Fundamental Code for Design on Railway Bridge and Culvert
铁路桥涵设计基本规范
1 General Provisions
1.0.1 This code is formulated with a view to unify the technical standard of railway bridge and culvert design, carry out the relevant laws and regulations and railway technical policies of the nation and make the design of railway bridge and culvert meet the requirement of safety and usability, advanced technology and economy and rationality.
1.0.2 This code is applicable to the design of railway bridge and culvert with Grade I, II standard gauge on the combine line of passenger and freight, passenger car that the design operation speed is or lower than 160 km/h, the freight car that the design operation speed is or lower than 120 km/h (80 km/h of 8 A Type freight car bogie) in railway network.
The applicable span of this code: the span of the concrete beam is or less than 96 m.
the span of the steel beam is or less than 168 m, the span of the steel sheet beam is or less than 40m,
1.0.3 The bridge is classified as the following according to its length:
Super major bridge — the bridge length is more than 500 m;
Major bridge — the bridge length is more than 100 m up to 500 m;
Medium bridge — the bridge length is more than 20 m up to 100 m;
Minor bridge — the bridge length is 20 m or lower.
Note: bridge length — means the length between the front ballast retaining wall of abutments for the beam bridge, the length between outside ends of two expansion and contraction joints between the upper side wall of arch and side wall of abutment for the arch bridge, the length between the outsides with the rigid frame along the span direction for the rigid frame bridge.
1.0.4 For the bridge and culvert design, the river history and actuality as well as its development trend shall be investigated concretely, the interrelation between bridge, culvert and water conservation, navigation, environment protection and industry and agriculture shall be taken into account, the geology condition of bridge site shall be explored so as to determine the correct design scheme.
1.0.5 During the design, fabrication, transportation, erection and operation, the bridge and culvert structure shall be possessed with specified strength, stiffness, stability and durability. The bridge structure shall be designed according to 100 years of service life. When designing the bridge and culvert structure, the computational check of the speed-limited passing for the long freight train shall be performed according to the relevant regulation in current Rating Code for Railway Bridge (file No. Tie Yun Han[2004] 120 ).
The structure shall be designed with advanced technology economically and rationally, the component shall be standardized as possible to be convenient to fabrication and mechanized construction, meeting the requirement of curing, emergency repair, detection and maintenance, equipped with the necessary facilities.
The bridge shall be designed combining with the environment and beautiful out-looking.
1.0.6 The dimensions and the construction materials adopted for the bridge and culvert structure shall be considered with the impact on the durability by the local temperature and environment.
1.0.7 The bridge and culvert shall be designed and checked according to the flood frequency standard in Table 1.0.7.
Table 1.0.7 Flood Frequency Standard for Bridge and Culvert
Railway grade Designed flood frequency Checked flood frequency
Bridge Culver Super major (or major) bridge with complex technology, repair difficulty or importance
Grade I, II 1/100 1/100 1/300
Notes: 1 If the observed flood (including investigated flood ) frequency is less than that in the above table, it shall be designed according to observed flood frequency, but when the observed flood frequency is less than the following, the design shall be performed as:
1 /300: for the super major, major, medium bridge of grade I, II railway, 1/100: for the minor bridge and culvert.
2 When the water level is not determined with the flow due to upwind, ice jam, tide, down-draught, river-bed evolution, reservoir impoundment and backwater of other hydraulic structures, the flow and the water level shall be determined respectively.
3 For the bridge and culvert built within the inundated limits of reservoir, the norm of flood frequency listed in the table can be held good. For the bridge and culvert built in downstream of dam, if the norm of designed flood frequency of reservoir is higher than that of flood frequency of bridge and culvert, the standard flood discharge of reservoir listed in the table plus the catchment quantity between bridge and dam shall be used as the flow quantity for design and calculation of bridge and culvert, if the norm of check flood frequency of reservoir is less than the norm of flood frequency of bridge and culvert, it shall be negotiated with the relevant department that the norm of check flood frequency of dam shall be uprated to make it conform to the norm of flood frequency of railway bridge and culvert. If difficult, the design can be done according to the natural status of the stream, furthermore, it shall be properly considered that the dam breaking is possible to have an unfavorable influence on the bridge and culvert.
