To Engineer Is Human Read online

  Oliver Wendell Holmes is remembered more widely for his humor and verse than for the study entitled “The Contagiousness of Puerperal Fever” that he carried out as Parkman Professor of Anatomy and Physiology at Harvard Medical School. Yet it may have been his understanding of the seemingly independent working of the various parts of the human body that helped him to translate his physiological experiences into a lesson for structural and mechanical engineers. Although some of us go first in the knees and others in the back, none of us falls apart all at once in all our joints. So Holmes imagined the foolishness of expecting to design a horse-drawn carriage that did not have a weak link.

  Although intended as an attack on Calvinism, in which Holmes uses the metaphor of the “one-hoss shay” to show that a system of logic, no matter how perfect it seems, must collapse if its premises are false, the poem also holds up as a good lesson for engineers. Indeed, Micro-Measurements, a Raleigh, North Carolina-based supplier of devices to measure the stresses and strains in engineering machines and structures, thinks “The Deacon’s Masterpiece” so apt to its business that it offers copies of the poem suitable for framing. The firm’s advertising copy recognizes that although “… Holmes knew nothing of … modern-day technology when he wrote about a vehicle with no ‘weak link’ among its components,” he did realize the absurdity of attempting to achieve “the perfect engineering feat.”

  In Holmes’ poem, which starts on here, the Deacon decides that he will build an indestructible shay, with every part as strong as the rest, so that it will not break down. However, what the Deacon fails to take into account is that everything has a lifetime, and if indeed a shay could be built with “every part as strong as the rest,” then every part would “wear out” at the same time and whoever inherited the shay from the Deacon, who himself would pass away before his creation, would be taken by surprise one day. While “The Deacon’s Masterpiece” is interesting in recognizing that breaking down is the wearing out of one part, the weakest link, it is not technologically realistic in suggesting that all parts could have exactly the same lifetime. That premise is contrary to the reality that we can only know that this or that part will last for approximately this or that many years, just as we can only state the probability that any one paper clip will break after so many bendings. The exact lifetime of a part, a machine, or a structure is known only after it has broken.

  Just as we are expected to know our own limitations, so should we know those of the inanimate world. Even the pyramids in the land of the Sphinx, whose riddle reminds us that we all must crawl before we walk and that we will not walk forever, have been eroded by the sand and the wind. Nothing on this earth is inviolate on the scale of geological time, and nothing we create will last at full strength forever. Steel corrodes and diamonds can be split. Even nuclear waste has a half-life.

  Engineering deals with lifetimes, both human and otherwise. If not fatigue or fracture, then corrosion or erosion; if not war or vandalism, then taste or fashion claim not only the body but the very souls of once-new machines. Some lifetimes are set by the intended use of an engineering structure. As such an offshore oil platform may be designed to last for only the twenty or thirty years that it will take to extract the oil from the rock beneath the sea. It is less easy to say when the job of a bridge will be completed, yet engineers will have to have some clear idea of a bridge’s lifetime if only to specify when some major parts will have to be inspected, serviced, or replaced. Buildings have uses that are subject to the whims of business fashion, and thus today’s modern skyscraper may be unrentable in fifty years. Monumental architecture such as museums and government buildings, on the other hand, should suggest a permanence that makes engineers think in terms of centuries. A cathedral, a millennium.

  The lifetime of a structure is no mere anthropomorphic metaphor, for how long a piece of engineering must last can be one of the most important considerations in its design. We have seen how the constant on and off action of a child’s toy or a light bulb can cause irreparable damage, and so it is with large engineering structures. The ceaseless action of the sea on an offshore oil platform subjects its welded joints to the very same back and forth forces that cause a paper clip or a piece of plastic to crack after so many flexures. The bounce of a bridge under traffic and the sway of a skyscraper in the wind can also cause the growth of cracks in or the exhaustion of strength of steel cables and concrete beams, and one of the most important calculations of the modern engineer is the one that predicts how long it will take before cracks or the simple degradation of its materials threaten the structure’s life. Sometimes we learn more from experience than calculations, however.

  Years after my son had outgrown Speak & Spell, and within months of his disaffection with the video games he once wanted so much, he began to ask for toys that required no batteries. First he wanted a BB gun, which his mother and I were reluctant to give him, and then he wanted a slingshot. This almost biblical weapon seemed somehow a less violent toy and evoked visions of a Norman Rockwell painting, in which a boy-being-a-boy conceals his homemade slingshot from the neighbor looking out a broken window. It is almost as innocent a piece of Americana as the baseball hit too far, and no one would want to ban slingshots or boys.

  I was a bit surprised, however, to learn that my son wanted to buy a slingshot ready-made, and I was even more surprised to learn that his source would not be the Sears Catalog, which might have fit in with the Norman Rockwell image, but one of the catalogs of several discount stores that seem to have captured the imagination of boys in this age of high-tech toys. What my son had in mind for a slingshot was a mass-produced, metal-framed object that was as far from my idea of a slingshot as an artificial Christmas tree is from a fir.

