tor Carl Brookes, who is head of Gifford's engineering analysis team, use of laser scanning is just one technology that is being used in efforts to increase use of 3D models for various building and bridge maintenance schemes. "This is the crucial breakthrough, the wider use of accurate 3D cad models," Brookes says. "The time involved to construct a finite element mesh can be reduced from hours to minutes with such a model, which can be used and added to by different dis-ciplines.

"This is our ultimate aim, for different parts of a project team to be able to push and pull data from the same model. The building of the 3D cad model is key to accuracy and efficiency and where an accurate model of an existing structure is needed, laser scanning is proving to be very quick and useful," says Brookes.

Gifford has used the services of survey specialist Plowman Craven. Its 3D team man-ager Duncan Lees says the biggest market for laser scanning is still the film industry; building electronic 3D models of sets and locations for digital effects. However, use of this technology in the infrastructure sector is 'hugely increasing', Lees says.

"We have done a whole series of bridges. People are specifying laser scan data for a variety of reasons, including for building more complete quality assurance informa-tion to demonstrate the quality of work for audit purposes." The deliverable can be an xyz point 'cloud' of data or fully-rendered drawings and elevations, says Lees. "The rail and tube networks are early adopters because the technology is very quick. Phase-based scanners can be moved to a new location every 10-12 minutes and large areas can be fully scanned in working hours."

The Balgay bridge was laser-scanned by Plowman Craven and modelled by the engi-neering analysis team for Gifford's civils infrastructure group, which is assessing the strength of the cast-iron structure for Dundee City Council. The historic footbridge is currently closed due to safety concerns, but the council wants to restore it. The arched main span and two side-spans cross a valley separating Balgay's cemetery from the old observatory. Historic Scotland is backing the project, initially with funding for the structural assessment.

The bridge was laser-scanned from several positions to build the point cloud of around 12 million points. Colour was also recorded which helped the team visualise the data as a 3D image using Pointools software.

"Results of the laser scan give comprehensive measurements including all surfaces and edges that could be seen from the scanning positions," says Brookes. "The sur-face modelling software Rhinoceros was then used to construct the 3D cad model. This involved wrapping 3D surfaces over the point cloud data by several construction methods to give a true and accurate shape of the bridge.

"One benefit of using true shape geometry in a strength assessment is that global con-struction tolerances are captured, which is particularly important for buckling analysis. After importing the 3D cad geometry to Ansys analysis software and producing a finite element mesh, running the analysis and post processing results was pretty straightforward. There is a time-saving element at each stage, particularly in generat-ing the FE model from the 3D cad model. Choosing shell elements for the FE model was also important. This allowed the process to be streamlined from the more tradi-tional and labour intensive line beam representation."

The time saving in building models from laser scanning is only theoretical at present, Brookes admits, as Gifford is only just getting used to the technology. But the poten-tial exists for analysts to save 'a lot of time' by working from a true 3D cad model, he says.

"We are also using 3D cad models in computational fluid dynamics. There are several classes of problem that we are interested in but modelling wind is our current priority. Models are imported into Ansys CFX and with additional boundaries, virtual wind tunnel tests are being developed to simulate air velocities and incident pressures in and around buildings," Brookes says.

"We are also looking at fluid/structure interaction problems where for example wind, through turbulent effects, causes movement in a structure which in turn alters the wind turbulence. Phenomena such as galloping and flutter, which can result in struc-tural instability, can be predicted in this way."

Gifford's work is concentrating on validating its CFD and FSI techniques against the results of wind tunnel tests. Some wind engineers remain sceptical about the accuracy of CFD, admits Brookes, but he believes these techniques have come of age.

"The advent of multi-processor 64 bit PCs has certainly helped. It is likely we will still undertake physical testing for critical structures, but this will be to confirm the effectiveness of final designs rather than for the design process."

Steel fabricators have been using 3D visualisation of designs for some time - effectively having all of a scheme's drawings in one place to allow errors or potential buildability problems to be spotted. Perhaps the ultimate model so far, Brookes says, is that of Heathrow Terminal Five in the UK. A complete virtual model of the project design has been built up, with construction phasing and every component of the buildings included. Eventually the model will form a vital part of the terminal's asset management system.

"Although 3D cad models are becoming invaluable for communication of designs within teams and disciplines, more common formatting of software is needed," Brookes says. "We overcome most format problems by using Rhinoceros 3D surface modelling software which brings different formats together and pipes data to other modelling applications. Rhinoceros is supported by the new 3D PDF file format, meaning 3D view-only models can be easily distributed to anybody with Acrobat Reader. This type of universal viewing is going to accelerate demand for 3D working."

Printer friendly version

Email this article to a friend