4 For the bridge and culvert built within the influenced limits of dam upstream and downstream. if the serious deposition of reservoir is likely to have an unfavorable influence on the bridge and culvert, the norm of flood frequency of bridge and culvert may be uprated according to the actual condition.
5 The aperture of submerged and inlet submerged culvert shall be calculated according to the flood frequency of design height of embankment.
6 The flood frequency of existing railway improvement or additional 2nd railway shall be considered according to the secular operating conditions and the specific status of flood hazard. The existing buildings shall be utilized as much as possible to avoid unnecessary dismantling and more relocating.
1.0.8 For the special structure and representative bridge, the train-bridge coupling dynamic response comprehensive analysis shall be performed with its train operating safety and stationarity index meeting the relevant regulations in current Code for Dynamics Performance Evaluation and Test Evaluation for Rolling Stock (GB 5599) and Dynamics Performance Test Evaluation Method and Evaluation Standard for Rail Locomotive (TB/T 2360).
The carbody vertical acceleration of train with the strong vibration frequency of the ballasted deck no more than 20 Hz: a≤0.35g.
1.0.9 For the design of the bridge operating double-layer container train, not only the requirements stipulated in this code, but also those in current relevant ones of the nation shall be complied with.
1.0.10 For the railway bridge laid with unballasted track or operating 120 km/h freight train, not only the requirements stipulated in this code, but also those in current relevant ones of the nation shall be complied with.
1.0.11 The railway bridge and culvert shall be designed with safety protection sign, caution sign and protection facility according to the relevant regulation in Railway Transportation Safety Protection Regulations (File No. 430 of State Department of P. R. C).
1.0.12 For the renovation of existing line or construction of additional second line, the increased section and rebuilt section shall adopt newly-built standard.
1.0.13 In the railway bridge and culvert design, not only the requirements stipulated in this standard code, but also those in current relevant ones of the nation shall be complied with.
2 Terms and Symbols
2.1 Terms
2.1.1 Railway bridge
The overhead structure where the railway crosses over the natural obstruction or man-made facilities.
2.1.2 Railway culvert
The structure going across the railway subgrade and used for flood discharge, irrigation or as passageway.
2.1.3 Jacked-in bridge or culvert
The bridge or culvert going across the existing railway subgrade with jacked-in method adopted for construction.
2.1.4 Bridge superstructure
The structure crossing over the bridge hole above the beam bridge support or spring line of arch bridge.
2.1.5 Carbody vertical acceleration
Ratio between carbody vertical vibration inertia and carbody weight.
2.1.6 Settlement after construction
The setting volume generated after the completion of infrastructure and commence of track-laying.
2.1.7 Vertical dynamic force of train
Vertical dynamic force action generated during train operation.
2.1.8 Centrifugal force of train
The horizontal force trending towards the curve outside generated from train operating on the curve.
2.1.9 Braking force of train
The horizontal force in running direction generated to the structure from the braking of the operating train.
2.1.10 Tractive force of train
The horizontal force against running direction generated to the structure from the train starting.
2.1.11 Lateral sway force of train
Left and right horizontal force generated to the rail top surface from the train operatioa
2.1.12 Longitudinal force due to temperature variation
The longitudinal force generated from the longitudinal relative displacement of the bridge and long rail due to the temperature variation.
2.1.13 Longitudinal force due to deflection of the structure
The longitudinal force generated from the longitudinal relative displacement of the bridge and long rail due to the bridge deflection under the action of the train load.
2.1.14 Breaking force of long rail
The longitudinal force generated from the longitudinal relative displacement of the bridge and long rail due to fracture of long rail.
2.1.15 Longitudinal force due to long rail
Generic terms of longitudinal force due to temperature variation, longitudinal force due to deflection of the structure and breaking force of long rail.
