Overview of tower cranes and their safety and management

A tower crane is a rotating crane with a boom mounted on the top of a tall tower. It has a large working range and is mainly used for vertical transportation of materials and component installation in multi-story and high-rise building construction. It consists of three parts: metal structure, working mechanism and electrical system. The metal structure includes the tower body, boom, base, attachment rod, etc. The working mechanism has four parts: lifting, luffing, rotation and walking. The electrical system includes motors, controllers, distribution boxes, connecting lines, signals and lighting devices, etc.

Tower cranes are divided into two categories: upper rotating type and lower rotating type:

1. Upper rotating tower crane: The tower body does not rotate, and the boom, balance arm, etc. above the slewing bearing make a full rotation around the center line of the tower body through the slewing mechanism. According to the requirements of use, it is divided into running type, fixed type, attached type and internal climbing type. Running tower cranes can run along tracks, have a large working range, and are widely used. They are suitable for multi-story building construction. If the crane base is fixed on the track or the tower body is directly fixed on the foundation, it becomes a fixed tower crane with a longer boom. If the tower body of a fixed tower crane is connected to the building with an attachment rod at a certain height, it is an attached tower crane. It uses a tower height connection device to make the upper rotating part of the crane increase accordingly with the height of the building, and is used for high-rise building construction. The crane is installed in a shaft such as an elevator shaft or a connected hole, and a hydraulic cylinder is used to make the crane climb up along the shaft according to the construction progress. It is called an internal climbing tower crane. It saves part of the tower body, has a large service range, and does not occupy the construction site, but has certain requirements for the structure of the building.

2. Lower rotating tower crane: The slewing bearing is installed between the base and the turntable. Except for the walking mechanism, other working mechanisms are arranged on the turntable and rotate together. In addition to the track type, there are also crawler and tire types with crawler chassis and tire chassis as walking devices. It has a low center of gravity, can be disassembled and transferred as a whole, is light and flexible, has a wide range of applications, and is suitable for multi-story building construction.

Origin

According to records, the first patent for a tower crane for construction was issued in 1900. In 1905, a crane with a fixed tower body and a boom was produced. In 1923, the prototype of a modern tower crane was made, and the first relatively complete modern tower crane appeared in the same year. In 1930, Germany had begun mass production of tower cranes and used them in construction. In 1941, the German industrial standard DIN8770 for tower cranes was announced. The standard stipulates that the product (tm) of the load (t) and the amplitude (m) is used together to express the lifting capacity of the tower crane as a gravity moment.

My country's tower crane industry started in the 1950s. Compared with the sluggish tower crane industry in Central and Western European countries due to the sluggish construction industry, my country's tower crane industry is in a period of rapid development.

However, driven by a series of favorable policies such as the urbanization strategy, the tower crane market has been guaranteed to a certain extent, and the overall development of the industry is approaching that of developed countries such as Europe and the United States. However, while tower crane technology is becoming more and more intelligent and green, and tower crane enterprise management is becoming more internationalized, the development of small and medium-sized tower crane enterprises in my country is not optimistic.

Large companies such as XCMG, Zoomlion and Sany have gone abroad to explore foreign markets, especially Zoomlion, which has become the world's largest tower crane enterprise. Without exception, these giant companies have very complete R&D, quality assurance systems and operation management mechanisms, and can participate in the competition for international market share. But for small and medium-sized enterprises, there is no such opportunity. Due to their own technical and financial constraints, coupled with the intensification of industry price wars and the serious arrears of payments, the situation facing the entire industry is not clear.

Features

From the perspective of the technical development of tower cranes, although new products are emerging in an endless stream, and new products have improved in production efficiency, ease of operation, easy maintenance and reliable operation, the technology of tower cranes has not changed fundamentally. The research on tower cranes is developing towards a combined type. The so-called modular type is to decompose the tower body into several parts according to the structural and functional characteristics with the tower body structure as the core, and then divide each part into several modules according to the requirements of serialization and generalization and the principle of modular system. According to the parameter requirements, appropriate modules are selected to form tower cranes with different technical performance characteristics to meet the specific needs of construction. The implementation of modular tower cranes helps to speed up the development of tower crane products, save product development costs, and better serve customers.

Classification

Tower cranes are divided into two categories: upper slewing tower cranes and lower slewing tower cranes. The former has a higher bearing capacity than the latter. What we see on many construction sites is the upper slewing tower crane with upper jacking and joint height. According to whether it can be moved, it can be divided into: walking type and fixed type. The tower body of the fixed tower crane is fixed and does not rotate. It is installed on a whole concrete foundation, or on a strip or X-shaped concrete foundation.

