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Covering all the angles

3D modelling - Strength assessment of a cast iron footbridge near Dundee is among several projects consultant Gifford is using to develop better processes for 3D modelling and structural analysis.

Benefits from advances in numerical structural analysis and geometry modelling are many, according to consultant Gifford's technical director Carl Brookes. The crucial factor is the wider use of accurate 3D CAD models.

Time needed to construct a finite element mesh, for example, can be reduced from hours to minutes. Once a 3D model is created, it can be reused and added to on different projects.

'Our plan is for various parts of a design and construction team to be able to push and pull data from the same model, ' says Brookes.

Gifford is using a number of projects to validate and put into practice advanced numerical methods for geometrical, structural and fluid modelling.

The building and use of the 3D CAD model is key to accuracy and efficiency, says Brookes.

Where an accurate 3D model of an existing structure is needed, laser scanning is proving to be very quick and useful.

Balgay Bridge near Dundee has been modelled by Gifford's engineering analysis team. The consultant's civils infrastructure group is assessing the strength of the cast iron structure for Dundee City Council.

The historic footbridge is currently closed because it is considered unsafe, but the council wants to restore it. The arched main span and two side spans cross a valley separating Balgay's cemetery and old observatory. Historic Scotland is backing the project, initially with funding for the structural assessment.

The bridge has been laser scanned from several positions by surveying specialist Ploughman Craven Associates to build an xyz point cloud of around 12M points. Apart from position information, 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 surface modelling software Rhinoceros was then used to construct the 3D CAD model. This involved wrapping 3D surfaces over the point cloud data to give a true and accurate impression of the bridge.

'One enefit of using true shape geometry in a strength assessment is that global construction tolerances are captured, ' says Brookes.

'This is important in buckling analysis. After importing 3D CAD geometry to our analysis software, in this case ANSYS, 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 generating the finite element model from the 3D CAD model. Choosing shell elements for the finite element model was also important.

This allowed the process to be streamlined even though the problem size grew from the more traditional, but labour intensive, line beam representation.' The time saving in building models from laser scanning is only theoretical at present, Brookes admits, as Gifford is currently just getting used to the technology.

But the potential 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 (CFD).

'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 (FSI) problems where, for example, wind turbulence causes movement in a structure which in turn alters the turbulence.

Phenomena such as galloping and utter, which can result in structural 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 are sceptical about CFD's accuracy, but Brookes 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 conrm the effectiveness of nal 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 so they can spot errors or potential buildability problems.

Perhaps the ultimate model so far, Brookes says, is Heathrow Terminal Five. A virtual model of the project design has been built up with construction phasing and every building component.

Eventually the model will form a vital part of the terminal's asset management system.

'Although 3D CAD models are becoming invaluable for communicating designs within teams and disciplines, more standard formatting of software is needed, ' Brookes says.

'We overcome most format problems by using Rhinoceros.

This 3D surface modelling software brings different formats together and pipes data to other modelling applications.

'It is supported by the D PDF le format, meaning 3D view-only models can be easily distributed to anybody with Acrobat Reader.

This type of universal viewing is expected to accelerate demand for 3D working.'

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