How accidents happen1 February 2003
Felix Weinstein explains how and why tower crane accidents happen
Tower cranes are one of the most useful tools on a construction site, and generally they are safe. Sometimes, however, accidents do happen, resulting in serious injuries and even deaths. Even where the outcome is not so tragic, tower crane accidents can result in property damage and/or costly disruption to the schedule of the project.
Nowadays advanced building technologies and building site logistics are based on the presence of a tower crane for all phases of the project.
Different types of tower cranes are used for different projects. The construction of private homes differs from the building of skyscrapers, where the tower crane increases in height together with the building. On greenfield sites the scope for catastrophe resulting from a tower crane collapse is restricted to the site itself. In city centres or at busy highway intersections, where tower cranes can most often be seen at work, and where several tower cranes are often working overlapped on the same building site, the scope for catastrophe widens considerably.
Tests have been carried out on thousands of tower cranes. This research includes periodical tests every six months, tests following structural changes according to project requirements, and tests after the completion of construction on site. From this, a picture has emerged of the supreme importance of the safety of the tower crane and its working environment.
Responsibility for safety extends from management level to all workers, including crane operators and the technicians that service the cranes. The tower crane operator carries the heavy responsibility of correctly operating the crane, of carrying out the building assignments at the site, and for daily and periodical tests of the various systems in the tower crane. The operator is also the first to detect or sense faults developing in the tower crane and irregular noises warning of the need for repairs or service by a professional. The technician that services the tower crane is responsible for the repair, maintenance and calibration of the command and control systems.
These people may sometimes be subject to various pressures that affect safety-critical decision making, which may lead to the collapse of the tower crane at the site and to casualties and damage to property. Sometimes their training and experience are not compatible with the servicings required or the performance of certain repairs on the tower crane.
Serious tower crane accidents often occur as a result of the accumulation of many small faults. Workers do not repair these faults properly because they believe that the faults can wait until the tower crane is dismantled.
Workers and managers must be alert and professional in order to detect changes or irregular phenomena in the tower crane area. They need the same attributes in order to recognise when those conditions will render the crane's structure unstable.
Safety can be compromised by activities on neighbouring sites, such as the operation of compaction machinery whose vibrations reverberate to the base of the tower crane, or an excavation that alters ground characteristics. Any other tower crane operating close enough for contact also poses risks - a collision might unsettle one or both towers. Engineers cannot make tower cranes collision-proof because they cannot calculate the effect of the forces acting on the anchorage system as a result of a collision. A load falling from a tear in the sling can also affect anchors and foundations incalculably.
Tower crane designers have to meet all relevant standards but also have to remain competitive in the market. This has an effect on the safety coefficients and dynamic coefficients that determine the form and weight of the structure.
The tower crane structure is simple, and assembly is much like Meccano or Lego, provided that the manufacturer's instructions for assembly and dismantling are strictly adhered to and performed by skilled professionals using the proper tools.
The lifespan of a tower crane structure can be 20 or 30 years, or more. Over the years, the dismantling, assembly and transportation take their toll and extensive repairs are likely to be conducted on the crane. The safety of the tower crane depends on repairs of the structural parts, the electrical systems and the command and control systems. Bad welding of a critical point, for example, can be the source of a tower crane collapse.
Old tower cranes develop corrosion within the structure that causes the width of the section area to diminish. The elemental strength of the tower crane may drop to such an extent that the lifting jib or counter jib can break under its own weight.
A first visit to a building site might suggest that the managers and workers are adhering to safe working practices with the tower crane. However, after a time it may become apparent that their principal motivation is to economise. Crane companies can economise on regular maintenance, repairs of faults, use of substitute spare parts and employee selection.
Although the tower crane site appears safe and conforms to requirements, it may still be dangerous because of the underlying approach to safety. Greater thoroughness is essential.
How the site managers seek to use the crane is also of the utmost importance. The operator may at times be required to perform lifting and pulling actions that are contrary to familiar safety regulations and which place the tower crane at risk. This type of work once again is derived from economic considerations at the site. Some crane managers decide to push a tower crane beyond its safe limits, rather than incur the additional expense of bringing in a more appropriate means of lifting. These managers act as if they have no concept of the risks entailed in such a work process.
A crane is only as good as the accessories that it employs. One sometimes sees a new and sophisticated tower crane on a site whose load is tied to the hoisting hook with old and defective cables or belts. A faulty sling is likely to result in a dropped load. The shock of dropping a load can have affect the crane's stability.
One of the major obstacles for tower cranes is the wind force. The wind regime changes, becoming more severe with height. Frequently, when ground conditions are calm, a tower crane operator will not be able to turn the jib in the direction requested by the site foreman on the ground because of a strong wind blowing at jib height.
Tower crane manufacturers take into account the impact of wind when designing their equipment, but they are unable to calculate the way the shape and size of the load affects the force of the wind acting on the crane. The surface area of the load is a considerable issue in determining the correct and safe working load for the crane under strong gusts of wind. It is interesting to note that a load weighing only 20% of the safe working load can jeopardise stability of a tower crane if it has a particularly large surface area. On several occasions a strong gusty wind caught a relatively light form being lifted and blew down the crane, with the operator in the cab.
Within this context, attention should be given to the advertising signs that are hung on the crane's counter jib. These have implications to stability when the crane is in service and out of service. The signs on cranes act as sails.
Of special importance are the technicians that perform regular maintenance and repair faults. The simple matter of tightening the bolts that join the tower mast sections requires the proper tools, such as a moment wrench. The technician is required to close the screw at the corner of the section when the crane's counter jib is above that corner. If such a simple task is not performed correctly, the crane's stability will be at risk.
Obviously, the foreman that calls upon the technician to tighten the crane bolts assumes that the technician is equipped with the proper tools and is well versed in the required work regulations.
Cranes have a tendency to self-dismantle. Tower cranes that have been working intensively for more than 10 years develop gaps in the tower mast section joints, in the jib section and in all joints that have bolts or pins.
The looseness of the joints causes the bolts to unscrew and the pin joints to dismantle while performing regular crane jobs on the site. The dynamic forces that develop in the joint areas increase far beyond those originally designed by the crane manufacturer.
To prevent damage and accidents as a result of the loosening of the sections and other elements of the crane, a daily comprehensive check of all the tower crane parts is required. Preventive maintenance and regular handling by professionals is required for as long as the crane is working at the site. If such daily checks and regular maintenance are not carried out, the crane may collapse on site.
Statistics from extensive investigation of work accidents with tower cranes show that a significant number of accidents could have been prevented by using the crane correctly, knowing its limitations, using the proper technicians to carry out maintenance and repairs, and properly planning and maintaining the foundations. It is not just the team operating and servicing the crane that need to understand the limitations of tower cranes; site managers must too.