2.1.16 Pier
The structure supporting the adjacent bridge superstructure and transferring its load to the foundation.
2.1.17 Abutment
The retaining structure connecting bridge superstructure with subgrade.
2.2 Symbols
2.2.1 External force and internal force
p——vertical earth pressure acting on the culvert by the embankment
e——horizontal earth pressure acting on the culvert by the embankment, horizontal earth pressure acting on the culvert by the live load
qh——vertical pressure acting on the culvert by the live load
ε——earth pressure coefficient
K——earth pressure coefficient, pier shape coefficient
1+μ——-dynamic coefficient
W——wing load strength, ship weight or catamaran weight, vertical static live load d of train
W0——foundational wind pressure
P——flowing water pressure
F——impact force of ship or catamaran
q1——vertical pressure acting on round culvert tube coupling by permanent load and
live load
q2——horizontal pressure acting on round culvert tube coupling by permanent load and
live load
2.2.2 Geometric parameter
L——bridge span, loading length of influence line
f——rise of arch
L0——calculated length of arch ring
△——horizontal displacement of pier and abutment top cap surface
r——average radius of round pipe section
3 Layout of Bridge and Culvert
3.1 General Requirement
3.1.1 Bridges and culverts shall not be arranged on arable lands or shall minimize the use of arable land. The layout shall take into consideration the land and water transportation, needs of irrigation and drainage, safety of dwellers at upper and lower reaches, housing and sheds as well as crops, and constitute the integral and well functional drainage system together with drainage facilities in urban and other areas.
3.1.2 The cross between Class I Railway and highway shall be in the form of grade separation, and others may have grade separation according to relevant stipulations in the current Code for Design of Railway Line.
3.1.3 It is a principle to build one bridge when one river is to be crossed.
If there are two or more stable channels at the bridge site, or the flow of beach land accounts for a large portion of the designed flow, and the water current is hard to be channeled into the same bridge, it is acceptable to build bridges on the main channel and branches or beach land separately, but a long and large-scale diversion dike shall not be used to concentrate the water flow forcibly.
3.1.4 For areas of plains, grasslands and overflowing areas, bridges and culverts shall be arranged according to flood diversion area, but it should not have an excessively big interval between two bridges and culverts.
3.1.5 Natural water courses shall not be rechanneled willfully. If the rechanneling of water course or channel cutoff will surely be able to improve the working conditions of bridges and culverts or bring considerable economic benefits, it is allowed to do so. Nevertheless, the consequences of the change on the hydraulic conditions shall be taken into consideration.
3.1.6 The centerline of bridge site should be orthogonal to the flow direction of floodwater, so that backwater at the bridge head and the resulted triangular backflows should be avoided, which are a threat to the safety of bridge on the line.
3.1.7 The roofs of diversion and protection structures that are not immersed shall be higher than the water level of designed flood frequency by at least 0.25 m (consideration shall be also given to the upstream face gradient). Where necessary, study into the effects of back water level, height of hitting waves, local flow uprush, partial skew flow uprush, cant of cove, river bed sedimentation etc.
The roofs of immersed diversion and control structures should be higher than normal water level. The water face of diversion and control structures that are not immersed shall be equipped with full-height protection. And the immersed diversion and control structures shall be protected at both sides and top side. The protection criteria for various diversion and control structures shall be determined by the impacts possibly from water streams, waves, ice drifts, running logs, drifters etc. Think about the impact of scouring when it comes to the design of slope toe.
3.1.8 The centerline of bridge site on the operational rivers shall be orthogonal to the shipping line. Expand the navigable clear opening if they have to obliquely intersect with each other. As for rechanneled water courses, take into account their effects on navigable spans.
3.1.9 Railway and highway bridges should be constructed separately. If it is necessary to build them altogether, submit the proposal to the Ministry of Railways for approval.
3.1.10 When building the bridge on the second line, many factors such as national defense requirements, hydrographic and geological conditions, working conditions of bridge on the existing line, status of infrastructure, navigation requirements, interferences in construction and traffic shall be taken into account to decide and arrange the bridge at the upper or lower reaches of the bridge on the existing line as well as the distance between two lines.
