Analysis and Prevention of Tower Accidents in Construction

In recent years, with the rapid development of urban construction and the increase of high-rise buildings, the use of tower cranes (hereinafter referred to as tower cranes) has become increasingly common, and the incidence of major injury accidents has also been constantly rising. Therefore, in light of the safety conditions of the use of lifting machinery, including those used on construction sites, all relevant units jointly conducted a comprehensive inspection and examination of the in-use tower cranes. They comprehensively summarized and analyzed the existing problems, potential hazards, and accidents that have occurred, and proposed corresponding remedial or preventive measures for reference.

Classification and prevention of tower crane accidents or hazards

1.1 Issues with manufacturing quality

(1) Material quality and welding quality issues of the structure The quality issues of structural components include the material quality of the components and the welding quality.

The quality issues of crane materials include the correct selection of materials and material quality assurance (macroscopic quality of materials and microscopic quality of chemical composition), especially the materials used for key components of the crane's metal structure, such as balance booms, lifting booms, standard parts of the tower body, tie rods, turntables, trolley frames and underframes, etc. In the material quality analysis conducted at the fracture point of the main chord of a certain QqZ25 tower crane in 2003, the thickness measurement of the Angle steel failed to meet the material thickness standards in many places. Moreover, the metallographic examination indicated that there were a large amount of silicate and oxide inclusions in the material. When these defects encounter specific factors such as heat-affected zones, high strain rates and high stress concentrations, these factors play an important role in the expansion of internal defects until material failure. In 2004, from the analysis of the fracture surface of the main limb Angle steel of the standard parts of a certain tower crane, it was found that there were serious problems with the material of the Angle steel: the metallurgical quality of the material used was too poor, with many inclusions, excessive impurity elements, interlayers and obvious longitudinal cracks. Due to multiple applications of paint, it is difficult for installers and inspectors to detect defects during the macroscopic visual inspection of installation and inspection.

② Welding quality issues of structural components. In the production of welded structures, due to unreasonable structural design, components, electrode (wire) materials and joints not meeting requirements, unreasonable welding processes or welders' operation skills, various defects often occur in the welded joints. Common welding defects include: The shape and size of the weld seam do not meet the requirements, as well as undercutting, weld beads, slag inclusions, gas holes, incomplete penetration and cracks, among which incomplete penetration and cracks are the most harmful. For instance, in 2003, during the construction of a QTZ60 tower crane produced by a certain factory in Sichuan, due to the fact that the high-strength bolt connection ear plates of the standard intervals were not fully welded (it was found that there were 8 dense pores in the weld metal at the weld separation points, with diameters ranging from 0.80 to 3.10 Innq). Among the two ear plate seats connected, no fusion zone was found on one side of the welding area, while a small amount of fusion zone was seen on the other side. Moreover, the root of the weld seam was not fully penetrated: length 3.5mm. There is a fusion zone at the welding part on one side of the other ear base, while there is no fusion zone on the other side. It led to the collapse of the tower crane, causing a major casualty accident. The main reasons for incomplete penetration are rust and oil stains on the workpiece surface, too small bevel angles or gaps, too fast welding speed and too small current. The most fundamental reason is the poor management level and technical quality of the enterprise and workers (welders), as well as their weak quality awareness.

(2) Manufacturing quality issues after tower crane assembly, welding and assembly: Any tower crane, regardless of its technical performance or structural differences, can always be decomposed into three parts: metal structure, working mechanism and drive control system.

