A certain company undertook a project with 30 floors above the ground, a building height of 95m, and a construction area of 32,000 square meters. A QTZ125(ZX6016) tower craneprovided by the developer was used. This tower crane did not come with a random operation manual. It was installed by embedding 16-M36 bolts for tensioning, and there was no counterweight on the base of the tower crane. When it was put into use, the actual installation height of the tower crane was 25m, the length of the jib was 42m, and the lifting weight at the end of the jib was 2.9t.
Due to the lack of a counterweight on the base, eltower craneinstalled the first attachment device at a tower height of 26m in advance. The project department designed, manufactured, and installed an ultra-long attachment device for the tower crane by itself according to the dimensions of the standard attachment device of the JL6018 tower crane. The distance between the building and the center of the tower crane was 9.3m, the distance between the two groups of strut rods attached to the wall was 8m, the longest extended attachment rod was 11m, and after the attachment, the tower crane climbed to 44m. On the 10th day of use, the pre-embedded steel bars of the 8-16mm wall attachment support seat broke. A group of attachment strut rods fell from the column of the wall-attached building to the 8th floor, narrowly avoiding a major accident of the tower crane collapse.
After the accident occurred, the project department installed a second attachment device with the same size at a tower height of 35m, and changed to use 6-20mm steel bars as the wall attachment pre-embedded parts. After completing the attachment, el tower craneclimbed to 50m. During the self-inspection and acceptance by the installation unit, the rotating jack hole of the M64 adjusting distance bolt on the attachment strut rod cracked again. Some chord angle steels of the lattice attachment strut rod could not be used due to compression deformation. At this time, the construction site stopped work and reported for handling.
Accident Analysis
1. Excessive attachment angle of the attachment strut rod
Due to the limitations of the site conditions and the insufficient consideration of the installation position of the tower crane, the attachment angle of the strut rod to the wall was too large (more than 70°). This caused the axial tensile and compressive stresses borne by the attachment strut rod to be too large. When subjected to external loads composed of unbalanced moments generated by the balance arm and the lifting arm of the tower crane, dynamic loads, wind loads, and rotational inertia forces, etc., the alternating tensile and compressive stresses made the weakest part of the entire attachment device fail.
In a tower crane, the attachment device plays a crucial role in ensuring the stability of the tower crane. When the attachment angle is too large, the force transmission path of the strut rod changes, resulting in uneven force distribution. This not only increases the stress on the strut rod itself but also affects the connection between the attachment device and the building structure. Como resultado, under the action of various external loads, the attachment device is more likely to fail, posing a serious threat to the safety of the tower crane.
2. Insufficient stiffness of the ultra-long attachment strut rod
The four lattice slender inner and outer attachment strut rods with a length of 8 to 11m had a cross-section of 300mm × 300mm, which was too small. This led to insufficient stiffness and poor stability.
The stiffness of the attachment strut rod is directly related to its ability to resist deformation. For ultra-long strut rods, if the cross-section is not designed reasonably, they will be more prone to bending and deformation under the action of loads. In this case, the small cross-section of the strut rod made it difficult to effectively bear the loads transmitted from the tower crane, resulting in a decrease in the overall stability of the attachment system. When the tower crane was operating, the strut rod might vibrate or deform excessively, which could eventually lead to the failure of the attachment device.
3. Uneven force on the double-ear single-hole support seat
Under the action of the inner and outer strut rods, the double-ear single-hole support seat was unevenly stressed. This led to asymmetric cyclic stress on the pre-embedded steel bars, increasing the impact frequency of the horizontal shear stress on the pre-embedded parts.
The support seat is an important part of the attachment device that connects the strut rod and the building. When the force on the support seat is uneven, it will cause uneven stress distribution on the pre-embedded steel bars. The cyclic stress generated by the repeated action of the strut rod will accelerate the fatigue damage of the pre-embedded steel bars. As the impact frequency of the horizontal shear stress increases, the pre-embedded steel bars are more likely to break, affecting the stability of the attachment device.
4. Improper selection of pre-embedded part materials
The pre-embedded parts used 16mm steel bars to pass through the holes in the bottom plate of the wall attachment support seat and were welded with a 90° elbow. This reduced the toughness of the 20MnSi steel and increased its brittleness, making it prone to excessive plastic deformation. The fracture surface of the on-site detected steel bars showed that the pre-embedded steel bars had overload and fatigue phenomena under the action of alternating tensile and compressive stresses and horizontal shear stresses. Under the condition of high cycle (high frequency), cold brittleness caused the fracture.
The selection of pre-embedded part materials is a key factor in ensuring the safety of the attachment device. The improper processing method of the 16mm steel bars, such as the 90° elbow welding, changed the mechanical properties of the steel. The reduction in toughness and increase in brittleness made the steel bars more sensitive to stress. Under the long-term action of alternating stresses, the steel bars gradually accumulated fatigue damage, and finally, the cold brittleness caused them to break, resulting in the failure of the attachment device.
