Part D – Capacity of Bridges

Appendix: Guideline for Bridge Safety Management with clarification comments on grey background

3.1 - Scope

Each railway authority’s BSMP should prevent the operation of equipment that could damage a bridge by exceeding safe stress levels in its components or by extending beyond the horizontal and vertical clearance limits of the bridge.

Section Analysis 3.1 – Scope

This section proposes that procedures be incorporated in BSMP to prevent the operation of equipment that could damage a bridge by exceeding safe stress levels in its components or by extending beyond the horizontal or vertical clearance limits of the bridge. Protection of bridges and their components from overstress is essential to the continued integrity and serviceability of the bridge. It is also essential that equipment or loads that exceed the clearance limits of a bridge not be operated owing to the potential for severe damage to the bridge.

3.2 - Determination of Bridge Load Capacities

  1. Each railway authority should determine the load capacity of each of its bridges. The load capacity is intended to be the safe load capacity not the ultimate or maximum load capacity.
  2. The load capacity of each bridge should be documented in the railway authority’s BSMP, together with the documented method by which the capacity was determined.
  3. The load capacity shall8 be determined by a Railway Bridge Engineer using engineering methods and standards applicable to the particular class, configuration, and type of bridges being evaluated.
  4. Bridge load capacity may be determined from existing design and modification records of a bridge, provided that the bridge substantially conforms to its recorded configuration. Otherwise, the load capacity of a bridge should be determined by measurement and calculation of the properties of its individual components, or other methods as determined by a Railway Bridge Engineer.
  5. Where a bridge inspection reveals that the condition of a bridge or its component might adversely affect the load capacity of the bridge to carry the traffic operated, a Railway Bridge Engineer should determine a new capacity.
  6. Railway bridge load capacity may be expressed in terms of numerical values related to a standard system of railway bridge loads, but should in any case be stated in terms of weight and length of individual or combined cars and locomotives, for the use of transportation personnel.
  7. Bridge load capacity may be expressed in terms of both normal and maximum load conditions. Operation of equipment that produces forces greater than the normal capacity should be subject to any restriction or conditions that may be prescribed by the Railway Bridge Engineer.

Section Analysis 3.2 - Determination of Bridge Load Capacities

a. Each railway authority should determine the load capacity of each of its bridges. It is essential that the railway authority know that loads operated over a bridge not exceed the safe capacity of that bridge. However, once it is determined that a bridge has adequate capacity to carry the loads being operated,  no additional effort is required  to precisely calculate the additional capacity of that bridge although that might well be useful from a planning or economic standpoint.

A railway authority should schedule the rating of bridges for which the load capacity has not already been determined. It is intended that the unrated railway bridges be given relative priority for rating, based on the judgment of the Railway Bridge Engineer. This prioritization can be accomplished either by observation or by evaluation of certain critical members of a railway bridge, as determined by the Railway Bridge Engineer using professional judgment.

b. The load capacity of each bridge should be documented in the railway authority’s BSMP, together with the method by which the capacity was determined. Once the load capacity is determined, the value should be recorded in order for it to be useful. Examples of methods of determination could be the original design documents, recalculation, or rating inspection.

c. Section 11 of the Railway Safety Act states” All the engineering work related to railway works, including design, construction, evaluation or alteration, shall be done in accordance with sound engineering principles. A professional engineer shall take responsibility for engineering work” Therefore, a professional engineer competent in the field of railway bridge engineering, must determine railway bridge capacity. Load capacity determination requires the education, experience and training of an engineer who is familiar with railway bridges and the standard practices that are unique to that class of structure.

While the present standard references for railway bridge design and analysis are found in the "Manual for Railway Engineering" of the American Railway Engineering and Maintenance of Way Association (AREMA), and that the chapters in this Manual dealing with Timber, Concrete and Steel structures, and Seismic Design, are under continuous review by committees consisting of leading engineers in the railway bridge profession, it is recognized that alternative methods and standards exist and are employed by some railway authorities to determine load capacity.

d. Bridge load capacity may be determined from existing design and modification records of a bridge, provided that the bridge substantially conforms to its recorded configuration. Determination of bridge load capacity requires information on the configuration of the bridge and the dimensions and material of its component parts. If the bridge is found to conform to the drawings of its original design and modifications, those drawings may serve as the basis for any rating calculation that might be performed, thus simplifying the process. Lacking that prior information, it may become necessary that the configuration, dimensions and properties of the bridge and its components be determined by on-site measurement of the bridge, as it currently exists.