  Stephen was incredulous as I took him into the woods behind our house looking for the proper fork with which to make what I promised him would be a real slingshot. We collected a few pieces of trees that had fallen in a recent wind storm, and we took them up to our deck to assemble what I had promised. Unfortunately, I had forgotten how easily pine and dry cottonwood break, and my first attempts to wrap a rubber band around the sloping arms of the benign weapon I was making met with structural failure. We finally were able to find pieces strong enough to withstand the manipulation required for their transformation into slingshots, but their range was severely limited by the fact that they would break if pulled back too far.

  My son was clearly disappointed in my inability to make him a slingshot, and I feared that he had run away disillusioned with me when he disappeared for an hour or so after dinner that evening. But he returned with the wyes of tree branches stronger and more supple than any I found behind our house. We were able to wrap our fattest rubber bands around these pieces of wood without breaking them, and they withstood as much pull as we were able or willing to supply. Unfortunately, they still did not do as slingshots, for the rubber bands kept slipping down the inclines of the Y and the bands were difficult to hold without the stones we were using for ammunition slipping through them or going awry.

  After almost a week of frustration trying to find the right branch-and-rubber band combination that would produce a satisfactory slingshot that would not break down, I all but promised I would buy one if we could not make a top-notch shooter out of the scraps of wood scattered about our basement. Stephen was patient if incredulous as I sorted through odd pieces of plywood and selected one for him to stand upon while I sawed out of it the shape of the body of a slingshot. He was less patient when I drilled holes to receive a rubber band, and I acceded to his impatience in not sanding the plywood or rounding the edges before giving the device the test of shooting. I surprised him by producing some large red rubber bands my wife uses for her manuscripts, and he began to think he might have a real slingshot when I threaded the ends of a rubber band through the holes in the plywood Y. With the assembly completed I demonstrated how far a little pebble could be shot, but I had to admit, at least to myself, that it was very difficult to keep th
e pebble balanced on the slender rubber band. My son was politely appreciative of what I had made for him, but he was properly not ecstatic. The pebbles he tried to shoot dropped in weak arcs before his target, and he knew that his slingshot would be no match for the one his friend had bought through the catalog.

  In my mind I admitted that the homemade slingshot was not well designed, and in a desperate attempt to save face with my son I decided to add a second rubber band and a large pocket to improve not only the range but also the accuracy of the toy. These proved to be tremendous improvements, and with them the slingshot seemed almost unlimited in range and very comfortable to use. Now we had a slingshot of enormous potential, and my son was ready to give it the acid test. We spent an entire weekend practicing our aim at a beer bottle a good thirty yards away. The first hit was an historic event that pinged off the glass and the second a show of power that drilled a hole clear through the green glass and left the bottle standing on only a prayer. As we got better at controlling the pebbles issuing from our homemade slingshot we changed from bottles to cans for our targets and hit them more and more.

  With all our shooting, the rubber bands began to break from fatigue. This did not bother my son, and he seemed to accept it as something to be expected in a slingshot, for it was just another toy and not a deacon’s masterpiece. As rubber bands broke, we replaced them. What proved to be more annoying was the slipping of the rubber band over the top of the slingshot’s arm, for we had provided no means of securing the band from doing so. In time, however, we came to wrap the broken rubber bands around the top of the arms to keep the functioning ones in place. This worked wonderfully, and the satisfaction of using broken parts to produce an improved slingshot was especially appealing to my son. He came to believe that his slingshot could outperform any offered in the catalogs, and the joy of producing it ourselves from scrap wood and rubber bands gave him a special pleasure. And all the breaking pieces of wood, slipping rubber bands, and less-than-perfect functioning gave him a lesson in structural engineering more lasting than any textbook’s—or any fanciful poem’s. He learned to make things that work by steadily improving upon things that did not work. He learned to learn from mistakes. My son, at eleven, had absorbed one of the principal lessons of engineering, and he had learned also the frustrations and the joys of being an engineer.

  APPENDIX

  THE DEACON’S MASTERPIECE

  Or, the Wonderful “One-Hoss Shay”

  A Logical Story

  By Oliver Wendell Holmes

  Have you heard of the wonderful one-hoss shay,

  That was built in such a logical way

  It ran a hundred years to a day,

  And then, of a sudden, it—ah, but stay,

  I’ll tell you what happened without delay,

  Scaring the parson into fits,

  Frightening people out of their wits—

  Have you ever heard of that, I say?

  Seventeen hundred and fifty-five.

  Georgius Secundus was then alive,—

  Snuffy old drone from the German hive.

  That was the year when Lisbon-town

  Saw the earth open and gulp her down,

  And Braddock’s army was done so brown,

  Left without a scalp to its crown.

  It was on the terrible Earthquake-day

  That the Deacon finished the one-hoss shay.

  Now in the building of chaises, I tell you what,

  There is always, somewhere, a weakest spot,—

  In hub, tire, felloe, in spring or thill,

  In panel, or crossbar, or floor, or sill,

  In screw, bolt, thoroughbrace,—lurking still,

  Find it somewhere you must and will,—

  Above or below, or within or without,—

  And that’s the reason, beyond a doubt,

  That a chaise breaks down, but doesn’t wear out.

