Foundation, Concrete and Earthquake Engineering

Design Criteria for Hydraulic Analysis and Design of Bridges

Structures with a centerline span of 20 feet or more can be defined as Bridges. But, Design criteria for structures designed hydraulically irrespective of span length are treated as bridges in this blog. The following general criteria shall be used in the hydraulic analysis and design of bridge:
a) The final design selection should consider the maximum backwater allowed by the National Flood
Insurance Program (NFIP) unless exceeding the limit can be justified by special hydraulic conditions;
b) For sites outside of Federal Emergency Management Agency (FEMA) regulation, the backwater shall not cause increased flood damage to property upstream of the crossing;
c) The final design should not significantly alter the existing flow distribution in the floodplai
d) The "crest-vertical curve profile" is the preferred highway bridge crossing profile when allowing for embankment overtopping at a lower discharge and for adequate deck drainage;

f) Sag vertical curves can cause deck drainage to pond and ice up on the bridges and should be avoided;

g) Horizontal curve transitions cause water to flow across lanes and should not be located on a bridge because of icing and hydroplaning problems;
h) Clearance or freeboard should be provided between the low girder and the design water surface to allow for the passage of ice and debris;
i) The design capacity of any bridge will be the flow that will pass through the bridge with adequate freeboard and without roadway overtopping;
j) Estimate all degradation and aggradation plus contraction scour and local scour for the design year and for the 500-year event. Indicate the total scour envelope with a continuous line drawn such that the structural designer may adequately design substructure components. Scour depths are to be estimated with consideration of the local geology;
k) Velocities through the structure(s) will not damage either the highway facility or increase damages to adjacent property;
l) Pier spacing and orientation, and abutment location shall be designed to minimize flow disruption and
potential scour. Bridge piers should not be placed in the main channel area;
m) Foundation design and/or scour countermeasures shall be made to avoid failure by scour. Typically, substructure components are designed to avoid failure by scour;
n) Although appropriate in some debris prone streams, connecting a discrete pier column to a debris-deflecting wall can significantly increase scour depths if the channel alignment ever shifts. A debris-deflecting wall can also greatly increase the stiffness of a pier that reduces the number of available design options. More preferably, a long span bridge design reduces the number of piers and therefore, reduces the benefits derived from debris deflecting walls. It is now often more efficient for a designer to simply design a pier (and if necessary the superstructure) for increased stream loads due to debris;
o) When two or more bridges are constructed in parallel over a channel, care should be taken to align the piers and to provide streamlined grading and protection for abutments. This abutment grading is to minimize expansion or contraction of flow between the two bridges;
p) Commercial mining of sands and gravel in streams is common because the material is clean and well graded and the stream replenishes the supply. Borrow pits, either upstream or downstream of a highway-stream crossing, can cause or aggravate scour at the bridge. This fact should be considered when calculating bridge scour, and should be estimated by sediment transport modeling;
q) Disruption of ecosystems is to be minimized. Consideration is to be given to the preservation of valuable characteristics that are unique to the floodplain and stream;
r) Economic analysis of the design shall include complete life cycle costs and benefits. Factors that should be considered are construction, maintenance, operation, as well as any potential liabilities;
s) Adequate right-of-way shall be provided upstream and downstream of structure for maintenance operation.

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