Key Control Points for Tower Crane Safety Supervision

An analysis of the causes of many tower collapse incidents reveals that overloading is the primary factor. Overloading occurs either due to the weight of the load exceeding the specified limit or because the horizontal distance of the load exceeds the operational radius. After installing a moment limiter, if the weight exceeds the limit or the operational radius is too great, causing the torque to surpass the tower crane's technical specifications, the lifting or jib drive will automatically be cut off, and an alarm signal will be triggered to prevent accidents.

Currently, there are two types of torque limiters: one is electronic, and the other is mechanical. The electronic type allows simultaneous reading of torque, operating radius, and weight data, features a warning signal when approaching the allowable torque of the tower crane, is user-friendly, but is highly affected by working conditions, has poor reliability, is prone to damage, and is difficult to maintain. The mechanical type lacks a display unit and warning signal but offers reliable performance, is well-suited to on-site construction work conditions, and has a simple structure with a low damage rate.

Cranes must adjust the moment limiter when reassembling on a new site, changing the magnitude, or altering the boom length. Electronic overload alarm points must also be recalibrated based on actual working radius and weight for trial lifts. For cranes with jib amplitude variation, when mechanical moment limiters are selected, they must be compatible with the crane (preferably from the same manufacturer). Cranes with mechanical moment limiters on the jib must adjust the overload limit tonnage after each amplitude change, according to the allowable load by the working radius. During safety inspections, if there's no way to test the reliability of the moment limiter, the trial run records after installation can be reviewed to confirm that the moment limit testing results meet requirements and the overall precision of the moment limiter system satisfies ±5%.

Overload Limiter (Lifting Load Limiter) – Some tower crane models are equipped with an overload limiter as per regulations. An alarm signal is emitted when the load reaches 90% of the rated lifting capacity; if the lifting weight exceeds the rated capacity, the power supply to the upward direction should be cut off, allowing the mechanism to move downward. During safety inspections, a test should be conducted simultaneously to confirm.

Limit Switch - Also known as the upper limit position limiter, it automatically cuts off the lifting mechanism's power supply when the crane hook reaches its maximum position. The mechanism can move downwards, and an operational test should be conducted during safety inspections to verify its function.

Variable Stroke Limiting Device: Includes carriage stroke and boom stroke. A test should be conducted for safety verification during inspections.

(1) Luffing of the Small Crane. The tower crane employs a horizontal boom, with the lifting load suspended from a crane trolley. The luffing is achieved by the trolley moving horizontally along the boom. The trolley luffing limiter utilizes two limit switches and a cushioning device, installed at the top and base of the crane boom, to restrict the trolley's position.

(2) Boom Luffing. The tower crane changes its operating radius (luffing) by altering the boom's inclination angle. The light signal is transmitted to the indicator panel in the operator's cabin through the change in the contact device, indicating the angle of inclination. When the control of the boom's inclination angle reaches the upper and lower limit positions, the limit switches are pressed to cut off the power supply, preventing the boom from exceeding the inclination angle and causing instability in the tower crane.

During on-site action verification, an experienced person should be in charge to supervise and direct, to prevent accidents from occurring.

Traveling Limit Switch – Prevents出轨 accidents during the operation of track-mounted cranes. During safety inspections, a travel movement test of the crane should be conducted, and the collision limit switch should be verified for reliability.

Insurance Device

Hook safety device. Primarily prevents equipment accidents caused by the hook continuing to descend while the heavy object is obstructed during tower crane operation. This device consists of a spring cover installed at the hook opening, which can only be pressed down and not opened upwards, preventing the rope from slipping out of the opening.

2. Coiler Safety Device. Primarily prevents wire ropes from being tangled on the reel when the transmission mechanism fails, thus avoiding accidents such as rope bite beyond the end flange of the reel.

3. Ladder Guard Ring

(1) When the ladderway height exceeds 5 meters, install the guard ring starting from 2 meters above the platform. The guard ring must remain intact without excessive deformation, missing rings, or weld cracks.