The walking type can be divided into four types: crawler type, car type, tire type and track type. In the construction of houses, fixed type is generally used. According to the way of changing the boom, it can be divided into two types: horizontal boom trolley changing the boom and boom changing the boom; according to the installation form, it can be divided into three types: self-lifting type, integral quick disassembly and assembly type. The most widely used are the bottom slewing, quick disassembly and assembly, rail-mounted tower cranes and self-lifting tower cranes that can be used for four purposes (rail-mounted, fixed, attached and internal climbing).

National Standard

Equipment Management

Equipment Features

The boom of a tower crane can be divided into two types: horizontal and lever-type. When the boom is horizontal, the load trolley moves along the horizontal boom to change the boom, and the boom movement is balanced. The boom is longer, but the boom has a larger deadweight. When the boom is a lever-type, the boom mechanism pulls the boom to change the boom in pitch and pitch, and the boom movement is not as stable as the horizontal type, but its deadweight is smaller.

The lifting capacity of a tower crane varies with the amplitude. The product of the lifting capacity and the amplitude is called the load moment, which is the main technical parameter of this crane. Through the slewing mechanism and slewing bearing, the tower crane has a large lifting height, and the inertial mass of slewing and walking is large, so it needs to have good speed regulation performance, especially the lifting mechanism is required to be able to move quickly with light loads, slow with heavy loads, and move slightly after installation. In addition to using resistor speed regulation, eddy current brakes, frequency modulation, pole changing, thyristor and electromechanical combination are also commonly used for speed regulation.

Data management

Construction companies or tower crane owners should report the tower crane's production license, product certificate, disassembly and assembly license, instruction manual, electrical schematic diagram, hydraulic system diagram, driver's license, tower crane foundation diagram, geological survey data, tower crane disassembly and assembly plan, safety technical disclosure, and main parts warranty (wire rope, high-strength connecting bolts, anchor bolts and main electrical components, etc.) to the tower crane inspection center. After passing the inspection by the tower crane inspection center, they will obtain a safe use certificate and a handover record with the project manager after installation. At the same time, in daily use, the dynamic tracking management of the tower crane should be strengthened, and the shift records, inspection records and maintenance records (including minor repairs, medium repairs and major repairs) should be made and signed by the relevant responsible persons. The materials and wearing parts replaced during the maintenance process must have a certificate of conformity or a quality guarantee, and the above materials should be sorted and filed in time to establish a one-machine-one-file account.

Disassembly and assembly management

The disassembly and assembly of tower cranes is a high-incidence stage for accidents. Safety accidents caused by improper disassembly and assembly and unqualified installation quality account for a large proportion. The disassembly and assembly of tower cranes must be carried out by qualified disassembly and assembly units, and the installation and disassembly must be carried out within the scope of qualifications. Disassembly and assembly personnel must undergo special business training, have certain disassembly and assembly experience and hold certificates to work. At the same time, all types of personnel must be complete, positions must be clear, each must perform their duties, and obey unified command. In the process of commissioning, the technical level and sense of responsibility of professional electricians are very important. Electricians must hold electrician certificates and crane operator certificates. We have conducted statistics on a large amount of tower crane inspection data and found that a disassembly and assembly unit in our city installed a total of 54 tower cranes, and 47 failed the first inspection, with a first inspection pass rate of only 13%. Most of them were due to the poor installation technology level of the installation electricians, the disassembly and assembly units' negligence in management, and the safety awareness needs to be further improved. Therefore, we carried out special governance to strengthen business training for the unit and achieved good results. In addition, the disassembly and assembly costs in the disassembly and assembly market are not in accordance with the budget price, and even 400 to 500 yuan are installed for a tower crane, which is also an important reason for the decline in installation quality. A special disassembly and assembly plan should be prepared for disassembly and assembly. The plan should be reviewed and signed by the technical person in charge of the installation unit, and the warning area and cordon for the disassembly and assembly unit should be provided. A special person should be arranged to command, and irrelevant personnel are prohibited from entering the site. The operation should be carried out strictly in accordance with the requirements of the disassembly and assembly procedures and instructions. When the wind force exceeds level 4, the disassembly and assembly should be stopped, and the tower crane should stop lifting operations when the wind force exceeds level 6. In special cases, there should be sufficient lighting for night operations. In special cases, it is necessary to fully negotiate and reach a consensus with the car crane driver on the procedures and precautions for disassembly and assembly.

