The History of StructuresA tribute to Professor J.E. GordonMichael Grantham, MG Associates

The Bricklayer's StoryFirst told by Gerard Hoffnung in his speech to the Oxford Union in 1958A wonderful introduction to stresses, strains and pulleys, which is where we all started learning about mechanicsIt refers to a letter from a Bricklayer to his boss, following an incident on a building site.

Professor J.E. GordonJames Edward Gordon was born in 1913. He took a degree in naval architecture at Glasgow University and worked in wood and steel shipyards, intending to design sailing ships. On the outbreak of the Second World War he moved to the Royal Aircraft Establishment at Farnborough, where he worked on wooden aircraft, plastics and unorthodox materials of all kinds. He designed the sailing rescue dinghies carried at one time by most bomber aircraft. He later became head of the plastic structures sections at Farnborough and developed a method of construction in reinforced plastics which is now used for a number of purposes in aircraft and rockets.

Professor J.E. GordonFor several frustrating years he worked in industry on the strength of glass and the growth of strong 'whisker' crystals. In 1962 he returned to government service as superintendent of an experimental branch at Waltham Abbey concerned with research and development of entirely new structural materials, most of which were based on 'whiskers'. He was Industrial Fellow Commoner at Churchill College, Cambridge, and became Professor of Materials Technology at the University of Reading, where he was later Professor Emeritus. He was awarded the British Silver Medal of the Royal Aeronautical Society for work on aircraft plastics and also the Griffith Medal of the Materials Science Club for contributions to material science.

Professor J.E. Gordon He wrote two famous books, The New Science of Strong Materials (or Why You Dont Fall Through the Floor!) and Structures, or Why Things Don't Fall Down, both published in Penguin.Professor Gordon died in 1998. In its obituary The Times wrote of him that he was 'one of the founders of materials science' and that he wrote 'two books of outstanding literary quality ... at once entertaining and informative, providing absorbing interest for both expert and student'.The first book was set reading, when I studied with the Open University.Reading his books, a twinkling sense of humour is apparent behind all the excellent information

So Why Dont We Fall Through the Floor?So how does steel or stoneor timber or plastic resist a mechanical force?The problem defeated Galileo!It was Robert Hooke (1635-1702) that first came up with a theory.He realised that if a material is to resist a load, it can only do so by pushing back at it with an equal and opposite force!

Action and ReactionSo if a Cathedral pushes down on its foundations, then the foundations must push up with an equal and opposite force.So forces cant get lost, they must be balanced wherever they occur in the structure or, according to Sir Isaac Newton, they are going to start accelerating somewhere! (or break of course)Good equation F=ma!Something being pushed accelerates!Something which isn't accelerating isn't being pushed, or is experiencing balanced forces!Heavy things (with a big mass) take more force to accelerate them!

Action and Reaction

TensionSo assuming the brick doesnt fall, then the weight of the box must be balanced by an equal and opposite tension in the rope!Of course it doesnt have to be a weight. Think of a child pulling on a cats tailDont pull pussys tail, darling.Im not pulling Mummy, Pussys pulling!

The Cats tailNow suppose the cats tail isn't attached to the cat but to a wallHow can something inert like a wall pullWell, of course it doesnt much matter whether the child pulls, or the wall passively reacts, the tension experienced in the tail is the same!So how do structures generate the large reactive forces that are needed?

Sir Robert Hooke 1635 to 1703Robert Hooke, played an important role in the scientific revolution, through both experimental and theoretical work. His father's name was John Hooke and his mother unknown.Born on the Isle of Wight, Hooke received his early education at Westminster School. In 1653, Hooke won a place at Christ's Church, Oxford. There he met the chemist (and physicist) Robert Boyle, and gained employment as his assistant. It is possible that Hooke formally stated Boyle's Law, as Boyle was not a mathematician. In 1660, he discovered his law of elasticity, which describes the linear variation of tension with extension in an elastic spring. In 1662, Hooke gained appointment as Curator of Experiments to the newly founded Royal Society, and took responsibility for experiments performed at its meetings.

Sir Robert HookeIn 1665 he published a book entitled Micrographia, which contained a number of microscopic and telescopic observations, and some original biology. Indeed, Hooke coined the biological term cell-- so called because his observations of plant cells reminded him of monks' cells which were called "cellula". The hand-crafted, leather and gold-tooled microscope Hooke used to make these observations for "Micrographia," is on display at the National Museum of Health and Medicine in Washington, DC. Also in 1665 he gained appointment as Professor of Geometry at Gresham College.

Robert HookeRobert Hooke also achieved fame as Surveyor to the City of London and chief assistant of Christopher Wren, helping to rebuild London after the Great Fire in 1666. He worked on designing the Monument, the Royal Observatory at Greenwich and the infamous Bethlem Royal Hospital (which became known as 'Bedlam').He died in London in 1703.

