EARLY FINITE ELEMENT RESEARCH AT BERKELEY

1 EARLY FINITE ELEMENTRESEARCH AT BERKELEY1byRay W. Clough Nishkian Professor of Structural Engineering, EmeritusUniversity of California, BerkeleyandEdward L. WilsonT. Y. Lin Professor of Structural Engineering, EmeritusUniversity of California, BerkeleyABSTRACTS ignificant FINITE ELEMENT RESEARCH was conducted at the University of California atBerkeley during the period 1957 to 1970. The initial RESEARCH was a directextension of classical methods of structural analysis which previously had beenrestricted to one-dimensional elements . The majority of the RESEARCH conductedwas motivated by the need to solve practical problems in Aerospace, Mechanicaland Civil Engineering.

2 During this short period the FINITE ELEMENT method wasextended to the solution of linear and nonlinear problems associated with creep,incremental construction or excavation, crack closing, heat transfer, flow of waterin porous media, soil consolidation, dynamic response analysis and computerassisted learning of structural analysis . During the last six years of this period thefields of structural analysis and continuum mechanics were computer programs developed during this EARLY period at BERKELEY were freelydistributed worldwide allowing practicing engineers to solve many new problems instructural mechanics. Hence, the RESEARCH was rapidly transferred to theengineering profession.

3 In many cases the RESEARCH was used professionally priorto the publication of a formal paper. 1 Present at the Fifth National Conference on Computational Mechanics, Aug. 4-6, 19992 INTRODUCTIONP rior to 1952 structural analysis was restricted to elements connected to only twopoints in space. Structural engineers used the lattice analogy, as developed byHrennikoff [1] and McHenry[2], to model membrane and plate bending parts of thestructure. However, this analogy could not be applied to nonrectangular areas. RayClough first faced this problem in the summers of 1952 and 1953 after joining theBoeing Summer Faculty Program. During this period he worked with Jon Turner,head of the Structural Dynamics Unit, and was asked to calculate the bending andtorsional flexibility influence coefficients on low aspect wings.

4 Static experimentalresults had been obtained for the swept-back box wing structure shown in Figure 1and they did not agree with the results produced by a structural analysis modelusing one-dimensional elements only. This significant historical work has beendocumented in detail by Clough [3] where Turner is given principal credit forconceiving the procedure for the development of the constant strain LoadsTorsional LoadsTriangular ELEMENT Required To Model WingFigure 1. Swept-back Box Wing Test StructureTurner presented the Boeing pioneering work at the January 1954 meeting of theInstitute of Aeronautical Sciences in New York. However, the paper was notpublished until September 1956 [7].

5 In addition to the constant strain triangularmembrane ELEMENT , a rectangular membrane ELEMENT , based on equilibrium stresspatterns, was presented which avoided shear locking. The node equilibrium3equations were formed by the direct stiffness method. The purpose of the two-dimensional ELEMENT development at Boeing was to accurately model the dynamicstiffness properties and displacements of the structure, but was not proposed as ageneral solution method for stress analysis of continuous 1956 and 1957 Clough was on sabbatical leave in Trondheim, Norway. Duringthis period he had time to reflect on his work at Boeing and to study the newdevelopments in the field.

6 The comprehensive series of papers by Argyris andKelsey, published in Aircraft Engineering between October 1954 and May 1955,unified many different approximate methods for the solution of both continuous andone-dimensional frame structures [6]. By using matrix transformation methods itwas clearly shown that most structural analysis methods could be categorized aseither a force or a displacement method. It was in Norway where Clough concluded that two-dimensional elements ,connected to more than two nodes, could be used to solve problems in continuummechanics. For the Turner triangular ELEMENT the stress strain relationship withinthe ELEMENT , displacement compatibility between adjacent elements , and forceequilibrium on an integral basis at a FINITE number of node points within thestructure were satisfied.

7 It was apparent that the satisfaction of these threefundamental equations proved convergence to the exact elasticity solution as themesh was refined. This discrete ELEMENT idealization was a different approach to the solution ofcontinuum mechanics problems; hence, Clough coined the terminology finiteelement method. Therefore, analysis models for both continuous structures andframe structures were modeled as a system of elements interconnected at joints ornodes as indicated in Figure 2. Other researchers in structural analysis may haverealized the potential of solving problems in continuum mechanics by usingdiscrete elements ; however, they were all using the direct stiffness terminology atthat Actual FrameFinite ELEMENT Model FINITE ELEMENT ModelActual DamFigure 2.

8 The FINITE ELEMENT IdealizationIt should be pointed out that during the nineteen sixties there were many differentresearch activities being pursued at BERKELEY . First, it was the height of the ColdWar and the Defense Department was studying the cost and ability to reinforcebuildings and underground structures to withstand nuclear blasts. Second, a verysignificant program on Earthquake Engineering RESEARCH , including theconstruction of the world s largest shaking table, was initiated by ProfessorsBouwkamp, Clough, Penzien and Seed. Third, the Federal Government and theCalifornia Department of Transportation were rapidly expanding the freewaysystem in the state and were sponsoring RESEARCH at BERKELEY , led by ProfessorsScordelis and Monismith, concerning the behavior of bridges and overpassstructures.

9 Fourth, the manned space program was a national priority and ProfessorsPister, Penzien, Popov, Sackman, Taylor and Wilson were very active conductingresearch related to these activities. Fifth, the offshore drilling for oil in deep waterand the construction of the Alaska pipeline required new technology for steelstructures, which was developed by Professors Popov, Bouwkamp and , the construction of nuclear reactors and cooling towers required thedevelopment of new methods of analysis and new materials. Also, ProfessorsPopov, Scordelis and Lin were consultants on the design and construction of many5significant long-span shell structures. To support this RESEARCH and development anew Structural Engineering Building, Davis Hall, was built on the BerkeleyCampus and a new shaking table, to simulate earthquake motions, was constructedat the Richmond Field Station.

10 The FINITE ELEMENT Method was an analysis toolthat complemented all of these analytical and experimental RESEARCH YEARS 1957 TO 1960 After Clough returned from sabbatical leave in Norway in 1957 he initiated a newstructural analysis RESEARCH program at BERKELEY . He applied for, and received, asmall NSF grant to support RESEARCH on computer analysis of structures. Inaddition, he initiated a new graduate course entitled Matrix analysis of the Fall semester of 1957 he listed several possible graduate studentresearch areas. This list contained RESEARCH topics on the FINITE ELEMENT Analysisof Plane Stress Structures, FINITE ELEMENT analysis of Plates, and FINITE ElementAnalysis of IBM 701 digital computer, with 4k of 16 bit memory, had been installed in theCollege of Engineering the previous year.