Attachments

15th July 2001    Back to Bridges    back to Home Page

For the want of a nail, the horse-shoe was lost . . . .

A part of the roof of a Boeing 737 blew off, after creeping failure of skin joints.  The plane landed safely.  One person was lost.  In 1974 the cargo door of a DC-10 blew off because a closure indicator showed that a bolt was home when it wasn’t.  Everyone died.   In 1979 an engine came off a DC-10 because of a faulty maintenance procedure on the connection between engine pylon and wing.  Everyone died.  In 1985 the rear dome of a Boeing 747 failed, destroying the fin and the control systems, because of a faulty repair to a joint in the dome.  Only four people survived.  Deadly and expensive failures can result from simple faults in joining parts together.

On a simpler level, if you make a bunt in a glider, and you aren’t strapped in, your next action will be to pull the rip-cord, assuming that you have a parachute.

Note, in the picture at the top of the page, how the joints are wider than the bars.  These are the chains of the Clifton bridge, near Bristol.  Look at the "knees" of a flamingo, or even your own ankles.  The difficulty of making joints, especially moving ones, was a factor in the long period which elapsed between the invention of variable sweep by Barnes-Wallis, and its use in actual aircraft, such as the Tornado, F111, F-18 and B-1.  However, numerous aircraft have had folding wings for use on aircraft carriers.

The jib of this crane is assembled by bolting together a series of short sections.  These are revealed by the widening of the top member, especially where the sloping bar holds the whole thing up.

In spite of all efforts in design and maintenance, pieces occasionally fall off aircraft.  If they fly clear, the aircraft will normally be able to continue, and the parts hardly ever hit anything significant on the ground.  Planes have flown with a piece of rudder or flap missing, or decompressed by ejection of a window or failure of a fuselage part, just as birds and butterflies can fly with bits missing.  But in the rare cases that a part falls on a runway, if the even more rare event happens where a wheel actually hits the part, a tyre can be damaged.  And in even more rare cases, this can damage the plane enough to bring it down.

No matter how strong the parts of a structure, they have to be joined to other parts, in a way that does not reduce their effectiveness.  This can present problems that are far from trivial.  Some airline passengers probably wonder what stops the wings falling off.  In fact, of course, the wing structure goes right across through, over, or under, the fuselage, forming a continuous beam, to which the fuselage is attached.  The attachment must transmit the forces in a way that does not produce too much stress concentration.  In fact, the two wings may be bolted to the central beam.  The bolts have to withstand enormous forces caused by the bending moment.  When a Boeing 747 exploded near Lockerbie, these bolts held, and although most of the fuselage was shattered, the wings were intact until they hit the ground.

If the wings are to move, as in some carrier-based aircraft, or in types with variable sweep, the weight penalty may be severe.  It may be partially offset by a reduction in the requirement for high-lift devices for take-off and landing.  Imagine the structural needs of the wing of a bird or beetle, both of which fold the wings when not in use.  To appreciate the difficulties faced by the designers of movable connections, we need only look at the human ankle, the legs of a horse or a flamingo, the universal joint in the transmission shaft of a truck, or the hinges of the blades on a helicopter.  And think about how many problems, especially in old age, and in people who take part in games and sports, are caused by wear and tear or disease of joints.

Here are some pictures taken on a very dark morning after a severe storm.  Most of the tree-branches broke at or near the point of attachment.  Why do you think that in so many cases, the branch brought away a long strip of the trunk, instead of just snapping off?

And here is the same phenomenon on a much larger scale, showing a common type of fracture.

This picture shows a view from above of a gate post made from a tree trunk.  Much of the middle has rotted away, but five conical parts remain, pointing inwards and downwards.  These are the remains of the anchorages of five branches that formed at that height on the tree, as can be seen from the "knots" on the outside.

This tree has split right down the trunk, which is seen to have been hollow.  The break probably started high up at a branch point.

The other pictures show pieces of wood cut from a place where two branches grew out, together with a computer simulation.  If you look at an old fallen tree you can often see clearly the flow of the stresses to which its growth was a response.  Given the enormous time-scale of evolution, we can assume that natural structures represent good compromises between all the requirements for survival and reproduction.

A plant or an animal is never strong enough to resist all possible forces, for it would be at a disadvantage in more usual circumstances.  The evolved structure is strong enough that on average the species continues.  As many as a third of adult gibbons have broken bones because of misjudgments or breaking branches.  Were they stronger, and therefore heavier, their speed would be reduced, and they would probably be less successful in feeding, reproducing and escaping.  On the other hand, some male animals, such as elephant seals, have gone to the other, massive, extreme, and have evolved suitable mating behaviour along with the increase in size.  

