History
The first bridges were naturally formed beam bridges such as a fallen log over. Then, when the first civilizations appeared, humans realized that they could use the resources around them to span water rivers, and thus, the first man-made bridges were formed. The earliest bridges were made from stone, dirt, and wooden logs. These bridges could not span large distances and they were usually temporary because there wasn't mortar to keep them together. Bridges like these were used for thousands of years until the Roman Empire came along. The Romans invented mortar which was used to make the bridges as permanent, but these bridges couldn't hold much weight because they were still beam bridges. Luckily, the Romans didn't have the weight of cars to put on those bridges so they held. Beam bridges last for centuries until 1801 when the first suspension bridge appeared, in Pennsylvania, even though the first drawing of a suspension design appeared in the late 16th century. Truss bridge designs first appeared in the early 19th century, but few were built before 1850. Truss bridges became a very popular bridge design between 1870 and 1930, though the design is slowly dying in modern times. Triangles are in most modern bridge designs such as the tied-arch bridge, cantilever bridge, and cable-stayed bridge. Bridge designs will keep evolving in the future, being able to hold larger and larger loads. One thing they will all have in common though is triangles.