 2019 [
11]. T/C accidents were officially announced during the first quarter of 2020, when five casualties had already occurred.
Figure 1 presents an image from January 2 of 2020, where a 30 m T/C collapsed at a construction site in Incheon, Korea. This accident caused two fatalities and one injury. Kim [
12] analyzed T/Cs at Korean construction sites and proposed the following main accident causes: (1) In the case of CC T/Cs, the main causes are equipment age, insufficient work management, violation of work guidelines and safety rules, and lack of communication. (2) In the case of RC T/Cs, the main causes are a lack of knowledge regarding work manuals for installation workers, insufficient checking of the cables used for fixing lifted objects, a lack of simultaneous checking of camera feeds during tying and lifting work, and insufficient checking of the specifications of heavy objects.
Therefore, this study aimed to identify whether ranking can be utilized for the accident causes’ management of T/C types by analyzing the importance of accident causes for each crane type. The results of this study can contribute to reducing construction accidents by identifying management causes for T/Cs that should be considered during the planning process for construction accident prevention and safety management activities.
3. Methods
This study aimed to derive the importance of T/C accident causes and the potential for T/C accidents by quantifying the experiences of experts related to construction accidents. Intensively managing such factors should aid in reducing T/C accidents. The analytic hierarchy process (AHP) technique was adopted to quantify the experiences of experts related to construction accidents in the field. The AHP is a structured decision-making technique developed by Saaty in the early 1970s. It can reflect the knowledge, experience, and intuition of respondents in pair-wise comparisons based on the elements of the hierarchy of decision-making [
27]. The experts related to construction accidents for the AHP are construction engineers, construction managers, safety managers, and T/C operators that have more than 10 years of field experience.
As shown in
Figure 3, this research process can be divided into the following 4 steps. (1) Identify major T/C accidents through a literature review. (2) Extract first-level phase/task and second-level structure accident causes for the AHP through interviews with related experts. After extracting 11 phases/tasks from the previous literature [
12,
14,
15,
18,
19,
21,
28], 5 items were selected by integrating 11 items through interviews with the related experts. For the second-level accident causes, 15 items were selected by conducting a preliminary survey of accident causes extracted from the previous literature (refer to
Table 2) as a Likert scale to the related experts, and the results are presented in
Table 3. (3) Provide an AHP questionnaire to a total of 44 related experts, such as construction managers, 10 safety managers, 14 construction engineers, and 10 T/C operators. (4) Follow the AHP to derive priority management accident causes for reducing T/C accidents.
5. Case Study
Since it is difficult to measure the rate of reduction in accident causes by applying the results of this AHP analysis to actual construction sites, we propose the reduction rate of accident causes through the scenario of installing a camera and wireless transmitter on the trolley of tower cranes and also placing a safety manager of tower crane (refer to
Figure 5). In this scenario, it is assumed that the tower crane operator (both CC T/C and RC T/C), the safety manager of tower crane, and the field office have installed monitors that can check the lifting work of the tower crane. As shown in
Table 6, the weight calculated in AHP analysis was converted into the probability of accident causes in order to calculate the reduction rate of accident causes. The probability of erection phase is 22% for CC T/C and 25% for RC T/C. Dismantling phase is 17% for both T/C and Lifting work; is 22% for CC T/C and 27% for RC T/C and prime contractor management; is 17% for CC T/C and 15% for RC T/C and T/C machinery; is 22% for CC T/C and 16% for RC T/C. As shown in
Table 6, in the case of installing the camera to tower crane for tower crane operator, the safety manager of tower crane and field office, most of the accident causes are eliminated, so that the reduction probability for CC T/C is 29% and RC T/C is 30%. In the case of placing the safety manager of tower crane, the reduction probability for CC T/C is 55% and RC T/C is 49%. Although this reduction probability is not the result obtained after applying two cases to the actual construction site, it is believed that the camera attached to tower crane and safety manager of tower crane can eliminate most of the actual causes of tower crane accidents.
