When working at height, weather conditions can be more severe than on the ground. Stephen Cousins reports on how new Met Office technology is helping contractors including the JV building the new Forth crossing.
Photo: David Holt
London is on the brink of what has been referred to as a “tsunami of towers”, with an updated survey commissioned by New London Architecture discovering that there are 263 buildings over 20 storeys either in construction, with planning permission or awaiting decisions. Among these are 25 at 50 storeys or above.
But building higher means entering a realm of more extreme and hard to predict meteorological conditions, which threaten to disrupt construction work, add to cost and programme, and potentially put workers’ lives in danger. The trend for building tall also puts extra demands on rope access workers, highly trained specialists who can take their skills to other sectors.
The term skyscraper is commonly applied to buildings above 150m, and any construction professional who’s worked on one will know that wind is the most obvious weather factor to consider above 100m. Although a range of factors are likely to affect the speed of wind, as a rule of thumb, meteorologists estimate that at a height of 100m on an average day, sustained wind speed is about one and a third times its speed at a ground level of up to 10m. And at 200m, it rises to roughly double that on the ground.
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.
Height safety fact file
- According to the HSE, working at height remains the biggest cause of fatalities and major injuries in construction. In its most recent report, for 2013/14, there were 19 fatalities caused by falling from height – 45% of all deaths in construction. A fall from height was also responsible for 31% of the major/specified injuries in construction, with 581 cases reported.
- Working at height in the UK is regulated by the Health and Safety at Work etc Act 1974 (HSW Act), which places general duties on what is reasonably practicable on a site. This is complemented by the Work at Height Regulations 2005, specifying duties on employers, the self-employed and any person that controls the work of others.
- In the past five years 15.5% of prohibition and improvement notices issued by the HSE in the construction industry were the result of breaches to the Work at Height Regulations, according to a search on its online database. Over the past five years there were 47,191 notices in total; 7,289 related to working at height.
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. 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, for working heights of up to 50-60m, and specific forecasts at any height or range of heights. The forecast and alert service VisualEyes provides a site-specific forecast with coloured warnings to signal when customer-specified thresholds for temperature, precipitation, winds at height, lightning or visibility are breached, so that customers are aware of risks as soon as they are forecast.
London’s skyline will be transformed by the 263 towers that now have planning permission. Below: The Met Office VisualEyes service can offer rainfall radar and customised alerts
“Customers define which conditions they are comfortable working in and which risks they want to keep an eye on, then the forecast provides a red, amber or green signal depending on whether those conditions are likely to be breached,” says Faragher. “For example, a 70% likelihood of a certain wind speed can give an amber alert, but a 90% chance can signal red.”
He adds: “Receiving a forecast designed for your specific needs, rather than just a general forecast, you get more nuanced information that will enable contractors to make much more effective decisions. Conditions like thunderstorms can emerge very quickly in a forecast, forcing you to make rapid changes to workloads, taking people out of harm’s way to minimise safety risks.”
The Met Office UK forecast model updates every three hours. However, a five-day forecast each morning is sufficient for most projects, says Faragher.
Worldwide, we’re experiencing rising temperatures linked to manmade activity (according to the Intergovernmental Panel on Climate Change) which many scientists believe is implicated in the frequency and severity of extreme weather events. So should UK contractors be factoring in risks associated with climate change?
Not necessarily, says Faragher, as it’s still impossible to determine what’s driven by climate change, and what is ever-variable “British weather syndrome”. “We are in a changing climate that has potentially impacted on the frequency and severity of extreme weather events, but at the moment the science is not complete enough to determine to what extent. The problem is that the natural variability of the climate makes longer term changes difficult to detect. There is a lot of research going on now to determine how extreme weather has been impacted by changes to the climate.”
But over the next decade, as the construction sector builds out the pipeline of 263 tall buildings proposed for the capital, it would be foolhardy to predict that our changing climate won’t impact on typical windspeeds, heavy rains and storm events. Whatever the nuances of the climate debate, contractors will increasingly need to model the weather as a variable in plans and programmes.
Forth workers are prepared for the haar
A range of Met Office forecasting packages are being used by the Forth Crossing Bridge Constructors consortium (of Dragados, Hochtief, American Bridge International and Morrison Construction) during the construction of the Queensferry Crossing, a new 1.7 mile-long cable-stayed road bridge across the River Forth in Scotland, which started on site in 2011 and is due to complete in 2016.
The Forth River flows into the cold North Sea, which has a considerable effect on local weather through the year. The area is subject to prevailing winds from the west (the influence of the Atlantic) but from April to September poor visibility caused by a fog from the North Sea, known locally as “haar”, can occur despite fine weather just a short distance away.
The project management team receives a regular five-day site-specific forecast and also uses the web-based planning tool Weather Windows to plan tasks up to 15 days ahead. This helps mitigate any likely weather impacts and identify periods when large and expensive equipment is hired.
The Weather Windows information is user-defined and specific to every job; its graphical display uses simple colour-coding to show the opportunities to carry out tasks, and an alert system highlights the degree of risk. The team also receives bridge wind speed, direction and maximum gust forecasts for elevations of 50, 100, and 200m when teams are working at height.
Andrew Price, marine liaison manager for the FCBC consortium, who relays forecasts to the construction teams, told CM: “The bottom line for this is scheduling work. We need to know if we can support and facilitate an operation prior to it commencing. This is a particular concern when planning the transportation of concrete used to build the bridge towers, which has a cure time from production to when it is pumped from barges on the river into the formwork. We need to know the wind speed is going to be below 35 knots, above which barge operations must be grounded, to give the concrete manufacturers a go/no-go to go into production.
“During November and December we had days when nothing could go out on the water, so the Met Office service proved critical in predicting that. It will also prove vital during later stages when road deck sections have to be lifted from the barges up to a height of 65m [the current height of the bridge] to be laid into position.”