If the bridge on the second line and bridge on the existing line are located in the scope of water current interference, the center lines of pier and abutment shall correspond to each other, roughly parallel to the flow direction of flood water.
3.1.11 The renovated bridge and culvert shall keep their previous center line and location if they have no major flaws and the coordination on route plan and profile is well planned.
It is allowed to enclose or newly build the bridge and culvert on the existing line only when sufficient evidences and approval from the consumer construction are gained.
3.2 Aperture of Bridge and Culvert
3.2.1 The aperture of bridge and culvert must be designed to ensure safe access for floods, ice drifts, mudflows and drifters etc. of the designed frequency, and the impacts of back water and scouring on upper and lower reaches shall be considered in the design to ensure the reliability of the embankment adjacent to the bridge and culvert and to facilitate the maintenance and repair.
3.2.2 The alteration of river bed shall be noted in the design of aperture of bridge and culvert. The natural status of water currents should not be changed.
In case the river bed is likely to be scoured, the allowable coefficient of scour (i.e. the ratio between required flow area and the supply area) should not exceed the values presented in Table 3.2.2.
After calculating the aperture of the bridge in the plain areas by coefficient of scour, the effects of back water in front of the bridge on the villages and farm lands at the upper reaches must be checking calculated If the hazard exists, the aperture of bridge needs to be enlarged
The bridge aperture on the artificial channel should not be narrowed. Instead, the number of piers in the middle shall be reduced.
The aperture of bridge in mudflow-prone areas shall be designed based on the basic river width of the section where the valley crosses through, which should not be narrowed or overly enlarged. It should adopt bridges of single aperture or multi-apertures with large spans, and excavation must not be performed under the bridge.
With regard to the aperture of bridge that may be influenced by the reservoir, its design shall respect not only the natural conditions of rivers, but also the changes of river conditions caused by the reservoir.
Fundamental Code for Design on Railway Bridge and Culvert
铁路桥涵设计基本规范
1 General Provisions
1.0.1 This code is formulated with a view to unify the technical standard of railway bridge and culvert design, carry out the relevant laws and regulations and railway technical policies of the nation and make the design of railway bridge and culvert meet the requirement of safety and usability, advanced technology and economy and rationality.
1.0.2 This code is applicable to the design of railway bridge and culvert with Grade I, II standard gauge on the combine line of passenger and freight, passenger car that the design operation speed is or lower than 160 km/h, the freight car that the design operation speed is or lower than 120 km/h (80 km/h of 8 A Type freight car bogie) in railway network.
The applicable span of this code: the span of the concrete beam is or less than 96 m.
the span of the steel beam is or less than 168 m, the span of the steel sheet beam is or less than 40m,
1.0.3 The bridge is classified as the following according to its length:
Super major bridge — the bridge length is more than 500 m;
Major bridge — the bridge length is more than 100 m up to 500 m;
Medium bridge — the bridge length is more than 20 m up to 100 m;
Minor bridge — the bridge length is 20 m or lower.
Note: bridge length — means the length between the front ballast retaining wall of abutments for the beam bridge, the length between outside ends of two expansion and contraction joints between the upper side wall of arch and side wall of abutment for the arch bridge, the length between the outsides with the rigid frame along the span direction for the rigid frame bridge.
1.0.4 For the bridge and culvert design, the river history and actuality as well as its development trend shall be investigated concretely, the interrelation between bridge, culvert and water conservation, navigation, environment protection and industry and agriculture shall be taken into account, the geology condition of bridge site shall be explored so as to determine the correct design scheme.
1.0.5 During the design, fabrication, transportation, erection and operation, the bridge and culvert structure shall be possessed with specified strength, stiffness, stability and durability. The bridge structure shall be designed according to 100 years of service life. When designing the bridge and culvert structure, the computational check of the speed-limited passing for the long freight train shall be performed according to the relevant regulation in current Rating Code for Railway Bridge (file No. Tie Yun Han[2004] 120 ).