5. Untreated bolt materials
Important bolts made of 45# steel must be quenched and tempered to eliminate the internal stress generated by mechanical processing and improve the comprehensive mechanical properties of the screw. The cracking of the rotating jack hole of the M64 adjusting distance bolt of the attachment strut rod was caused by the use of 45# steel without quenching and tempering treatment.
The mechanical properties of bolts have a direct impact on the connection strength and stability of the attachment device. El 45# steel bolts, if not properly treated, will have internal stress residues, which will reduce their strength and toughness. When the bolts are subjected to loads during the operation of the tower crane, the internal stress may cause the bolts to crack or break. In this case, the cracking of the rotating jack hole of the M64 bolt was a direct consequence of the lack of quenching and tempering treatment, which affected the normal operation of the attachment device.
Accident Handling
First, the installation unit prepared a technical scheme for the installation of the ultra-long attachment of the tower crane, including treatment measures and attachment arrangements.
The treatment measures were checked according to the tensile force when the 8-16mm pre-embedded steel bars of the attachment plate broke, based on the above accident analysis.
1. Determine the specifications of the attachment strut rod
It was determined to use ∠75 × 8 angle steel as the chord rod and ∠40 × 4 angle steel as the web rod to manufacture an attachment strut rod with a cross-section of 400mm × 400mm. According to the attachment rod angle > 70°, the stiffness and stability of the attachment rod were checked to ensure that they could meet the maximum load of the tower crane in both working and non-working states.
The selection of the cross-section and materials of the attachment strut rod is crucial for improving its bearing capacity and stability. By using larger-sized angle steel and a more reasonable structure, the stiffness and strength of the strut rod can be enhanced. Checking the stiffness and stability under different working conditions can ensure that the strut rod can safely bear the loads transmitted from the tower crane, reducing the risk of failure.
2. Modify the attachment support seat and pre-embedded parts
The attachment of the strut rod to the wall was changed to a single-ear double-hole support seat and 10-M24 anchor bolt wall attachment pre-embedded parts.
Changing the structure of the support seat and pre-embedded parts can improve the force distribution of the attachment device. The single-ear double-hole support seat can make the force on the pre-embedded parts more uniform, reducing the risk of uneven stress. The use of 10-M24 anchor bolts can increase the connection strength between the attachment device and the building, ensuring the stability of the attachment.
3. Modify the bolts
The M64 adjusting distance bolt of the attachment strut rod made of 45# steel was changed to M72 and quenched and tempered treatment was carried out. At the same time, the rotating jack hole was cancelled.
Increasing the diameter of the bolt and carrying out quenching and tempering treatment can improve the strength and toughness of the bolt. Canceling the rotating jack hole can avoid the potential risk of cracking caused by the hole. These measures can effectively improve the reliability of the bolt connection and ensure the normal operation of the attachment device.
Attachment Arrangement
The first attachment device was installed at a tower height of 32m. At the same time, the original two failed attachment devices were removed. The second attachment was made at a tower height of 50m, the third attachment was made at 68m, and the fourth attachment was made at 86m. The total height of the tower crane was 107m, and the independent height above the attachment device was 21m, meeting the construction requirements.
The reasonable arrangement of the attachment device can effectively ensure the stability of the tower crane during the construction process. By installing the attachment devices at appropriate heights, the tower crane can be better supported, reducing the risk of collapse. At the same time, removing the failed attachment devices can eliminate potential safety hazards and ensure the safety of the tower crane operation.
En conclusión, when the tower crane has an ultra-long attachment, in addition to having technical materials such as the operation manual, the installation must be carried out by a unit with installation qualifications, and a safety technical scheme must be formulated. The tower crane must be accepted before use after the attachment installation and climbing. The failure to implement these tasks is the fundamental cause of this accident. The direct cause of this accident is that the pre-embedded parts of the ultra-long attachment device of the tower crane underwent excessive plastic deformation under the high-cycle impact of alternating tensile and compressive stresses and horizontal shear stresses, and broke due to fatigue. At the same time, the design of the ultra-long attachment device of the tower crane should fully consider that the attachment angle of the attachment rod should be as close to 45° as possible, the stiffness and stability of the attachment rod should be checked to meet the maximum load of the tower crane in both working and non-working states, and comprehensive factors such as the selection of appropriate materials and manufacturing processes should be considered.
During the construction process, the safety of the tower crane is of great significance. Any negligence in the design, installation, and use of the tower crane may lead to serious accidents. Therefore, it is necessary to strengthen the management and supervision of the tower crane, improve the safety awareness of construction personnel, and ensure the safe operation of the tower crane through scientific design, strict installation, and regular maintenance. Only in this way can we effectively prevent similar accidents from happening and ensure the smooth progress of construction projects.
