Suspension bridge

A suspension bridge is a type of bridge that has been made since ancient times. Simple suspension bridges, for use by pedestrians, are still constructed in some places worldwide for use by pedestrians and livestock using designs based upon the ancient Inca rope bridge. Suspended from two high locations over a river or canyon, simple suspension bridges follow a shallow downward arc and are not suited for modern roads and railroads. Over the centuries, advancements in materials and design led to the development of the suspended-deck suspension bridge, a modern bridge capable of carrying vehicles and light rail. Instead of the deck following a downward arc of the main load bearing cables (or chains) these cables are suspended between towers and the cables support vertical suspender cables to carry the weight of a deck below, upon which the traffic crosses. This arrangement allows the deck to be level or to arc slightly upward for additional clearance.

Construction sequence

• Where the towers are founded on underwater piers, caissons are sunk and any soft bottom is excavated for a foundation. If the bedrock is too deep to be exposed by excavation or the sinking of a caisson, pilings are driven to the bedrock or into overlying hard soil, or a large concrete pad to distribute the weight over less resistant soil may be constructed. The foundation piers are then extended to above water level.
• Where the towers are founded on dry land, deep foundations or pilings are used.
• From the tower foundation, towers of multiple columns are erected using concrete, stonework, or steel structures. At some elevation there must be a passage for the deck, with the columns extending high above this level.
• Smooth open cable paths called saddles are anchored atop the towers. These allow for slight movements of the cable as the loads change during construction. The top of these saddles may be closed with an additional part after completion of the bridge.
• Anchorages are constructed to resist the tension of the cables. These are usually anchored in good quality rock. The anchorage structure will have multiple protruding open eyebolts enclosed within a secure space.
• A temporary suspended walkway supported by wire rope follows the curve of the cables to be constructed, mathematically described as a catenary arc.
• Another set of wire ropes are suspended above the walkway and are used to support a traveler that has wheels riding atop these cables. There will be one set of wire ropes and a traveler for each cable to be "spun"
• Pulling cables attached to winches are capable of pulling the traveler from one anchorage to the other, traveling in arcs to the tops of the two towers.
• High strength wire, typically less than 10 mm in diameter, is pulled in a loop by pulleys on the traveler, with one end affixed at an anchorage. Workers stationed along the walkway attach the passing cable to a bundle with a temporary binding. When the traveler reaches the opposite anchorage the loop is placed over an open anchor eyebar.
• The traveler is returned to the start point to pick up another loop or it is used to carry a new loop from this side.
• As loops are placed, corrosion proofing may be applied.
• In this way a complete sub-cable is created linking the eyebar (or a set of eyebars) from one anchorage to the other. The sub-cables will have a hexagonal cross section and are held together with the temporary bindings.
• Multiple adjacent sub-cables are placed adjacent to each other. While these are on a hexagonal grid, the general form for the larger cable is circular.
• The entire cable is then compressed by a traveling hydraulic press into a closely packed cylinder and tightly wrapped with additional wire to form the final circular cross section.
• Saddles to carry the suspender cables are clamped to the main cables, each with an appropriate shape to conform to the ultimate slope of the main cables. Each saddle is an equal horizontal distance from the next, with spacing appropriate to the design of the deck.
• Suspender cables engineered and cut to precise lengths and carrying swedged ends are looped over the saddles. In some bridges, where the towers are close to or on the shore, the suspender cables may be applied only to the central span.
• Special lifting hosts attached to the suspenders or from the main cables are used to lift prefabricated sections of bridge deck to the proper level, provided that the local conditions allow the sections to be carried below the bridge by barge or other means, otherwise a traveling cantilever my be used to extend the deck one section at a time. During the construction the finished portions of the deck will appear to pitch upward rather sharply, as there is no downward force in the center of the span. Upon completion of the deck the added load will pull the main cables into an arc mathematically described as a parabola, while the arc of the deck will be as the designer intended - usually a gentle upward arc for added clearance if over a shipping channel, or flat in other cases such as a span over a canyon,
• With completion of the primary structure various details such as lighting, handrails, finish painting and paving are added.