e. It is proposed that a Railway Bridge Engineer determine a new capacity when a bridge inspection record reveals that the condition of a bridge or its component might affect its load capacity. Accurate determination of current bridge capacity depends on accurate information about the current configuration and condition of the bridge. The Railway Bridge Engineer might determine that a change in condition or configuration calls for a revised rating calculation.

f. The railway bridge load capacity may be expressed in terms of numerical values related to a standard system of railway bridge loads, but should in any case be stated in terms of weight and length of individual or combined cars and locomotives, for the use of transportation personnel. Railway Bridge Engineers use standard definitions of loading combinations for design and rating of railway bridges. Common among these standard definitions is a series of proportional loads known as the Cooper System. The capacity of a railway bridge and its components can be described in terms of a Cooper Rating, and the effect of a load on a railway bridge can also be related to a Cooper System value. Proper application of this system requires a full understanding of its use and limitations. However, the results of its application can be translated into terms of equipment weights and configurations that can be effectively applied by persons who manage regular transportation operations of the railway company. This enables them to determine if a given locomotive, car or combination can be operated on a railway bridge with no further consideration, or if the equipment must be evaluated as an exceptional movement.

g. The bridge load capacity may be expressed in terms of both normal and maximum load conditions. Normal bridge ratings generally define the loads that can be operated on a bridge for an indefinite period without damaging the bridge. In some cases, mostly involving steel or iron bridges, a higher rating, up to a maximum rating, can be given to the bridge to permit the operation of heavier loads on an infrequent basis. These heavier loads should not, in themselves, damage the bridge, but the cumulative effect of the higher resulting stresses in its members could cause their eventual deterioration.

This paragraph also proposes that operation of equipment that produces forces greater than the normal capacity should be subject to any restrictions or conditions that may be prescribed by a Railway Bridge Engineer. A Railway Bridge Engineer can often prescribe compensating conditions that will permit the movement of equipment that is heavier than normal. Examples include speed restrictions to reduce the impact factor of the rolling load, the insertion of lighter weight spacer cars between the heavier cars in a train, or the installation of temporary bents or other supports under specific points on the railway bridge.

3.3 - Protection of Bridges from Over-weight and Over-dimensional Loads

A railway company should know what rolling stock is allowed to operate on its network, their equipment rating, and restrictions required. A railway company should have, and ensure the implementation of documented procedures for the operation of equipment exceeding the normal weight or dimension restriction on a bridge. Equipment exceeding the normal weight or dimension restriction should only be operated under conditions determined by the Railway Bridge Engineer, who has properly analyzed the stresses resulting from the proposed loads.

The railway authority is expected to advise other railway companies operating over a railway bridge of the normal loads permitted over it. Railway companies should develop, maintain, and enforce written procedures to restrict a load that exceeds those limits, unless specific authority has been granted and in accordance with restrictions placed by the railway authority.

Each railway company should issue instructions to the personnel who are responsible for the configuration and operation of trains over its railway bridges to prevent the operation of cars, locomotives and other equipment that could exceed the capacity or dimensions of its railway bridges. The Railway Bridge Engineer should be informed of any substantial change in train operation or traffic patterns, which may affect bridge safety. These instructions should:

  1. be expressed in terms of maximum equipment weights, and either minimum equipment lengths or axle spacing.
  2. be expressed in terms of feet and inches of cross section and equipment length, in conformance with common railway industry practice for reporting dimensions of exceptional equipment in interchange in which height above top-of-rail is shown for each cross section measurement, followed by the width of the car of the shipment at that height.
  3. apply to individual structures, or to a defined line segment or group(s) of line segments where the published capacities and dimensions are within the limits of all structures on the subject line segments.

