Load Testing for Rating Bridges 145

The performance and structural capacity of a bridge may be reduced during the designed service life due to damage, corrosion, loss of section and material deterioration. Field measurements of a bridge involve assessments of section properties such as the actual member size, deterioration and the uncertainties of the position of internal components. These can be used for rating bridges.

The objective of a non-destructive loading test is to determine the load capacity for the load rating of a bridge. The performance and structural capacity of a bridge can be quantified by the loading test. When a controlled and predetermined load is applied to a bridge, the actual structural response can be evaluated. Results should take into account the rating of the bridges, which can be determined by comparison of the previous test results. The principle of the loading test is to compare the field measurement, such as load versus deflection or load versus strain, of a bridge, with its theoretical performance. When the actual deflection of a bridge subjected with a controlled load is measured, the capacity of the bridge can be inversely evaluated by

comparing the theoretical result. This will enable to estimate the capacity of the bridge to carry live loads.

Load testing can be classified as static load tests and dynamic load tests. AASHTO (2011) defines that a static load test is conducted using stationary loads to avoid bridge vibrations. The intensity and position of the load may be changed during the test. A dynamic load test is conducted with time-varying loads or moving loads that excite vibrations in the bridge. Dynamic tests may be performed to measure modes of vibration, frequencies, dynamic load allowance, and to obtain load history and stress ranges for fatigue evaluation.

There are mainly two types of non-destructive static load tests which are the proof load testing and performance load testing ("Standards Australia AS 5100.7," 2004). Both tests are based on the measurement of the bridge response to the vehicle loading.

Proof load testing involves loading the bridge incrementally, using a load close to the ultimate limit state where the bridge behaviour is within the linear-elastic range. The aim of the test is to proof the ability of the carrying capacity of the bridge. A satisfactory proof load test usually provides higher confidence in the load capacity than a calculated capacity (AASHTO, 2011). However, proof load tests are often considered as a high risk approach and may lead to a rapid deterioration and collapse (Mehrkar-Asl & Brookes, 1997). Proof load testing should not be used unless the bridge is fully instrumented to measure strains in steel and concrete, and rotation and deflection at critical positions (Mehrkar-Asl & Brookes, 1997).

A static performance loading test that is a serviceability limit state test, involves monitoring a structure using normal road or railway traffic loads, or specific vehicles loaded to pre-determined weights to determine specific responses, such as vertical and horizontal forces, deflection and strains, to assist in assessing load distribution, to identify weak or failed components and to understand the structural performance. Load versus deflection ratio can be used as an indicator to access the current performance of the bridge. The test may also be employed regularly to monitor the degradation of structural performance and assist in detecting defective

components. Comparing the load versus deflection ratio of the original design and the current performance, the capacity of the bridge can be evaluated ("Standards Australia AS 5100.7," 2004).

The proposed vertical displacement measurement approaches are applicable for static load testing of a bridge. The measurements for load testing are usually strain, vertical displacement and slope (2011). In the proposed methods, strain/curvature and slope will be measured and the vertical displacement is determined by the implementation of the approaches. An extra instrument for the vertical displacement measurements is not required when the proposed methods are implemented. The cost of the loading tests can be reduced.

Although load testing can determine the actual load carrying capacity of the bridge, the tests are not suitable to be conducted when the cost of testing reaches or exceeds the cost of bridge strengthening, and the test may be impractical because of access difficulties or site traffic conditions. As mentioned in Chapter 2, existing vertical displacement measurement methods are sometimes impractical because a specialised operator is required or measurements can be affected by weather. For these instances, the proposed methods are useful, as they are inexpensive and a stationary reference is not required.

The proposed methods can measure the required parameters from the beginning of the test to continuously monitor the responses of the bridge under incremental loading in order to keep it under linear-elastic behaviour and limit distress due to cracking or other physical damage.

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Appendix

Appendix A

Matrix Structural Analysis4

The MATLAB code of the stiffness method for structural analysis of two and three dimensional frame structure is given as follows,