Bridge Testing and Load Ratings by BDI: Overview
No matter how sophisticated, modern structural analysis techniques cannot replace field testing for the determination of the live-load performance of a bridge. Only by recording field measurements as the structure is subjected to live load can the engineer gain an understanding of the bridge's actual structural response. Furthermore, the field data can be used to “calibrate” an analytical model that closely represents the behavior observed in the field. This "Integrated" technique (the method using field measurements to modify an analytical model) provides an approach that can be used for detecting damage, general evaluations, and for developing load ratings for all types of steel, concrete, and timber structures.
The Integrated method was pioneered by the members of BDI and is based on a "semi-static" live load test that consists of instrumentation installation, several load passages, and instrumentation removal. Up to 64 re-usable strain transducers are mounted to the structural members of the bridge (no holes or welds are required) and strains recorded as a loaded vehicle with known weight crosses at crawl speed. Each strain sensor requires only about five minutes to install. Only one lane needs to be closed at a time, minimizing traffic interruptions. High-speed passes are often also conducted in order to determine actual impact factors.
Depending on access, a typical short or medium-span bridge is usually instrumented and tested with up to 64 channels in one working day with a two-to three-person crew.
The subsequent analytical modeling techniques range from a simple planer grid model to a three-dimensional finite element representation. The response of the model is systematically compared with the field test results using multiple gage locations and load configurations. Structural parameters such as lateral deck stiffnesses and rotational restraints are then modified through an iterative process until the analytical responses closely match the field measurements. This calibrated model can then be used to predict stress levels at critical locations due to rating and overloaded vehicles. Rating factors can then be developed using either the Allowable Stress method (ASD) or the Load Factor method (LFD).
This approach has been used on highway bridges, railroad bridges, lock gates, amusement park rides, and other structures where the live load can be easily applied and the live load stresses are significant. Steel, pre-stressed concrete, reinforced concrete, and timber structures have all been tested successfully.
Based on the experience of more than 250 field tests and subsequent analysis comparisons since 1989, Bridge Diagnostics, Inc. (BDI) provides efficient and inexpensive load testing services as well as structural analysis and rating . Since BDI does not offer inspection or design services, we make an excellent team member for design firms and researchers interested in acquiring field data. In addition, BDI manufactures the Structural Testing System (BDI-STS) to allow agencies or private consultants to perform their own efficient load testing and rating procedures.