The metal structure part accounts for approximately 70% to 80% of the tower crane's own mass and bears various working loads. After the assembly and welding of each metal structural component of the tower crane, it is not only necessary to ensure the welding quality, but also to strictly control the shape and position of each structural component to meet the requirements of the drawings and relevant standards and specifications, such as straightness, perpendicularity, waviness, coaxiality, parallelism and distortion, etc. Otherwise, it will cause severe damage to the crane structure, and even lead to catastrophic accidents such as tower crane overturning, breaking, and trolley frame falling, resulting in casualties and damage to equipment and buildings. During the inspection and testing of tower cranes, it is often found that there are potential safety hazards in important components. For instance, the wave degree (straightness) of the square steel or channel steel of the main boom of the tower crane (especially for small-tonnage tower cranes such as QT/AO and QTZ31.5) exceeds the tolerance, which causes severe shaking when the trolley moves and is very likely to cause the trolley to derail or fall. The crane boom and tie rods may be distorted or deformed during manufacturing or transportation, which can prevent them from being installed in place and affect the overall installation quality of the crane, etc.

② During the assembly process of each working mechanism of the tower crane (hoisting, luffing, slewing, jacking), its relative independence and the overall consistency of the tower crane must be taken into account; otherwise, it is easy to cause damage to its main components and structural parts. For example: For a high-quality QTZ63 tower crane, during the positioning process of the balance arm of its hoisting mechanism, due to the inconsistency between the centerlines of the drum or guide pulley and the center axes of the front and rear booms of the tower crane, the maximum Angle of deviation of the steel wire rope when it enters or exits the drum or guide pulley is much greater than 4°, causing the rope arrangement on the drum to be messy. The wear and broken wires between each layer and adjacent steel wire ropes intensify and increase, reducing the service life of the steel rope. When it accumulates to a certain extent, it leads to the sudden breakage of the steel wire rope, causing major personal and equipment safety accidents. For example: In the actual manufacturing and installation process of tower cranes, the wire rope disconnection stop and anti-jump devices of the hoisting mechanism and luffing mechanism are often overlooked. As a result, the gap between the wire rope disconnection device of the pulley and the outermost edge of the pulley exceeds the specified specifications. When the hoisting steel rope or luffing steel rope vibrates, loosens or disconnects from the side of the pulley, The steel wire rope is damaged and even breaks due to compression with the sharp corners of the side plate or the shaft of the fixed pulley.

(3) Other issues

If there are no toe guards under the railing or its height does not meet the specification requirements; The anti-slip mesh area of the walkway exceeds the standard. The width of the ladder and the distance between the step stops do not meet the specification requirements. The small rest platform is not set up as required. Some even lack amplitude indication signs and so on.

1.2 Installation quality issues

Tower cranes are frequently moved and installed many times. The quality of their installation directly affects the service life and safety of use of the tower cranes themselves. Tower cranes are highly dangerous during disassembly and assembly. Therefore, great attention must be paid during installation, erection and dismantling. Only by fully understanding the performance and characteristics of the crane can installation, erection and dismantling be carried out to avoid accidents.

(1) Installation and connection of metal structural components of tower cranes, overall shape and position deviations, and installation and fixation of each working mechanism. The steel structure of a tower crane is the framework of the crane, and the quality of its installation directly affects the safety of the tower crane's use. For example: Installation control of the verticality of the tower body (connection between standard parts); Selection of the number of counterweights for different boom lengths; All components are correctly and reliably connected. The pre-tightening force of high-strength bolts, the fit clearance of pins and shafts, the fixation of split pins, as well as the threading of steel wire ropes, which are known as the "meridians" of cranes, the fixation and protection of rope ends, should not cause hard bends, cage distortion, loose strands, broken wires, exposed cores or other phenomena after installation. In previous tower crane inspections, the following problems were mainly found:

① The high-strength bolts between the standard parts of the tower crane tower body are loose or there are no double nuts for anti-loosening, etc.

② The anti-loosening cotter pins connecting the standard parts of the boom, the front and rear tie rods, and the tie rods and the boom were not opened, and even the cotter pins were not inserted and locked.

③ The number of balance counterweights does not match the length of the assembled boom (this phenomenon is relatively rare);

④ The steel wire rope is flattened, cage-shaped deformed, with strands coming out or broken by spot welding, etc. (damaged during installation);

⑤ The rope end alignment of the lifting wire rope at the boom end is not standardized: the rope clamps are in opposite directions, the two strands of rope are not joined together (the requirement for the number of rope clamps is ineffective), and the number of rope clamps is missing.

