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Tension revisited –2: IDN

News | September 1, 2008 | By:

An ongoing review of tension structure software.

NDN is a new software package that has been developed specifically for the tensile membrane engineering industry. It combines model building, form finding, analysis, member sizing and patterning into one program. It has video game style graphics and a friendly user interface. I had the pleasure to test early versions of the software while it was under development. At FTL Design Engineering Studio, we now use the completed package on a daily basis.

It is not an exaggeration to say that — at last — the industry has a software package that provides what it needs. The program provides engineers with everything they need for the design and optimization of membrane, cable and steel structural systems. It also does many things that people only dreamed about in the past. As an example, it can automatically load the structure, does the static analysis at the touch of a button, and automatically sizes the members. In addition, it can actually automatically pattern the structure based on a given fabric width.

To simplify operation for the user, NDN is organized into several different sections called “Environments.” Each Environment contains the functions required during a particular phase of the design process: model building, form finding, analysis, and so on. The Environments appear graphically as a “tab” on the display. A few of these Environments are described below:

Model: The Model Environment contains everything that is required to build, edit, and manipulate models. Figure 1 shows an image of FTL’s Arizona State University “Sky Song” model. This unique PTFE membrane project was recently designed by our office and is being built by FabriTec Structures of Dallas, Texas.

Shape: The Shape Generation Environment performs form finding of the membrane system. It operates on either the membrane components alone, or on the full system, as directed by the user. It can be used to produce shapes with either defined stress fields or defined force densities. All element types — including cables, struts, and beams — can be actively and simultaneously used during the form-finding process.

Load: This Environment is used to generate the data for externally applied loads such as wind, snow and live load. It contains time-saving commands and algorithms for automatic loading of the models.

Analysis: The Analysis Environment performs the large deflection FEM analysis based on the Stiffness Method. The internal solver is an optimized sparse matrix solver. It is not limited by bandwidth and uses minimum memory. It runs very fast, even with models having thousands of nodes and elements.

Post Process: The Post Process Environment allows the user to examine and graphically display analysis results, such as deflections, stresses, member forces and moments. It currently performs automatic steel and cable member sizing using either British, Australian, or U.S. design codes. It generates output and calculation reports to several different levels of detail. Figure 2 illustrates an example of some of the graphical displays available in the Post Processor.

Pattern: The Pattern Environment allows patterns to be defined individually by the user or generated automatically based on an available fabric width. DXF files are generated yielding the cutting patterns as an “exploded assembly” complete with marking plans. Pattern edges are splined and seam allowances (adds and subtracts) are automatically included. Figure 3 shows an image during use of the “Pattern Review” command.

As mentioned earlier, FTL had the opportunity to review and suggest improvements to NDN during its development. We now use the software to assist us in the engineering of our tensile membrane projects on a daily basis. We believe that it is superior and more comprehensive than anything else available. We also believe that it will become the industry standard in the near future. I believe much of its strength comes from the fact that the software author is not only the developer but also an experienced tensile membrane engineer. The author makes sure that users always have direct contact and input on the continued development.

Joe Schedlbauer is a licensed professional engineer with more than 20 years’ experience in the tensile membrane industry. He can be reached at FTL Design Engineering Studio, New York City, where he practices as the engineer principal.

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