How does a bridge hold up heavy trucks?

1. 1

   Roo says

   The simplest bridge is a long, flat beam resting on strong posts called piers. When a truck drives across, its weight presses down on the beam. The beam passes that push straight down the piers into the ground. So in the end, the solid ground holds the truck up!

   For ages 4–7
   Hear Roo

   Checked against Bridge basics (PBS Building Big)

2. 2

   Why can't a beam bridge be really long?

   Press the middle of a long ruler and it bends. A bridge beam bends a little under weight too. The farther apart its piers are, the weaker the beam gets. So long beam bridges need lots of piers, or a stronger shape like an arch.

   For ages 6–9
   Hear Roo

   Checked against Bridge basics (PBS Building Big)

3. 3

   What makes an arch so strong?

   An arch is a curve. When weight presses down on top, the push spreads out along the curve, all the way to the ground at each end. That squeezes the arch, and arches are great at being squeezed. The strong banks at each end push back and keep it in shape.

   For ages 8–11
   Hear Roo

   Checked against Shapes lab (PBS Building Big)

In Lil' Phil, reaching the bottom of this wonder earns a sticker for Roo's collection.

• 

  Why does a ball bounce?

  Ages 4–11Roo
• 

  How do magnets stick?

  Ages 4–11Roo