The structure shall be designed with advanced technology economically and rationally, the component shall be standardized as possible to be convenient to fabrication and mechanized construction, meeting the requirement of curing, emergency repair, detection and maintenance, equipped with the necessary facilities.
The bridge shall be designed combining with the environment and beautiful out-looking.
1.0.6 The dimensions and the construction materials adopted for the bridge and culvert structure shall be considered with the impact on the durability by the local temperature and environment.
1.0.7 The bridge and culvert shall be designed and checked according to the flood frequency standard in Table 1.0.7.
Table 1.0.7 Flood Frequency Standard for Bridge and Culvert
Railway grade Designed flood frequency Checked flood frequency
Bridge Culver Super major (or major) bridge with complex technology, repair difficulty or importance
Grade I, II 1/100 1/100 1/300
Notes: 1 If the observed flood (including investigated flood ) frequency is less than that in the above table, it shall be designed according to observed flood frequency, but when the observed flood frequency is less than the following, the design shall be performed as:
1 /300: for the super major, major, medium bridge of grade I, II railway, 1/100: for the minor bridge and culvert.
2 When the water level is not determined with the flow due to upwind, ice jam, tide, down-draught, river-bed evolution, reservoir impoundment and backwater of other hydraulic structures, the flow and the water level shall be determined respectively.
3 For the bridge and culvert built within the inundated limits of reservoir, the norm of flood frequency listed in the table can be held good. For the bridge and culvert built in downstream of dam, if the norm of designed flood frequency of reservoir is higher than that of flood frequency of bridge and culvert, the standard flood discharge of reservoir listed in the table plus the catchment quantity between bridge and dam shall be used as the flow quantity for design and calculation of bridge and culvert, if the norm of check flood frequency of reservoir is less than the norm of flood frequency of bridge and culvert, it shall be negotiated with the relevant department that the norm of check flood frequency of dam shall be uprated to make it conform to the norm of flood frequency of railway bridge and culvert. If difficult, the design can be done according to the natural status of the stream, furthermore, it shall be properly considered that the dam breaking is possible to have an unfavorable influence on the bridge and culvert.
4 For the bridge and culvert built within the influenced limits of dam upstream and downstream. if the serious deposition of reservoir is likely to have an unfavorable influence on the bridge and culvert, the norm of flood frequency of bridge and culvert may be uprated according to the actual condition.
5 The aperture of submerged and inlet submerged culvert shall be calculated according to the flood frequency of design height of embankment.
6 The flood frequency of existing railway improvement or additional 2nd railway shall be considered according to the secular operating conditions and the specific status of flood hazard. The existing buildings shall be utilized as much as possible to avoid unnecessary dismantling and more relocating.
1.0.8 For the special structure and representative bridge, the train-bridge coupling dynamic response comprehensive analysis shall be performed with its train operating safety and stationarity index meeting the relevant regulations in current Code for Dynamics Performance Evaluation and Test Evaluation for Rolling Stock (GB 5599) and Dynamics Performance Test Evaluation Method and Evaluation Standard for Rail Locomotive (TB/T 2360).
The carbody vertical acceleration of train with the strong vibration frequency of the ballasted deck no more than 20 Hz: a≤0.35g.
1.0.9 For the design of the bridge operating double-layer container train, not only the requirements stipulated in this code, but also those in current relevant ones of the nation shall be complied with.
1.0.10 For the railway bridge laid with unballasted track or operating 120 km/h freight train, not only the requirements stipulated in this code, but also those in current relevant ones of the nation shall be complied with.
1.0.11 The railway bridge and culvert shall be designed with safety protection sign, caution sign and protection facility according to the relevant regulation in Railway Transportation Safety Protection Regulations (File No. 430 of State Department of P. R. C).
1.0.12 For the renovation of existing line or construction of additional second line, the increased section and rebuilt section shall adopt newly-built standard.
