WIND SPEED RISKS IN STEEL CONSTRUCTION

The British Constructional Steelwork Association (BCSA) is an industry organisation of UK steel contractors. One of their publications is a guide for steel construction under wind speed conditions, applicable for structures up to 10 storeys high. Two important aspects are emphasised in the guide:

• Wind Gusts & Turbulences are the main risk factor, not the average wind speed normally reported.
• Conventional weather forecasts cannot be counted on for gust data: Only a weather monitoring system installed on-site can detect these dangerous winds.

When cranes operate under unsuitable weather conditions, two main types of accidents can occur: the load can be dropped, or the crane can collapse completely. Material damage is certain in both cases, and human consequences are very likely.

According to the UK Health & Safety Executive, there have been 61 tower crane accidents in the country since 2001, with 25 serious injuries and 9 casualties.

The BCSA guide has been accepted and endorsed by the Health & Safety Executive, and it provides useful information to mitigate wind risks in construction sites. The recommendations can also be used to guide important decisions, based on data provided by a reliable wind monitoring system. Good decisions require accurate data, so the role of a monitoring system cannot be overlooked when operating tower cranes:

• Gusts are impossible to predict, so the best approach is monitoring them and reacting as fast as possible.
• Weather forecasts provide a general idea of the wind conditions that can be expected and should be used as a guideline but are not reliable for crane operations.

WHY GUSTS ARE CONSIDERED THE MAIN RISK FACTOR

A useful “rule of thumb” to understand wind risks is the following: If wind speed increases by 2, the pressure exerted increases by 4. For example, if a crane is working under a 12 mph wind and there is a sudden gust of 24 mph, it must resist a sudden load that is four times as high.

• The BCSA recommends stopping all crane activity if wind gusts exceed 27 mph.
• Above 40 mph, the recommendation is to stop all outdoor construction, even activities that don’t involve a crane.

Keep in mind that strong gusts can occur even if the predicted wind speed is 10 mph, and a 20 mph forecast does not guarantee 40 mph gusts. Site-specific wind data is essential for efficient and safe project management:

• You can lose valuable time if you stop work in advance, based only on the assumption there will be a gust of wind.
• You are exposing your personnel and equipment if you assume there will be no wind gusts on a certain day, just because the average wind speed predicted is low.

The shape of structural elements being lifted should also be taken into account, since some geometries are more susceptible to the wind. Objects with a large area facing the wind can experience significant drag forces even with a mild breeze and at the same time create vortex of wind around its surroundings (turbulences)

WIND GUST & TURBULENCE MONITORING SYSTEM

The guidelines provided by BCSA can increase project site safety dramatically, but there is one important factor to consider – good decisions need accurate data, and also a reliable method to access it in real time.

Accurate wind speed monitoring is necessary for safe operation. Consider that an accurate wind speed monitoring system creates a historic wind speed data file for the site, which is useful when contractors need to justify delivery delays due to the weather.