5 November 2008

The Water Cube design was based on a natural pattern of organic cells and the formation of soap bubbles

Arup and its design partners used Bentley MicroStation and Microsoft Visual Basic for Applications (VBA) scripts to produce a 3D model and drawings in the shortest possible time to compete in the Beijing Olympics venue design competition.

Securing the design contract for the National Swimming Centre for the Beijing 2008 Olympics was a task worthy of gold medal distinction. But with the help of MicroStation, which runs on Windows, and Bentley Structural software, Arup and architectural firm PTW of Sydney, Australia, together with China State Construction Engineering Corporation (CSCEC) from Beijing and Shenzhen, beat out ten qualifiers for this prestigious, world-class project. Arup’s ‘Water Cube’ design was based on a natural pattern of organic cells and the formation of soap bubbles. Contained within the Centre’s blue bubble walls are the pools for the Olympic swimming and diving competitions, along with seating for 17,000 spectators.

The project had two stages: competition and design. During the competition stage, a large portion of the design was created, but the main task was to develop a method that would enable Arup to produce a 3D model and drawings in the shortest time possible prior to the presentation. During the second stage, Arup incorporated the final features, calculations, and design that would complete the project.

The building structure is a 3D Vierendeel space frame 175 metres on each side and 35 metres high, based on a geometric cell made up of 12 pentagons and two hexagons, which is repeatable in 3D without leaving any empty spaces. Covering this frame are the translucent bubbles, or ETFE pillows.

Using Bentley Structural and MicroStation TriForma, Arup generated a 3D array of the cell, rotated it about two axes, and then sculpted the building. The cut surface planes of the remaining elements form the flanges of the composite structure, while the internal elements form the webs.

Stuart Bull, Arup senior 3D technician, describes the modelling and documentation process as a “daunting proposal”. Arup created a 3D centreline wire-frame and exported it to a structural analysis program for engineering. The analysed model was output to a text file containing geometric and structural member design data.

Next, Arup wrote a MicroStation VBA routine that used the text file to create a complete 3D model of the steel structure. By enabling MicroStation Development Language (MDL) functions, the model could be created as surfaces, solids, or structural elements as appropriate.

“The ability to use the VBA scripts to create our geometry, which gave us the link from the engineering and analysis model to our working 3D CAD model, was very important,” said Bull. With more than 22,000 beams and 12,000 nodes in the structure, the automation by the VBA routine saved Arup months of manual 3D modelling.

As part of the competition, Arup, PTW, and CSCEC also had to provide a physical model of its design. This was done by saving a copy of the 3D structural model as a stereolithography (STL) file to create a rapid prototype model. The physical model was created from a process called SLA, in which liquid epoxy resin is solidified by a laser following the STL file information to make a semitransparent plastic. This model was presented in the final competition stages.

Reflecting on what made the Beijing Swimming Centre project a success, Bull says: “Bentley Structural’s capabilities, such as automatic drawing extraction, dramatically reduced the time needed to produce 2D documentation. Since we didn’t have to worry about that part of our workload, we could focus on the 3D model. And using a MicroStation VBA routine to automatically model the structure saved us quite a bit of time.”

He continues: “Also, being able to save files in other formats let us quickly issue drawings to clients and consultants in the formats they needed. Yet we didn’t have to give up the enhanced capabilities that Bentley solutions offer us. That was quite important. If we had been using any other software package, it’s unlikely we could have produced such complicated geometry and documentation, and integrate with structural analysis, especially in the time frame available.”

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