Week 9 - Steel

• Steel

  The most widely used structural metal
• Structural properties of steel

  • Possesses great strength
  • Exhibits good ductility
  • Has high stiffness
  • Fabrication is easy
  • Relatively cheap
• Advantages of steel's structural properties:
• Steel structures vs concrete structures

  Steel is in competition principally with reinforced and prestressed concrete, timber and brickwork
• Reasons for selecting steelwork

  • Ease and speed of erection
  • Requirement of large clear spans and/or subsequent modifications
• Advantages of steel structures

  • No formwork
  • Minimum cranage
  • Much of the structure can be prefabricated away from the site
  • Largely self-supporting during erection
  • Extensions/strengthening relatively straightforward
  • Permits large clear spans
  • Prefabrication in the shop ensures accurate work
• Basic Oxygen Steelmaking (BOS) Process

  1. Iron ore is fed into the top of the blast furnace

  2. Air is blown through to increase the temperature - molten iron is achieved

  3. Molten iron is poured into the top of the BOS vessel

  4. Oxygen is blown in at great speed using a water-cooled lance, which combines with excess carbon and other impurities and then float off as slag

  5. The molten metal is cast into moulds where it solidifies into the ingots to be rolled into plates, bars

  
• Steel is composed of about 98% of iron with carbon and silicon being the main alloying elements
• Rolling

  1. Reheating, together with actual mechanical working received during rolling modifies the steel in such a way that its tensile strength is enhanced considerably

  2. The most common treatment is hot rolling in which the steel is squeezed between a pair of rotating cylinders termed rolls

  3. The sequence of operations involved in hot rolling commences with the ingot being heated in a soaking pit for between two and eight hours
• Material Strength

  The main concern when assessing the load-carrying capacity of a particular member
• Tensile test

  A small coupon of the material is pulled in a testing machine until it fractures, furnishing useful information on material stiffness and ductility
• Stress-strain curve

  Typical for structural mild steel: Modulus of elasticity, Es is about 205 kN/mm2 (GPa), Typical yield stress, y is 250 MPa, Strain at yield stress is about 0.0012, Plastic flow occurs when strain is increased above this value

  
• Effect of loading rate

  • Tensile tests performed by the manufacturer are frequently referred to as "mill tests" and are conducted at a fairly high rate of loading (a strain rate of 0.0025/s)
  • The static yield stress is the most appropriate basis for normal design calculations

  
• Residual stresses

  • Caused by uneven heating and cooling during rolling and welding
  • Those parts of a section which cool first will be left in residual compression, while those that cool more slowly will contain residual tensile stresses
  • Causes the member to behave as if it possesses a non-uniform distribution of yield stress over its cross-section, particularly important for compression members and members in bending
• Brittle fracture

  • Caused by the combination of low temperature, thick plates with stressed welds and fast rates of loading
  • Toughness is necessary in structural steel to avoid brittle fracture
  • Chemical composition affects a steel's suitability for welding, as measured by the carbon equivalent (C.E.)
• Fire protection

  • Steel's strength may be reduced substantially by high temperatures
  • Sufficient protection must be provided for the main skeleton of the building to stand up long enough for people inside to escape
  • Fire protection is provided by encasing the steelwork in a suitable fire-resistant medium
• Fatigue

  • Failure may occur by the continued growth of cracks in the material at stresses well below those necessary to cause ordinary static yielding or collapse
  • Fatigue is unlikely to be a problem for ordinary building structures but more significant for some heavily loaded bridges
• Corrosion protection

  1. Involves covering the exposed steel, either with paint or with a metallic coating, or possibly with a plastic coat

  2. Satisfactory painting results are best achieved at air temperatures between 5°C and 30°C and when relative humidity does not exceed 85%

  3. Hot dip galvanising is a two-step operation involving degreasing, rust removal, and immersion in molten zinc