Civil's Guide

Structural Steel

Structural Steel

Structural Steel is a commonly used construction material which generally forms the ‘skeleton’ of a building. Steel is made by heating iron ore. This is usually done in a blast furnace by blowing oxygen into into the iron and lowering the carbon content (this affects the strength and ductility of the steel). Also, depending on what elements you combine the steel with, witll affect its properties(i.e, stainless steel, weathering steel). Cast iron is an iron-carbon element which contains more than 2% carbon content. This type of material was used in the late 1700s and through the \(18^{th}\) and \(19^{th}\) century during the industrial revolution. However, due to it’s high carbon content, the material is brittle compared to modern day steel. Wrought iron is another material used in the 1900s, but due to it’s brittleness, it was eventually replaced by mild steel. A famous example of a buidling made of wrought iron is the Effiel tower

The advantages of using steelwork as a construction material are listed below:

  • Strength – high strength/weight ratio
  • High aesthetic potential
  • Works well with other materials
  • Good environment performance – can be recycled or re-used
  • Ability to be prefabricated off-site
  • Constructed quickly and accurately on site

The disadvantages of using steelwork as a construction material are listed below:

  • Needs corrosion protection (and maintenance)
  • Cost of steelwork may vary (depends on international price as RC frames may sometimes be cheaper)
  • Flexibility and stability (stability usually achieved through steel bracing, but bracing locations will most likley have issues with architects plans and layouts)
  • Vibration – especially for hospitals, stadiums and gyms

Hot Rolled Steel

Hot rolled steel is formed in a mill process by rolling steel at a high temperature of up to \(1700^\circ\). The steel can then be shaped and formed easily. When the steel cools off, it will slighly shrink which results in less control over the final shape (i.e, check allowable tolerances). Most steel frames are designed with hot-rolled steel members.

Cold Rolled Steel

Cold rolled steel involves an additional process ater hot rolling, where the material is cooled, followed by annealing (heat treatment to remove internal stresses and toughen it. This increases the yield stress but comes at the expense of ductility and toughness. General materials which are cold rolled are purlins on a roof.

Steel material properties

Density 78.5 \(kN/m^3\)
Tensile Strength 275-460 \(N/mm^2\) yield stress and 430-550 \(N/mm^2\) ultimate strength
Poisson’s ratio 0.3
Modulus of elasticity, E 200 – 210 \(kN/mm^2\) (GPa)
Modulus of rigidity, G 79 – 81 \(kN/mm^2\) (GPa)
Linear coefficient of thermal expansion 12 x \(10^{-6/\circ}C\)