Page T1.2 . 29 September 2004                     
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    Texturing Granite

    continued

    But early on, the available architectural engineering software was found wanting. How do you manufacture a freeform shape from granite? How can you cut a stone structure accurately? How do you smoothly align slices of granite cut by the stonemason's computer-numerical-control (CNC) machines?

    To produce data files compatible with the CNC cutting machine's software, the design team needed to define the form in 3D space at every point along the fountain. Enter two automotive designers, Neale Williams and Mark Mason, from Surface Development & Engineering Ltd. (SDE).

    Mason explains that he and Williams are former Jaguar designers, so they are experienced in the highly refined surface modeling of flowing shapes. They use ICEM Surf, advanced modeling, analysis, and visualization software which offers more precision than that found in most architectural CAD applications. Unix and PC based, the software uses files so large they require a reasonably robust workstation network. Also unlike most architectural CAD, their system models surfaces, not solids.

    Fine-Tuning the Models

    When SDE was invited into the design team in February 2003, they realized that the digital models that had already been made could not provide the precision required for the stonecutter's software.

    Mason explains: "Due to proximity of the general public to the fountain, we realized that surface irregularities would be entirely unacceptable because people would be able to view and interact with the fountain from very close quarters."

    The files already developed by Arup consisted of the ground contours, 47 sections, and a surface model of the structure. This surface model had been created by making interpolating sweeps between the sections. Mason explains: "This produced a surface model that contained many irregularities that would spoil the flowing lines."

    But a key characteristic of the Diana Memorial was to be its freeform organic shape. Williams says that to ensure a smoothly flowing shape, they had to apply Bezier algorithms which can be controlled more effectively than nonuniform rational B-spline (NURBS) surfaces. NURBS algorithms, which are more common to CAD packages, create curves indirectly by manipulating control points. The file that SDE eventually created was very different from the engineer's original file.

    Texturing the Stone

    The next step was to digitally create the three-dimensionally textured surfaces that produce the desired variations in the fountain's water flow. The challenge was not only to visualize them as 3D objects but also to physically produce them.

    London-based Barron Gould, a specialist in the design of textured surfaces, came up with the solution. They customized a process that produced surface pattern files covering one third of the fountain's surface. Rather than rendered simulations of surface textures, these are true 3D surface variations. This texture file was merged with the block file to create a CAD model for machining.

    Once the all-encompassing smooth-surface model was complete, it was divided into 545 individually shaped stones. The individual virtual stones were produced taking into account foundation levels, aesthetics of joint lines, drainage locations, stonecutters' machining techniques and size limits, and the quarry's daily output capacity.

    "It was a cross fertilization of information from architects, engineers, and stonecutters," says Williams. "Not only was this to help the stonecutters but also to provide information to the quarry as to how much stone to produce on a daily basis." By adding engineering criteria to the surface model, the designers ensured economies of scale, reduced granite waste in the cutting process, and minimized subsequent job-site adjustments.

    Placing the Stones

    Finally, the virtual stone files were released to the stonecutters for milling on their CNC machines. Over a period of five months, the stones varying in width from 10 to 26 feet (3 to 8 meters) were cut, finished to 0.2-inch (5-millimeter) tolerances, delivered to the site, and slotted into their allocated positions.

    "We could not afford to have more than a 0.1-inch (2.5-millimeter) slip on any stone as this would completely close the 0.2-inch (5-millimeter) gap, which never happened. This validated the whole process," Williams says. Mason adds proudly: "Such was the accuracy of the 3D CAD model that instead of making the last block fit the gap, it was cut as per the file."

    Because of the successful development between the software developers and stonemasons, an alliance was set up in January 2002 to provide a "one-stop design shop" for architectural freeform surface design. Named Texxus, the alliance includes SDE, Barron Gould, and S. McConnell & Sons, the stonemasons.

    Texxus has built a start-to-finish 3D tool that is both creatively intuitive and seamless with the numerous engineering processes of real-world manufacture. Being fully digital, the working files can be used and viewed at any stage of the design process and communicated and reproduced as models in a range of materials, of any scale, for evaluation.

    As the Diana Memorial project has demonstrated, texturing technology can be applied to large-scale stonecutting very successfully. The outcome provides a glimpse into a future of truly freeform architectural design.

    Discuss this article in the Architecture Forum...

    Don Barker is a freelance writer and photographer in London, who has lived and worked in Europe, Australia, Thailand, Sri Lanka, Hong Kong, and Singapore. He is a contributing editor to ArchitectureWeek and writes for several periodicals in the United Kingdom.

     

    AW

    ArchWeek Image

    The Diana Memorial in Hyde Park, West London.
    Photo: SDE

    ArchWeek Image

    Idiosyncratic forms and textures were executed through a collaboration of designers, engineers, software developers, and computer-aided stonecutters.
    Image: SDE

    ArchWeek Image

    Cutting stone with computer-driven equipment.
    Photo: SDE

    ArchWeek Image

    Overall model of the fountain.
    Image: SDE

    ArchWeek Image

    The digital model for the Diana Memorial changes at every turn.
    Image: SDE

    ArchWeek Image

    Varying forms in the fountain create differences in the appearance of the flowing water.
    Image: SDE

    ArchWeek Image

    Samples of textures in the digital model.
    Image: SDE

     

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