⑥ Perpendicularity out of tolerance (this phenomenon is relatively rare);

(2) Installation quality issues of safety devices for tower cranes

The safety devices of tower cranes used in construction mainly consist of moment limiters, overload limiters, and travel limiters (including lifting height limiters) It is composed of electrical protection devices such as luffing stroke limiters, operation safety stoppers and buffers, wire rope disengagement stoppers and anti-jump devices, trolley luffing rope breakage protection devices, trolley shaft breakage anti-fall devices (anti-derailment devices), as well as emergency switches, zero position protection, and undervoltage protection. All these safety devices should be provided in accordance with relevant professional standards and installed in compliance with the requirements of national standards and norms to prevent unexpected situations that may occur during the operation of the tower crane, protect the crane equipment and prevent personal accidents. According to the "Regulations for Supervision and Inspection of Lifting Machinery" and the "Safety Technical Specifications for Special Equipment" issued by the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China, safety devices are an important part and key link in all crane inspection items. Based on the cases investigated in the past, the problems with safety devices mainly occur in the following aspects:

① The torque limiter failed. The mechanical lifting moment limiters of the full moment method are more commonly used in construction tower cranes. The main reasons for their failure are as follows: First, the elastic failure or insensitivity of the bow-shaped plate is the most common, mainly in the small tower cranes of the QT/AOT series produced in Shandong. Second, the electrical trigger switch has no corresponding protective measures or the measures are not effective, causing short circuit failure, such as The phenomenon of no waterproof or moisture-proof devices is quite common.

② The lifting height limiter has failed. The main reasons are electrical issues (moisture exposure) and the setting of the safety distance being out of tolerance. For tower cranes with trolley luffing, there are regulations on the minimum distance from the top of the hook device to the lower end of the trolley frame. When installing, it should not be less than the relevant minimum safety distance to prevent the hook from hitting the top.

③ Failure of the steel wire rope being pulled out of the groove and the anti-jump device. Although the wire rope out-of-groove blocking and anti-jump devices have been installed before leaving the factory, the installers are obligated to point them out and make rectifications until they meet the specification requirements. The three accidents of tower crane hoisting wire rope breakage and lifting gear fall that occurred in a certain place in the south in 2004, as well as the tower crane luffing wire rope breakage accident that occurred in the same place in 2004, were all caused by the disconnection from the trough and the blocking and the installation of anti-jump devices not meeting the requirements of the specifications. In some tower crane cases, there are even more severe cases where the steel wire rope jumps out of the pulley and directly rubles against the shaft, causing severe wear on the fixed pulley shaft and posing a huge safety hazard.

④ Failure of the protection device for the breakage of the trolley luffing rope. The failure of this protective device is mainly manifested in the fact that the installers bind the (left and right) blocks on the trolley frame or their supports together, thus losing the function of stopping the trolley from sliding when the luffing wire rope breaks. The failure of the trolley luffing rope breakage protection device is due to the fact that some tower crane luffing mechanisms are not equipped with brakes and position the lifted object by the tension of the luffing wire rope. Therefore, the tension of the luffing wire rope in use needs to be frequently adjusted, increasing the tensile stress of the wire rope and reducing its service life. At the same time, because of this setting, the luffing wire rope is prone to jumping. This causes the stop block fastened at the end of the steel wire rope to rise and fall, colliding with the horizontal chord on the upper boom. To ensure the normal operation of the trolley, the stop block is often bound, and the rope breakage protection device thus loses its function.

⑤ Failure of other safety devices. The failure of the luffing stroke limit and the lifting capacity limit; There is a lack of awareness of the function of rotary limiters, and the failure of rotary limiters is also a relatively common case. Excessive wear of the flange of the small wheel or the absence of the horizontal guide wheel can cause the luffing trolley frame to sway during movement, posing an unsafe factor of derailment and falling.

