A bridge is a structure that connect two points of a road, rail, or track, it suspends the roads from two sides of land usually over a body of water. Bridges are a commonplace item to see while driving so people don't realize how complex building a bridge really is. Architects and engineers have to incorporate how forces like tension (the force of pulling apart two points of an object), torsion ( the force of twisting two points of an object oppositely), compression( the force of pushing together two points of an object), and shear effect (the force of pulling one point of an object upwards and one point downwards) effect their bridge to make sure it doesn't buckle, curve, or snap.
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The forces of Tension, Compression, Shear, and Torsion on a block of wood.
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| A box girder bridge a larger form of a beam bridge |
The first bridge is a beam bridges the simplest type of bridges and can be found in a variety of sizes like a small wood bridge to cross over a stream or a large one that can stretch over a river, large beam bridges have pillars that hold up segments of the bridge they are called box girder bridges. The beam bridge is the most common bridge used today because it is simple and doesn't need that much resources and is thought to be the first type of bridge ever made. A single beam spanning any distance has compression and tension acting on it so if enough weight is added to a length of a beam bridge it starts to bend because the force of compression on top of that segment and the force of tension on the bottom of it increases and if the forces get too great the bridge will buckle and snap
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| Cartoon shows compression acting at the top of bridge and tension acting on the bottom |
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| A Deck and Through Truss Bridge |
The second type of bridge is a truss bridge, which is a bridge that has a structure that forms triangles which is called a truss. This structure can go above (through) or below (deck) the roadway, and the truss allows the bridge to become more rigid and prevent compression and tension. The truss is able to make a more rigid bridge because the triangles allow forces like compression and tension to spread over a larger area and not just on one beam like how a snowshoe spreads your weight over a larger area so you don't sink in the snow. Truss bridges are reliable and use up minimal resources which makes them one of the most common forms of bridges and one of the oldest.
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| A tied arch bridge (above) and can be held up by wires |
The third type of bridge is an arch bridge like the truss bridge it helps spread out the weight so it doesn't buckle or snap and the arch can be above (tied arch) or below the roadway. A tied arch bridge though does not
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| An arch bridge that is connected over a large gap |
spread the forces out instead the bridge is restrained by tension. A regular arch bridge has abutments, which is the arched structure like the trusses of a truss bridge and they transfer the weight. Like beam bridges it has repeated arches creating pillars.
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| A Cable Stayed Bridge |
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| A Suspension Bridge |
The forth and final bridge is the suspension and cable stayed bridge which instead of spreading the forces of the bridge. Suspension bridges are suspended from cables and towers the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river. A cable stayed bridge like suspension bridges, are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately higher.
