In the design of a new bridge, provision must be made for maintenance inspection. For example, plate girders can be provided with safety handrails, safety railings can be specified on top of wide piers for inspectors to check bearings, and safety ladders can be installed to provide access to elements of the bridge otherwise difficult to reach. For deck-type bridges of moderate span and width, it will be possible to access the superstructure from special bridge inspection vehicles operating on the deck. For longer spans where the depth of girder exceeds the vertical capacity of a boom, and for wider bridges where the horizontal reach of the boom is not adequate, it may be necessary to provide catwalks or permanent movable inspection platforms. These devices are becoming increasingly popular as inspection and maintenance requirements are given the attention they deserve in the design process.
Stream scour can undermine bridge piers or abutments, resulting in collapse of spans and loss of life. Several such incidents, including collapses in Alabama, New York (Schoharie Creek bridge on the New York Thruway, 1987), and Tennessee, caused FHWA to mandate the evaluation of all highway bridges for scour vulnerability by 1997.
An insidious aspect of scour is that soil around a foundation can be removed and redeposited during a flood without leaving clear evidence that this has occurred, so that material may be present but may not provide the required support. Beyond surveying the stream bottom for local lowering of the flow line and inspecting around the pier by visual, manual, and remote means, current techniques for determining whether a loss of support has occurred are limited. They include physical probing and use of ground-penetrating radar.
Bridge scour evaluation requires input from hydraulic engineers as well as from structural and geotechnical engineers. Following the determination of a total scour prism, all three disciplines should be involved in providing structural stability.
Scour Study. A scour study at an existing bridge will include some or all of the following:
• A channel bottom physical inspection
• A channel bottom topographic inspection
• A localized scour evaluation conducted around each substructure element
• Photographic or video recording of observations
• Hydraulic analysis
• Soils investigation including laboratory testing
Hydraulic Analysis. In the hydraulic analysis, depth of scour is calculated for 100-year and 500-year floods. Inclusion of the 500-year flood calculation reflects a change of thinking in regard to bridge hydraulics that has taken place in the last 20 or 30 years. Previously, it was thought acceptable to have a very small percentage of bridges wash out in a severe flood, and if this did not occur the hydraulic design requirements were considered excessive. The current thinking is that a complete washout should be avoided, even in very extreme floods because total cost of a bridge failure would be more than design for scour. A difficulty in implementing this policy, as in earthquake engineering, is that the hydrologic database has been developed over a relatively short period of time in the United States.
AASHTO LRFD specifications require the bridge foundations to be investigated for the following two conditions:
• 100-year design flood for scour or an overtopping flood of lesser recurrence interval
at strength and service limit states
• 500-year check flood for scour or an overtopping flood of lesser recurrence interval
at extreme event limit state
AASHTO Model Drainage Manual contains guidance on design procedures and computer software for hydrologic and hydraulic design.
A series of hydraulic analysis computer programs are available to assist in scour analysis. They include HEC-18, Evaluating Scour at Bridges; HEC-20, Stream Stability at Highway Structures; HEC-23, Bridge Scour and Stream Instability Countermeasures; HEC-RAS, River Analysis System; HY-8, Culvert Hydraulics Analysis; and FHWA’s water surface modeling program, WSPRO. These programs are primarily for inland streams. In coastal areas, tidal velocities and hurricane surge velocities may also cause scour. To perform the hydraulic analyses for these conditions, it may be necessary to obtain data from the Federal Emergency Management Agency (FEMA) and the National Oceanic and Atmospheric Administration (NOAA) and to use other analysis techniques.
Soils Investigation. The objective of the soils investigation is to determine whether and to what degree the soils are subject to being eroded. Grain size is of particular interest.
Countermeasures. Where a potential for undermining is found, countermeasures will be required to ensure the stability of the bridge. Countermeasures include riprap, poured-concrete protective aprons with keyed edges, cabled-concrete sections, precast-concrete units, rock-filled basket mattresses, and protective piles or sheet-piling. In the case of new bridges, where more opportunities for preventing scour exist, some of the available options are a larger waterway opening that reduces stream velocity, location of piers out of the scour-vulnerable zone, use of deeper piles, and selection of a different pier shape.
Design Information on Plans for New Bridges. Information from the scour analysis for a new bridge should be placed on the construction drawings so that a permanent record of scour estimates, and their effect on design, is readily available for future inspections and for improvement of this design process.
