Civil Structures
Bridges are used by pedestrians, animals and vehicles to span gaps. They make the journey safer, quicker or shorter. Bridges may be loaded with compressive forces, tensile forces, torsional forces, and shear forces. The forces may be applied as dead load (weight-force of bridge), live load (load frequently changes, i.e. traffic), impact load, and environmental load.
Historical and societal influences
· Historical developments
- Beam bridges:
o Can be simply supported or cantilevered
o Truss bridges are composed of connected members which are stretched in compression or tension, triangles are constructed as they are most rigid shape
o Opening bridges used to allow water vehicles to pass
- Arch bridges
o Forces members into compression therefore directing the forces onto abutments
- Suspension bridges
o Support the deck on tensioned cables strung between support towers
o Tension members don’t buckle so they can be thin and light, enabling longer spanning bridges
o Fin-shaped box-section deck enabled less wind impact and lighter bridge
· Engineering innovation in civil structures and effect on peoples lives
- Primitive beam and suspension bridges in jungles
- Beam ridges in ancient times for human and animal transport
- Romans revolutionarily developed arch bridges which was more stable and secure, improved transport, did not impede water traffic
- Truss girder in 16th century improves beam bridge design which was longer spanning, greater safety, less pylons
- Modern suspension bridge in 19th century which was even longer spanning and little impediment to water traffic
- 20th century first box girder which enabled freeway construction, beneficial for road freight
- First modern balanced cantilever bridges which limited water traffic impediment and had an effective use of materials
- Steel cables in suspension bridge (Brooklyn Bridge)
- Concrete very commonly used for arch bridges, altering bridge design significantly
· Construction and processing materials used in civil structures over time
Material
/ Used in / Advantages / DisadvantagesTimber
Rope
Stone
Bricks
Cast iron
Wrought iron
Steel
Concrete / Early beam bridges, from logs or trees, deck for early suspension bridges
Early suspension bridges
Roman arch bridges
Arch bridges, viaducts
All bridges, frames, since 18th century. Replaced stone arch bridges
Early suspension bridge chains to suspend deck
All bridges, cables, 19th century. Replaced cast/ wrought iron and enabled reinforced concrete
Beam and arch bridges, also used as deck and abutments. 20th century, replaced nearly all old materials / Readily available, easy manufacture, environmentally friendly
Easy to use; distributes load; cheap
Doesn’t rot; strong in compression; availability
Doesn’t rot; strong in compression; easily shaped
Strong in compression; easily melted and casted saving time and introducing pre-fabrication; hollowable; light
Used for cabling
Malleable, ductile, tough
Equally strong in tension and compression, relatively cheap, cables enabled longer spans, creates reinforced, pre/post stressed concrete
Strong in compression – reinforcing with steel gives strength in tension; easy to transport/shape, fabricated on site / Rots
Rots, relatively weak
Heavy; time-consuming; expensive to manufacture
Not strong in tension
Rusts; have to paint; weak in tension and shear
Unreliable material due to fibrous structure therefore weakening it and limiting length
Rusts, corrodes, quite heavy
Shrinks and can crack, heavy, weak in tension
· Environmental implications from the use of materials
- Timber – deforestation of areas, old growth forests hard to recover
- Stone – digging large quarries scar landscape and severely impact on flora and fauna
- Bricks – need vast amounts of clay and shale and therefore large pits were dug, sometimes causing subsidence
- Cast/Wrought iron – require fossil fuel combustion, and iron mining and smelting caused processing plants and transport facilities (railways), producing emissions
- Steel – impacts similar to iron but much more profound as steel is more widely used
- Concrete – obtaining minerals and aggregate impacts on landscape
Engineering Materials
· Testing of materials
- Non-destructive testing
o X-ray: X-rays passed through object and exposes a photographic film on opposite side. If cavity present, the rays will pass through that area easier and thus more x-rays will reach the film and will show a darker patch, revealing possible cavity
o Dye penetrant: determines crack in metallic objects. Surface prepared by removing oil or surface contaminants, area heated, dye placed over surface and excess wiped off, a developer (white powder) sprinkled over surface, object cooled. Cracks revealed under UV light as they will squeeze dye out due to contraction of cooling object
