It's my pleasure to try to sum up my thoughts on the philosophy of engineering and to give some examples of what I believe is good engineering in my own field, structural engineering. The tasks of the structural engineer are to conceive, design, and execute those load resisting constructs needed by a civilized society to enclose or bridge space. I'd like to emphasize the words conceive and design. They imply imagination, the imagination to conceptualize actual things that are to be constructed or manufactured. This is the essential feature which distinguishes engineering from science. Engineering is not merely applied science. Engineering may utilize the results of science, but engineering design is a separate and wholly distinct intellectual activity.

What are the responsibilities of the structural designer? He or she has to identify technologically feasible options-again, using imagination-to select that one which maximizes safety and durability and minimizes cost-continuing cost as well as construction cost.

So, engineering involves developing options and making decisions. In that respect, it is like the other learned professions. What does the physician do? A physician makes practical decisions; he decides upon a therapy based upon his diagnosis.

Like other professionals, engineers do not operate in a vacuum. Their activities are subject to constraints. Those impinging upon the work of a structural engineer are: function, site, strength, durability, constructability, economy, and aesthetics.

What is good? We're not going to have a Socratic discussion here-what is true? what is beautiful? and what is good? So, I will just give a definition of the word "good:" For the purposes of this talk, "good" will mean a state approaching a preconceived and established condition or quality in terms of form. It presupposes an idea more or less intuitively recognized as such or one that has previously been found to have worked for the protection or for the advantage of people.

I stressed the word "form" because it seems to me that form, at least form in my profession and the artifacts that my profession produces, is something that I can reach this whole audience with. So, we are going to concentrate on form for the remainder of this talk. Form implies: strength or security, clarity of function, solidity or gracefulness, appropriateness or incongruity, tradition or innovation, order or variety, simplicity or complexity. These are some of the attributes of form. Form, therefore, implies aesthetics.

What is an aesthetic object? An aesthetic object is anything with enough vividness and poignancy to make us appreciate it as given. What are the aesthetic choices that have to be made? Here structural engineering overlaps a bit with architecture. Form, proportion, refinement, and balance: these are some of the aesthetic choices, the decisions that both engineers and architects have to make.

To illustrate my ideas about good engineering I shall, very briefly, trace some of the history of one of the basic structural forms important in my profession.

The dome was carried to a high degree of development by the ancient Romans. Even though they did not have access to the modern technology of materials or to the sciences of statics, dynamics, and solid mechanics, Roman engineers were able to construct stupendous domed buildings, many of which still stand and function 2000 years after they were originally built. How were the ancients able to do this? Is there something timeless about engineering with as much meaning today as it had ages ago?

Vitruvius wrote this at the time of Augustus: "In fact, all kinds of men, and not merely architects [who included the engineers of his day], can recognize a good piece of work, but between laymen and the latter there is this difference, that the layman cannot tell what it is to be like without seeing it finished, whereas the architect, as soon as he has formed a conception, and before he begins the work, has a definite idea of the beauty, the convenience and the propriety that will distinguish it."

In other words, the ancient engineer, just as his modern counterpart does, first conceives of the work in great detail, then he meticulously plans it to fulfill its intended function subject to the various constraints of aesthetics and economy, and then, finally, he constructs the work almost exactly as it first appeared to him in his imagination.

Are there some examples from the past of good engineering or of bad engineering that are instructive? Consider the domes covering: the Pantheon in Rome, built by Hadrian; St. Peter's Cathedral in the Vatican, designed by Michelangelo; and the Schott works of Zeiss in Jena, Germany, built in 1923. All three domes are approximately 130 feet in diameter. The Pantheon dome is approximately 3 feet thick and made of light weight "sandwich" concrete. It weighs approximately 3000 tons. St. Peter's dome is almost 10 feet thick and made of brick and stone masonry surrounded by iron chains. It weighs 11,000 tons. The Schott dome is only 2.36 inches thick and made of steel reinforced concrete. It weighs only 347 tons.

In my opinion, good engineering implies the application of state-of-the-art knowledge. Hadrian's engineers (including Hadrian himself who was trained as an architect) used every bit of significant engineering knowledge and observation that had accumulated to the time when the Pantheon was conceived, designed, and built. It is, perhaps, the most remarkable building ever built. It has endured 2000 years during which it has suffered several earthquakes, many fires, and assorted vandalism. Considering the state of knowledge of the ancient Romans, surely everyone will agree that the Pantheon's design is engineering at its best.