1.3 Improper use

The management of the in-use tower cranes is a key responsibility of the user units, including the correct provision and acceptance of concealed works, the management of tower crane drivers, crane operators and other operators, as well as the implementation and supervision of protective measures against potential hazards in the surrounding environment of the tower cranes.

(1) Correct provision and acceptance of concealed works. The foundation of tower cranes used in construction is generally a fixed reinforced concrete structure. The foundation drawing is provided by the tower crane manufacturer after verifying the overall anti-overturning stability and ground pressure. The drawing specifies the quantity, model and specification of reinforcement, as well as the minimum requirements for the depth of reinforcement embedded in the tower body of the foundation or anchor bolts. The user unit should, based on the geological conditions and in accordance with the requirements of the drawings provided by the manufacturer, strictly control the quality of the foundation pouring, prevent the settlement of the foundation after the tower crane is used, so as to avoid the tower crane reaching the critical state of overturning, the destruction of stability and the occurrence of the whole machine overturning accident.

(2) Management of tower crane drivers and crane operators (command and rigger).

Statistical data show that most tower crane accidents are caused by violations of operating procedures by the user. Among the numerous violations, the driver and the crane operator (the commander and rigger) often bear inescapable responsibility. A competent driver not only meets the operational technical requirements of "stability, accuracy, speed, safety and rationality" during the use of tower cranes, but also bears the responsibility for the daily inspection and maintenance of tower cranes. While fully understanding the operational performance of tower cranes, they also check the damage of the metal structure and major components of tower cranes, as well as the failure of electrical protection and safety devices.

① Forcibly binding the torque-limiting bow-shaped plate and short-circuturing the torque-limiting electrical switch, artificially causing the torque-limiting to fail;

② To increase the lifting height of the lifting gear, manually adjust the safety distance setting or short-circuit the electrical connection;

③ Daily inspections are ineffective: The aggravation of broken wires in the steel wire rope and the deformation of the waviness of the steel wire rope are ignored, and the torque limiters, height limiters, rotation limiters and other travel limiters are also not inspected on a daily basis.

The strictly prohibited "Ten Nos" have not been implemented.

(3) Implementation and supervision of protective measures against potential hazards in the surrounding environment of tower cranes

Tower cranes are used in the construction of high-rise or small high-rise buildings. Due to the geographical environment of their construction (such as bustling city centers or building Spaces with interlaced roofs), some safety protection and supervision during the operation of tower cranes are inevitably involved. For example: Safety canopy protection when the operating radius of a tower crane (during rotation) passes over buildings, streets and high-voltage lines; A concrete protective net to prevent the detachment of crushed stones is set on the balance boom. Supervision of adjacent tower cranes during operation on the same horizontal working surface, etc.

1.4 Prevention of Accidents or potential Hazards

The accidents or potential hazards of tower cranes exist in the risk factors such as design, manufacture, installation and use (including repair). To a large extent, preventing the occurrence of accidents lies in controlling the generation of the above factors, especially strengthening: The control over manufacturing, installation and use requires the user units to select products produced by manufacturers with manufacturing qualifications, allows qualified units to install tower cranes, and enables trained drivers and crane operators to operate them. All these are aimed at controlling the occurrence and aggravation of dangerous factors from the source. However, to prevent the occurrence of the above-mentioned risk factors, the most important thing is to earnestly implement relevant standards and norms, establish and strictly enforce rules and regulations, carry out regular inspections, and with the high attention of the manufacturing, installation and usage units, as well as the high sense of responsibility and safety awareness of the employees of these units, only by doing so can dangerous accidents be avoided or reduced.

2. Conclusion

Safety is one of the important performances of tower cranes. Due to its significance in the construction industry, the requirements for its safety are getting higher and higher. While strengthening the number of legal personnel, new technologies should also be adopted to enhance its safety and reduce production costs. While achieving remarkable economic benefits, further enhance the technological manufacturing level of the tower crane industry and the overall performance of tower crane products.