o Ultrasonic: like x-rays, though high frequency radio waves used instead. Transmitter sends ultrasonic waves through object and reflected back, reflected signals are recorded by a display. If cavity present, the sound wave will be reflected early and the display will show lower reading, meaning less thickness
- Destructive Testing
o Tensile: used to determine the tensile strength of materials. Test piece is stretched and load and extension are recorded
o Compressive: used to determine the compressive strength of materials. Test piece is compressed and load and deformation are recorded
o Transverse: used to determine a materials performance when undergoing bending and shear
o Torsion: done to see how a material will cope with twisting forces (couples)
· Ceramics
- A ceramic is any hard, brittle, heat-resistant, and corrosion-resistant material made by shaping and then firing a nonmetallic mineral at high temperature
o Stone: Sandstone widely used in early construction because of its availability. Others include granodiorite (in Harbour Bridge’s pillars), slate, and granite. All stone is weak in tension, strong in compression, and brittle (therefore easy to chisel but had to be in compression). Sedimentary rocks look grainy. Igneous and metamorphic are very dense
o Glass: is amorphous (no crystalline structure). Used widely in modern days for transparency. However, the amorphous structure disallows shaping by force, as atom slipping cannot occur. Shaping must be done at high temperatures where viscosity is reduced. Glass is brittle, weak in tension (prompting development of toughened glass).
o Cement: can be hydraulic cement (harden in water) or non-hydraulic. Portland cement (hydraulic) widely used. Cement strong in compression, weak in tension, brittle, can be casted and shaped very easily. Cement hardens quickly, but doesn’t reach full bond strength for many years.
· Composites
- Made of different materials combined together to capitalize desirable properties of each
o Timber: cellulose fibres are held together by a natural resin. Tracheids in a lignin matrix. Can be pored (hardwood) and non-pored (softwood). Hardwood has pores/vessels running through the structure while softwood has a neater, more uniform structure without pores. Timber has excellent strength/weight ratio, can bend, good stiffness (high Young’s modulus). However, it is affected by weather and pests.
o Mortar: material used between bricks in construction. It is a paste that slowly hardens into a hard solid. It contains Portland cement, sand and lime in the ratio of 3:2:1. Mortar has good workability (smooth, easily spread, sticks), and good bond strength which is durable.
o Asphalt: widely used for road surfacing. It is hard aggregate and bitumen as the matrix. Asphalt is tougher than concrete (therefore can deal with slight movements better), crack resistant (due to bitumen), hard to wear, and impervious to oil contamination.
o Concrete: very widely used today. Consists of cement, aggregate, sand, and water. The sand fills in the gaps between the aggregate while the cement is the matrix. The sand and aggregate contribute to concrete’s great strength. With less cement needed it is cheaper and used extensively. The ratio of aggregate, sand, cement and water is 4:2:1:0.5. Concrete is strong in compression, weak in tension, brittle, fireproof, corrosion resistant.
· Reinforced concrete: To overcome tension failure in the lower section, rods or steel mesh is used to take the tensile load, making the concrete more failure resistant. To improve performance of reinforced concrete, two methods are used that place the concrete into compression prior to loading
§ Pre-tensioned (Pre-stressed): steel rods tensioned, concrete poured over and sets and cures, cables released trying to return to unstrained state, compressive stress set up in concrete beam
§ Post-tensioned (post-stressed): tubes running through slab, concrete poured and sets and cures, steel wires pulled through tubes and anchored at one end and tensioned at the other, this pulls and compresses slab, once required tension is gained the wires are anchored, then cement slurry injected into tubes to stop corrosion
o Geotextiles: woven polymers or ceramic fibres, used for stabilizing road base. Geotextile sheet placed between subsoil and road base, stablising road surface and making it less likely to form potholes
· Corrosion
- The chemical deterioration of a material
- Oxidation: occurs when the metal loses electrons, and it occurs at the anode. Oxidation = Loss
- Reduction: consumption of electrons and occurs at the cathode.