(2) When the ladder is installed inside the structure, if the distance between the ladder and the structure is less than 1.2 meters, a guard ring may not be required.

Wall Mounting Device and Clamping Rail Clamp

The free-standing height of the self-elevating tower should be as per the instructions manual. When exceeding the specified limit, it should be attached to the building to ensure the stability of the crane.

2. Wall Mounting Device

The tensile strength must meet design requirements when attached to a building.

(2) When attaching, use a theodolite to check the verticality of the tower and make adjustments as necessary. The arrangement of the bracing rods, the spacing between them, and the vertical distance of the wall attachment devices should comply with the specifications in the manual.

(3) When the specific nature of the project requires altering the length or angle of the attachment rod, a calculation of the attachment device's strength, rigidity, and stability should be performed to ensure it meets or exceeds the safety level of the original design.

(4) When using the track-type crane in an attached mode, the load-bearing capacity of the track foundation must be increased, and the power supply to the travel mechanism must be disconnected.

3. Track Clamp. A wind-resistant track clamp should be installed on overhead cranes operating on tracks.

4. The track clamp device must ensure effective braking after clamping. When the operator needs to leave the crane tower for lunch, to clock out, or for a temporary stop, the operator must secure all the track clamps of the tower crane according to regulations before leaving. Stainless steel welding rod

Installation and Dismantling

1. The installation and dismantling of tower cranes is a complex and dangerous task. With various types of tower cranes, differing working environments, and varying levels of familiarity among installation teams, it is imperative to develop a detailed construction plan prior to work. This plan should include: the operational procedure, number of personnel and their positions, types and positions of cranes working in conjunction, the burial of ground anchors, preparation of rigging equipment, and protection of the on-site working environment. For the hoisting work of self-raising towers, specific requirements for maintaining balance with the boom and counterweight must be met, along with the hoisting steps during the process and reliable measures to prevent rotation.

2. The installation and dismantling of cranes must be carried out by a professional team, with personnel holding a certificate issued by the municipal relevant departments. A dedicated person must be in charge of the operation.

Crane Controller

1. Tower crane operators are classified as special operation personnel, and must undergo formal training and pass an examination to obtain a qualification certificate. The content of the certificate or training examination must match the type of crane being operated by the driver.

2. The signal handlers for tower cranes should have undergone formal training and passed the assessment, obtaining a qualification certificate. Their signals must comply with the national standard GB5052-85 "Command Signals for Lifting and Hoisting."

3. In cases where multi-tower operations interfere with each other on-site, or when operators of tall towers cannot clearly hear the whistle signals from the signal officers or see their hand signals, on-site conditions should be considered and flag signals or two-way radios should be used for guidance.

Subgrade and Track

The foundation and track laying for tower cranes must be strictly followed according to the manufacturer's instructions. Generally, the soil bearing capacity of the foundation: medium-sized towers (3~15t) is 0.12~0.16MPa; for heavy-duty towers (above 15t) is >0.2MPa. The foundation should be leveled and compacted, with sand and gravel laid on top, and drainage measures in place.

2. Sleepers can be made of wood, reinforced concrete, or steel, with cross-sectional dimensions as specified in the manual (e.g., 16×240, 180×260, etc.). The length of the sleepers should be at least 1,200 mm longer than the gauge size. When using a layout of one long and two short sleepers, a channel steel tie rod should be installed every 6 meters or so to maintain the gauge. The spacing between sleepers is 600mm. When using a prefabricated subgrade box, it must be inspected and confirmed to meet requirements before use.

3. Both sides of the track should be securely fastened with spikes (or clamps) onto each tie, without any missing or loose fasteners. The track joints should be staggered, with the joints resting on the ties, with a height difference not exceeding 2mm between the ends. The joint plates should match the track and all bolts should be fully installed and tightened.

4. Track level deviation is not greater than 1/1000 in both longitudinal and transverse directions. (A level should be used, measuring at three points within a 10m range on both tracks, and taking the average value).

