Why Buildings Fall Down by Matthys Levy and Mario Salvadori

• The accidental death of a building is always due to the failure of its skeleton, the structure.
• A dome may be naively thought to be a series of vertical arches (its meridians) rotated around a vertical axis and sharing a common keystone, and in fact, a dome does carry to earth its own weight and the additional weights on it by such a mechanism. But these imaginary arches are not independent of each other; they are, so to say, glued together and, hence, work together.
• In practice, all structural failures may be considered due to a lack of redundancy.
• THe simplest example of the difference between a stable and an unstable mechanical situation is demonstrated by a marble resting at the bottom of a bowl as against one balanced at the top of the same bowl turned upside down.
• Redundancy implies that a structure can carry loads by more than one mechanism--that is, that the forces on it can follow alternate paths to the ground. It guarantees that if one mechanism fails, loads can still be carried by other mechanisms.
• The earth’s crust is like a broken eggshell floating on the viscous inner magma of melted rock. Plate tectonics, the recently develop theory of movements of the earth’s crust, offers the most plausible explanation for the existence of bands all over the globe along which most earthquakes occur.
• Bricks and concrete blocks joined with a cement mortar are very strong in compression but posses little tensile strength.
• Metal fatigue [is] the weakening of metals subjected to frequent reversal of stresses from tension to compression (from pulling to pushing) and vice versa.
• Aircraft structures are particularly subject to fatigue, caused by the alternative pressurization and depressurization of the hull and the bending of the wings up and down as the plane flies through varying meteorological conditions.
• The danger of fagieu is increased even by the unavoidable microscopic imperfections of the metal, such as minute pinholes or cracks, because stress concentration occurs at all these discontinuities.
• Soils often act as the very worst structural materials, yet we must entrust the support of all our buildings to their strength.
• Only five basic factors influence every structural design and, hence, the safety of all built structures, and each may be totally or partly responsible for a failure. They are:
• Structural theories
• Calculation techniques
• Material properties
• Communication procedures
• Economic factors
• The ancient world achieved amazing feats of structural virtuosity on the basis of a limited knowledge of structural theory.
• A frame structure is one in which beams and columns are rigidly connected, either welded or bolted together.
• With the exception of wood, natural materials suffer from being strong in only tension, like vegetable fibers, or only in compression, like stone.
• The invention of reinforced concrete by French engineers in the middle 1800s produced the first all-purpose artificial material, used today all over the world to build some of our tallest buildings, our largest roofs, and our most economical housing.
• As noted, steel’s increase in strength is all too often accompanied by an increase in brittleness. This brittleness will limit steel’s strength much above that already reached.
• Economic factors have always been of the greatest importance in structural design.
• Ambition, one of the prime movers of human activity, may push us to erect new towers of Babel or to devise better design and construction methods. We must conclude that in the field of structure, as in any other field of human endeavor, technological improvements alone cannot guarantee a decrease of failures and may even increase it. Only a deeper consciousness of our human and social responsibilities can lead to the construction of safer buildings.
• When all is said and done, most of the time structural failures flow from human error, always in concert with physical forces or loads acting on structures. If the earth did not attract, the wind did not blow, the earth’s crust did not shake or settle unevenly, and temperature did not change, there would be no need for today’s structure.
• Architectural structures consist of massive elements, like columns, beams, arches, and domes, and their own load, the so-called dead load, is most of the time the heaviest they must support.
• The evaluation of the dead load of a structure presents the engineer with a paradox: It cannot be computed until the structure is designed, but the structure cannot be designed until the dead load is computed and added to all the other loads.
• The gravity loads the structure must support in addition to its own dead load are called live loads and include the weight of the furniture, people, goods, fixtures, snow, etc.
• Slowly growing loads are called static loads or said to act statically. Other loads, like those caused by winds and earthquakes, grow rapidly or even suddenly; they are called dynamic loads or said to act dynamically. There are the cause of many disastrous structural failures and high losses of life.
