Feature Working at height
> Short bursts of fast-moving air, or gusts,
can have significantly greater speeds, and can be impacted by surrounding buildings. For example, a phenomenon known as “canyoning” occurs when wind is channelled horizontally between two buildings, speeding it up. Strong updrafts occur when the wind runs against a building facade and upwards, potentially impacting on the stability of maintenance cradles or working platforms. By the same function, downdrafts can run down a building, blowing debris or construction materials around at street level. Wind forces exerted on a tower crane and its suspended load can be quite large and affect safe handling. It is not always understood that these forces are due to wind pressure, not wind speed, which varies as the square of the wind speed. So if the wind speed doubles, the wind pressure increases by a factor of four. “Even with large light loads, such as shuttering, a situation may occur some way below the tower crane’s design wind speed,” says David Dursley, principal safety adviser at the Building Safety Group. “For example, at a wind speed of 14m/s (31mph) the wind load on an 8ft x 4ft sheet of ply will be 38kg. If the wind speed increases to 20m/s (45mph) the wind load will rise to 76kg. So it is important that all lifts are planned and not started in rising winds.” At the top and side edges of buildings
the Venturi effect can occur where, similar to water flowing through a narrowing channel, air is compressed and wind speed increases as it blows up and over, or around a building’s sides.“If someone is working on the facade, this effect may not be noticeable until they reach the edges or top,” says John Faragher, construction business manager at the Met Office, which offers a range of forecasting services for the sector. “Another phenomenon is vortex shedding, where as air moves extremely fast over the top of a building, a small amount of calm air is created, followed by very turbulent air behind it, similar to turbulence along back edge of a plane wing. If working on leeward side of building at top you might encounter this.” Above 100m, visibility can be reduced
as a result of fog or mist, particularly in cities at high altitudes, and at 200m it is possible you will be above the cloud base.
36 | APRIL 2015 | CONSTRUCTION MANAGER
”Even with large light loads, such as shutters, a situation may occur some way below the tower crane’s design wind speed” David Dursley, Building Safety Group
When mist is noticeable on the ground, it tends to be much denser at height. Lightning is another significant risk, particularly on buildings taller than the surrounding structures. This is highlighted by the Met Office as a particular danger in the renewables industry, where workers on wind turbines must down tools during any period when lightning is forecast. Wind turbines are also subject to a phenomenon known as shedding, where movement in high humidity or freezing fog builds up a layer of ice which can be thrown off, causing a risk to those close by.
Bespoke site-specific forecasts When planning the construction of high- rise buildings, many contractors choose a bespoke weather forecasting service to
provide wind speed, visibility or other data for sites at specific heights and times. These help projects keep to schedule by averting the need to call off materials deliveries or heavy plant hire at short notice, while reducing the possibility of accidents and helping site managers comply with health and safety regulations. The Met Office offers a range of
forecasting services that cover the entire lifecycle of a project. During the pre- construction phase it can supply data on weather conditions expected, predictions of days lost due to severe weather, and any considerations contractors should be aware of related to working at height. Services for the operational phase include a tower crane wind forecast,
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