Tower crane foundation

The tower crane foundation is the foundation of the tower crane. Practice has proved that many major safety accidents are caused by problems with the tower crane foundation. It is an important factor affecting the overall stability of the tower crane. Some accidents are caused by the construction site hastily installing the concrete in order to rush the construction period, some accidents are caused by insufficient ground endurance, some are caused by excavation near the foundation, which leads to landslide displacement, or uneven settlement due to water accumulation, etc., all of which will cause serious safety accidents. We must pay great attention to it, and there can be no ambiguity. The stability of the tower crane is its ability to resist overturning, and the biggest accident of the tower crane is overturning and collapse. When making the foundation of the tower crane, we must ensure that the ground endurance meets the design requirements, and the strength of the reinforced concrete reaches at least 80% of the design value. Special treatment measures must be taken for the foundation of the tower crane with basement projects: some piles must be driven under the foundation, and the steel bars at the pile ends must be firmly welded to the foundation anchor bolts. The bottom surface of the concrete foundation must be flat and compacted, and the bottom of the foundation cannot be made into a pot bottom shape. The size error of the foundation anchor bolts must be strictly constructed in accordance with the requirements of the foundation drawing. The anchor bolts must maintain a sufficient length exposed to the ground, and each anchor bolt must be pre-tightened with double nuts. The foundation surface must be processed before installation to ensure that the horizontality of the foundation does not exceed 1/1000. At the same time, the tower crane foundation must not be waterlogged, which will cause uneven settlement of the tower crane foundation. No pits or trenches shall be dug or dug near the tower crane foundation.

Safety distance

When arranging the tower crane, a plan should be drawn, especially in real estate development areas, where there are many residential buildings and tower cranes, and the safety distance between adjacent tower cranes should be considered. A safety distance of no less than 2m should be ensured in both horizontal and vertical directions. The tower bodies and lifting arms of adjacent tower cranes should not interfere with each other, and the tower cranes should be able to rotate freely when the wind is too strong. The safety distance between the rear arm of the tower crane and the adjacent buildings should be no less than 50cm. The safety distance between the tower crane and the transmission line meets the requirements.

If the safety distance between the tower crane and the transmission line does not meet the specified requirements, a protective frame should be installed on the tower. In principle, the protective frame should be erected during power outages, and metal materials should not be used. Bamboo poles and other materials can be used. The distance between the bamboo pole and the transmission line should not be less than 1m, and it should have a certain stability to prevent it from being blown down by strong winds.

Safety devices

In order to ensure the normal and safe use of tower cranes, we must compulsorily require that tower cranes must be equipped with specified safety devices during installation, mainly including: lifting torque limiter, lifting weight limiter, height limiter, amplitude limiter, slewing limiter, hook safety device, drum safety device, wind direction and anemometer, wire rope slip-out safety device, trolley anti-rope breaking device, trolley anti-shaft breaking device and buffer, etc. These safety devices must ensure that they are intact, sensitive and reliable. If damage is found during use, they should be repaired and replaced in time, and they must not be removed or adjusted arbitrarily without permission.

Stability

The ratio of the height of the tower crane to the bottom support size is large, and the center of gravity of the tower body is high, the torque is large, the starting and braking are frequent, and the impact force is large. In order to increase its stability, we need to analyze the main reasons for the overturning of the tower crane. There are several reasons:

1. Overload. Cranes of different models usually use lifting torque as the main control. When the working range increases or the weight exceeds the corresponding rated load, the overturning torque of the weight exceeds its stability torque, which may cause the tower crane to collapse.

2. Oblique lifting. When the heavy objects are lifted obliquely, the overturning moment will be increased, and horizontal and vertical forces will be generated at the lifting point. An additional overturning moment will be generated at the bottom support point of the tower crane, thereby reducing the stability coefficient and causing the tower crane to collapse.

3. The uneven foundation of the tower crane, insufficient ground bearing capacity, and excessive verticality error will also increase the overturning moment of the tower crane and reduce the stability of the tower crane. Therefore, we must strictly inspect and check based on these key factors to prevent major equipment and personal safety accidents.

Electrical safety

According to the requirements of the "Construction Safety Inspection Standard" (JGJ59-99), the dedicated switch box of the tower crane must also meet the requirements of "one machine, one switch, one leakage, and one box", and the tripping rated action current of the leakage protector should not exceed 30mA, and the rated action time should not exceed 0.1s. The switchboard in the driver's cab must not be exposed. The electrical cabinet should be in good condition, tightly closed, with complete door locks. The electrical components in the cabinet should be in good condition, the lines should be clear, the operating control mechanism should be sensitive and reliable, the limit switches should perform well, and professional electricians should be arranged regularly for inspection and maintenance.

