Tri-Chord Systems cut the time required to evaluate alternate steel framing designs from 4 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 from Tri-Chord provide the most structurally sound, cost effective and easy to use alternatives to wood. The company's designs are based upon a roll formed 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 about 4 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.

Nearly all previous steel construction methods have used the C-channel beam. 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 of assembly because the fastener has been rigorously vibration and stress tested over the past 40 years by the automotive industry worldwide.

The triangles are fastened at intervals that typically range from 4 to 6 inches. Special crimping tools are used in the fabrication plant to crimp the beams and at the job site to assemble the trusses. Gusset plates are used to join nonparallel joints such as at the peak of the roof truss.

For wall framing, Tri-Chord has developed a load bearing stud that approximates the thermal value of dimension lumber and can be used with either steel or wood top and bottom plates. Furthermore, when used with steel plates walls are framed without any screws or nails whatsoever and when using the wood plates the stud is end nailed from the top and bottom. These framing methods allow the studs to be used with minimal training, no new tools and in the same time frame required with wood. A telescoping sleeve connection allows rake walls to be framed without having to cut a single stud.

A key advantage is that steel is considerably more regular than wood thus offering more consistent performance. Because of irregularities, wood trusses may sag or bow, making it necessary for a pickup crew to plane the roof or replace certain members. Steel on the other hand, provides well known properties that prevent such irregularities. Steel framing also eliminates the possibilities of infestation by insects.

Tri-Chord is a research and development company that is licensing the new design to fabrication shops that are setting up to produce the new framing design. The device will be test marketed in four Western states, California, Nevada, Arizona and Utah, beginning early in 1996. The first structure built with the new technology is an apartment building in Las Vegas which was completed in late 1995. Tri-Chord is enlisting wood truss fabricators, of which there are about 2000 in the United States, to fabricate the new frames.

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. It would be simple to use handbook formulas to calculate section properties for a perfect equilateral triangle. Unfortunately, the flange that is required to close the beam, moves the center of gravity and complicates the analysis to the point that hand calculation was virtually out of the question.

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 effective 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 to four days for each of the approximately 40 iterations which needed to be evaluated. The reason that this number of iterations was required 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, Computerized Application and Reference Systems (AISI/CARS), was developed by Desktop Engineering Int'l Inc., Woodcliff Lake, New Jersey in cooperation with the Auto/Steel Partnership, a consortium of 10 North American steel companies and Ford, Chrysler and General Motors.

The effective properties of the section are computed according to the element effective width calculation results. The element effective width calculations are based on the Cold-Formed Steel Design Manual and Automotive Steel Design Manual published by the American Iron & Steel Institute. The program includes a graphical preprocessor which allows the user to create and edit the cross section graphically using a keyboard, mouse or digitizing tablet. Among other uses, this tool makes it easy to calculate the properties of a complex section prior to finite element analysis thus greatly reducing the time required to perform the analysis.

In the AISI Cold-Formed Steel Design Manual, the effective width concept is used to account for the post-buckling strength of a thin-walled section. The post-buckling strength is the additional load carried by elements of the cross section, after they have buckled locally, by means of a redistribution of stress. Under the effective width concept, only certain portions of the plate width are considered to be effective in carrying loads after exceeding the local buckling stress and the nonuniformity of stress distribution in the element is accounted for by replacing the actual plate width by a reduced effective width such that the total area under the stress distribution curve is the same.

The following procedure is developed and implemented in Geometric Analysis of Sections (GAS) module of CARS to compute the effective cross section properties: 1) Compute the normal stress distribution assuming all the segments are fully effective. 2) Calculate the effective width of each segment. 3) Calculate the cross section properties of the section based on the effective portion of each segment. 4) Compute the stress distribution using the calculated effective section. 5) Calculate the effective width of each segment based on the revised stress distribution. 6) Calculate the cross section properties of the section based on the effective portion of each segment. 7) Repeat Steps 4 and 6 until the area and moments of inertia of the effective section converge.

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 taken 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 the 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 comes from the roll forming 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 a 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.