Hooke againBy about 1676, Hooke saw that not only do most solids resist weights or other loads by pushing back at them, but also that solids change shape, by stretching or contracting when forces are applied to themIt is this change of shape which enables the solid to push backThis, we now know, happens due to interatomic forces and chemical bonds

Hookes LawThe power of any spring is in the same proportion as the tension (i.e. extension) thereof: That is, if one power stretch or bend it one space, two will bend it two, three will bend it three and so forward. And this is the Rule or Law of Nature, upon which all manner of Resistuent or Springing motion doth proceedRobert Hooke

Load and DeflectionSo it is this deflection or extension that provides the restoring force.In other words, all structures deflect under load, if they didnt they wouldnt work!Bent Columns at Salisbury CathedralBent ground at Pisa!!

The Power of Chemical BondsHooke made a further important step in his reasoning.He realised that if a structure was to resist a load, then each part of the structure would have to resist it in proportion to its size, by stretching or contracting down to a very fine scale, (molecular in fact, as we now know)These strong and stiff bonds, together provide the restoring force by stretching or compressing, as appropriate.

Springy Chemical Bonds!

The Science ProgressesHooke knew nothing in detail about chemical bonds, but he understood that something of that kind was happening inside the structure.It was in a series of experiments on different materials that he realised that extension (or tension as he called it) was proportional to loadIn Hookes experiments, most solids recovered their original shape

So Are Steel and Concrete Springy?The answer, of course, is yes, up to a point.When you exceed their elastic limit, they experience a permanent stretch, and ultimately fracture as the load exceeds the strength of the chemical bonds.It may seem blindingly obvious to us, but Hooke went through great mental effort to get here and had many doubts!

Robert HookeAfter he had discussed his ideas with Sir Christopher Wren, Hooke published his experiments in 1679 in a paper called De Potentia Restitutiva (of a Spring)This paper contained the famous statement ut tensio sic vis (As the extension, so the force!)This became known as Hookes law, which of course is still investigated by every youngster for their GCSEs

After HookeSurprisingly, following Hookes discoveries, little progress was made until some 120 years after Hookes death.Part of this was due to the fact that some scientists saw themselves as philosophers and way above the mundane, sordid level of industry and commerce!

Sir Isaac Newton 1643 - 1727Sir Isaac Newton was an English natural philosopher, and generally regarded as the most original and influential theorist in the history of science. In addition to his invention of the infinitesimal calculus and a new theory of light and color, Newton transformed the structure of physical science with his three laws of motion and the law of universal gravitation. .

Isaac NewtonAs the keystone of the scientific revolution of the 17th century, Newton's work combined the contributions of Copernicus, Kepler, Galileo, Descartes, and others into a new and powerful synthesis. Three centuries later the resulting structure - classical mechanics - continues to be a useful but no less elegant monument to his genius

Hooke and NewtonHowever, we cannot leave out the enmity that existed between Hooke and NewtonThey were probably of about equal intellect, but Hooke was touchy and vainNewton was a snob. Hooke, despite his friendship with Charles II, wasntNewtonHooke

Hooke and NewtonHooke occupied himself in the pursuit of practical problems about springs and clocks and buildings and microscopes and the anatomy of the flea!He invented the universal joint and the iris diaphragm, still used in cameras today. His carriage lamp, which used a spring feed to move a candle as the wick burned down, went out of use only in the 1920sWhen he wasnt inventing, and between pursuing servant girls, he lived in sin with his niece!!!

Sir Isaac NewtonNewtons ideas and vision of the universe may have been wider than Hookes, but his interest in science was much less practicalIn fact it was quite anti-practical!He spent much time theorising about such theological ideas as the number of the beast and ended up detesting Hooke, elasticity and everything Hooke stood for!As a result, and owing to the high standing of Newton, subjects such as structures suffered heavily in popularity.

The following yearsSo the usefulness of theoretical elasticity in engineering was limited.French Engineers tried to build structures with what theory was available to them. They often fell down!English Engineers, indifferent to theory, tended to work with rules of thumb.Their structures probably fell down nearly, but perhaps not quite, as often!

The way forwardAll through the 18th century and into the 19th century, scientists got bogged down with understanding elasticity.They tended to focus on the whole structure rather than the behaviour of its component parts and considering the forces and extensions which existed at any point in the structure.

The 18th and 19th CenturiesAll through the 18th and well into the 19th Century, people like Leonhard Euler (1707-83) and Thomas Young (1773-1829) struggled with the concept of stress and strainIt was Augustin Cauchy (1789-1857) who presented a paper to the French Academy of Sciences in 1882, who first came up with the modern idea of stress and strainEnglish Engineers eventually caught on when they read Cauchys workLeonhard EulerAugustin Cauchy

Stress and StrainSurprisingly Galileo almost cracked it!In The Two New Sciences, a book he wrote in his old age, he stated very clearly that a rod which is pulled in tension has a strength directly proportional to its cross-sectional area!Why it subsequently took 200 years for someone to get the idea of dividing the breaking load by the cross sectional area, to get what we now call tensile stress almost beggars belief!

Measuring StressSo going back to our bricks hanging from the tree, if our load was 5kg and the cross sectional area of the string 2mm2 then the stress would be 2.5 Kgf/mm2We can use any old units for stress, but to avoid confusion, the SI system uses MN/m21 MN/m2=10.2 kgf/mm2 = 146 psiRemember that the stress exists at any point in the structure and isn't especially related to any particular cross-sectional area.

