When the wind blows

When looking at the actual or potential causes of lifting accidents, the power of wind is often underestimated or neglected. Most people’s experience is that the wind is a nuisance, blowing litter and leaves around rather than causing significant damage, except in severe storm conditions. This is because humans are, in the physical sense, quite dense, with a high weight in relation to their surface area. However with cranes and lifting operations there is a significant potential for disaster due to wind loads. The effects of wind on cranes can be both direct and indirect. The direct effect is the potential to affect the stability and/or structural integrity of the crane, while the indirect effects are wind forces causing the load on the hook to move suddenly. This in turn may lead to material falling or people being knocked over, perhaps from a height.

Before looking at these effects a short explanation of the principles behind wind forces and their effects on structures, including cranes, is useful. Air is a combination of various gases that have a certain density, which is small but not zero. (If it was, the earth’s atmosphere would float off into space!). When the wind blows, the molecules of the gases in the air are moved at a certain velocity and gain energy. When the wind meets a surface the air is slowed down or stopped and the energy of the molecules is translated into wind pressure or force per unit area. In general terms the relationship between wind pressure (p) and wind speed (vs) is p = K vs2 where K is a factor related to the density of air, which for design purposes is assumed to be constant. Where the wind pressure (p) is in N/m2 and wind speed (vs) in m/s, K is 0.613, giving p=0.613 vs2. This squared relationship between wind speed and wind pressure is not always appreciated: if the wind speed doubles the wind pressure increases four times. One can see that a small increase in wind speed can have a significant effect on the wind force and hence the stability or strength of a crane.

Once the wind pressure for a given wind speed is known, the wind force on a particular crane can be calculated by multiplying the pressure by the area of the parts of the crane structure exposed to the wind. A crane structure is made up of many components of different shapes and sizes, each having a different resistance to the wind and hence wind load. Consequently, each differently shaped component is assessed separately, with its area square to the wind being multiplied by the appropriate force coefficient (Cf). This process is covered in detail in the International Standard ISO 4302-1981 Cranes – Wind load assessment.

Height plays an important part in the effect of wind on crane stability (see figure 1). As we have seen, the force exerted by the wind on a structure is the wind pressure multiplied by the area of the structure. This force acts at the centre of area or centre of pressure of the structure and creates an overturning moment that affects stability. This moment is the wind force multiplied by the distance from the centre of area to the ground. Height is important in two ways. Firstly, the greater the distance between the ground and the centre of pressure, the greater the overturning moment. Secondly, wind speed rises with height above ground level with the increase depending on a number of factors such as the presence of surrounding buildings and hills. One can see that with very tall cranes there is a double effect. Not only is the overturning moment due to wind greater because the distance from the ground to the centre of pressure is greater, but the wind speed, and hence wind force, will also have increased due to the height of the jib above the ground.

Most cranes are designed to national or international standards that lay down minimum in-service wind speeds that the crane must be able to withstand safely. These are typically 14m/s (31mph) for mobile cranes, 20m/s (45mph) for tower cranes and 28.5m/s (64mph) for dockside and container cranes. In addition the standards specify minimum out of service wind speeds for those cranes which cannot be easily lowered to the ground such as tower, dock and offshore cranes. These wind speeds are typically 36m/s (80mph) onshore and 44m/s (98mph) offshore.

The important point here is that most wheeled and crawler mobile cranes are not designed with an out of service wind speed greater than their in-service limit. They are designed on the assumption that their jibs will be lowered when the wind speed rises above the specified limit. This point is sometimes not appreciated and it is not uncommon to see mobile crane jibs, particularly those of crawler cranes, permanently in the air on a site. Indeed, crawler cranes are often working in a position where it is not possible to lower the jib without travelling the crane some distance to a clear area of the site. This lack of appreciation of the need to lower jibs when a mobile crane is out of service has caused several spectacular failures.

Wind not only has a significant effect on the crane, it can also affect the load in several ways. Relatively light loads with large wind areas, such as formwork, can have significant horizontal forces imposed on them which can adversely affect a crane. For example, with a wind speed of 14m/s (31mph) the wind load on an 2.5m x 1.3m (8ft x 4ft) sheet of ply will be 372 N or 38kg. With wind blowing from behind the crane, the load radius can be increased significantly, inadvertently overloading the crane and perhaps causing it to overturn. For example a shutter weighing 750kg with an area of 3.25m2 and suspended on a 27m hoist rope will move 1.4m from the vertical when subjected to a 14m/s (31mph.) wind. Moving the load radius by this distance on a 35t capacity truck crane with a 34m main boom working at 18m radius would reduce the rated capacity from 950kg to 640kg.

If the wind is blowing from the side this will put a significant side load on the jib for which it is not designed. This may cause it to collapse and the load to fall, with potentially disastrous consequences. A well known example of this is ‘Big Blue’, the Lampson Transilift which fell over in 1999 while lifting a 450 US ton, 100ft x 180ft section of the retractable roof of the new Miller Stadium in Milwaukee, Wisconsin. The subsequent investigation found that wind forces on the huge roof section, which had not been properly calculated, played a major part in the collapse.

In the case where the wind on the load does not move it sufficiently to affect the stability or strength of the crane, it may well move enough to endanger people. An example of this is where a tall concrete column was being poured on site using a bottom discharge skip. The skip was being guided into position by two men standing on a platform at the top of the column formwork. A gust of wind caught the skip and moved it sideways, knocking the men off the platform.

Wind forces are also particularly important when considering tower cranes, as they are generally erected for periods of months on construction sites and must therefore withstand any wind that may blow during this time. To do this, when out of service, the crane is allowed to free slew with the slew brake held off mechanically, allowing the crane to weathervane and present the minimum area to the wind. As the wind area of the front jib is always larger than the rear, or counter jib, the crane top will always slew to present its back to the wind. It is for this reason that one must be very careful not to upset the in-balance between the wind areas by adding advertising signs or decorations to the back jib. If this happens the crane will not weather vane and will present a much larger wind area, which will in turn give rise to larger forces than the crane is designed to cope with. This delicate balance of wind areas is also a significant issue on luffing jib tower cranes where the temptation is to leave the crane out of service with the jib luffed up to minimum radius to avoid other cranes or structures. But this minimises the crane’s ability to weather vane. The effect of wind forces on tower cranes during erection and, in particular, climbing operations should also not be overlooked as any change in wind strength or direction can alter the balance of the crane top significantly with a consequent effect on crane stability.

From this brief overview one can see that the effect of wind on cranes and their loads is not to be taken lightly and it must not be overlooked when carrying out the risk assessment and planning that is an essential part of all effective lifting operations.