Why materials matter
A good structure needs the right material in the right place. We compare materials by their properties: how strong, how heavy, how they react to fire and water, and how much they cost. No material is best in every way, so engineers choose by the job.
Strength has two main kinds. Compressive strength is the ability to resist being squeezed. Tensile strength is the ability to resist being pulled apart. A column mainly squeezes; a hanging cable only stretches; a beam does both, squeezed on top and stretched at the bottom.
Timber, steel, concrete, brick and RCC
Timber (wood): light, easy to cut and join, good in both squeeze and stretch along the grain, renewable. Weak points: it burns, can rot when wet and can be eaten by termites.
Steel: very strong in both squeeze and stretch, so thin parts carry big loads and long spans are possible. Weak points: it rusts and loses strength in a hot fire, so it is covered with fire-protective coating.
Concrete (cement, sand, stones and water): strong in squeeze, cheap, shapeable when wet, fire-resistant. Weak in stretch and heavy.
Reinforced concrete (RCC): concrete with steel bars placed where it would stretch. Steel takes the stretch, concrete takes the squeeze and protects the steel from fire and rust.
Brick and masonry: good in squeeze, good fire and heat resistance, low cost, easy to lay by hand. Weak in stretch, so tall brick walls need support.
Weight and density
Density is mass per volume: density = mass ÷ volume. Approximate values: timber 500 kg/m³, brick 1,800 kg/m³, concrete 2,400 kg/m³, steel 7,850 kg/m³. Steel is heavy per cubic metre, but because it is so strong we use much less of it, so a steel frame is often lighter than a concrete one for the same load.
Weight matters because the structure must also carry its own weight (dead load), and a heavy building needs bigger foundations.
Fire, durability and cost
Fire: concrete and brick resist fire well. Timber burns but a thick beam chars on the outside and keeps its core for a while. Bare steel can bend in a hot fire, so it needs a coating or concrete cover.
Durability: steel needs protection from rust, timber from water and termites, concrete from cracks that let water reach the bars.
Cost and environment: brick and concrete are cheap and local in many regions, steel and cement need much energy to make, and timber from well-managed forests is renewable. Think about the whole life of the building, not only the purchase price.
Choosing for the job
Method: list what the job needs, give each property a weight from 0 (does not matter) to 4 (very important), then multiply each material level by the weight and add. The highest score fits best. Example: a long beam needs stretch strength and lightness, so steel, timber and RCC score high. A fire-safe wall needs fire safety, so concrete, brick and RCC win.
Remember that the real choice also depends on local supply, skilled workers, climate and rules, not just numbers.
Try it: pick a material
In the 3D, go to the last step and press each job button. Which material gets the "Best" tag for each job? Predict first, then check. Then look at step 2 again and explain why RCC beats plain concrete for a beam.
At home: bend a dry biscuit (weak in stretch, like concrete) and then hold a pencil lengthwise to a strong ruler (like steel bars in RCC). Or compare a wooden stick, a steel spoon and a brick in your hands: which feels lightest, which strongest?
Key formulas and definitions
- Density = mass ÷ volume, so mass = density × volume
- Stress = force ÷ area
- Material score = sum of (property level × job weight)
- Beam: top squeezes, bottom stretches
Worked examples
1. Find the mass of 0.2 m³ of steel (density 7,850 kg/m³).
Mass = 7,850 × 0.2 = 1,570 kg.
2. Find the mass of the same volume of timber (500 kg/m³). How many times lighter than steel?
Mass = 500 × 0.2 = 100 kg. Steel ÷ timber = 1,570 ÷ 100 = 15.7, so timber is about 16 times lighter for the same volume.
3. A concrete column is 0.4 m × 0.4 m × 3 m. Its density is 2,400 kg/m³. Find its mass.
Volume = 0.4 × 0.4 × 3 = 0.48 m³. Mass = 2,400 × 0.48 = 1,152 kg.
4. Why can steel bars make concrete strong in a beam?
The bottom of a beam is stretched. Concrete is weak in stretch, so it cracks. Steel bars are strong in stretch and take that pull. Concrete still takes the squeeze at the top.
5. A material scores (squeeze, stretch, light, fire, cost) = (5, 5, 2, 2, 2) and a job has weights (1, 3, 3, 1, 0). Find the score.
5×1 + 5×3 + 2×3 + 2×1 + 2×0 = 5 + 15 + 6 + 2 + 0 = 28.
6. A wall near a kitchen stove needs fire safety most. Pick between timber, steel (bare) and brick and explain.
Brick. It resists fire and heat well. Timber burns and bare steel loses strength in a hot fire.
Common mistakes
- Thinking steel is best for everything. It rusts and softens in fire, and can cost more than brick or concrete.
- Saying concrete is strong, so no steel is needed in beams. Concrete is weak in stretch and cracks without bars.
- Mixing up weight and strength. Steel is heavy per m³ but we use much less of it.
- Ignoring fire safety when choosing a material for a kitchen or stair.