  But the Deacon swore (as deacons do,

  With an “I dew vum,” or an “I tell yeou”)

  He would build one shay to beat the taown

  ’N’ the keounty ’n’ all the kentry raoun’;

  It should be so built that it could n’ break daown:

  “Fur,” said the Deacon, “’t ’s mighty plain

  Thut the weakes’ place mus’ stan’ the strain;

  ’N’ the way t’ fix it, uz I maintain, Is only jest

  T’ make that place uz strong us the rest.”

  So the Deacon inquired of the village folk

  Where he could find the strongest oak,

  That couldn’t be split nor bent nor broke,—

  That was for spokes and floor and sills;

  He sent for lancewood to make the thills;

  The crossbars were ash, from the straightest trees,

  The panels of white-wood, that cuts like cheese,

  But lasts like iron for things like these;

  The hubs of logs from the “Settler’s ellum,”—

  Last of its timber,—they couldn’t sell ’em,

  Never an axe had seen their chips,

  And the wedges flew from between their lips,

  Their blunt ends frizzled like celery-tips;

  Step and prop-iron, bolt and screw,

  Spring, tire, axle, and linchpin too,

  Steel of the finest, bright and blue;

  Thoroughbrace bison-skin, thick and wide;

  Boot, top, dasher, from tough old hide

  Found in the pit when the tanner died.

  That was the way he “put her through.”

  “There!” said the Deacon, “naow she’ll dew!”

  Do! I tell you, I rather guess

  She was a wonder, and nothing less!

  Colts grew horses, beards turned gray,

  Deacon and deaconess dropped away,

  Children and grandchildren—where were they?

  But there stood the stout old one-hoss shay.

  As fresh as on Lisbon-earthquake-day!

  EIGHTEEN HUNDRED; it came and found

  The Deacon’s masterpiece strong and sound.

  Eighteen hundred increased by ten;—

  “Hahnsum kerridge” they called it then.

  Eighteen hundred and twenty came;—

  Running as usual; much the same.

  Thirty and forty at last arrive,

  And then come fifty, and FIFTY-FIVE.

  Little of all we value here

  Wakes on the morn of its hundredth year

  Without both feeling and looking queer.

  In fact, there’s nothing that keeps its youth,

  So far as I know, but a tree and truth.

  (This is a moral that runs at large;

  Take it.—You’re welcome.—No extra charge.)

  FIRST OF NOVEMBER,—the earthquake-day,—

  There are traces of age in the one-hoss shay,

  A general flavor of mild decay,

  But nothing local, as one may say.

  There couldn’t be,—for the Deacon’s art

  Had made it so like in every part

  That there wasn’t a chance for one to start.

  For the wheels were just as strong as the thills,

  And the floor was just as strong as the sills,

  And the panels just as strong as the floor,

  And the whipple-tree neither less nor more,

  And the back crossbar as strong as the fore,

  And spring and axle and hub encore.

  And yet, as a whole, it is past a doubt

  In another hour it will be worn out!

  First of November, ’Fifty-five!

  This morning the parson takes a drive.

  Now, small boys, get out of the way!

  Here comes the wonderful one-hoss shay,

  Drawn by a rat-tailed, ewe-necked bay.

  “Huddup!” said the parson.—Off went they.

  The parson was working his Sunday’s text,—

  Had got to fifthly, and stopped perplexed

  At
what the—Moses—was coming next.

  All at once the horse stood still,

  ’Close by the meet’n’-house on the hill.

  First a shiver, and then a thrill,

  Then something decidedly like a spill,—

  And the parson was sitting upon a rock,

  At half past nine by the meet’n’-house clock,—

  Just the hour of the Earthquake shock!

  What do you think the parson found,

  When he got up and stared around?

  The poor old chaise in a heap or mound,

  As if it had been to the mill and ground!

  You see, of course, if you’re not a dunce,

  How it went to pieces all at once,—

  All at once, and nothing first,—

  Just as bubbles do when they burst.

  End of the wonderful one-hoss shay.

  Logic is logic. That’s all I say.

  4

  ENGINEERING AS HYPOTHESIS

  Every issue of The Structural Engineer, the official journal of the British Institution of Structural Engineers, carries prominently displayed in a box on its contents page this definition of its subject:

  Structural engineering is the science and art of designing and making, with economy and elegance, buildings, bridges, frame-works, and other similar structures so that they can safely resist the forces to which they may be subjected

  Since some engineers deny that engineering is either science or art, it is encouraging to see this somewhat official declaration that it is both. And indeed it is, for the conception of a design for a new structure can involve as much a leap of the imagination and as much a synthesis of experience and knowledge as any artist is required to bring to his canvas or paper. And once that design is articulated by the engineer as artist, it must be analyzed by the engineer as scientist in as rigorous an application of the scientific method as any scientist must make.