Given that most animals have to reproduce by sexual means, and that many, from spiders to tigers, are fierce predators, ingenious means have evolved to achieve coupling without suicide.  Nevertheless, in some species such as mantids, male promiscuity is not usually possible.  Plants, being mainly immobile, have evolved an enormous range of mechanisms to get their gametes together, often using animals to transport pollen.  In fact, many flowers have probably evolve together with the corresponding insects and birds.

The same is true of many mammals and their fleas, which have also evolved together, so that the fleas cannot live apart from their mammal.  Lichens are symbiotic pairs of fungi and algae.  We ourselves contain mitochondria, which may represent ancient species which have become almost a part of ourselves.  These species bonds may not be physical, but they are strong nevertheless.

Returning to more tangible ideas, the difficulty of transmitting huge forces can be appreciated if we think of an elephant, the Eiffel tower, a Saturn rocket, or a Boeing 747.  In each case, huge forces are at some places transmitted through quite small areas.

Stress concentrations at bolts, rivets, rivet holes, welds, and other joints, are major sources of concern.  Deformed rivets from the Eiffel tower are on display in the shops, showing dramatically the effects of continuous long term stress.  Look at pictures of the wreckage of the first Tay bridge to see the effects of poor connections between the iron piers and the masonry below.  Connections do indeed begin with the foundations, and end only at the top.  Examples – Tay bridge 1 and Tay bridge 2.

Watch a weight-lifter and look at the great care taken to ensure the best possible connection of the hands to the bar.  Anyone who has carried heavy shopping bags will know about this.  The connection formed by a handshake is in fact a symbol of goodwill between people, and the term is even used in electronics and communication to denote the correct exchange of information.  We sometimes speak of marrying parts together.  Other forms of touching, such as holding hands, kissing and hugging, are also used by people.

Some common phrases referring to joining are – "coming apart at the seams", "a screw loose", "hold on", "losing his grip", "buttoned up", "unbuttoned", "unhinged", "a stitch in time saves nine", "stitched up", "Let these persons be joined together in holy matrimony".

Here are some common requirements –

Within materials –

Cohesion – cast iron, spider web, steel, etc

Within parts –

Reinforced concrete, pre/post-stressed concrete, fibreglass, tufnol, and other composites – avoidance of separation, delamination and cracking (see web-page about cracks, in this web-site).

Between parts –

Joints – Compression, Tension, Sliding fit, Rotating joints

Here are some commonly used ways of joining and holding things –

Anchor, Arc welding, Ball and socket, Ball race, Belt, Bluetack, Bobby pin, Bolt, Brazing, Buckle, Bulldog clip, Bush, Button, Cement, Chain, Chuck, Circlip, Cleat, Clevis pin, Clip, Clip-board, Clothes peg, Collet, Contact adhesive, Contact welding, Cotter-pin, Crimp, Crocodile clip, Cyano-acrylate adhesive, Door-bolt, Double sided tape, Dovetail, Dowel, Drawing pin, Drift, Duct tape, Electromagnet, Electrostatics, Epoxy, Expansion bolt, Explosive bolt, Eye-bolt, Flange, Friction welding, Gasket, Gecko feet, Glue, Grub-screw, Gummed paper, Hasp, Hat-pin, Hinge, Hook, Hook and eye, Impact adhesive, Jubilee clip, Jumar, Karabiner, Kirby grip, Knot, Lace, Latch, Lock, Lock-washer, Magnet, Magnetic chuck, Match casting, Mooring rope, Mortar, Mortice and tenon, Nail, Nut, Olive, O-ring,  Paper-clip, Passepartout, Peg, Picture hook, Pin, Piton, Plain bearing, Plug and socket, Popper, Pop rivet, Post-it, Rawlplug, Rawlbolt, Redux bonding, Rivet, Rope, Rubber band, Safety belt, Safety pin, Screw, Sealing wax, Self-tapping screw, Sellotape, Set screw, Shrink fit, Shrink wrap, Solder, Split pin, Spot welding, Stamp hinge, Staple, Stitching, Strap, String, Sucker, Superglue, Surface tension,  Tenon, Tendon, Tent peg, Thermite welding, Tie-clip, Tie-pin, Toggle, U-bolt, Universal joint, Velcro, Vice, Wedge, Weight, Welding (arc, contact, friction, spot, etc), Wire, Woodscrew, Wringing, Yorkshire, Zip.

