Feature Working at height

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for working heights of up to 50-60m, and specifi c forecasts at any height or range of heights. The forecast and alert service VisualEyes provides a site- specifi c forecast with coloured warnings to signal when customer-specifi ed 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. “Customers defi ne which conditions

“Customers defi ne which risk they want to keep an eye on and the forecast provides a red, amber or green signal ” John Faragher, Met Offi ce

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 specifi c 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 Offi ce UK forecast model

updates every three hours. However, a fi ve-day forecast each morning is suffi cient for most projects, says Faragher.

Calculating climate change 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 diffi cult to detect. There is a lot of research going on now to determine how extreme weather has been impacted by changes to the climate.“

> 38 | APRIL 2015 | CONSTRUCTION MANAGER

Forth workers are prepared for the haar

A range of Met Offi ce 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 1.7mile-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 fl ows 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 infl uence 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 fi ne weather just a short distance away. The project management team

receives a regular fi ve-day site-specifi c 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-defi ned and specifi c 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 35knots, 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 Offi ce 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.”

IMAGE: DAVID HOLT

IMAGES: TRANSPORT SCOTLAND