Reduction = Gain
- Some metals tend to be anodic while others more cathodic. Cathodic metals tend to be more stable and less affected by corrosion
o Dry corrosion: occurs through chemical reactions of metals/alloys with gases, at high temperatures. The metal reacts with oxygen and other molecules in flue gases
o Wet corrosion: occurs when metal is placed into fluid, usually and electrolyte (a solution containing ions which carry electric charges)
· Uniform attack: when metal placed in electrolyte, some parts will become anodic and other cathodic. The locations of the anode and cathode continually changes, resulting in uniform corrosion. E.g. steel sometimes produces uniform layer of rust
· Galvanic attack:
§ Galvanic corrosion: when dissimilar metals are plaed together in the presence of a corrosive environment. One metal becomes anodic, the other cathodic.
§ Concentration cells: occur when there’s different electrolyte concentration in the fluid. E.g. when water has settled for a period of time, O2 is unbalanced
§ Stress cells: result of high residual stress in parts of a metal. The high stressed areas become anodic and lower stressed areas become cathodic. Grain boundaries tend to be high stress locations, thus finer grained metals is more likely to corrode than a course grained metal
- Corrosion can cause major harmful effects such as the following:
o Reduced metal thickness, leading to loss of mechanical strength and structural failure/breakdown – e.g., if steel truss girders in a bridge corrode, there’s a high chance that trusses will collapse
o Hazards/injuries to people arising from structural failure/breakdown - Metals that corrode can cause certain parts of that metal to break off and fall increasing the potential for injury for people under the bridge
o Time used up when manufacturing industrial equipment and materials for replacement – Valuable time is spent to re-manufacture the materials that have corroded in the bridge
o Reduced value of materials due to deterioration of appearance – The corrosion can be un-aesthetically appealing for people like tourists
o Added complexity and expense of equipment needs to be designed to withstand a certain amount of corrosion – A demand in technology and money is used to find ways of combating corrosion in materials
- Preventing corrosion includes:
o Use of proper design: design to avoid various forms of corrosion, e.g. galvanic and stress corrosion cracking can be controlled by proper design. This includes adequate ventilation to disallow concentration. Overdesign is used to add thickness to materials to allow corrosion but it is expensive. Materials selection includes knowledge of factors affecting corrosion, using the right materials, and maintenance. Passive metals include aluminium, titanium, chromium, stainless steel. Metals exhibiting passivity (inert) have a non-porous film protecting it from corrosion
o Painting and galvanizing: Coatings control corrosion by placing a barrier between the material and the surrounding environment. Galvanizing involves dipping in molten zinc which covers it and forms a passive layer. Alclad is duralumin coated with pure aluminium and is also useful.
· Recycling of materials
o Steel: almost all steel used in bridges can be recovered and recycled. Manufacture of steel from scrap metal is more advantageous as it reduces mining extracts. Steel scraps from bridge members and from reinforced concrete can be remade into new pieces
o Concrete: recycled concrete is weaker than original due to exposure to the elements, structural stress, strain and subsequent crushing. The rubble is used as aggregate for new concrete. Repeatedly recycling steel has less of environmental impact than concrete
o Wood: needs to be screened to ensure that chemicals used in the wood will not make it unsuitable for recovery. Some woods may still be sound enough to be reused as other wooden products, e.g. furniture. Unsuitable wood can be recycled into garden mesh, playground covering, paper and cardboard production
o Glass: recycled glass, known as cullet, is used to make new glass. Benefits include preserving countryside by reducing quarrying, and because cullet melts easily, it saves energy and reduces emissions
Personal and Public Transport
Historical and societal influences
· Historical developments
- Bicycle:
o
- Car:
- Train:
· Effects of engineering innovation in transport on people’s lives
Bicycles
- Pedal powered velocipede greatly improved bicycle usability
- Penny Farthing enabled faster transport but more dangerous
- Rover Safety Cycle was safer with similar speeds
- Pneumatic tyres made smoother ride
- Mass production of bicycle in early 1900s made it popular
- Freewheeling hubs made cycling much more safer
- High strength alloys developed and made bike lighter thus faster
- BMX became popular with children
- Mountain bike in 1980s, most popular bike, specialized components developed
- Modern days with more better materials improving performance
Cars
- Early cars were slow, noisy, expensive
- Early 1900s production increased, cars cheap and available
- Development of four-wheel brakes made cars safer
- Cheaper cars enabled access to remote areas
- Improvements in handling and braking, suspension and disc brakes more safer