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 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 heavy rail traffic in which high concentrated 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. Variations Underspanned In an underspanned suspension bridge, also called under-deck cable-stayed bridge, the main cables hang entirely below the bridge deck, but are still anchored into the ground in a similar way to the conventional type. Very few bridges of this nature have been built, as the deck is inherently less stable than when suspended below the cables. Examples include the Pont des Bergues of 1834 designed by Guillaume Henri Dufour; James Smith's Micklewood Bridge; and a proposal by Robert Stevenson for a bridge over the River Almond near Edinburgh. Roebling's Delaware Aqueduct (begun 1847) consists of three sections supported by cables. The timber structure essentially hides the cables; and from a quick view, it is not immediately apparent that it is even a suspension bridge. Suspension cable types The main suspension cables in older bridges were often made from a chain or linked bars, but modern bridge cables are made from