The simplest cross section of the pier is square, or rectangular, but other shapes are also common. In medieval architecture, massive circular supports called drum piers, cruciform (cross-shaped) piers, and compound piers are common architectural elements.
Single-span bridges have abutments at each end that support the weight of the bridge and serve as retaining walls to resist lateral movement of the earthen fill of the bridge approach. Multi-span bridges require piers to support the ends of spans between these abutments. In cold climates the upstream edge of a pier may include a starkwater to prevent accumulation of broken ice during peak snowmelt flows. The starkwater has a sharpened upstream edge sometimes called a cutwater. The cutwater edge may be of concrete or masonry, but is often capped with a steel angle to resist abrasion and focus force at a single point to fracture floating pieces of ice striking the pier. In cold climates the starling is typically sloped at an angle of about 45° so current pushing against the ice tends to lift the downstream edge of the ice translating horizontal force of the current to vertical force against a thinner cross-section of ice until unsupported weight of ice fractures the piece of ice allowing it to pass on either side of the pier.
In the Arc de Triomphe, Paris (illustration, right) the central arch and side arches are raised on four massive planar piers.
St Peter's Basilica
The original plan by Donato Bramante for St Peter's Basilica in Rome has richly articulated piers, rendered in solid black (illustration, left). The vaulting they support is in double lines, a familiar representation convention in architectural plans. Four piers support the weight of the dome at the central crossing. These piers were found to be too small to support the weight and were changed later by Michelangelo to account for the massive weight of the dome.
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