• Generally name given to a large family of alloy of iron with carbon and a variety of different elements
• The strongest, the most resistance to ageing, and generally the most reliable in quality
• Composed of about 98% of iron with the main alloying elements of carbon, silicon and manganese

• Strong under compression and tension compare to concrete and timber

  
  

• Can resist excessive load due to wind, earthquake, vibration & impact transfer load

• Strength level required
• Mechanical properties required together with strength
• Steel making, heat treatment and other plant available
• Arbitrary local condition and code of practise

• Consist of skeletal framework which carries all the load
• Steel members are used to carry :
  i) Lateral load when acting as beams and girders
  ii) Axial loads when acting as struts and ties
• Steel frame are made of load bearing elements

Carries axial stress load Shell serves a use function in addition to participation in carrying loads

• The main stress in tension is the containment vessel

• Axial tension to support system
• Major supporting element in tension cable
• Tension element is the most efficient way of carrying load, structure utilization this concept are coming into increasing use

1) Carbon steel
a) Low carbon steel

• Up to 0.15% carbon
• Soft
• Suitable for wire and thin sheet
  b) Mild carbon steel
• 0.15 - 0.29% carbon
• Strong
• Ductile
• Suitable for rolling into sections, sheet and strip
• Easily worked and welded
  c) Medium carbon steel
• 0.3 - 0.59% carbon
• For general engineering purpose
  d) High carbon steel
  *0.6 - 1.7% carbon
• Suitable for casting

2) Alloy steel

• Contain either silicon or manganese or any other element or elements with deliberate alloying additions.
• Alloying addition are made to steels to enhance the properties

3) Weathering steel

• Containing 0.2% copper
• It has better resistance to corrosion than mild steel

4) Stainless steel

• Contain chromium
• Classified as :
• Martensitic (About 13% chromium)
• Ferritic ( about 17% chromium)
• Austenitic ( 16 - 19% chromium together with 6 - 14% nickel)

• Strong under compression and tension compared to concrete & timber
• Can resist excessive load due to wind, earthquake, vibration & impact transfer load

1) Primary steel making
-Refers to the fining processes used to produce liquid steel
-The aim is to produce a melt of the required composition
-Typical ranges required for structural steel are :

• Carbon 0.15 - 0.25%
• Manganese 0.5 - 1.5%
• Sulphur and phosphorus < 0.05%
  -Type of refining process:
  *The bessemer/ Thomas process
• The open-hearth process
• The basic oxygen process
  *Electric arc steelmaking
  

2) Secondary steel making
-The liquid steel process is not finished when it is tapped from the furnace
-It must undergo further treatment
-This is done by adding manganese and silicon, which react with dissolved oxygen to form insoluble particles of oxide
-It may be necessary to adjust the carbon and manganese content to give the grade of steel required, because with carbon it will improves strength and manganese gives low temperature toughness

3) Mechanical forming process
-Are used to continuously cast material into the shapes and sizes required
-It can be done either by hot rolling or cold rolling
i) Hot rolling

• All constructional steel are hot rolled
• These steel are heated to temperature of 1200 - 1300 C to make them soft enough to deform and shape
  ii) cold rolling
• Cold rolling cannot be used to the same degrees as hot working for shaping purposes, because of the increase strength of the cold work piece
• Modest reduction can be achieved by rolling to give better size tolerances and surface qualities
• Mainly for light weight sections

3) Elasticity

• Steel behave closer to design assumption than most materials because it follow Hooke's law up to fairly high stresses

4) Ductility

• The property of a material by which it can withstand extensive deformation without failure under high tensile stresses

2) Uniformity

• The properties of steel do not change appreciably with time, as do those of a reinforced concrete structure

5) Toughness

• It has large deformations will still be able to withstand large forces

1) High strength

• The high strength of steel per unit of weight means that the weight of structure will be small.

1) Maintenance cost

• Most steels are susceptible to corrosion when freely exposed to air and water and therefore must be painted periodically

2) Brittle fracture

• Under certain condition steel may lose its ductility and brittle fracture may occur at places of stress concentration

3) Fatigue

• Strength may be reduced if it is subjected to a large number of stress or even to a large number of variation of tensile stress

i) Metallic coating, either:

• A sprayed-on coating of aluminium or zinc is used
• Dipping it in a bath of molten zinc in the galvanizing process
  ii) Zinc coating
• The most commonly used coating for steel building
• Zinc is many time more resistance to corrosion than mild steel, it is not immune to corrosion

• Steel sheet or strip can be surfaced with PVC, acrylic, epoxide or phenolic coating applied as liquid or laminates
• These product can be pressed and otherwise fabricated without damage to the coating

• Various system are used. such as
• Using a primer of zinc chromate followed by finishing coats of micaceous iron oxide
• In special case:
  i. Plastic paint
  ii. bituminous paint