2See a demonstration of these forces! Important Terms:Squeezing (Compression) Compression is a force that squeezes a material together. When a material is in compression, it tends to become shorter.Stretching (Tension) Tension is a force that stretches a material apart. When a material is in tension, it tends to become longer.See a demonstration of these forces!
31. Truss BridgesConsist of vertical, lower horizontal and diagonal membersTypically composed of triangular units that are connected at joints
4Characteristics of Truss Bridges The “truss” (which is usually a triangular unit) creates both a very rigid structure and one that transfers the load from a single point to a considerably wider areaTruss bridges are commonly made from a series of straight, steel bars.
12The larger the H/S ratio, the greater the strength Every bar in this bridge experiences either a pushing or pulling force. The bars rarely bend.
13Pros & Cons of Truss Bridges Economical to constructSpans longer distances than beam bridgesAbility to support weight relies on the strength of the joints
14Also known as a “girder” bridge Simplest type of bridge 2. Beam BridgesAlso known as a “girder”bridgeSimplest type of bridge
15Characteristics of Beam Bridges Consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers.The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet.
22Under load, the beam's top surface is pushed down or compressed while the bottom edge is stretched or placed under tension
23When something pushes down on the beam the beam bends When something pushes down on the beam the beam bends. Its top edge is pushed together, and its bottom edge is pulled apart.
24Pros & Cons of Beam Bridges Needs to resist twisting and bending under loadSimple design: Less expensive and requires little maintenanceOnly suitable for short spans (about thirty to six hundred feet)
25Cables are hooked on vertical suspenders that support the load 3. Suspension BridgesCables are hooked on vertical suspenders that support theload
26Characteristics of Suspension Bridges These bridges can span 2,000 to 7,000 feet -- much farther than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.
33In all suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. Cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight.
34Pros & Cons of Suspension Bridges Allows for longer spans than other bridge typesRequires stable ground for the “anchorages” at either endMay withstand earthquakes better than other bridge typesAesthetically more beautiful than other two types of bridge