Brick, stone, cast iron, wrought iron, steel, mild steel, high-tensile steels, alloy steels, aluminium, steel-reinforced concrete, pre-stressed concrete, post-stressed concrete, glass-reinforced plastic.  These are some of the materials that are available for bridge building.

Bending, compression, impact, oscillation, pressure, tension, torsion, vibration; contraction, corrosion, expansion, fatigue, friction, rain, river flow, sea-water, scouring, temperature, tidal flow, turbulence, waves, wind erosion, wind pressure.  These are some of the stresses that materials must withstand, in a variety of combinations.

What do we want from materials?  Strength?  But what is strength?  It is a word with many meanings.  Here are some attributes of a material that might be important, and which might come under the heading of strength.

Amount of force in tension that it will withstand without breaking.  In order to standardize on size, we use the concept of stress, which is force per unit area.  So we refer to the ultimate tensile strength.

But if the material stretches a lot before breaking, we might consider the stress that it can take while retaining the ability to regain its original condition.  And we need to know how much it stretches for a given force, which is the modulus of elasticity, or Young's modulus.  After all, for any given piece of steel, we can find a big enough piece of rubber that will be as strong, but we wouldn't get much traffic on a rubber bridge.  A related quantity is the amount of energy that a material can absorb.  Organic materials such as tendon are superb in this respect, and only recently have engineers had at their disposal similar artificial materials such as Kevlar.

As well as tension we must consider compression, torsion and bending.

And still we are not finished.  We can combine materials to use their best properties.  Fibreglass, or glass-reinforced plastic, is an example.  So is reinforced concrete.  We can take this further by employing pre-stressing, which enables concrete beams to withstand tension, which plain concrete, like cast iron, is very ill suited to take.

We have considered strength and elasticity.  What else?  Cost is extremely important.  Some very strong and light materials are simply too expensive to use, except in specialist projects in which raw cost is over-ridden by some great necessity.

Assembly is more than a simple matter of connecting all the parts.  Welding produces high temperatures, which produce expansion and distortion.  Management of the cooling process is vital.  Poor quality control of welding may allow detrimental changes to the properties of the metal.  The weight distribution of a structure changes during assembly, requiring precautions such as adjustable jacking.  Forcing two parts into alignment produces undesigned and undesirable stresses, which can start cracks.

We are still not done.  Once we have bought our material and fashioned into the structure we want, we have to maintain it.  Air contains mainly nitrogen and oxygen, but it contains much smaller quantities of gases which were generally not present 500 years ago.  Sulphur dioxide and sulphur trioxide, and nitrogen oxides, are some of these.  These gases are constantly available at at every surface, aided on occasion by rain, which wets the surface and increases the intimacy of contact.  What we are talking about is corrosion.  Riveted or bolted joints may be penetrated by thin films of liquid, which may do its insidious work unseen.  There is an entire industry which provides solutions such as painting and anodizing.

Mechanical problems too, do not cease at the end of construction.  Traffic may increase ten-fold or more within twenty years.  Fatigue may change the internal structure of a metal or alloy.  Creep may change dimensions.

The page about problems provides more detail about some of these topics.

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