Civil Structures Flashcards
historical developments of civil structures
Designs - columns, arches, beams, suspensions and cantilevers etc
New materials and technologies changed designs.
Testing of materials, processing, design and drawing technology.
Industrial Revolution - wood to iron/steel
engineering innovation in civil structures and their effect on people’s lives
buildings, bridges, roads, dams, sewerage plants, canals and water supplies
Bridges - enabling more efficient travel by shortening distances and allowing vehicles and pedestrians.
Examples - river thames (sewer), golden gate bridge, snowy hyrdo scheme
construction and processing materials used in civil structures over time
timber: originally was just a tree but developed to having pieces of timber cut to form beams of a truss. ease of manufacture and readily available. although it rots over time.
rope: used in early suspension bridges. rots away and could only carry small loads
stone: more permanent material. strong in compression, weak in tension. perfect for arches.
bricks: similar to stone, but are made from clay.
cast iron: readily available in industrial revolution. strong in c weak in t. can be melted and casted. can be pre-fabricated. made frames in bridges.
wrought iron: early suspension bridges, made the chains. unreliable material due to fibrous structure present in the ferrite, which weakened it. made bridges limited in length.
steel: equally strong in c and t. could have longer spans on bridges. used to reinforce concrete.
concrete: weakness in tension, had to be reinforced.
environmental implications from the use of materials in civil structures
Minimise impact on environment - production of materials, construction of structures, maintenance and demolition. CO2
steel - large amounts of water, quenching and pollution control, production of toxic materials, abundance of toxic chemicals, galvanised steel (zinc contamination in soil)
Concrete - only recycled as aggregate, 6 times more material than steel.
Wood - denudation of olf growth forrests, leaching of chemicals
Maintenance
testing of materials - specialised testing of engineering materials and systems
Bridges contain a large number of joints, welds and connections that are potential initiation points for fatigue and cracks. Critical bridge members need to be regularly examined and monitored.
testing of materials - X-ray
what is it?
advantages?
safety?
radiation goes through an object it is differentially absorbed by variations in density, thickness, chemistry and defects etc.
highly portable, high quality films, images of higher resolution and greater sensitivity can be produced. major advantage because hard copy results are available. Capture image digitally too, better because the image will not deteriorate. image can be sent all over the world and digitally enhanced. bridge girders, pressure vessels, piping and welding etc.
safety about radiation, license to use, special outifts
testing of concrete
The concrete slump test measures the workability of fresh concrete before casting. It measures the consistency of the concrete in and between batches.
The process involves packing fresh concrete into a cone-shaped container. Careful removal of the cone allows the concrete to “slump”.
The slump is the distance that the centre of the cone top settles. In a so-called “true” slump test, the base of the concrete does not spread excessively. If the concrete collapses or shears to one side, the test results will prove to be unreliable. Unfortunately, the simplicity of the test and variations in the operators procedures can lead to a wide range of results.
crack theory - crack formation and growth
things that cause cracks?
Dependent on the material selected and the method of component manufacture, stress concentrations may occur due to:
weld defects, quench cracking, corrosion pitting, machining marks, mishandling damage, arc strikes from welding, inadequate radii at section changes, casting defects such as porosity, shrinkage and inclusions.
Cracks formed at the surface may be detected using simple visual inspection techniques such as magnetic particle and dye penetrant tests while sub-surface cracks may require ultrasonic or radiographic methods.
crack theory - failure due to cracking
Types?
Process?
Cracking of a component occurring under cyclic loading can occur at stresses well below yield stress. Cracking produced in this manner is called fatigue. Fatigue is the most common form of materials failure.
A fatigue fracture always starts as a small crack, which under repeated application of the stress grows in size with little macroscopic ductility or distortion. As the crack expands the load carrying cross-section of the component is reduced, with the result that the stress on thus section is raised. Failure, therefore, occurs progressively over a number of stress cycles and may take from several hours to years.
One of the most characteristic features usually found on fatigue fracture surfaces is the presence of “beach” marks. These marks represent the successive positions of the advancing crack and are centred on a common point that corresponds to the fatigue-crack origin.
crack theory - repair and/or elimination due to cracking
controlled with a number of mechanisms such as:
- keeping threshold stresses below a pre-calculated value
- transformational toughening involving localised structural change of material by inducing volumetric change thus squeezing cracks shut.
ceramics - structure/property relationships and their application to civil structures
combination of 1 or more metallic elements with a non-metallic element. they form ionic/covalent bonds giving them unique properties.
silicate ceramics, oxide ceramics, non-oxide ceramics, conventional ceramics, glasses and advanced ceramics.
generally hard, brittle, chemically inert materials, good electrical conductors and exhibit high temperature resistance.
display good compressive strength yet have little tolerance for cracking. processing techniques are often directed to producing a compressive surface layer that must first be overcome before fracture will occur.
ceramics - glass
Structure?
Types?
Properties?
non crystalline fashion. their constituent atoms are arranged in irregular random patterns,
lead glass, glass fibre, borosilicate glass, commercial glass (soda-lime glass).
commercial glass - main constituent is sand, sand is fused to make glass. 1700 degrees. adding other chemicals can considerably reduce the temperature of fusion, soda carbonate (or soda ash) will reduce the fusion temp to 800 degrees.
chemicals like calcium oxide and magnesium oxide give glass stability.
commercial glass is colourless, which allows different varieties of light, visible, ultraviolet, and infa-red.
ceramics - cement
recognised as a bonding material used to hold aggregate particles together in a solid form.
the romans made mortar around 300BCE consisting of sand, burnt lime volcanic ash and pulverised bricks.
modern cement ins calcium aluminosilicate that forms a binding matrix when combined with water.
CONCRET AND CEMENT ARE NOT THE SAME
ceramics - bricks
block or single unit of ceramic used for construction. stacked and joined together with mortar, used for walls in buildings and other structures.
strong, reliable durable building material dating back to 7500 BCE
composites - timber
composition?
hard and softwood? what plants?
properties?
composite because of the cellulose fibres, tracheids, are held together by a natural resin.
pored (hardwood) and non-pored (softwood). hard wood has pores or vessels running through the structure between the tracheids to carry nutrients, while softwood have a neater, more uniformed structure without pores. hardwood comes from flowering plants (angiosperms) whereas softwood comes from pines and conifers (gymnosperms).
excellent strength to weight ratio, reasonable performance in bending, relatively high youngs modulus, adversely affected by weather and susceptible to attacks from pests (termites). not many old buildings made from timber nowadays.