6. Discussion and Conclusions
Despite various efforts to reduce accidents related to T/Cs, the number of accidents is still increasing. This study was conducted to help prevent T/C-related accidents by ranking the accident causes related to CC T/Cs and RC T/Cs, as well as the weights of each cause to be used as a reference for management. As shown in the results of our AHP questionnaire, various accidents occur when an operator cannot directly check the status or ties when a lifting object is hidden by other structures or objects, and is largely dependent on the signals and radio communications of other workers. The result of this study is “operator visibility” as first ranking cause in case of CC T/C, and is “operator unable to check the weight and specification of the lifting objects” as second ranking cause in case of RC T/C. In previous study [
17], the operator’s impact such as “operator proficiency”, “operator character”, and “employment source (operator)” was dominated. The collective weight of these causes is nearly 24%. The cause of the tower crane “operator proficiency” was suggested as the second-level accident cause. Especially, the comprehensive cause of operator impact was suggested. The operator proficiency is an ambiguous cause of tower crane accidents. In other words, the way to solve this cause is ambiguous. To solve this cause, the operator training cycle in Korea has recently been shortened. In this study, operator visibility and to check lifting objects are very specific causes that can be managed and are suggested solutions at construction sites. Therefore, one could prevent tower accidents by attaching a device like RPSA to the jib or hook for both CC T/Cs and RC T/Cs. Such device could help T/C operators monitor their work from the cabin and make decisions based on signals from other workers and their own judgment. Additionally, it is necessary to train managers and workers continuously to help them maintain and comply with guidelines and manuals related to erection work, lifting work, and operation.
The major accident cause for CC T/Cs were ranked in descending order of “poor operator visibility”, “bad bolting on the brace/mast/telescopic element”, and “poor subcontractor technology”. Such causes like “bad bolting on the brace/mast/telescopic element” and “poor subcontractor technology” are combined various factors such as the management problem of the prime contractor, the management problem of the subcontractor, and the skill of erecting/dismantling worker. The major accident cause for RC T/Cs were ranked in descending order of “poor management of lifting objects and control of surroundings”, “operator unable to check the weight and specifications of lifting objects”, and “failure to comply with safety rules and work guidelines”. In particular, in the case of poor management of lifting objects and control of surroundings, there is a problem in the function of properly controlling and managing T/Cs because cranes are controlled and managed by numerous operators who have completed the required training for each type of construction work. Therefore, it is recommended to have a separate manager in charge of supervising work using T/Cs at a construction site. Furthermore, the second major accident cause of checking the weight and specifications of lifting objects can be addressed by attaching RPSA to help operators to make informed decisions. This situation is similar to that of a CC T/C.
In previous study [
17], the “site-level safety management” is the highest weight cause affecting safety on construction sites with tower crane. Especially, the superintendent effect has “superintendent character” and “site-level safety management” to influence the safety of the crane-related site, a total of over 23%. In other previous study [
20], as a result of analyzing the previous literature, the causal factors belonging to “site personnel management” are also very important as the frequency occupies the first to third place. In this study, most accident causes are related to situation/surrounding control using T/Cs and management issues that require workers to comply with work guidelines and rules of erection, dismantling, and lifting works that reflect reality. Therefore, it is important to designate a safety manager of tower crane with sufficient experience and knowledge regarding T/Cs to train T/C operators, engineers, and managers, and to revise, supplement, and manage various instructions and manuals. Additionally, the safety manager of tower crane is expected to provide sufficient help in terms of reducing accidents through consistent safety-based management and exercising practical control authority over T/C erection, telescoping, and lifting operations. In the case of an RC T/C, plans for supplemental education should be prepared as it becomes increasingly easy to obtain an operating license. In the field, it is necessary to establish a reinforced training plan for safety rules during erection, operation, and dismantling.
To prevent T/C accidents, we derived the priorities of accident causes for different T/C types using an AHP questionnaire. It is crucial to reduce accidents for sustainable management at construction sites. If the priorities of T/C accident causes presented in this paper are utilized in various checklists or management plans, and construction site management is conducted accordingly, then sustainable construction management can be realized. In the future, if additional research is performed by narrowing the scope of AHP questionnaire respondents to workers who directly use T/Cs and experts familiar with T/Cs, more realistic results can be derived, which will further reduce T/C accidents.