Structures: Or Why Things Don't Fall Down (Da Capo Paperback)

by J.E. Gordon

Most engineers have had no aesthetic training at all, and the tendency in the schools of engineering is to despise such matters as frivolous.

Many people – especially English people – dislike theory, and usually they do not think very much of theoreticians.

Hooke realized that, if a material or a structure is to resist a load, it can only do so by pushing back at it with an equal and opposite force.

2. It is this change of shape which enables the solid to do the pushing back.

provide the basis of the modern science of elasticity. Nowadays, and with hindsight, the idea that most materials and structures, not only machinery and bridges and buildings but also trees and animals and rocks and mountains and the round world itself, behave very much like springs

the stress in any direction at a given point in a material is simply the force or load which happens to be acting in that direction at the point, divided by the area on which the force acts.

stress tells us how hard – that is, with how much force – the atoms at any point in a solid are being pulled apart, so strain tells us how far they are being pulled apart

This stress-strain diagram, which may look something like Figure 4, is very characteristic of any given material, and its shape is usually unaffected by the size of the test-piece which happens to have been used.

the graph is a straight line. When this is so we speak of the material as ‘obeying Hooke’s law’ or sometimes of a ‘Hookean material’.

The strength of a structure is simply the load (in pounds force or Newtons or kilograms force) which will just break the structure.

The strength of a material is the stress (in p.s.i. or MN/m2 or kgf/cm2) required to break a piece of the material itself. It will generally be the same for all specimens of any given solid.

strength for weight, metals are not too impressive when compared with plants and animals.

Strength. By the strength of a material we usually mean that stress which is needed to break it.

Anything which is, so to speak, elastically out of step with the rest of the structure will cause a stress concentration and may therefore be dangerous.

Therefore little energy is wasted or left behind to provide a recoil or to damage the weapon. In this respect, at least, bows and catapults are a great improvement on guns.

This is that one must never, never, never ‘shoot’ a bow or a catapult without a proper arrow or other appropriate missile.

High-quality swords are expected to be able to recover, elastically, after they have been bent so that the tip touches the hilt.

According to the pure and simple Griffith doctrine, a crack shorter than the critical length should not be able to extend at all,

cracks of less than the critical length do manage to extend themselves,

the failure of a structure may be controlled, not by the strength, but by the brittleness of the material.

a more important cause of fracture in old people is the progressive loss of nervous control over the tensions in the muscles.

The battens which cross the sails are attached to the masts and, since the whole rig is constructed from flexible materials, as the wind increases, the sail bows out between the battens after the fashion of Figure 8 without much loss of aerodynamic efficiency.

A riveted joint is reliable and easy to inspect, and in a large structure it acts to some extent as a crack-stopper:

nearly every substance will continue to extend or creep under a constant load with the passage of time.

Poisson was made an Academician – one of the highest honours France has to offer – at the age of thirty-one for his work on elasticity.

It is for this reason that, if a surgical wound has to be reopened within two or three weeks of the original operation, it may be dificult to get the new stitches to hold.

it is collagen which makes meat tough.

the failure was due to lack of stability and not to lack of strength.

The many leisure hours which are at the command of females in the superior orders of society may surely be appropriated, with greater satisfaction, to the improvement of the mind and to the acquisition of knowledge than to such amusements as are only designed for facilitating the insipid consumption of superfluous time...

Contrary to what one might suppose, weight at the top is likely to make a wall more, and not less,

am afraid that it seldom is. Anyone who looks at a modern housing estate or a new university cannot help seeing that the walls are full of cracks, and, where there is a crack, there must once have been a tension stress.

However, although these cracks do a good deal of damage to the plaster-work and interior decoration,† they seldom constitute any danger to the stability of the main structure.

The basic condition for the safety of masonry is that the thrust line should always be kept well inside t...

Although poets, politicians and other non-technical people have attributed special qualities to real and figurative keystones, in fact the keystone is functionally no different from all the other voussoirs, and its distinction, if it has any, is purely decorative.

All this means that arches are extraordinarily stable and are not unduly sensitive to the movements of their foundations.

the weight of the structure will increase as the cube of the dimensions;

The cross-sectional areas of the various parts which have to carry this load will, however, increase only as the square of the dimensions,

Unlike most other structures, buildings fail because they become unstable and tip up; and for any size of building this can be predicted from a model.

The stability of a building is like that of a balance; it is not affected by scaling up.

Therefore, for middle-aged people, it is wise to keep the thrust line as near the middle of the backbone as possible.

If we are concerned with the plain, semi-circular masonry arch which was widely used in Roman and medieval times, then one of the facts of life is that the rise of the arch must be about half its span.

A solid roof over one’s head is one of the prime requirements of a civilized existence, but permanent roofs are heavy and the problem of supporting them is really as old as civilization itself.

the beam is one of the most important devices in the whole of structural engineering.

The ceiling-joists, in particular, may be barely stiff enough to support the ceiling without causing the plaster to crack. If we are tempted to indulge in the fashionable activity of turning a modern attic into an extra bedroom, the most serious problem is likely to be the stiffness of the floor. Although the roof-truss is unlikely to break, the deflections caused by the extra weight of people and furniture may well cause serious and expensive damage to the house.

In the Land of the Free, where every man was an amateur, skilled craftsmen of the European type scarcely existed.

If tension is about pulling and compression is about pushing, then shear is about sliding.

Such materials, which include the metals, brick, concrete, glass and most kinds of stone, are called ‘isotropic’, which is Greek for ‘the same in all directions’.

The fact that metals are isotropic (or nearly so) and have the same properties in all directions makes life somewhat easier for engineers and is one of the reasons why they like metal.

Cloth is one of the commonest of all artificial materials and it is highly anisotropic.