5. At a distance of 1m from the track terminal, install an ultimate position stopper (shutoff stopper), which should be taller than the radius of the walking wheel to prevent the crane from derailing after power failure.

6. The foundation construction of fixed tower cranes should be carried out according to the design drawings. The design calculations and construction details should be included as one of the contents of the special construction organization design for tower cranes. After construction, it should be inspected and documented.

Electrical Safety

1. Crane cables must not drag on the ground; they should be equipped with a tensioning device on the cable drum. As the crane moves, the drum automatically winds the cable, preventing accidents caused by friction between the cable and sleepers or entanglement with debris on the tracks.

2. The safety distance between overhead lines at construction sites and tower cranes is specified in the temporary power supply regulations: "The minimum horizontal distance from the edge of any part of the rotating jib crane or the edge of the suspended object to the overhead line edge below 10kV shall not be less than 2 meters." When the distance is less than this, protective scaffolding must be erected as required. For night construction, 36V colored bulbs (or red bulbs) should be used. When the crane's operating radius passes over the overhead lines, protective measures should also be in place above the lines.

3. When using the TT system on-site, the tower crane must be grounded with a resistance not exceeding 4Ω; when using the TN system, in addition to the protective grounding, it should also be grounded repeatedly according to the temporary electrical power regulations, with a resistance not exceeding 10Ω.

4. Tower cranes should have repeated grounding at both ends of the track. For longer tracks, an additional grounding device should be installed every 30 meters. A circular electrical connection should be made between the two tracks using rebar or flat iron, etc. The joints between the tracks should be bridged with wire to form an electrical connection.

5. The protection zero and grounding wire of the tower crane must be separated. The power cable can be sent to the end of the track of the tower crane to a distribution box, where the PE wire is connected to the repeated grounding wire of the track, effectively connecting the PE wire to the equipment housing through the track wheel.

Multi-tower operation

When two or more tower cranes are operating on adjacent tracks or on the same track, they should maintain the minimum distance between the two machines.

(1) Mobile Cranes. The distance between any part (including the load) must be no less than 5 meters. The distance between the crane hook and the lower part of the tower must be no less than 2 meters.

2. When construction is limited by site operation conditions and cannot meet the requirements, two measures should be taken simultaneously.

(1) Organizational Measures. Specify the operation of tower cranes and their pathways, and supervise their execution by designated supervisors.

(2) Technical Measures. Limiting devices should be installed to shorten the boom, elevate (lower) the tower, and other measures to prevent overstepping the specified operational range due to improper operation, which could lead to collision accidents.

Installation Acceptance

1. Tower crane trial operation and acceptance are divided into three scenarios: pre-delivery, after major overhaul, and after reinstallation for reuse. Here, we mainly refer to the trial operation and acceptance after reinstallation for reuse. It should include the following aspects:

(1) Technical Inspection. Check the stability of the tower crane, the contact between the pulley and the steel wire rope, electrical circuits, safety devices, and the installation accuracy of the tower crane. In the unloaded state, the deviation of the tower's verticality to the ground should not exceed 0.3%.

(2) No-load Test. Perform action tests separately for lifting, rotation, amplitude change, and walking mechanisms, and conduct combined tests for lifting, walking, and rotation. During the test, collide with all limit switches to check their sensitivity.

(3) Rated Load Test. Lift the boom to its minimum working radius, hoist the rated maximum lifting capacity, with the heavy object 20cm above the ground, hold for ten minutes without changing the height (at this point, the torque limiter should emit an alarm signal). Upon passing the test, perform lifting, walking, and rotation tests at maximum, minimum, and intermediate working radii, as well as combined action tests.

During the above tests, use a theodolite to observe the deformation and recovery of the tower crane in both directions, monitor for any abnormal phenomena during the test process, such as temperature rise, oil leakage, paint peeling, etc., and record and measure these. Finally, confirm that it meets the qualifications for operation.