Stability of cranes under wind loads: a strong demand for expertise
CSTB’s expertise in assessment of the risk of construction tower cranes tipping over under strong wind loads started in 1997, and this activity has been growing quickly over the last few years.
Since the CNAMTS (Caisse Nationale d'Assurance Maladie des Travailleurs Salariés) (National Health Insurance for Salaried Employees) recommendation R406 was adopted in 2004 recommending assessment of this type of risk, several accidents have encouraged Mayors to oblige site managers to get these expertises done before the cranes are installed. Thus, CSTB carried out 1200 theoretical expertises and 25 wind tunnel tests on the stability of site tower cranes under wind loads, in 2007. "In practice, the inspection offices ask us to perform preventive studies on risks related to wind loads", explains Christian Barré, CSTB "Wind and structures" expert.
Taking account of the near environment
The expertise begins with a theoretical study of the predicted wind speed at the jib height. It takes account of the crane characteristics and the site environment. The study includes the crane near environment (nearby building heights) that can significantly modify the wind speed and effects, in addition to the reference wind speed values given by meteorological stations and the Eurocode regulatory approach. "If there is a risk of a strong interaction, we suggest a detailed wind tunnel expertise integrating physical simulation of the crane and its environment at a 1:80 scale", continues Christian Barré. Forces applied by the wind on the crane are measured in the absence and then in the presence of the near environment. The results are expressed in the form of increased forces due to the site effect.
More specific studies may be undertaken on exceptionally tall sites (towers in La Défense district in Paris, etc.). They include construction phases and the possibility of cranes telescoping as the tower is being raised. CSTB thus carries out five to six studies of this type per year.
Physical simulation of wind effects
The physical simulation approach to building/crane interaction effects benefited from the work done in Dimitri Voisin’s thesis during the year 2000. Crane mock-ups and their environment were installed in a 15-meter long, 4-meter wide and 2.5-meter high wind tunnel, and the wind speeds referenced in the Eurocode base and local and regional climatological data are applied to them.