What if a bridge were built with different materials?

Bridges are made of different things. Some are made of steel. Some are made of wood.

What if we built the same bridge out of wood instead of steel? Would it bend? Would it break?

Look at bridges near you and you might see steel, concrete, stone or wood. Engineers choose a material for each bridge, and the choice matters.

In this lab, a 20-tonne truck stops in the middle of a beam bridge 20 meters long. The beam starts as solid steel. Then you swap it for wood or aluminum, keeping exactly the same size and shape. Will the new beam bend more, bend less, or break?

A beam has two separate jobs: it must be stiff enough not to sag too much, and strong enough not to crack or yield. Different materials score very differently on those two tests, and on a third that matters for long spans: how much they weigh.

This lab keeps the geometry fixed (a solid beam 20 m long, 1 m deep and 0.5 m wide, supported at both ends, with a 20 t truck at midspan) and changes only the material, so you can see which property controls which result.

Our bridge starts out made of steel. A big truck drives on.

The bridge bends just a tiny bit. It is strong and safe.

In the model, the steel beam sags only about 14 millimeters in the middle when the truck stops on it. That is a tiny bend for a beam 20 meters long: less than one thousandth of its length.

The steel is also working at only about one seventh of its strength, so the model calls it safe. But steel is heavy: this solid beam weighs about 78 tonnes, much more than the truck it carries.

With steel (E = 200 GPa, yield strength 250 MPa, density 7,850 kg/m³), the model gives a maximum sag of 13.6 mm and a peak bending stress of 34.9 MPa, a safety factor of 7.2. The beam itself is 78.5 t of steel.

Much of that stress comes from the beam's own weight. Switch self-weight off and the truck alone causes only 3.92 mm of sag and 11.8 MPa of stress. On long spans, a heavy material spends much of its strength just holding itself up.

Now Olly makes the bridge out of wood. It is the same size.

The wooden bridge bends much more! But it does not break. It still holds the truck.

Choose wood and the same-sized beam sags about 68 millimeters, about five times as much as steel. That's because wood is much less stiff: it bends more easily under the same push.

But look at the safety readout. It only drops from about 7 to about 6, so the model still calls the wooden bridge safe. Wood is weaker than steel, but the wooden beam weighs only about 5 tonnes, so it has much less of its own weight to carry. Aluminum lands in between: about 21 millimeters of sag.

With clear Douglas-fir (E = 13.4 GPa, bending strength 85 MPa, density 510 kg/m³) the sag rises to 67.9 mm and the peak stress falls to 13.3 MPa, a safety factor of 6.4. With aluminum alloy 6061-T6 (E = 68.9 GPa, yield 276 MPa, 2,700 kg/m³) the sag is 21.0 mm and the safety factor 14.0, the highest of the three.

The stress drops for the lighter materials because self-weight is a large share of the load. For the truck alone, wood sags 58.6 mm against steel's 3.92 mm, a factor of about 15: exactly the ratio of their stiffnesses, 200 ÷ 13.4.

Some things bend easily, like a ruler made of soft plastic. Some things are very stiff, like a metal spoon.

Steel is stiffer than wood. So the wooden bridge bends more.

Every material has a stiffness, which scientists measure as the elastic modulus. A stiff material needs a big push to bend even a little. Steel is about 13 to 15 times stiffer than wood, so a wooden beam of the same size bends far more.

Strength is different: it is how much squeezing or stretching a material can take before it breaks. A beam can bend a lot and still be safe, or barely bend and still crack. Our model uses values for perfect, knot-free wood. The USDA Wood Handbook says wood with knots is weaker, so real timber would not do as well.

Sag goes as load × L³ ÷ (E·I), so for a fixed shape it is inversely proportional to E. Stress, M ÷ S, depends on the load and the shape but not on E; the safety factor then compares that stress with the material's strength. That is why stiffness controls the bend and strength controls the verdict.

Model limits: the wood values are averages for small, clear specimens, and the USDA Wood Handbook notes that most mechanical properties are lower in wood containing knots. The beam is one solid rectangle (real beams are I-sections, boxes or trusses), and the model does not check buckling, twisting, shear, fatigue, wind or rot.

Long ago, many bridges were made of wood and stone.

Today, most big bridges are made of steel and concrete with steel inside.

Wood, stone, iron and concrete have long been the main bridge materials. Britannica explains that wood is cheap and easy to find but suits small bridges with light loads. Stone is strong when squeezed but weak when stretched, and it has mostly been used in arches and piers.

Today, steel, reinforced concrete and prestressed concrete are used almost everywhere. Concrete is strong when squeezed but weak when stretched, so engineers put steel bars inside it to take the stretching. A wooden beam in our model holds the truck only because it is a big, perfect block.

Britannica names wood, stone, iron and concrete as the four primary bridge materials. It notes that wood is relatively weak but widely available and inexpensive, and has served small bridges with light loads, such as footbridges, while modern bridges are almost exclusively built with steel, reinforced concrete and prestressed concrete. Steel is stronger than iron in both tension and compression; concrete is strong in compression but weak in tension, so reinforcing bars carry the tension.

OpenStax's table of elastic moduli lists steel at 20 × 10¹⁰ Pa, aluminum at 7.0 × 10¹⁰ Pa and hardwood at 1.5 × 10¹⁰ Pa, the same ordering the model uses. Choosing a material is a trade between stiffness, strength, weight, durability and cost.