with bits about dragonflies, fields, frogs, glaciers, pottery, railways, roads and trees
For Cracks Part Two - Click Here
Robert Maillart and the Zuoz bridge
In 1903, Robert Maillart observed that the spandrel walls of his Zuoz bridge had cracked near the abutments. A lesser man might have made sure that later bridges were stronger in that region, but Maillart saw that the material was actually not needed, since the bridge was still there. This logic led him to create some very economical designs, which were also very elegant, and eventually he invented the deck stiffened arch, in which the functions of stiffness and thrust resistance were separated.
Discovery by chance observation is not uncommon in the history of engineering and science, but it has to come to the attention of a prepared mind. The discovery of radioactivity by Henri Becquerel is an example.
Cracks caused by paint?
Do you think that painting something could cause cracks? Look at the next picture.
About a kilometer of this blacktop path runs along the side of a new road. Although it is not old, it has cracked along much of its length. The cracks wander about, and sometimes fade out, to replaced by others offset by as much as tens of cm. It is if a crack is needed, but the exact position is not important. The white line is a thick layer of paint, intended to divide the path into a cyclists' part and a pedestrians' part, though only a few people use the path each day. In some places the irregular cracks fade out and are replaced by almost straight ones along the edges of the line.
How does the line "attract" the cracks? It is much more reflective of light than the tarmac is, and will suffer much smaller changes of temperature with the coming and going of the sun. It may also have a different thermal expansion coefficient to the blacktop. The differential expansion has been enough to initiate the cracks that were already destined to occur because of the poor foundation. So the paint did not cause the cracks, but it certainly fostered their beginnings in particular places.
These cracks are not especially important, but cracks, or the avoidance of them, can be supremely important in the design, use and maintenance of structures. Large structures, and many lives, have been lost through cracks that were initially very small. "Papering over the cracks" is a well-known saying in places where wallpaper is used: it refers to any behaviour which avoids actions which are necessary to rectify a problem. "Brushing the dust under the carpet" has a similar meaning.
Here is a cantilever bridge of a type which is found in several places in Gloucestershire and Wiltshire. This one crosses the Barnwood bypass, east of Gloucester. There is another one about a kilometre to the east. The narrow roads leading up to these bridges exhibit cracks which are typical of slumping on embankments. The local ground is largely clay. Pictures at right show some of these cracks. The last picture shows soil nearby, cracking during drying out and shrinking. Why do you think that the cracks follow the line of the footpath through the crop?
Shortly after the pictures were taken, some of the cracks were "repaired" by covering them with tar and gravel. Shortly after that, the holes began to appear again. Some weeks later, an entire new layer of tarmac was placed.
These two pictures show sections of a tarmac footpath that runs alongside a tarmac road. Because the road is slightly elevated, there is a tendency for the edge of the path to slide away. The cracks in one picture show that the sliding has begun, but in the other picture, the cracks are few in number, and have not opened up. What is the difference? The cracks on the inside of the curved road have not opened. If the footpath is to slide inwards, its radius, and hence its circumference, would decrease, but this is not possible: there is no means of the path becoming shorter. In effect, the curved section of the path is acting as an arch against a horizontal force.
Here a crack that has been marked for "repair" has been populated by small plants. Larger plants would follow, with the build-up of soil. In winter, the freezing of moist soil would exert pressure on the sides of the crack. Over the years, a neglected structure is gradually taken over by vegetation.
The picture shows the join between one cantilever and the suspended span of the bridge mentioned above. Moss is beginning to grow in the space. Eventually soil will build up, enabling small plants to grow.
The diagram below shows how slumping can occur on a hillside or an embankment. The ground often breaks along a roughly cylindrical surface.
Here, the pressure of the soil, and the waterlogged ground have caused a large retaining wall to move. A crack has opened from top to bottom. Apart from that, the integrity of the wall has not been compromised, but it will need watching from now on.