• Weight applied dynamically can be equivalent to twice its static weight.
• Loads applied suddenly (like the blow of a hammer on a nail) are called impact loads and can be equivalent to many times their static values; they can be very dangerous unless their dynamic effects are taken into account.
• The effect of a force changing in value depends not only on how fast it changes but on the structure it is applied to. This is so because each structure has a characteristic time of vibration, called its fundamental period or simply its period, and each force will have its own static or dynamic effects on the structure depending on one thing: Does the force reach its maximum value in a time longer or shorter than the structure’s period?
• A tall building oscillates in the wind like an upside down pendulum.
• Varying forces may have a different type of dynamic effect if they are repeatedly applied in rhythm with the period of sthe structure. Such forces are said to be in resonance with the structure and called resonant forces. These rhythmic forces are particularly dangerous, because with repeated rhythmic application the effects accumulate and can reach large values.
• The oscillations of a tall building must sometimes be damped to avoid the inconvenience of airsickness to the occupants. This has recently been done by the use of a gadget first introduced to damped machinery oscillations, called a tuned dynamic damper. The damper action is based on the Newtonian concept of inertia: that a mass tends to stay put (or move at a constant velocity) unless acted upon by a force.
• Violent motions of the earth’s crust, the quaking of the earth, shake buildings and generate high dynamic loads in their structures.
• Loads caused by changes in temperature, thermal loads, and those resulting from uneven settlements of the ground, settlement loads, are particularly insidious because they are not visible, like those caused by gravity, and may be most damaging if neglected.
• We learn in school that when temperature increases, bodies expand and that they contract when temperature decreases.
• The forces acting on a structure can only pull or push on its elements. To speak of these forces in engineering parlance, structural elements can only be put in tension or in compression by tensile (pulling) or compressive (pushing) forces.
• All structural materials are strong i either tension or compression, and one--steel--is equally strong in both.
• Concrete is [...] weak in tension, and this is why French engineers suggested in the 1850s that steel bars be embedded in areas of concrete beams and other structural elements eher loads could develop tension. They thus invented reinforced concrete, which today is the most economical and widely used structural material in the world over.
• A material’s strength in tension or compression is determined by testing how much load each unit area can resist before breaking.
• Engineers are very conservative, as they should be to avoid failure. They will not allow a structural material to “work” at more than a fraction of its ultimate strength. When dealing with static loads, they adopt safety factors on the order of 2--that is, they use working stresses about one-half of the ultimate strength. When considering dynamic loads, they may required coefficients of safety as high as 4 or even larger.
• The greater the uncertainty about the material’s strength, the values of the loads, or the behavior of the structural system, the higher the coefficient of safety.
• In addition to the property of strength, a structural material under load must exhibit two behaviors called elasticity and plasticity. The first requires that when a load is removed from a structural element, the element returns to its original unloaded shape. The need for this requirement is fairy obvious: if, upon unloading, the element remained deformed, the next time it is loaded an additional deformation would appear, and after a number of loadings and unloadings the element would be so deformed as to be unusable. Most structural materials not only behave elastically as demanded by the requirement we have just discussed but also deflect under load in proportion to the load and are said to be linearly elastic. [...] This property is essential since if the deflection is larger than the proportional deflection you expect from linearly elastic behavior, you know that the material is overstressed.
• When a material behaves elastically, the stress is also proportional to the deformation (elongation or shortening) of a unit length of material, which we call the strain.
• Materials that exhibit a permanent deformation after a certain load is reached (called the yield point) and are said to have a plastic behavior above the yield point are preferred because a permanent deformation is the loudest alarm a material can give that it is ready to fail and should not be subjected to additional loads.
• We build structural systems by putting together structural elements made out of structural materials.
• The first requirement of any architectural structure is to stay put, not to move, or as engineers say, to be stable.
• A structure not only must be stable--that is, not be subjected to large displacements--but, except for the tiny changes in the shapes of its parts caused by the forces acting on it, must not move at all; it must be in equilibrium. This requirement implies, of course, that each element of a whole structure must also be in equilibrium so that the structure will stay together.
