Editing Suspension bridge (section)

==Structure==
===Bridge main components===
{{wide image|Severn bridge-panoramic.jpg|800px|align-cap=center|The slender lines of the [[Severn Bridge]]}}

Two towers/pillars, two suspension cables, four suspension cable anchors, multiple suspender cables, the bridge deck.<ref>diagram</ref>

===Structural analysis=== <!-- Please see the article discussion if you think the terms parabola and catenary should be swapped. -->
[[File:Comparison catenary parabola.svg|thumb|upright=1.25|Comparison of a catenary (black dotted curve) and a parabola (red solid curve) with the same span and sag. 
The main [[force (physics)|force]]s in a suspension bridge of any type are [[tension (mechanics)|tension]] in the cables and [[compression (physical)|compression]] in the pillars. Since almost all the force on the pillars is vertically downwards, and the bridge is also stabilized by the main cables, the [[column|pillars]] can be made quite slender, as on the [[Severn Bridge]], on the Wales-England border.

In a suspended deck bridge, cables suspended via towers hold up the road deck. The weight is transferred by the cables to the towers, which in turn transfer the weight to the ground.

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The catenary represents the profile of a simple suspension bridge or the cable of a suspended-deck suspension bridge on which its deck and hangers have negligible mass compared to its cable. The parabola represents the profile of the cable of a suspended-deck suspension bridge on which its cable and hangers have negligible mass compared to its deck. The profile of the cable of a real suspension bridge with the same span and sag lies between the two curves.
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The main cables of a suspension bridge will form a [[catenary]] when hanging under their own weight only.  When supporting the deck, the cables will instead form a [[parabola]], assuming the weight of the cables is small compared to the weight of the deck. One can see the shape from the constant increase of the gradient of the cable with linear (deck) distance, this increase in gradient at each connection with the deck providing a net upward support force. Combined with the relatively simple constraints placed upon the actual deck, that makes the suspension bridge much simpler to design and analyze than a [[cable-stayed bridge]] in which the deck is in compression.

=== Comparison with cable-stayed bridge ===
[[Cable-stayed bridge]]s and suspension bridges may appear to be similar, but are quite different in principle and in their construction.

In suspension bridges, large main cables (normally two) hang between the towers and are [[Earth anchor|anchored]] at each end to the ground. The main cables, which are free to move on bearings in the towers, bear the load of the bridge deck. Before the deck is installed, the cables are under [[tension (mechanics)|tension]] from their own weight. Along the main cables smaller cables or rods connect to the bridge deck, which is lifted in sections. As this is done, the tension in the cables increases, as it does with the [[live load]] of traffic crossing the bridge. The tension on the main cables is transferred to the ground at the anchorages and by downwards [[compression member|compression]] on the towers.

<gallery class="center" caption="Difference between types of bridges" widths="250px" heights="75px">
File:Bridge-suspension.svg|Suspension bridge
File:Bridge-fan-cable-stayed.svg|Cable-stayed bridge, fan design
</gallery>

In cable-stayed bridges, the towers are the primary load-bearing structures that transmit the bridge loads to the ground. A [[cantilever]] approach is often used to support the bridge deck near the towers, but lengths further from them are supported by cables running directly to the towers. By design, all static horizontal forces of the cable-stayed bridge are balanced so that the supporting towers do not tend to tilt or slide and so must only resist horizontal forces from the live loads.

===Advantages===
[[File:Suspension bridge in Bairnsdale, Australia.jpg|thumb|A suspension bridge can be made out of simple materials such as wood and common wire rope.]]

* Longer main spans are achievable than with any other type of bridge.
* Less material may be required than other bridge types, even at spans they can achieve, leading to a reduced construction cost.
* Except for installation of the initial temporary cables, little or no access from below is required during construction and so a waterway can remain open while the bridge is built above.
* They may be better able to withstand earthquake movements than heavier and more rigid bridges.
* Bridge decks can have deck sections replaced in order to widen traffic lanes for larger vehicles or add additional width for separated cycling/pedestrian paths.

===Disadvantages===
* Considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck from vibrating under high winds.
* The relatively low deck stiffness compared to other (non-suspension) types of bridges makes it more difficult to carry [[Passenger rail terminology#Heavy rail|heavy rail]] traffic in which high concentrated [[Structural load#Types of loads|live loads]] occur.
* Some access below may be required during construction to lift the initial cables or to lift deck units. That access can often be avoided in [[cable-stayed bridge]] construction.