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Cast iron evidence

Surveying the world's first iron bridge was impossible without the latest CAD developments. Alan Sparks reports.

Iron Bridge is the engineering jewel in the crown of English Heritage (EH). Spanning the River Severn at Coalbrookdale in Shropshire, the Ironbridge was the first industrial-scale iron structure when built in 1779.

EH's job is to ensure the preservation of 450 sites, like Ironbridge, around the country.

These range from Stonehenge through to the recently restored Osborne House on the Isle of Wight. Extensive knowledge about each structure and its particular construction sequence is fundamental.

Ironbridge was assembled from nearly 1,700 cast iron components, the largest weighing in at 5.5t. These were cast, positioned and fitted under the direction of master iron founder, Abraham Darby III and his pattern maker, Thomas Gregory.

However, until recently, little was known about how the bridge was designed, how the parts were cast and how it was constructed. Then, three years ago, out of the blue an Elias Martin watercolour sketch was discovered in a private collection. Although there are plenty of early views of the bridge by various artists, this one showed it actually under construction.

The finding spawned a chain of events including a BBC Timewatch documentary programme.

The culmination was the creation of a half-size physical model that reproduced the construction method illustrated in the watercolour and tested its credibility as an engineering solution.

But first, English Heritage had to 'reverse engineer' the bridge, creating drawings and assembly details so that the model could be cast and assembled in exactly the same way, as the original was 223 years ago. 'The bridge is old, cracked, and appears to be distorted. Before we could carry out any work, we had to fully understand the structure, how it was made and how it was assembled.

The problem was, we had no consistent 3D information to help us, ' says English Heritage metric survey team manager Bill Blake.

Even with paintings and sketches to work from, the task of creating actual working drawings is long, arduous, error prone and expensive.

Three techniques are generally available to generate the required information. Photo- grammetry is the primary source of 3D CADready data for large-scale structures. Measured drawing from photographs using pencil and paper produces details of small, complex and easily accessible items such as sills and mouldings.

In recent years, electromagnetic distance measurement (EDM) with tripod mounted equipment and prisms has been used to produce building plans.

But this needs two people, and is too cumbersome for restricted access sites. Readings could not be checked on site and the measurement had to be plotted manually in the drawing office to produce useable drawings.

Even with computer aided design, the process remained lengthy and expensive. The metric survey team needed a method that was quick, accurate and used just one person.

All three needs were answered by the arrival of a new generation of Reflectorless EDM equipment such as the Leica Disto GSI, says Blake. Accurate measurements could now be made right into vertices and along complex curved intersections - locations where a prism just would not fit.

Still though, there was no realtime on-site verification, just a list of co-ordinates. The surveyor had no confirmation of the survey's quality until it was plotted, dot to dot in the office. A data capture application, rather than design application was called for. This had to be small enough and simple enough for use by non-surveyors. It also had to produce files in the industrystandard AutoCAD DWG format, to interface directly with the Reflectorless EDM equipment Software manufacturer, LatimerCAD, studied the surveying process in consultation with EH. The result was TheoLT, the world's first real-time CAD capture platform for Reflectorless EDM.

TheoLT takes the measurements from the survey process and automatically creates plans, elevations and isometrics so that any changes and additional measurements can be carried out on the spot.

'At last we are able to develop effective 3D survey strategies.

And because we had accurate 3D data capture, we were able to create proper working drawings in real time, on site, so ensuring that everything was satisfactory before leaving, ' he says.

EH worked with Plowman and Craven Associates to produce a high quality 3D wire frame model of the bridge using photogrammetry. Fine detail was provided by EH's archaeological team using traditional drawing methods and Reflectorless EDM. TheoLT was used to finish the job.

INFOPLUS

www. autodesk.co.uk www. ironbridge. org. uk www. latimer-cad. com

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