Why Buildings Stand Up by Mario Salvadori

• Science and technology at their best are motivated to satisfy genuine human needs.
• The purpose of a building is to perform a function. The function of most buildings is to protect people from the weather by creating enclosed but interconnected spaces.
• The development of structural material has not kept pace with the needs for the realization of advanced theoretical concepts Except for reinforced and prestressed concrete and high-strength steel, the materials we use today are very similar to those used by our forefathers.
• Cultures are immortalized by monuments, which express their conception of the world, of life and death.
• The collapse of the Meidum pyramid demonstrates that the problem of loads, of weight distribution, even in apparently simple geometric structures, is a complex, ever-present concert for builders. If the earth did not pull, the wind did not blow, the earth’s surface did not shake or sink, and the air temperature did not change, loads would not exist and structure would be unnecessary.
• Structure supports all the loads that act, unavoidably, on buildings.
• The engineer’s first job is to determine which loads will act on a structure and how strong they might be in extreme cases.
• A structure consists of heavy elements like columns, beams, floors, arches, or domes which must, first of all, support their own weight, the so-called dead load.
• The dead load is a load “permanently there”. In some structures built of masonry or concrete it is often the heaviest load to be supported by the structure.
• In addition to its dead load, a structure must support a variety of other weights--people, furniture, equipment, stored goods. These impermanent or live loads may be shifted around and they may change in value.
• Concern for safety suggests that live loads must be established on the basis of the worst loading conditions one may expect during the entire life of the structure.
• The dead load is permanent and unchanging and the live loads have been tacitly assumed to change slowly, if at all. Together, these unchanging or slowly changing loads are called by engineers static loads, loads that stay.
• But other loads change value rapidly and even abruptly, like the pressure of a wind gust, or the action of an object dropped on the floor. Such loads are called dynamic and may be exceedingly dangerous because they often have a much greater effect than the same loads applied slowly.
• Under the wind pressure the building bends slightly and its top moves. Its movement may be small enough not to be seen by the naked eye, or even sensed, but since structural materials are never totally rigid, all buildings do sway in the wind.
• The time it takes a pendulum to complete a full swing, from extreme right to extreme left and back, is called the period of the pendulum. Similarly, the time it takes a building to swing through a complete oscillations is called its period.
• The action of the gust depends not only on how long it takes to reach its maximum value and decrease again, but on the period of the building on which it acts. If the wind load grows to its maximum value and vanishes in a time much shorter than the period of the building, its effects are dynamic. They are static if the load grows and vanishes in a time much longer than the period of the building.
• Interestingly enough, there are loads which, though not growing rapidly, do have dynamic effects increasing, not instantaneously, but progressively in time. This phenomenon, called resonance, is one of the most dangerous a structure may be subjected to.
• Resonant forces do not produce large effects immediately, as impact forces do, but their effects increase steadily with time and may become catastrophic if they last long enough.
• THe forces exerted by winds on buildings have dramatically increased in importance with the increase in building heights. Stati wind effets rise as the square of a structure’s height.
• One of the basic questions to be resolved before designing a building is often: “What is the strongest wind to be expected at its site?”
• It is wiser to design buildings so that they will be undamaged by a wind with a chance of occuring once in, say, 50 years, but to allow minor damage under the forces of a 100 year wind.
• Besides depending on wind speed and building height, wind forces vary with the shape of the building. The wind exerts a pressure on the windward face of a rectangular building because the movement of the air particles is stopped by this face. The air particles, forced from their original direction, go around the building in order to continue their flow, and get together again behind the building. In so doing, the air particles suck on the leeward face of the building and a negative pressure or suction is exerted on it. The total wind force is the sum of the windward pressure and the leeward suction, but each of these two forces has its own local effects.
• In designing for wind, a building cannot be considered independent of its surroundings. The influence of nearby buildings and of the land configuration can be substantial.
• The swaying of the top of a building due to wind may not be seen by the passerby, but it may feel substantial to those who occupy the top stories of a high building.
• To avoid excessive wind deflections (or wind drift as it is technically called) buildings should be stiffened so that their tops will never swing more than 1/500 of their height.
• The earth’s crust floats over a core of molten rock and some of its parts have a tendency to move with respect to one another. This movement creates stresses in the crust, which may break out along fractures and faults. THe break occurs through a sudden sliding motion in the direction of the fault and jerks the buildings in the area. Since the dynamic impact forces due to this jerky motion are mostly horizontal, they can be resisted by the same kind of bracing used against the wind.
• The last category of loads the engineer must worry about consists of those caused by daily or seasonal change in air temperature or by uneven settlement of the soil under a building. These are sometimes called hidden or locked-in loads.
• On a summer day, when the air temperature reaches 90-degrees, the bridge lengthens, since all bodies expand when heated.
