Why does a balcony crack on top?

01

The idea

A balcony is a diving board

It bends down. The top gets stretched, and concrete hates being stretched. So it cracks on top, at the wall.

02

Play with it

Load it, stretch it, move the steel

Make it longer, add people, and try the steel on top, at the bottom, or not at all.

Try this · 0/3Get hairline cracks that the steel holdsMake it snapHold a full crowd on a 2.5 m balcony

03

The rule

Biggest moment at the wall, steel on the stretched side

Moment at the wall

\mom{M} = \dfrac{\force{w}\,\len{L}^2}{2}

Where the steel goes

On the face that gets stretched: on top for a balcony, at the bottom for a beam between two supports.

Hairline cracks are normal: concrete cracks, the steel takes over. Real balconies also limit how much they droop and keep the bars covered against rust.

04

Your turn

Prove it

▲ Build1/3

Fix this balcony

To do

A 2 m balcony, full crowd. The builder put the steel at the bottom. Move it so the balcony holds.

= Solve2/3

How big is the moment at the wall?

To do

A balcony strip sticks out $\len{2}\,\mathrm{m}$ and carries $\force{10}\,\mathrm{kN/m}$ along its length. What is the moment at the wall?

A balcony strip sticks out $\len{6}\,\mathrm{ft}$ and carries $\force{0.2}\,\mathrm{kip/ft}$ along its length. What is the moment at the wall?

◆ Pick3/3

Make the balcony stick out twice as far, same crowd. The moment at the wall becomes…

To do

05 · Takeaway

Balconies are stretched on top. The steel goes there.

\mom{M} = \dfrac{\force{w}\,\len{L}^2}{2}

Related: why concrete needs steel bars · why beams break in the middle.