Case Study: Aging Steel Bridges Load Assessment using FEA Plus – Sensors and FL

One of the major railway asset owners operates and maintains a series of steel truss bridges over major water crossings on the east coast of Australia. The bridges vary in age, from circa 1880’s to 1960’s.

As structures age and their loadings change, managing asset integrity becomes critical for the asset owners. Normally, traditional engineering methods are used for assessing a bridge’s load capacity, but these method scan sometimes be overly conservative and not cost-effective for asset owners.

To help the asset owner improve the load capacity of bridges and carry out more cost effective maintenance work, Rockfield was engaged to conduct a detailed load capacity assessment and determine the fatigue and remnant life of 13 bridges. The company used finite element modeling and combined it with field data obtained from structural health monitoring, resulting in a more accurate assessment and load rating of the bridges.

Project overview:

One of the 13 bridges was a heritage listed bridge that was constructed in 1925. The bridge is approximately 170 m long, consisting of 1 x 80 ft, 1 x 200 ft, 2 x 120 ft truss spans and 1 x 47 ft girder span, with 3 x concrete filled cast iron piers and 2 x concrete abutments.

This project consisted of the following tasks:

1. Instrumentation and structural health monitoring.
   An instrumentation system was installed to measure strains in truss members at key locations on each span. Load cases of the locomotive for the static test were developed in the FE model.
2. Structural modelling and analysis.
   Our team used the ANSYS software package to determine the truss load rating/ capacities and determine the fatigue/remnant life. Non-linear stress-strain, eigenvalue buckling and pre-deformed/displaced geometries form the basis of the analysis process. The reported corrosion was reviewed and rationalised/ simplified for modelling purposes. Non-linear elastic-plastic material properties have been considered for the ultimate limit state analysis and allowing for a more accurate and reliable evaluation of a structure’s capacity.
3. Develop method of rational.
   The rational method for the braking and traction forces were determined using first-principles approach based on bridge-specific design parameters specified by the railway asset owner.
4. Substructure analysis including detailed headstock FEA assessment.
5. Bearing replacement concept design and 3rd party review of detailed design.
6. Corrosion and repairs procedures.
   Rockfield developed repair and replacement procedures and drawings for defects that were determined through assessments, inspections, and previous work, as well as those that were specifically identified by the client. Additional goals included ensuring repairs are practical, cost-effective, and maximise capacity.
7. Probabilistic assessment for allowable corrosion and load rating.
   This assessment utilises a philosophy similar to the limit states assessment but allows for a more thorough/detailed determination of load/resistance variability and calculation of the Probability of Failure.