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13 Computer-Aided Design

13.1 GENERAL REMARKS

During the past 35 years, computers have been widely used in research, struc-tural analysis, and design. There is no exception for cold-formed steel struc-

tures.Because the research work on cold-formed steel structures usually involves

studies of the structural behavior and instability of plate components, indi-vidual members, and/or the entire assembly, hand calculations are excessivelylengthy and extremely difficult. Computers have been used to great advantagein obtaining solutions for complicated problems involving these structuresunder various boundary and loading conditions.

As discussed in the preceding chapters, the formulas used for the design

of cold-formed steel structural members are quite complicated, particularlyfor those members having unusual cross sections. It may be found that eventhe determination of sectional properties requires burdensome calculations,which may involve the use of successive approximations. For this reason,various institutions and companies have used computers to develop the datanecessary for the preparation of design tables and charts.

In addition, computers have also been used for the development and designof industrialized buildings, minimum-weight design of structural members,

minimum-cost design of structural systems, and special structures.Article 13.2 contains a brief review of some computer programs used for

the analysis and design of cold-formed steel members and structures in thepast.

13.2 COMPUTER PROGRAMS FOR THE DESIGN OF

COLD-FORMED STEEL STRUCTURES

13.2.1 Sectional Properties

Computer programs have been used extensively for the preparation of thedesign tables included in Ref. 1.159 and in many of manufacturerspublications. The equations needed for the calculation of sectional propertiesof angles, channels, hat sections, I-sections, T-sections, and Z-sections are

summarized in Part I of Ref. 1.159.

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In some cases the calculation of the sectional properties of various typesof structural members has been included as a subroutine in complete computerprograms for the analysis and design of cold-formed steel structures.13.1

13.2.2 Optimum Design

The minimum-weight design of cold-formed steel members has been studiedby Seaburg and Salmon1.247 on the basis of the AISI Specification.1.4 It hasbeen found that the gradient search method requires less time than the directsearch method. This optimization technique has been illustrated for the se-lection of the most favorable cross section of the hat-shaped roof deck.

In 1971 a computer program, DOLGAS, was developed by Klippstein for

the design of steel trusses fabricated from cold-formed steel members.13.1 Itprovides for the selection of minimum-weight members (channels, Z-sections,hat sections, sigma sections, or tubular sections) to meet the requirements ofthe AISI Specification.1.4 This program uses STRESS to compute memberforces and then check the design of members on the basis of the AISI designcriteria. The output includes structural design information and the necessarydata for fabrication.

In some cases, emphasis has changed to the minimum-cost design to con-

sider the costs of material, fabrication, and erection. For example, the mini-mum-cost design of composite floor systems using cold-formed steel deckinghas been conducted by Nicholls and Merovich.13.2 In the design, the gridsearch procedure has been used. This study includes the costs of cold-formedsteel decking, concrete slab, rolled beams, shoring of decking as necessary,temperature mesh in slab, and the necessary fireproofing. In addition, theoptimization of cold-formed steel shapes is presented by Douty in Ref. 13.3.

13.2.3 Special Structures

In Chap. 9 it was mentioned that the accurate analysis and design of specialstructures subjected to unsymmetrical loading and nonuniform support con-ditions can be achieved by using computers. For example, the analysis andthe design of the worlds largest cold-formed steel primary structure1.82 havebeen completed by using the finite-element method.13.4 The hypar module asshown in Fig. 9.24 has been analyzed under four different loading conditions.

Reference 13.4 indicates that the computer program specially developed byTezcan, Agrawal, and Kostro can be used to determine the deflections andstresses of any general multiwing hyperbolic paraboloid shell with any typeof support and beam arrangement under any arbitrary load condition.

13.2.4 Industrialized Buildings

The conventional optimum design of industrialized buildings is usually ac-

complished by repeated analysis, modification, and reanalysis of redefinedstructural systems. The use of computers increases the design flexibility of

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the building system and results in a minimum-weight or a minimum-costdesign.13.5 Computers may also be used for other operations concerning man-ufacturing, distribution, and administration work.

13.2.5 Decision Table and Flow Charts

A decision table is a concise tabular display of the logical conditions and theappropriate actions to be taken as the result of these conditions. It is usefulin preparing computer programs which utilize the specification provisions andin identifying all combinations of factors not specifically covered by the spec-ification.

The decision table formulation of the AISI Specification for the design of

cold-formed steel structural members has been originally developed by Sea-burg and modified to conform to the 1986 edition of the specification byMidgleyClauer Associates (Ref. 13.7). The table can be converted directlyinto programming language.

The 1986 edition of the AISI Cold-Formed Steel Design Manual containeda series of flow charts. These charts proved to be excellent means of helpingthe user to understand the design provisions and to provide a clear picture ofthe items that need to be considered in design. They were very useful guide-

lines for computer programmers.

13.2.6 Computer Programs

As computer technology advances, the use of computers for the design ofcold-formed steel structures is unlimited, as demonstrated by Zuehlke (Ref.13.8) and others.

In 1991, the American Iron and Steel Institute published a document,Cold-

Formed Steel Design Computer Programs, which contained descriptions of12 computer programs using the AISI Specification. This document was up-dated by the Center for Cold-Formed Steel Structures in 1993 and 1996 toinclude 30 programs from the United States, Canada, Australia, and SouthAfrica.13.9 Most of the computer programs are now (1999) available on theCenters website.13.10

Recently, Helen Chen developed a set of computer programs on the basisof the 1996 edition of the AISI Specification. The example problems are

dealing with (a) beam lateral buckling strength, (b) shear strength of thestiffened web, (c) web crippling strength, (d) compression members, (e) cy-lindrical tubes, (f) brace force for C- and Z-sections, (g) welded connections,(h) bolted connections, and (i) screw connections.13.11 These programs can befound on the AISI website at www.steel.org.

13.2.7 Numerical Solutions for Cold-Formed Steel Design

In Chap. 3, the effective width design approach of individual elements wasdiscussed on the basis of the current AISI Specification. It can be seen from

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Eq. (3.39) that the reduction factor used for calculating the effective widthof a given element is a function of the critical local buckling stress

cr, and

the maximum compressive stress. Instead of using the AISI equation to de-termine the

cr value for an individual element, Schafer and Pekoz indicate

that the critical elastic buckling stress of an element or a member can bedetermined by numerical solutions using (a) finite element method such asABAQUS, ANSYS, and STAGS or (b) finite strip method such as THIN-WALL13.9 and CUFSM.13.1213.14 The program CUFSM was developed bySchafer and can be obtained from http://www.cee.cornell.edu/schafer.13.15

The numerical solution can provide a proper handling ofelement interactionfor general cross sections. It can be used to investigate various possible buck-ling modes for a given structural member and to determine the lowest buck-

ling stress for design purpose. The advantages of such an approach areincreased accuracy and flexibility. References 13.13 and 13.14 discuss theapplication of the proposed Direct Strength Method for the design of cold-formed steel structural members.