Buy a pack of index cards, 3 inches by 5 inches, at an office supply store. Pick out one card. Using a ruler and a pencil, draw a line on the 3-inch side, 1 inch away from the edge and parallel to it. Do the same for the other side. Fold on the lines, so the card has a shallow capital "U" shape. Next, obtain a handful of small rocks, each rock roughly about the size of a child's marble.
Place the card on a table so the card looks like an upside down "U." You have just constructed a beam bridge. Place some rocks in the middle of the card, and observe closely what happens. Notice that first, the middle of the card bows down. Second, the legs "kick out," becoming wider at the bottom
What happened to the index card bridge is exactly what happens to a large beam bridge. Rocks represent cars and trucks. When a car travels across the road, the beam tries to bow downward. When the road bends, the top part tries to squeeze close together. This is called compression. The lower part of the road tries to pull apart. This is called tension. The support pillars, called posts, try to become wider at the bottom because the road is bending, as evidenced by your index card.
Run a second experiment to understand how a truss beam bridge works. Take a new index card. Lay two 1-inch thick books on a table. Space them 4 inches apart. Place an index card across the books longways, so about 4 inches of the card is unsupported. Place rocks on the card in the middle. Notice it bends down. This is a non-reinforced bridge. The same thing that happened to your index card happens to a real bridge. The roadway tries to bend down when cars travel across it.
Draw a line 1 inch from the 5-inch side, and parallel to the side. Do the same for the other 5-inch side. Fold on the lines, so the card has a shallow "U" shape along its long sides. Replace your bridge on the books, with the legs of the "U" pointing up. Load down the bridge with rocks. Notice the bridge does not bend downward. This is the guiding principle behind a truss beam bridge. The side supports hold up the road, so it does not bend.