1.0.13 In the railway bridge and culvert design, not only the requirements stipulated in this standard code, but also those in current relevant ones of the nation shall be complied with.
2 Terms and Symbols
2.1 Terms
2.1.1 Railway bridge
The overhead structure where the railway crosses over the natural obstruction or man-made facilities.
2.1.2 Railway culvert
The structure going across the railway subgrade and used for flood discharge, irrigation or as passageway.
2.1.3 Jacked-in bridge or culvert
The bridge or culvert going across the existing railway subgrade with jacked-in method adopted for construction.
2.1.4 Bridge superstructure
The structure crossing over the bridge hole above the beam bridge support or spring line of arch bridge.
2.1.5 Carbody vertical acceleration
Ratio between carbody vertical vibration inertia and carbody weight.
2.1.6 Settlement after construction
The setting volume generated after the completion of infrastructure and commence of track-laying.
2.1.7 Vertical dynamic force of train
Vertical dynamic force action generated during train operation.
2.1.8 Centrifugal force of train
The horizontal force trending towards the curve outside generated from train operating on the curve.
2.1.9 Braking force of train
The horizontal force in running direction generated to the structure from the braking of the operating train.
2.1.10 Tractive force of train
The horizontal force against running direction generated to the structure from the train starting.
2.1.11 Lateral sway force of train
Left and right horizontal force generated to the rail top surface from the train operatioa
2.1.12 Longitudinal force due to temperature variation
The longitudinal force generated from the longitudinal relative displacement of the bridge and long rail due to the temperature variation.
2.1.13 Longitudinal force due to deflection of the structure
The longitudinal force generated from the longitudinal relative displacement of the bridge and long rail due to the bridge deflection under the action of the train load.
2.1.14 Breaking force of long rail
The longitudinal force generated from the longitudinal relative displacement of the bridge and long rail due to fracture of long rail.
2.1.15 Longitudinal force due to long rail
Generic terms of longitudinal force due to temperature variation, longitudinal force due to deflection of the structure and breaking force of long rail.
2.1.16 Pier
The structure supporting the adjacent bridge superstructure and transferring its load to the foundation.
2.1.17 Abutment
The retaining structure connecting bridge superstructure with subgrade.
2.2 Symbols
2.2.1 External force and internal force
p——vertical earth pressure acting on the culvert by the embankment
e——horizontal earth pressure acting on the culvert by the embankment, horizontal earth pressure acting on the culvert by the live load
qh——vertical pressure acting on the culvert by the live load
ε——earth pressure coefficient
K——earth pressure coefficient, pier shape coefficient
1+μ——-dynamic coefficient
W——wing load strength, ship weight or catamaran weight, vertical static live load d of train
W0——foundational wind pressure
P——flowing water pressure
F——impact force of ship or catamaran
q1——vertical pressure acting on round culvert tube coupling by permanent load and
live load
q2——horizontal pressure acting on round culvert tube coupling by permanent load and
live load
2.2.2 Geometric parameter
L——bridge span, loading length of influence line
f——rise of arch
L0——calculated length of arch ring
△——horizontal displacement of pier and abutment top cap surface
r——average radius of round pipe section
3 Layout of Bridge and Culvert
3.1 General Requirement
3.1.1 Bridges and culverts shall not be arranged on arable lands or shall minimize the use of arable land. The layout shall take into consideration the land and water transportation, needs of irrigation and drainage, safety of dwellers at upper and lower reaches, housing and sheds as well as crops, and constitute the integral and well functional drainage system together with drainage facilities in urban and other areas.
3.1.2 The cross between Class I Railway and highway shall be in the form of grade separation, and others may have grade separation according to relevant stipulations in the current Code for Design of Railway Line.
3.1.3 It is a principle to build one bridge when one river is to be crossed.
If there are two or more stable channels at the bridge site, or the flow of beach land accounts for a large portion of the designed flow, and the water current is hard to be channeled into the same bridge, it is acceptable to build bridges on the main channel and branches or beach land separately, but a long and large-scale diversion dike shall not be used to concentrate the water flow forcibly.