Section Analysis 3.3 - Protection of Bridges from Over-weight and Over-dimensional Loads

Bridges can be seriously damaged by the operation of loads that exceed their capacity.
Movement of equipment that exceeds the clear space on a bridge is an obvious safety hazard. This section proposes that each railway company should issue instructions to personnel who are responsible for the consist and operation of trains over railway bridges to prevent the operation of cars, locomotives and other equipment that would exceed the capacity or dimensions of bridges. Transportation personnel of a railway company are ultimately responsible for the movement of trains, cars and locomotives. It is essential that they should know and follow any restrictions that are placed on those movements.

a. The instructions regarding weight should be expressed in terms of maximum equipment weights, and either minimum equipment lengths or axle spacing. Transportation personnel have information on the weights and configuration of cars and locomotives, and they must be able to relate that information to any restrictions placed on the movement of that equipment.

b. The instructions regarding dimensions should be expressed in terms of feet and inches of cross section and equipment length, in conformance with common railway industry practice for reporting dimensions of exceptional equipment in interchange in which height above top-of-rail is shown for each cross section measurement, followed by the width of the car or the shipment at that height. In the industry, a standard format exists for the exchange of information on dimensions of railway equipment. This standard practice is practical, even if it is not intuitive. Use of the industry practice is necessary to avoid error and confusion.

c. The movement instructions may apply to individual structures or to a defined line segment or groups of line segments where the published capacities and dimensions are within the limits of all structures on the subject line segments. Railway authorities commonly issue instructions related to equipment weights and dimensions to be effective on line segments of various lengths. It is not necessary that transportation personnel be advised of the capacity of every bridge as long as each railway bridge in the line segment has the capacity to safely carry the loads permitted on that line.

When there is a change proposed in train operation or traffic patterns, which may affect railway bridge safety, the transportation department should have a procedure in place for ensuring that the designated Railway Bridge Engineer is advised of the change so that the safety of the railway bridges can be verified.

3.4 - Protection of Safe Railway Operations during Repairs or Modifications

Each railway authority’s BSMP should specify procedures for any repair or modification that materially modifies the capacity of a bridge or the stresses in any primary load-carrying component of a bridge.  At a minimum, design for repairs or modifications shall9 be performed under the direction of a Railway Bridge Engineer. The design should specify the manner in which traffic or other live loads may be permitted on the bridge while it is being modified or repaired.

Designs and procedures for repair or modification of bridges of a common configuration, such as timber trestles, or instructions for in-kind replacement of bridge components, may be issued as a common standard. Where the common standard addresses procedures and methods that could materially modify the capacity of a bridge or the stresses in any primary load-carrying component of a bridge, the standard shall10 be designed under the direction of a Railway Bridge Engineer.

Each repair or modification pursuant to this part should be performed under the direction of a Railway Bridge Engineer who is designated and authorized by the railway authority to supervise the particular work to be performed. He should ensure that the repair or modifications were completed in conformity with the design.

Section Analysis 3.4 - Protection of Safe Railway Operations during Repairs or Modifications

Minimum standards to be incorporated in railway authority’s BSMP to provide for adequate design and effective supervision of those bridge modifications and repairs which will materially modify the capacity of the bridge or the stresses in any primary load-carrying component of the bridge. This section provides for correct design and adequate supervision of repair and modification of bridges where the work could materially affect the capacity of the bridge, or its continued integrity. TC does not intend that minor repairs that do not affect the capacity of the bridge must be designed by a Railway Bridge Engineer, but the supervision of that work should be performed by a person who is competent to assure that the work does not inadvertently compromise the integrity of the bridge. For instance, arc welding handrails to the members of a through truss might appear to some to be a minor repair, but it could seriously compromise the structural integrity of the bridge.

Design of entire railway bridges, modifications and repairs which materially modify the capacity of the bridge or the stresses in any primary load-carrying component of the bridge require the intelligent application of the principles of engineering and can be performed only by a Railway Bridge Engineer with training and experience in the field of railway bridges. Railway authorities have typically issued standard instructions for the performance of common maintenance repairs, such as replacement or upgrading of components of timber trestles. This section specifically permits such a practice. For purposes of this part, a primary load-carrying component is a railway bridge component, the failure of which would immediately compromise the structural integrity of the bridge.

Designs and procedures for repair or modification of bridges of a common configuration, such as timber trestles, or instructions for in-kind replacement of bridge components, may be issued as a common standard. Although it may be a standard procedure, the standard should be designed and issued by a qualified Railway Bridge Engineer.

8 Section 11 of the Railway Safety Act
9 Section 11 of the Railway Safety Act
10 Section 11 of the Railway Safety Act

Date modified: