Colorado State University

Abstract:

Bridge waterways are flow openings designed to pass design water flows. However, bridge waterways often face potential problems that should be considered and accommodated in bridge design. The common causes of the problems are the fixed elevation of the bridge deck, the erodibility of the river boundary, blockage of flow, and designs combining “hard” structural components (bridge beams and deck, abutment columns, piers) and “soft” structural components (earthen-embankment approaches). The problems frequently cause bridges to fail.
This seminar examines the problems, indicates some design solutions, and points out the similarities between a bridge waterway and the bridged entrance to a dam’s spillway. However, complicating a bridge waterway in comparison to spillway entrance is the erodible nature of the river’s boundary, at least for most rivers. Usually, the bottom of a bridge waterway is erodible, unlike the fixed elevation of the entrance crest of a spillway. Erodibility of boundary leads to concerns regarding boundary scour and possible loss of foundation support at piers and abutments. In these respects, water-flow and geotechnical considerations combine; water content affects geotechnical strength; and geotechnical strength affects scour depth. Blockage of a bridge waterway reduces the waterway’s capacity to pass a design flow and, consequently, affects water levels upstream of a bridge. Several factors may cause blockage, including undue contraction of flow owing to the waterway’s design; for example, as soft structural components are typically cheaper than hard components, designers may seek to minimize the length of a bridge’s deck, thereby narrowing a bridge waterway. Also, accumulations of woody or vegetation debris, or ice rubble, may clog a bridge opening, thereby elevating water levels upstream of the bridge, and complicating scour assessment and load estimation for bridge beams. The combination of hard and soft components may complicate bridge-waterway performance, because the strength characteristics of soft (earthen) components are difficult to control (especially close to a hard component) and may vary with flow elevation and duration. Common weak locations of a bridge waterway (same for a dam) are where hard and soft components adjoin at a bridge’s abutment, for example.

Bio: https://www.engr.colostate.edu/ce/robert-ettema/

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