3.1.4 For areas of plains, grasslands and overflowing areas, bridges and culverts shall be arranged according to flood diversion area, but it should not have an excessively big interval between two bridges and culverts.
3.1.5 Natural water courses shall not be rechanneled willfully. If the rechanneling of water course or channel cutoff will surely be able to improve the working conditions of bridges and culverts or bring considerable economic benefits, it is allowed to do so. Nevertheless, the consequences of the change on the hydraulic conditions shall be taken into consideration.
3.1.6 The centerline of bridge site should be orthogonal to the flow direction of floodwater, so that backwater at the bridge head and the resulted triangular backflows should be avoided, which are a threat to the safety of bridge on the line.
3.1.7 The roofs of diversion and protection structures that are not immersed shall be higher than the water level of designed flood frequency by at least 0.25 m (consideration shall be also given to the upstream face gradient). Where necessary, study into the effects of back water level, height of hitting waves, local flow uprush, partial skew flow uprush, cant of cove, river bed sedimentation etc.
The roofs of immersed diversion and control structures should be higher than normal water level. The water face of diversion and control structures that are not immersed shall be equipped with full-height protection. And the immersed diversion and control structures shall be protected at both sides and top side. The protection criteria for various diversion and control structures shall be determined by the impacts possibly from water streams, waves, ice drifts, running logs, drifters etc. Think about the impact of scouring when it comes to the design of slope toe.
3.1.8 The centerline of bridge site on the operational rivers shall be orthogonal to the shipping line. Expand the navigable clear opening if they have to obliquely intersect with each other. As for rechanneled water courses, take into account their effects on navigable spans.
3.1.9 Railway and highway bridges should be constructed separately. If it is necessary to build them altogether, submit the proposal to the Ministry of Railways for approval.
3.1.10 When building the bridge on the second line, many factors such as national defense requirements, hydrographic and geological conditions, working conditions of bridge on the existing line, status of infrastructure, navigation requirements, interferences in construction and traffic shall be taken into account to decide and arrange the bridge at the upper or lower reaches of the bridge on the existing line as well as the distance between two lines.
If the bridge on the second line and bridge on the existing line are located in the scope of water current interference, the center lines of pier and abutment shall correspond to each other, roughly parallel to the flow direction of flood water.
3.1.11 The renovated bridge and culvert shall keep their previous center line and location if they have no major flaws and the coordination on route plan and profile is well planned.
It is allowed to enclose or newly build the bridge and culvert on the existing line only when sufficient evidences and approval from the consumer construction are gained.
3.2 Aperture of Bridge and Culvert
3.2.1 The aperture of bridge and culvert must be designed to ensure safe access for floods, ice drifts, mudflows and drifters etc. of the designed frequency, and the impacts of back water and scouring on upper and lower reaches shall be considered in the design to ensure the reliability of the embankment adjacent to the bridge and culvert and to facilitate the maintenance and repair.
3.2.2 The alteration of river bed shall be noted in the design of aperture of bridge and culvert. The natural status of water currents should not be changed.
In case the river bed is likely to be scoured, the allowable coefficient of scour (i.e. the ratio between required flow area and the supply area) should not exceed the values presented in Table 3.2.2.
After calculating the aperture of the bridge in the plain areas by coefficient of scour, the effects of back water in front of the bridge on the villages and farm lands at the upper reaches must be checking calculated If the hazard exists, the aperture of bridge needs to be enlarged
The bridge aperture on the artificial channel should not be narrowed. Instead, the number of piers in the middle shall be reduced.
The aperture of bridge in mudflow-prone areas shall be designed based on the basic river width of the section where the valley crosses through, which should not be narrowed or overly enlarged. It should adopt bridges of single aperture or multi-apertures with large spans, and excavation must not be performed under the bridge.
With regard to the aperture of bridge that may be influenced by the reservoir, its design shall respect not only the natural conditions of rivers, but also the changes of river conditions caused by the reservoir.