Within these groups, think how many types of plugs and sockets, and how many types of screws, bolts, nuts and washers there are. 

Some of these types of fastenings may require water tightness, permeability, gas tightness, electrical insulation, electrical conduction, heat insulation, heat conduction, stiffness, flexibility, corrosion resistance, vibration resistance, pressure resistance, damp resistance, temperature resistance, sliding, rotation, inspection, reliability in inaccessible places, and so on.  And all parts of a joint must be compatible with each other, without unwanted effects, which may be binding, slipping, chemical action, electrolytic action, and other problems.  A joint may need to be reliable for many years, yet demountable for inspection, modification, or repair.

Parts of the famous iron bridge in Coalbrookdale are connected in ways which would not be used today, and were probably hardly ever used again.  The designer may have decided that innovation in materials was a sufficient leap of faith in itself, without inventing new jointing methods.  Certainly the history of projects that have tried to innovate in every possible way has not always been happy.  On the other hand, trying to invoke a new technology without supporting it with new design and construction techniques may nullify some of the potential gains.

Here are some examples of attachments in bridges –

Attachments that we don’t see are the roots of a tree, and the foundations of structures, which have to withstand the steady weight of the dead loads, and intermittent live loads from wind or traffic.

Joints

Here is a diagram which highlights the problem of joining two narrow members.  The gusset plate points the way to the idea of triangulation, which is the basis of trusses.  Without the plate, the large ratio of L and W means that an applied force produces a much larger force at the joint.

If joints are required only to provide connection, and not rigidity, they can be hinged or pinned, as here.  This is one of a row of wooden pillars which support a canopy on a building near the docks in Bristol.  These pillars remind us of the masts of Brunel’s ship, the Great Britain, which is being restored nearby.  The ship provides a wealth of technical and social insights.  Masts can be hinged like this, and held in place by rigging, or stepped into the hull, in which case they can be self supporting, though support from rigging is desirable.  On board the Great Britain you can see hinged masts. 

Foundations

Every bridge has to rest on the ground in at least one place.  The supports have to be placed so as not to move unacceptably, which means that the stresses must be reduced to values that can be supported by the ground.  On hard, strong rock, the supports can be narrow, but in weaker ground, a wider foundation may be necessary.

These three pictures show a piece of foam plastic which has been strained by pushing objects against it, to represent a pillar resting on the ground.  The strains are revealed by the square graticule that was drawn with a fibre pen.  From the distortions we can deduce the following facts –

The strains are concentrated near the point of application.

The strains are spread over a large area.

There is tension, as revealed by the curved upper edge, which is longer than the original straight edge.

There is shear, as revealed by the angles which are no longer ninety degrees.

And of course there is compression.

The final configurations are those which minimise the total strain energy.

This picture tries to give a rough idea of the way that pressure diffuses through the ground under a heavy weight.  The ground has to able to withstand the stresses at all points without giving way, either quickly or by creep.  In any volume where this is not the case, the ground must be replaced by a structural material.

Click here for a web-site dealing with earth structures and related matters.  It also includes numerous links to web-sites about engineering and science.

Similar considerations apply at the top of a structure.  Here the load on a roof beam is spread by means of a piece of wood.  Old Chinese and Japanese buildings often have complicated arrangements of brackets which spread the loads at the tops of the pillars.

The currently smallest known means of joining is represented by gluons, which are considered to transmit the interaction which holds quarks together in particles such as protons, which have a dimension of about 10^-15 m.  In spite of the immeasurably small "size" of the quarks, the force is reckoned in tonnes weight.

The largest known means of joining is represented by gravitons, if indeed they exist, mediating the gravitational force, which is believed to reach across the entire universe.  

The strongest known means of joining are probably the forces between quarks, mediated by gluons, and the forces holding everything inside a black hole.  The calculation of the results of gluon forces is complicated immensely by the fact that gluons themselves are held together by gluons, leading to the possibility of "glueballs" – particles containing gluons without quarks.  Calculation with gravity is also complicated by the fact that the field itself has energy.  Some discussion of these points can be found in the pages on physics.

The weakest known force is gravity, at the energies with which we are familiar.  But some physicists believe that at high enough energy, or temperature, the strong, electromagnetic and weak forces reach comparable strengths, though at lower energies they span a range of  about 10¹³ in magnitude.  Perhaps gravitation will one day be joined in this unification.

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