Wall-mounted device

When the tower crane exceeds its independent height, a wall-mounted device should be installed to increase the stability of the tower crane. The wall-mounted device should be installed in accordance with the requirements of the tower crane manual. The wall-mounted spacing and the free height above the wall-mounted point cannot be arbitrarily extended. The overlong wall-mounted support should be designed separately and have a calculation book to verify the strength and stability. The attachment frame should be kept horizontal and fixed firmly on the same horizontal plane as the attachment rod, and the connection with the building should be firm. After attachment, the verticality of the tower body below the attachment point is not more than 2/1000, and the verticality above the attachment point is not more than 3/1000. The connection point with the building should be selected on the concrete column or concrete ring beam. Use embedded parts or through-wall bolts to effectively connect with the building structure. Some construction companies use expansion bolts instead of embedded parts, and use cable wind ropes instead of attachment supports, which are very dangerous.

Safe operation

The key to tower crane management is the management of drivers. Operators must be in good health, understand the mechanical structure and working principle, be familiar with the mechanical principle and maintenance rules, and hold a certificate to work. Drivers must maintain the crane according to regulations, have a high sense of responsibility, and carefully clean, lubricate, tighten, adjust, and prevent corrosion. They must not work after drinking, or work with illness or fatigue. They must strictly follow the tower crane mechanical operation procedures and the "Ten Don'ts and Ten Don'ts" of the tower crane. They must not operate in violation of regulations or operate in a barbaric manner. They have the right to refuse illegal commands, and there must be sufficient lighting for night operations. The key to the safe use of tower cranes is the technical level and sense of responsibility of the operators, and the key to inspection and maintenance is the mechanical and electrical maintenance workers. We must firmly establish the people-oriented idea.

Safety inspection

Tower cranes must be inspected before and after installation and during daily use. The welds of metal structures shall not crack, metal structures shall not be plastically deformed, the quality of connecting bolts and pins shall meet the requirements, and measures shall be taken to prevent retreat and loosening. Personnel shall be arranged to pre-tighten the connecting bolts regularly, the wire rope shall be lubricated and well maintained, the number of broken wires shall not exceed the standard, broken strands shall not be allowed, plastic deformation shall not be allowed, rope clamp joints shall meet the standards, the reduction box and oil cylinder shall not leak oil, the hydraulic system pressure shall be normal, the brakes and limit insurance shall be sensitive and reliable, the transmission mechanism shall be well lubricated, the safety devices shall be complete and reliable, and the electrical control circuits shall be well insulated. In particular, tower crane drivers, maintenance electricians and mechanical maintenance workers shall be urged to conduct regular inspections, especially the wire ropes, hooks, various transmission parts, limit insurance devices and other wearing parts. Problems shall be dealt with immediately, and specific personnel, time and measures shall be set to strictly prevent mechanical Operation with illness.

Exit mechanism

The state has clearly ordered that the use of obsolete models must be resolutely prohibited. After identification and repair, the use of old and dilapidated tower cranes must be restricted. There is no unified standard for the service life of tower cranes. There are many different opinions and different regulations in different places.

Some users excessively pursue profit and benefits, do not pay attention to safety, and use small horses to pull big carts. Serious overloading is one of the main causes of high incidence of tower crane accidents. In order to meet the requirements of construction companies, expand sales, and occupy the market, some manufacturers increase the independent height and lengthen the boom to increase the coverage of the tower crane. This will inevitably reduce the stability of the tower crane, reduce the rated lifting capacity, and increase unsafe factors. There are also some privately modified tower cranes and privately assembled tower cranes. These tower cranes are older and cheaper to purchase second-hand. They are unwilling to invest more money in maintenance, so they have frequent failures. These should be highly valued by us. We should seek truth from facts, adapt to local conditions, and introduce relevant supporting policies to solve this problem on the basis of extensive consultation with all parties.

Conclusion

By strengthening the safety management of the above aspects of tower cranes, various accidents during the use of tower cranes can be effectively prevented, achieving the purpose of preventing accidents before they happen. Practice has proved that as long as each construction company, manufacturer, construction administrative department, and tower crane inspection agency can follow the above links and strengthen the special safety management of tower cranes, the occurrence of major tower crane safety accidents can be effectively controlled.