By Curt Kinney

Tri-Chord Systems has cut the time required to evaluate alternate steel framing designs from four days to 15 minutes with a software package that automates the process of determining effective properties of sections with arbitrary shapes. New light-gauge steel designs provide the most structurally sound, cost effective and easy to use alternatives to wood.

The company's designs are based upon a rollformed equilateral triangle shape that provides standard setting strength-to-weight ratios. Optimizing this design required evaluating 40 alternate materials and geometries in order to provide the necessary physical properties at minimum cost. Building and testing models would have taken four days per iteration. A computer software program that uses an iterative procedure to compute the stress distribution and corresponding effective properties provided accurate answers in just 15 minutes.

The new design includes a flange that provides a closed section which provides considerably higher strength than the C-channel approach. It is based on a press joint invented in 1895 that is used by several leading automobile manufacturers to replace threaded joints and friction welds.

By using automotive-type press joining to fabricate roof and floor trusses on fully automated assembly fixtures, the new design advances steel framing to the same production efficiency as wood. With press joining instead of screws, fabricators are assured of consistent quality assembly because the fastener has been rigorously vibration and stress tested over the past 40 years by the automotive industry worldwide.

Implementing an entirely new section required a lengthy optimization procedure and the generation of a considerable amount of engineering detail to support the new approach. Effective section properties take into account the fact that post-buckling stress will render a certain portion of the cross section ineffectual.

The difficulty in calculating effect section properties arises from a classic "chicken-and-egg" scenario. For example, the moments of inertia are needed to calculate the stress conditions under moments to determine the effective width of each segment, while the effective width of each segment is needed to determine the moments of inertia. This means that an extremely tedious and time-consuming series of calculations are required to iterate to a solution despite the fact that the equations involved are straight-forward.

This left the alternative of building physical models and testing them, which would have taken three or four days for each of the approximately 40 iterations which needed to be evaluated. The reason for this number of iterations was that Tri-Chord had to develop its frames in a large number of sizes to meet the full range of builders' requirements.

Before this process was started, a Tri-Chord engineer saw an ad for a computerized version of the American Iron & Steel Institute's Automotive Steel Design Manual. The program, Computer Application and Reference Systems (AISI/CARS), was developed by Desktop Engineering International, Woodcliff Lake, NJ in cooperation with the Auto/Steel Partnership, a consortium of 10 North American steel companies and Ford, Chrysler and General Motors.

Tri-Chord engineers evaluated a range of flange heights and materials for each size of wall stud and truss. The ability to provide section properties within 15 minutes for each iteration made it possible to accomplish in hours what would otherwise have take months and delayed the introduction of the product. All that is normally required is picking the material from the program's archives, filling in flange dimensions and starting the analysis.

The program generates complete section properties including stress and deflection of the loaded member. Once the design was optimized on the computer, tests were run and the results matched the analysis within a small margin of error. This error was caused by the fact that he crimping process does not provide a perfectly closed section. A wood engineering program was used later in the process to take the effects of crimping into account.

Tri-Chord Engineers were also able to incorporate the strength improvements that come from the rollforming process in the analysis. CARS also made it possible to optimize the radii of the corners of the triangle which the analysis showed had a big impact on the strength of the section.

Generally, the program showed that the best approach was to use a relatively high grade of material with a relatively thin section. The ability of the program to optimize the material section made it possible to use ASTM A653 Grade 50 in some applications at considerable savings. The section properties generated by the software were also used as input to a structural analysis program that was used to analyze complete structures.

For More Information Contact:

Desktop Engineering Int'l Inc.
172 Broadway
Woodcliff Lake, NJ 07677
USA
Tel: 201-505-9200
FAX: 201-505-1566
info@deiusa.com