Once cracks have opened, they can let in water, which in frost conditions can cause more damage by expansion. In spite of their name, the saxifrages cannot actually break up rocks: they only grow in existing cracks, like the plant shown here (not a saxifrage). But once a rock has cracked, the development of plants, especially trees, can accelerate the process of breaking up, as the roots spread and more soil is generated. The soil holds water which can further damage the rock. Fungi, too, can generate enormous pressure. So can the freezing of water into ice.
The cracks shown in the first four pictures below, developed in a flat footpath on flat ground, within a year of construction. The fourth picture shows red seeds that fell into the crack in the first autumn. These seeds will probably not develop, because of lack of soil, but smaller species certainly will, allowing the eventual formation of soil.
The fifth and other pictures show a piece of tree bark. The picture has been stretched lengthways by about 30 %. The pattern seems to be somewhat similar to the patterns of the cracks in the path.
The last pictures show plants that have taken root in the crack between kerb-stones and pavement, and moss, spreading from cracks in the pavement on a bridge. Without maintenance, many structures will slowly degenerate into wildness. That doesn't mean that we cannot allow pockets of wildlife habitats here and there.
These plants take advantage of cracks in rocks in the Alps. No plant can break a rock, not even, in spite of its name, a saxifrage. What they can do is to help the process of colonization by living things. This helps the process of increasing the amount of soil. Moisture in the soil can freeze, exerting forces on the rock as it expands, thus making cracks wider. Over a long period, rocks and walls can slowly disintegrate. During this period they can be a home for many different kinds of animals, from woodlice to snakes.
Crack Generation and Propagation
Banging nails into wood introduces tensile stresses around them. Eventually the wood may crack. The last picture shows the final result on the right.
These two pictures show the stump of a door handle which broke while a swollen door was being pulled shut. The differently coloured area at the bottom in both pictures suggests that a crack may have been present already, perhaps caused by repeated application of excessive force. The structure of the surface seems to radiate from that area.
These two pictures show a similar handle, unbroken, and a handle in a different style. The first one has a sharp transition between the lever and the cylinder, leading to a stress concentration. The second has a lever which is faired into the cylinder, leading to a much reduced concentration of stress.
A potential problem with stress concentrations occurs in designing the fuselage of a pressurised aircraft, or the deck of a ship, when material has to be removed from the ideal tube in order to provide access. The fuselage of an aircraft has to be pierced by various holes for doors, windows, undercarriage, antennas, and so on. The openings have to be designed carefully, to prevent stress concentrations. The Comet 1 airliner suffered explosive decompression when fatigue, starting at a hole, resulted in catastrophic spreading of cracks. This phenomenon is now much better understood, and all designs would now include measures to reduce the probability of cracks being generated, and also measures to prevent their propagation over long distances in the structure.
Sharp cornered hatches in the deck of a ship can result in stress concentrations which can be the source of cracks, which can propagate if not stopped. No cracked ship was available for photography, so a switch was photographed instead. The laws of engineering and physics work just was well for switches as they do for ships. The reason for stress concentration around cracks is that any object will deflect until it is in a state of minimum energy. It turns out that a few small volumes of high stress and a large volume of low stress often provide a lower total energy than a more uniform distribution of stress.
The three pictures above show damage caused by an earthquake. In all cases, cracks caused by failure in shear have gone to corners of windows and doors. Throughout the village, the damaged walls were all more or less parallel to each other, showing that the movement of the ground was coherent over the whole area. Few of the perpendicular walls were damaged.
As a result of considerations about construction, round tubes are not employed very often in bridges. What is beautiful to the engineer, the aesthete, and the financier may differ quite strikingly. Fritz Leonhardt, in his book "Bridges", explains the desirability of reaching a satisfactory resolution of these questions.
Nature does not experience the same constraints as people. Nature's constraint is that each step in evolution be attainable from the previous one, and that it should be a slight improvement in some way. The improvement need not be one that can be recognised millions of years later, when the use of an organ may be completely different from a previous use. Improvements that require a temporary set-back, however small, in overall probability of reproduction, cannot happen. Evolution has no foresight.
The next pictures show cracks in an old building which has been gutted for redevelopment. Two of the cracks began at the bottom corners of a rectangular window.