Why does concrete need steel bars?

01

The idea

Concrete is strong squeezed, weak stretched

Pulled apart, concrete snaps at about a tenth of the force it takes squeezed. Steel bars do the pulling for it.

02

Play with it

Load it, then add steel

The bottom of the beam is stretched. Load it with no steel, then add bars at the bottom.

Try this · 0/3Snap a beam with no steelCrack it, but keep it standingCarry 50 kN

03

The rule

Concrete takes the squeeze, steel takes the pull

Stretching stress at the bottom

\sigma = \dfrac{\mom{M}}{W}, \quad W = \dfrac{b h^2}{6}

What the bars hold

\mom{M} \approx A_s\, f_y \cdot 0.9\,\len{d}

Cracks are normal in reinforced concrete. They tell you the steel is working. Plain concrete gives no warning: it cracks and snaps at once.

04

Your turn

Prove it

▲ Build1/3

The fewest bars for 50 kN

To do

Steel costs money. Use as few bars as you can, but the beam must hold.

= Solve2/3

How hard is the bottom stretched?

To do

The beam above carries $\force{6}\,\mathrm{kN}$ in the middle of its $\len{4}\,\mathrm{m}$ span. Its $W = 3 \times 10^6\,\mathrm{mm^3}$ . What is the stretching stress at the bottom?

The beam above carries $\force{2}\,\mathrm{kip}$ in the middle of its $\len{12}\,\mathrm{ft}$ span. Its $W = 192\,\mathrm{in^3}$ . What is the stretching stress at the bottom?

◆ Pick3/3

A floor beam rests on two walls. Where do the main steel bars go?

To do

05 · Takeaway

Concrete squeezes. Steel pulls.

\mom{M} \approx A_s\, f_y \cdot 0.9\,\len{d}