• Unfortunately, steel is so stiff that the compressive load exerted by the abutments uses up half the strength of the steel. There is only one way of avoiding this dangerous overstress: one of the bridge ends must be allowed to move to permit the thermal expansion to occur. While gravity loads must be fought by increasing the strength and stiffness of a structure, thermal loads must be avoided by making the structure less rigid.
• It must be emphasized that most damage to buildings is caused by foundation problems.
• The purpose of structure is to channel the loads on the building to the ground. This action is similar to that of water flowing down a network of pipes; columns, beams, cables, arches, and other structural elements act as pipes for the flow of the loads. Obviously, this becomes a complex function when the structure is large and the loads numerous.
• THe remarkable, inherent simplicity of nature allows the structure to perform its task through two elementary actions only: pulling and pushing. Many and varied as the loads may be and geometrically complicated as the structure may be, its elements never develop any other kind of action. They are either pulled by the loads, and then they are stretched, or are pushed, and then they shorten.
• In structural language, the loads are sid to stress the structure, which strains under stress.
• With a judicious sense of economy, or intelligent laziness, a structure will always choose to channel its loads to the ground by the easiest of the may paths available. This is the path requiring the minimum amount of work on the part of the structural materials and is a consequence of what is termed in physics “the law of least work”.
• When a material is pulled, it is said to be in tension. Tension is easy to recognize because it lengthens the material. [...] We can detect tension easily in elements made of very elastic materials, like a rubber band. Pull on a rubber band and it easily becomes twice its original length.
• When a material is pushed it is said to be in compression. Compression, in a sense, is the opposite of tension, since it shortens the material. If we push on a rectangular sponge, the sponge becomes shorter.
• Structural materials are much stiffer than rubber bands or sponges. Their lengthening under tension or shortening under compression may not be seen by the naked eye, but it always occurs, since there are no perfectly rigid structural materials.
• The tiny changes in length due to tension and compression when divided by the original length of the element are called strains.
• The pull or push on an element, divided by the area it is applied to, is called stress.
• Since all structural actions consist of tension and/or compression, all structural materials must be strong in one or both.
• Strength is not the only property required of all structural materials. Whether the loads act on a structure permanently, intermittently, or only briefly, the lengthening and shortening of its elements must not increase indefinitely and must disappear when the action of the load ends. The first condition guarantees that the material will not stretch or shorten so much that it will eventually break under the working loads. The second insures that the material and , hence, the structure will return to its original shape when unloaded.
• A material whose change in shape vanishes rapidly when the loads on it disappear is said to behave elastically. A rubber band is correctly called an elastic, since it returns to its length when we stop pulling on it. All structural materials must be elastic to a certain extent, although none is perfectly elastic under high loads.
• Most structural materials not only behave elastically, but within limits show deformations that increase in proportion to the loads.
• All structural materials behave elastically if the roads are kept within given limited values. When the loads grow above these values, materials develop deformations larger and no longer proportional to the loads. These deformation, which do not disappear upon unloading, are called permanent or residual deformations. When this happens, the material is said to behave plastically. If the loads kept increasing after the appearance of plastic behavior, materials soon fail.
• If we progressively load a structure and measure its increasing deformations, we are warned that the structure is in danger of collapse as soon as we notice that these deformations grow faster than the loads. IN other words, materials that behave elastically under relatively small loads and plastically under higher loads do not reach their breaking points suddenly. Once they stop behaving elastically, they keep stretching (or shortening) under increasing loads until they continue to do so even without an increase in the loads. Only then they fail.
• Materials which do not yield are called brittle and cannot be used in structures, because they behave elastically up to their breaking point and fail suddenly without any warning.
• Thus, strength, elasticity, and plasticity are all necessary to good structural behavior.
• Steel, the strongest structural material available to man, becomes plastic at high temperatures and loses its strength at 1,200 degrees. Steel buildings must be fireproofed to retard the heating of its columns and beams in a fire. Concrete, instead, is a particularly good insulating material and prevents for a long time the heating and yielding of its reinforcing steel bars. Reinforced concrete buildings do not have to be fireproofed.
• On the other hand, at a temperature of minus 30 degrees, called its transition temperature, steel becomes brittle and breaks suddenly, particularly under impact, or suddenly increased, loads.
• The strength of  a structural material is measured by the number of pounds each square inch of material will carry before it breaks. This number, similar to hose measuring stress, is called its ultimate strength and varies from material to material and even in the same material depending on how it is stressed.
• Pound per pound, steel is the material with the greatest strength obtainable at the lowest price.
• Steel is an alloy of iron and carbon, with very tiny amounts of other metals to give it particular properties.
• In welding, the steel of the two parts to be connected is melted at high temperature and a welding metal is deposited at the joint. WHen a well-executed joint cools, the connection becomes as strong as the steel of the jointed pieces. The high temperatures used in welding, if reached or cooled too rapidly and concentrated in too small an area around the joint, may produce thermal locked-in stresses, which the steel is unable to resist.
