Mete A. Sozen
Kettelhut Distinguished Professor of Structural Engineering
Purdue University

A Way of Thinking


Structural engineering is a profession.  A profession is characterized by the need to exercise judgment within a domain defined by a set of canons.  The structural-engineering canons have developed sometimes as a result of and sometimes despite the interaction between science and experience.  That structures were built successfully without science and, worse yet, even with the wrong science for hundreds of years and that science has, so far, been insufficient to guarantee predictability (ecce engineering for earthquake response), emphasize the challenge in discriminating between the poles:  “Science is all!"  and  “Experience is all!"  The territory in between is vast and slippery. It is difficult to navigate a true course without giving in to the lure of one pole or the other.  Today, ready access to versatile and powerful software enables the engineer to do more and think less. It is not often questioned whether the exact analysis of the approximate model qualifies as an approximate analysis of the structure itself. To contemplate once again the role of analysis in design is not a waste of time. 

A way of thinking about structural design was developed in a series of iterations in mid 20th century by three engineers. Their goals were not global. The whole was made up of specific solutions for specific problems. But when their contributions are viewed together, a complete way of thinking about structural design becomes discernible. The goal of this talk is to encourage engineers to review the works of Harald M. Westergaard, Hardy Cross, and Nathan M. Newmark as a whole not for the specific processes but for the general principles of their art of thinking about structural design.

Of the three, Harald M. Westergaard, was the immaculate and painstaking scholar with a penchant for simple and direct expression.  His vision of the relationship between theory and design is captured by his statement “.. a simple device can yield perhaps 80 percent of the truth whereas the next 10 percent would be difficult to obtain and the last 10 percent impossible…."

Hardy Cross was the creative genius more interested in the engineering artifact than its analysis.  Though intensely interested and eminently successful in the development of analytical methods for structures, he kept his students’ eyes on the prize by repeating that a building is to build and not to analyze. Perhaps he said it all about the role of analysis in design when he wrote  “All analyses are based on some assumptions which are not quite in accordance with the facts.  From this, however, it does not follow that the conclusions of the analysis are not very close to the facts.”

As a graduate student in 1930’s at Urbana, Illinois, Nathan  M. Newmark worked with Professors Cross, the artist, and Westergaard, the analyst.  In retrospect, he appears to have excelled them both in scholarship and creativity.  It is difficult to capture Newmark’s bent of mind with a single quote.  Things came easily to him and, in general, he did not take the time to write to be understood.  But his flashes of genius in applied mechanics demonstrate his thinking was a perfect fusion of his gifted teachers.  His quick grasp of the important and his ability to cut through seemingly complex problems using sophomore-level mechanics is at once astounding and delightful. He was a master of the synecdoche. A dam would be modeled as a rigid object with a single degree of freedom. A multidimensional ground motion became a pulse. His understated apology after having represented one component of an entire ground motion by one half of a square wave, ”It would be possible to consider a sinusoidal pulse but this complicates the expressions unnecessarily,” is enough to make one stop in one’s tracks.  Because of the pinnacles of his achievements in simplifying geotechnical and structural design, it is easy to miss that his strongest contribution was to expand and deepen the way of thinking set into motion by Cross and Westergaard. 

The thinking of Cross, Westergaard and Newmark did not always intersect completely. But when it came to the relationship of structural mechanics to design (all three had design experience and remained active and interested in structural design throughout their careers), they were completely together. To them structural mechanics was perfect as long as it was not applied. When it was applied, it had to be applied with judicious care to maximize return in relation to investment. As long as one was going to be wrong anyway, one might as well be wrong the easy way.   

The talk is an attempt to derive from the works of the three engineers a perspective of their way of thinking about structural design. 


Friday, April 5th, 2002


2:00 PM EST

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Seminar sponsored by MCEER Networking and Education Programs, MCEER SLC and hosted by the Department of Civil, Structural and Environmental Engineering and the EERI Student Chapter at UB. For further information please contact the UB-EERI Student Chapter at or at (716) 645-2114 (ext.2437)


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