• A straight element in pure tension is called a tension bar or simply a bar and is used in many industrial buildings and roof designs.
• A cable is a stable structure when used as a vertical hanger pulled by a single load but is unstable when hanging from two points and carrying moving or variable loads because as the loads change position or value, the cable must change shape in order to be able to carry them only by means of tension.
• Cable instability limits the use of cables in architectural structures despite the enormous strength of modern steel cables.
• Cables are the most essential element of large structures like suspension bridges, suspended roofs, balloons, and tents.
• A straight element under pure compression is called a strut and is used mostly in bridges and roofs. When used vertically, a struct is called a column. The column has the basic function of transferring loads to earth.
• The more you pull a bar, the straight it becomes, but if you compress a strut too hard, something unexpected takes place. A thin strut submitted to an axial compressive load will not remain straight but bent out suddenly, or buckle, at a specific value of the compressive load, called its critical or Euler value.
• Today buckling is considered a very dangerous structural phenomena because our strong materials allow us to design thin elements in compression (columns, struts, arches, and domes) that buckle without giving notice.
• If an axe is pushed into a piece of wood, it splits it by pushing out the wood fibers. By the same kind of a wedge-shaped stone pushes out on two adjacent wedge pieces of stone with the force of its own weight and the loads on it. This is why an arch can built with materials strong in compression by means of wedge-shaped stones; it works in compression and stands up, provided its ends are prevented from moving outward by stones anchored in the soil, called abutments.
• Since the inward push, or thrust, of the abutments on the arch is essential to arch action, weak abutments are the most common cause of arch bridge failures.
• A beam is a straight, usually horizontal element capable of transferring vertical loads to its supports horizontally. [...] If such a beam is set on two end supports and loaded at midspan, it deflects in a curved shape, and the distance between the vertical line segments shrinks at the top and lengthens at the bottom. SInce lengthening is always due to tension, and shortening to compression a beam on two supports, or simply supported, develops tension below its neutral axis and compression above it; it is said to work in bending.
• Beams of reinforced concrete must have steel reinforcing bars located where nerve tension may develop under load.
• In steel construction, two basically different types of joints are used, depending on whether one wishes to allow or prevent the relative rotation of adjoining elements are called hinges, while those preventing it are said to be rigid or moment-resisting joints.
• In steel elements, hinges, also called shear joints, are usually obtained by connecting only the web of a steel beam to the supporting columns. Moment connections are obtained by connecting both the web and the flanges.
• The behavior of structures joined by shear connections is totally different from that of structures joined by moment-resisting connections. Moment-resisting joints give the structure monolithicity and greater resistance to lateral forces, like wind and earthquakes, but increase the value of stress caused by changes in temperature and soil settlements. Shear joints weaken structural resistance to lateral loads while reducing the values of thermal and settlement stress.
• Trusses are obtained by joining tension bars and compression struts by means of hinged joints. The variety of such structures is obviously great, but all consist of combinations of the same type of rigid element, a hinged triangle, the simplest rigid shape a structure can have.
• Because moment-resisting connections are more costly than shear connections, many modern high-rise buildings have hinged frames carrying the gravity loads and a central core of reinforced concrete walls, resisting the lateral forces of wind and earthquakes. The hinged frames lean on the core for support against lateral forces and would collapse like a house of card were it not for the bending resistance of the core, which acts like a stiff, tall, thin tower.
• Ballon roofs consist of large plastic membranes, stiffened by steel cables, that are curved in the shape of tensile arches by low air pressure that keeps the membrane up.
• All loads on buildings, including the preponderant dead load, must be supported on earth. If the earth’s surface, as most of us subconsciously expect it to be, were evenly strong and stable, and all soils equally consistent and resistant to compression, the design and construction of foundations would be an easy task. Unfortunately soils have different and variable consistencies so that even in the absence of earthquakes they move.