Tower cranes are miraculous tools of efficiency, allowing for construction work at extreme heights. In many cases, during the construction of skyscrapers and high-rise buildings, a tower crane actually paves its own sky-bound path: as it creates higher levels of a building above itself, the crane is elevated to the newly constructed higher levels via a hydraulic cylinder at its base. This continues until the tower crane reaches the building’s peak, where it constructs smaller derricks that disassemble it so it can return to the ground.
However, at high elevations, tower cranes are exposed to a vast amount of hazards that make them prone to accidents. Between 2000 and 2010, there were 1125 tower crane accidents reported worldwide, resulting in over 780 deaths. One of the main culprits behind these tragedies was exposure to wind, which caused 23 percent of all accidents.
In the lower atmosphere, wind speed generally increases along a gradient as height increases. However, the primary concern related to the forces that wind exerts on a tower crane’s load is not the speed of the wind, but its pressure. Wind pressure varies as the square of the wind speed, so, as wind speed doubles, the wind pressure increases by a factor of four times. This means that a small increase in wind speed can severely threaten the safe performance of a tower crane.
In their provided manuals, most tower cranes indicate the speed in which they must cease operations to prevent unwanted hazards. The maximum value for this is generally 20 m/s (45 mph), but this amount can vary. A review of in-service wind speeds by the CPA Tower Crane Interest Group concluded that the maximum recommended wind speed for towers in the UK should be 18.5 m/s.
According to ISO 4302:2016 – Cranes – Wind load assessment, the maximum wind speed for all types of cranes installed in the open should be 20 m/s, but this amount can be higher in certain transporter type uploaders that are meant to be exposed to high winds, for which it is at 28.5 m/s.
However, as previously noted, understanding the wind pressure around tower cranes can be more important for comprehending potential hazards to the aerial worksite. ISO 8686-3:2018- Cranes – Design principles for loads and load combinations – Part 3: Tower cranes establishes appropriate maximum values for wind pressure, at no more than 0.125 kN/m2 during the erecting of the crane, and 0.25 kN/m2 for when it is in service. This value is also based off inertia forces, which are also covered in the document.
Since the value of maximum permissible speed and pressure can vary depending on the specific tower crane use, it is essential that manufacturers properly convey that information to the purchaser and user of the crane. As stated in ISO 9374-3:2014 – Cranes – Information to be provided for enquiries, orders, offers and supply – Part 3: Tower cranes, manufacturers should calculate the maximum wind speed in which the crane is resistant to sliding, in addition to precautions that could be made during higher speeds, and hand that information over to the customer.
Wind speed is also one of the information labels that manufacturers should include on tower cranes in compliance with ISO 9942-3:1999 Cranes — Information labels — Part 3: Tower cranes guidelines. This should include symbols from ISO 7296-3:2006 – Cranes – Graphical symbols – Part 3: Tower cranes to simplify and standardize their safe use. Graphics from this document directly relating to wind speed include:
However, while a manufacturer’s guidelines can help to prevent wind related tower crane accidents, the actual user and installer of the crane must make efforts to guarantee its safe operations. ISO 12485:1998 – Tower cranes – Stability requirements (note: this standard has been revised by ISO 8686-3:2018 – Cranes – Design principles for loads and load combinations – Part 3: Tower cranes) includes wind as one of its many considerations for the assurance for a stable crane, for which W1is included as the value of the in-service wind effect.
The statistic of worldwide tower crane accidents that was mentioned at the beginning of this article came from a variety of factors that can limit the safety of crane use. While wind was only directly responsible for 23 percent of all accidents, it was likely a contributing factor in the other primary causes. For example, 38 percent of all accidents have occurred in-operation. ISO 9926-3:2016 – Cranes – Training of operators – Part 3: Tower cranes helps to reduce this value by defining competencies for persons operating tower cranes. Among these competencies, the standard states that a “trainee shall learn how to use an anemometer and to estimate the wind speeds by sight.”
In addition, among the different tower crane accidents, 31 percent occurred as they were being climbed during assembly and disassembly. While climbing is hazardous by itself, it can be impacted by other factors, such as weather. To limit accidents related to the climbing of tower cranes, it is explicitly specified in ISO 12480-3:2016 – Cranes – Safe use – Part 3: Tower cranes that “climbing should not be undertaken when wind speeds exceed 12m/s.”
Closely following international guidelines that address both tower cranes and wind pressure, in addition to awareness of manufacturer’s instructions, can save lives during the construction of tall buildings.
1. International Organization for Standardization (ISO), ISO 7296-3:2006 – Cranes – Graphical symbols – Part 3: Tower cranes (Geneva: ISO, 2006), 4.
2. International Organization for Standardization (ISO), ISO 7296-3:2006 – Cranes – Graphical symbols – Part 3: Tower cranes (Geneva: ISO, 2006), 4.