Construction – Bridges and Dams
Bridges, dikes, and dams are among the most demanding challenges of structural engineering. They are often subject to extreme and variable forces from tides, winds, or moving masses of snow and earth.
Bridges are built to span rivers, valleys, and crossroads. All possess a horizontal span and a means of vertical support, such as vertical piers, arches, or towers called pylons. The design of a bridge must take into account the form of the surrounding terrain, the depth and solidity of the bed- rock or soil for supporting piers or pylons, the load of traffic that the bridge will bear, climate conditions, and forces such as wind and water currents.
Other factors include the bridge’s construction costs and long-term maintenance costs, its overall appearance, and locally available building materials.
Arched bridges can be erected with steel as the main supporting material, but for massive bridges a pressure-resistant material such as concrete must be used. The arch can withstand tremendous pressure, and, as a structural component for building bridges, allows pressure to be distributed across its curve. Arched bridges made of steel arches can have a span of 1,640 feet (500 m).
Suspension bridges are constructed to span more than 2,620 feet (800 m). These have two high pylons with two horizontal steel supporting cables attached to them. Underneath, a concrete roadway is attached with vertical cables called hangers. The entire load and stress is transferred to the supporting cables via the hangers and directed to the pylons as a vertical force. Suspension bridges are susceptible to damage from vibrations triggered by wind.
Dams, dikes, and retaining walls
Canals and dams control and block the flow of rivers. Reservoir dams provide drinking water or power for industrial areas and protect against floods. Dikes and retaining walls are built mainly to protect against floods or landslides.
Most artificial dams are embankment dams, made of compacted earth. Embankment dams use their weight to hold back the force of the water. They can be covered with concrete and filled with earth or rock. Retaining walls in hilly areas hold back water, earth, rocks, or snow. In Peru, the Inca built terraces on otherwise unusable slopes. They also built canals and aqueducts to irrigate the soil for agriculture.
When a large dam is built, thousands of people may need to relocate from areas that will be flooded. Because of this, the construction of dams canals, and dikes often has great environmental impact on a region.
MOVABLE BRIDGES
When ships and roadways cross, a movable bridge is often the most functional and economic solution, particularly in harbor cities visited by large ocean liners.
Depending on the design of the bridge, the roadway, in parts or as one, may be raised at an angle, hoisted whole, or swung to the sides. Road traffic must wait until the ship has passed.
ISSUES TO SOLVE
HUGE DAMS like the Three Gorges Project in China or the High Aswan Dam in Egypt often have great environmental impact. When a large dam is built in a highly populated area,
natural and cultural heritage may be lost.