S t r a i n Stress tells us how hard something is, i.e. with how much force the atoms in a solid are being pulled apart.Strain tells us how far they are being pulled apart.So going back to our string and the brick. Suppose it started at 2m long and stretched under load by 1cm, then the strain would be 1/200 = 0.005 or 0.5%Strain is just a ratio, it doesnt have units

Bored Stiff?So we come to stiffness!Nowadays we measure this with Youngs Modulus, the ratio of stress divided by strainWe now regard Youngs Modulus as pretty fundamental in engineering, but it took the first half of the 19th century for the penny to drop in the minds of engineersThomas Young1773-1829

Youngs ModulusYoung had a very severe intellectual struggle with this conceptHis own definition in 1807 was The modulus of elasticity of a substance is a column of the same substance, capable of producing the same pressure on its base which is to the weight causing a certain degree of compression as the length of the substance is to the diminution of its length!!!After that, Egyptian Hieroglyphics, which he also studied must have been simple!To be fair, he was struggling with a concept of stress and strain, without them being defined, which didnt happen for another 15-20 years

French Theory and British PragmatismWork then commenced in the first part of the 19th Century, mainly by the French. Much of this was highly mathematical!In England practical men were regarded as greatly superior to mere theoreticians

Thomas Telford 1757-1834The name of Thomas Telford, from Westerkirk, Dumfries, is held in awe whenever road engineering and bridge building is discussed. If it is not, it ought to be, for his contributions to the art and science of crossing mountains and rivers in the most efficient, economical and speediest ways possible are legend. Telford's accomplishments include his early work as surveyor of Shropshire, the county that straddles the English-Welsh border: the bridges over the River Severn at Montford, Buildwas and Bewdley all completed in the 1780's. In 1793, Telford began work as engineer for the Ellesmere Canal Company, completing his monumental aqueducts that carried the canal over the valleys of the rivers Ceiriog and Dee in North Wales.

In the early days of the industrial revolution canals were built to transport raw materials and newly manufactured goods to all parts of the British Isles. William Telford solved what seemed to be the insurmountable problem of taking the Shropshire Union Canal across the narrow, steep-sided Dee valley in North Wales. His answer was the justly famous Pontcysyllte Aqueduct, the longest and highest in Britain. The name, unpronounceable to most English visitors, simply means, "connecting bridge."

Completed in 1805, one month after the Battle of Trafalgar, the 121-foot high aqueduct is 1007 feet in length, carrying the canal in a completely water tight, cast-iron trough supported by 18 piers. It is a bit of a shock to see barges merrily and magically glide across an expanse of sky high above the valley below and its road to Chirk (where another Telford masterpiece, the Chirk Aqueduct, takes the canal across the River Ceiriog).

Pontcysyllte AqueductChirk Aqueduct

Telford then left for Scotland, where he was responsible for the Caledonian Canal that opened up the lowlands to industry; the harbour works at Aberdeen, Dundee and other rapidly growing port cities. In his native Scotland, he was responsible for building more than 900 miles of roads and their attendant bridges. He then returned to Wales, managing to engineer the main highway from Shrewsbury and Chester all the way to Holyhead in northwest Wales by carefully selected routes through the mountains that would provide the least gradient.

Thomas Telford 1757-1834Telford had a singular distaste for mathematical studies, and never even acquainted himself with the elements of geometry. When a young man was recommended to work in his office, who had distinguished himself in mathematics, Telford said that he thought that rather disqualified him from the post rather than made him more fitting!

Fathers of Modern Flexural Theory

Factors of SafetyAfter about 1850, even British and American Engineers began to calculate the stresses imposed on their structures. They went to great efforts to ensure that the values were less than the officially quoted Tensile Stress of the materialThey often used factors of four to eight times less!

Problems with ShipsThe demand for fast, light ships began to cause problems. In 1901, the HMS Cobra, one of the fastest ships in the world, suddenly broke in two and sank in the North Sea during quite ordinary weather!At that time, stresses and strains were understood in only the broadest terms. The concept of stress raising was first coined by C.E. Inglis, who became a professor of Engineering at CambridgeHe realised that holes and cracks and sharp corners could raise local stresses, often over a very small area, dramaticallyHMS Cobra

ShipsIn 1928, the White Star liner Majestic had an additional passenger lift installed. This involved cutting through the ships strength decks. Somewhere between New York and Southampton a crack started from the lift opening, ran to the rail and down the side of the ship where, luckily, it was stopped by a porthole!The ship and its 3000 passengers arrived safely at Southampton!

The presentDespite these problems continuing, notably with the famous Comet disasters, we now understand the subject sufficiently to build, we hope, safely.

Beautiful Structures

Stress TestThe following photo has 2 identical dolphins in it. It was used in a case study on stress level at Saint Mary's Hospital.Look at both dolphins jumping out of the water. The dolphins are identical.A closely monitored, scientific study of a group revealed that in spite of the fact that the dolphins are identical, a person under stress would find differences in the two dolphins. If there are many differences found between both dolphins, it means that the person is experiencing a great amount of stress.

Stress Test