Monday, 24 August 2009

BullAnt takes on the WaterCube

So here's my take on to model the Watercube, a stunning stadium if you're ever lucky enough to see it in person.

Download the basic Rhino file from here.

It would have been brilliant to work on a structure such as this. The design team included Arup, PTW Architects, CSCEC and CCDI

There's reports on how the geometry was derived here and here that form the basis of my demonstration.

Thursday, 20 August 2009

Interpreting Structural Models with Curved and Tapered Members

The theory and formulation behind structural analysis algorithms and solvers are based on theory of straight elements with constant cross-section.

This means that models representing structural elements curved or tapered in nature are "meshed" or broken into straight segments with a representative cross section property. To my knowledge, any analysis program featuring "curved or tapered members" is simply automating this process behind the scenes.

This results in a side effect that when exporting the structural model to CAD format, that this approximation although fine for the structural assessment, results in jagged and problematic geometry when sharing the model with other designers/clients that don't appreciate this process. (Ie, remarks such as "I like that design but I'd like it without the steps"). For curved elements the segments result in underlap/overlap of the cross section, which is a weak spot if you wished to directly 3d print/prototype the geometry. To date this has generally needed manual remodelling of the structure.

Whilst I believe the better process is to model the curved/tapered structure in CAD and then derive your structural analysis model from this, it's possible to automatically interpret the segmented data (although if precise results are required, you should carefully check the interpretation).

The SSI Rhino plug-in will check each section name for "tagged" descriptions that instruct the plug-in how to "interpret" that cross section. This post looks at the -rhl tag that instructs the plug-in to loft the cross sections.

Download this GWA file representing the summit of the Eiffel tower Download.

Now to create the Mast, simply type -rhl in beam properties 20,21 and 22. The plugin will act on the loft instruction and "join" any sequential elements that have "the same property (or 1 section property either side." The plug-in will fit a curve through the the sequential element nodes, and then orient the cross section curve for each element on this curve at the point closest to the mid-point of the element.
If properties differ by 1 at the ends of the element, the cross-section curve is extrapolated at the start and end of the curve. By adding the "-rhl" tag to property 13, the curved support element for the mast will also be lofted.

Try importing the model into Rhino before and after making the changes, and note the difference. If you haven't had it work as per these images, try downloading this file and check what is different in the section properties. Download

Note that the default iso-curves shown for the nurbs surface are the cross-section shape determined location and orientation.

Monday, 17 August 2009

User Defined Profile Libraries

I've just uploaded a new version of the Structural Drawing Plug-in with plenty of improvements. I'm preparing to post a detailed tutorial showing how these new tools could be implemented to model the Beijing Water Cube, so watch this space.

The plug-in can now use profile libraries defined by the user. These are simply created by creating (or copying) rhino files into the plug-in sub-folder at C:\Documents and Settings\USER\Application Data\GeometryGym\UserProfiles\ or C:\Users\USER\Application Data\GeometryGym\UserProfiles\  (Note folder may be hidden, and is now reported on the Manage Tab of the main StructDrawRhino dialog). There's an example file there showing some numbers and letters as example profiles (you'll find them now listed in the selection drop down boxes). You can create as many collections of profiles as you like, and they will be grouped by filename (First Selection Box), layer name (Second Selection Box) and object name (Third selection box). The profiles must be located in the model YZ plane, and the origin is used as the default insertion point. A curve can be used to define a simple singular profile, or if it has voids, add it as a planar surface.

The files should allow easily to share and collate your profiles, let me know if you have any suggestions for improvements. Note that if you later rename either the file, layer or object, it will have to be reassigned. Another quick tip, the profiles are now assigned by dragging the first button over your model view (instead of clicking the button as per the previous release).