Now let us examine St. Peter's dome. It was built in the 16th century by Michelangelo who certainly had every opportunity to study the nearby Pantheon. Yet, to the best of our knowledge, neither he nor his associates made a thorough examination of the Pantheon when designing St. Peter's dome. The dome was designed without having as clear an idea of how such a structure functions as the ancient Romans had. The design was clumsy, enormously wasteful, and not very durable. It has been substantially rebuilt three times during its 400 year life. In short, the dome of St. Peter's Cathedral represents bad engineering, primarily because it did not take advantage of information about dome construction easily obtained from a careful study of the nearby Pantheon. The engineering that went into the design and construction of St. Peters's Dome was not state-of-the-art.

What about the dome over the Schott works at Jena? Was it good or bad engineering? It is, quite obviously, an efficient structure because it weighs so little. It covers the same enormous span as the Pantheon or St. Peters', yet it weighs only one tenth as much as the Pantheon dome and only one thirtieth as much as St. Peter's dome. How did this order of magnitude increase in efficiency come about?

The design of the Schott dome exhibits several features of 20th century engineering that did not exist in earlier times. First, it was based on a detailed understanding of the strengths of its materials. Second, the design proceeded from a clear idea of how a dome bears and distributes the loads applied to it. Third, and last, the Zeiss engineers succeeded in constructing and solving a mathematical model to predict accurately the stresses in such structures. This accomplishment enabled the Zeiss designers to optimize their design by minimizing the weight of the structure. Interestingly, the mathematical model used in the dome design was first formulated by Zeiss's engineers to enable them to design certain optical lenses.

So, good 20th century structural engineering implies: (1) application of state-of-the-art knowledge of the strength of materials and the mechanics of structures, (2) construction and solution of an appropriate mathematical model capable of accurately predicting the stresses generated by the applied loads, and (3) the search for an optimal construct (the "elegant solution") which minimizes materials, construction costs, continuing costs, or all of these. Certainly the design of the Schott dome is an elegant solution which may be regarded as an exemplar of 20th century engineering. It is, therefore., my conclusion that the Schott dome was just as good an engineering job for its day as the Pantheon's dome was for its day.

As a final note, I would like to explain yet another facet of the intellectual and professional activities that we today call engineering. Engineering has long used a systematic approach to the solution of problems that the 19th-century author, John Stuart Mill, called the "method of detail." For example, when the noted 18th-century French engineer, Mariotte, was asked to determine appropriate dimensions for the pipes to be installed at Versailles, he split the problem into several more detailed subsidiary questions: First, how freely will water flow in pipes of various diameters? Second, what is the bursting strength of various pipes when filled with water under pressure? Third, what is the bending strength of various pipes when carrying water over a gap between two supports? The original problem, considered as a whole, seemed impossible to solve. Yet, if the component parts of the problem are well-enough defined, and if each of these parts is attacked and solved in turn, the problem as a whole can be solved.

Galileo also was a practitioner of the method of detail. When asked by the ship builders of Venice whether ships twice as large as those currently under construction could be built, he advocated treating several subsidiary questions of detail before rendering an opinion on the original question. First, he advocated that tests be made of various woods to determine their tensile and compressive strengths. Second, he began an investigation into the basic mechanics of the bending of beams to determine the stresses engendered by external loads. Third, he attempted to correlate the stresses which occur in beams with the tensile and compressive strengths of the woods used in shipbuilding. No modern engineer would have the slightest difficulty in following the reasoning processes of Mariotte and Galileo.

Beyond the initial step of conceptualization of a work, the engineer must be concerned with its "buildability" or "manufacturability" That is, if the engineer's conception is to be realized, it must be capable of being built by real people using real materials and real techniques not substantially beyond the existing state-of-the-art. We at IIT have often heard repeated the words of Mies Van Der Rohe, "God lies in the details." Engineers sometimes make the same point in different words: "Any fool can tighten a nut with a pencil." While anyone can make a pencil sketch of a connection assembly showing the needed bolts and nuts, it takes an engineer's attention to detail to ensure that a worker will actually find the clearances needed to enable him to tighten the real nut properly with his wrench.