What is a composite structure?
A composite is something made of two or more materials working as one. A composite structure combines steel and concrete in one part.
Remember the two materials. Concrete is strong when squeezed but heavy and brittle. Steel is strong when pulled or pushed but slim parts bend and it softens in fire.
Put them together and each one covers the other's weak point. The two must stay stuck together and move as one, so bars, studs or the tube shape make sure they cannot slip.
Steel-reinforced concrete (SRC)
In SRC (in Japanese, 鉄骨鉄筋コンクリート) a steel frame (H-beams and columns) is placed inside the concrete, and ordinary rebar and ties are added too.
- Steel gives extra strength, so the part can be slimmer.
- Concrete covers the steel. It keeps the steel from buckling and protects it from fire and rust.
- The mix is very tough and bends without sudden breaking, which is good in earthquakes.
It is used in tall buildings and big beams, but the work is complex and costly, because we place steel, bars and concrete together.
Concrete-filled steel tube (CFT)
In a CFT (コンクリート充填鋼管) a steel pipe, round or square, is filled with concrete.
- The tube confines the concrete. When squeezed, concrete tries to bulge sideways. The tube holds it, so the concrete can carry much more load.
- The concrete inside stops the thin tube wall from folding inward (local buckling).
- The tube works as the mould, so we do not need wooden formwork, and building is quick.
CFT columns are slim, strong and tough. They are popular in tall buildings.
Why composite is better
Compare the options:
- RC: cheap, fire safe, heavy.
- Steel: light and fast, but needs fireproofing and braces.
- Composite: strong and slim, fire protected, tough in earthquakes, and quicker than RC.
The price is a more complex design and joints between the steel and the concrete. The designer must make sure the two share the load.
Try it
In the 3D: use the load slider with each button. Concrete only and steel only fail early. SRC and CFT stay safe.
At home: take a paper tube and fill it with dry sand. Stand it up and press on top with a book. A sand-filled tube holds a book. An empty tube folds. That is the idea of CFT: the filling stops the tube folding, the tube holds the filling.
Key formulas and definitions
- Composite = steel + concrete working as one part
- SRC = steel frame inside reinforced concrete
- CFT = concrete inside a steel tube
- Confinement: the tube stops concrete from bulging
- Concrete stops the tube wall from buckling inward
Worked examples
1. In a CFT column, what stops the concrete from bulging sideways?
The steel tube around it. This is called confinement. Confined concrete carries a larger load.
2. A designer wants a slim column for a skyscraper which also resists fire. Which is better: RC, steel only or SRC?
SRC. The steel makes it strong and slim, and the concrete cover protects the steel from fire.
3. Why do CFT columns need little formwork?
The steel tube is itself the mould. Concrete is poured into it. That saves time and wood.
Common mistakes
- Saying composite means any two materials. In buildings it means steel and concrete working together.
- Mixing up SRC and CFT. SRC has steel inside concrete; CFT has concrete inside steel.
- Thinking the tube carries all the load in CFT. Both carry it, and the concrete also keeps the tube from folding.
- Believing composite parts are always cheaper. They are strong and quick, but design and joints are more complex.