Why builders choose steel
Steel is iron mixed with a little carbon. It is very strong for its weight.
It can be pulled (tension) or pushed (compression) with great force. It can also bend a little and spring back (ductile), so it warns before it fails. Concrete does not do this.
Because it is strong, steel parts can be slim and light. This lets us build long spans like bridges and big halls with few columns.
Shapes of steel parts
Steel is rolled into shapes called sections: angle (L), channel (C), pipe, plate and the very common H or I section.
In an H-beam the two flanges (top and bottom plates) are far from the middle. When the beam bends, the top and bottom are squeezed and stretched the most, so we put the steel there. The thin web in the middle joins the flanges and fights cutting (shear). So an H-beam is strong but uses little steel.
A truss is a frame of straight bars joined in triangles. Each bar is only pulled or pushed, so a truss is light and can cover big roofs.
Joining steel: bolts and welds
Parts are made in a factory, which gives neat and exact work. Then they are carried to the site and joined there.
- Bolts (high-strength bolts): fast, easy to check and can be undone.
- Welding: melts the steel together. It is strong and neat but needs a skilled worker.
Joints must be designed with care, because many steel failures start at a joint.
Weak points: buckling, fire and rust
Buckling: a long slim part that is pushed from the ends suddenly bows sideways. It fails before the steel itself is crushed. A thicker part, a shorter part or braces between columns raise the buckling load.
Fire: above about 500 °C steel loses about half of its strength. So we cover it with fireproof paint, boards or concrete.
Rust: iron plus water and air makes rust. We stop it with paint, zinc coating (galvanising) and good drainage.
Compare with RC: steel is lighter and quicker to build; RC is cheaper, fire safe and does not rust easily.
Try it
In the 3D: step 4 shows a thin column. Raise the load until it buckles. Then make it thicker and try again. In step 5 raise the heat and tick "Fireproof coat".
At home: push a plastic ruler from both ends. It bows out. Hold a ruler by its end and push on its flat face, then on its edge: the edge is stiffer. That is why the H-beam web stands upright.
Key formulas and definitions
- Steel: strong in tension and compression
- H-beam: flanges take bending, web takes shear
- Buckling load rises fast with thickness; falls fast with length
- Steel loses about half strength at about 500 °C
- Truss: bars joined in triangles carry only pull or push
Worked examples
1. Why is an H-beam better than a solid square bar of the same weight?
The H puts the steel far from the middle, in the top and bottom plates, where bending forces are largest. It is much stiffer and stronger for the same weight.
2. Two columns are the same height and steel. One is slim, one thick. Which buckles first?
The slim one. Thicker parts resist sideways bowing much more, so the thick column carries a larger load.
3. A steel office building has a fire for 90 minutes. Why is the steel frame wrapped in boards?
Steel gets soft when hot and loses strength. Boards or paint keep it cooler for longer so the frame does not sag or fall.
Common mistakes
- Thinking a thin steel column is as safe as a thick one because "steel is strong". Slim columns buckle.
- Saying steel cannot burn so it needs no fire safety. It does not burn but it softens.
- Calling the middle of an H-beam the main strength. The flanges give most of the bending strength.
- Ignoring rust. Water and air slowly eat steel unless it is painted or coated.