• It must be remembered that steel is “fatigued” by reversal of stress from tension to compression and vice versa, when this cycle is repeated many times. We use this phenomenon ourselves to break wire by bending it back and forth a number of times.
• Possibly the most interesting man-made structural material is reinforced concrete. Combing the compressive strength of concrete and the tensile strength of steel, it can be poured into forms and given any shape suitable to the channeling of loads. It can be sculpted to the wishes of the architect rather than assembled in prefabricated shapes. It is economical, available almost everywhere, fire-resistant, and can be designed to be lightweight to reduce the dead load or to have a whole gamut of strengths to satisfy structural needs.
• Concrete is a mixture of cement, sand, crushed stone or pebbles, and water. The water and cement past fills the voids between the grains of sand and these fill the voids between the stones. After a few days the cement paste starts to harden or set and at the end of four weeks it gives concrete its nominal ultimate strength, which is as good as that of some of the strongest stones.
• Portland cement, as modern cement is called, is a mixture of limestone and clay, burned in a furnace and then pulverized. IMpervious to water, it actually becomes stronger if submerged after it hardens.
• IN reinforced concrete, bars of steel are embedded in the concrete in those areas where pulls will develop under loads, so that the steel takes the tension and concrete the compression.
• Plastics can be made as strong as steel in both tension and compression, can be given an elastic or a plastic behavior, and are practically indestructible. AMong the most useful plastics are those reinforced by glass fibers, like Fiberglass, which are shatterproof because glass, extremely strong in tension, has its brittleness cushioned by the plastic matrix in which it is embedded.
• Since we want our structures not to move, except for the miniscule displacements due to their elasticity, Newton’s laws of rest are the fundamental laws ruling the balance that must exist between all the forces applied to a structure.
• IN physics a body at rest is said to be in equilibrium, from the identical Latin word which means “equal weights” or balance. An understanding of two particularly simple aspects of the laws of equilibrium is essential to an insight into how structures work.
• Simple as this may seem, the task of the structure is to guarantee translational and rotational equilibrium of the building under the action of any and all forces and reactions applied to it, including, of course, its own weight. The task of the engineer is to shape and dimension the chosen structural materials so that the structure may produce equilibrium without breaking up, and with acceptably small elastic displacements.
• I-beams of steel with wide flanges, called wide-flange sections, are obtained by rolling heated and softened pieces of steel between the jaws of powerful presses and have flanges much wider than the top and bottom segments of a capital ‘I’. This is the most efficient shape a beam can be given to carry vertical loads horizontally from one point to another. One may think of a beam as a structural element that transfers vertical loads to the end supports along is horizontal fibers, as if the beam deflected the vertical flow of the loads by ninety degrees only to turn them around again in a vertical direction at the beam supports.
• Deep beams are stiffer than shallow beams. ON the other hand, the beam stiffness diminished dramatically with increases in length; doubling the length of a beam makes it sixteen times more flexible.
• When a beam’s ends curve up, as in a beam supported at its ends, its lower fibers are in tension and its upper fibers in compression. Whenever a beam curves down, like a cantilever, the upper fibers are in tension and the lower in compression.
• The moment a column bends out, the compressive force acquires a lever arm with respect to its axis and bends it progressively more. THis is a chain reaction where the more the column bends, the larger the lever arm becomes. This increases the bending action of the force, which increases the lever arm, and so on. Very soon the column fails in bending. The column is said to become unstable when the load reached its critical value.
• Since buckling is a phenomenon involving bending, it becomes clear why modern steel columns have the shape of wide-flange beams. Their resistance to buckling is magnified by this shape without a costly increase in material.
• Buckling is one of the main causes of structural failure.
• One of the most dangerous characteristics of a buckling failure is its suddenness, which gives no warning. Whenever a structure under load chooses the easy path of benign rather than the foressen path of compression, the structure may fail.
• Very thin, flexible elements can work only in tension. Strings and cables are so flexible that they cannot resist compression or bending, as a beam does. They can only resist pulls, and since they straighten when pulled, they are always straight between hanging loads. This is why, in order to carry loads by tension only, cables must change shape whenever loads change in location or number.
• Suspension bridges are the kings of the bridge world. No other method of construction can span greater distances. Their use of materials is totally logical.
• No large roof can be built by means of natural or man-made compressive materials without giving the roof a curved shape, and this is why domes were used before any other type of cover to achieve large enclosed spaces.
• Genius often consists of an ability to take the next step.
• Whether supported over the crossing of a church or directly on the ground, the dome must carry its own weight and the weight of the live load, including the pressure and suction of the wind and, in northern climates, the weight of snow.
• What makes the dome behave differently is the fact the the hypothetical arches it consists of are joined together along the vertical sections of the dome, making it a monolithic structure.
• Most domes are stiffened at their bottom by a strong ring, which to all practical purposes restrains the motion there.
• The compression strength of concrete--the most commonly used structural material the world over--has increased dramatically in the last few decades.
• Most limitations in construction are due to economic rather than technological factors.