Instrumentation and Response Measurement of a Double‐Lane Bailey Bridge During Load Test

Abstract

During the past decades very significant advances in analytical techniques have led to the development of extremely powerful and versatile methods of structural analysis that are generally computer-based. Within the linear elastic range, these methods are supposed to make predictions with reasonable accuracy. These advanced methods of analysis (finite element, finite strip, grillage analysis, etc.) are used for both the design and strength evaluation of bridges. It has frequently been found that significant discrepancies exist between the predicted and observed responses, even when the loading is within the linear elastic range of the bridge. The discrepancies between the analytical and measured responses were subsequently found to be not due to inadequacies of the methods of analysis, but rather due to the presence of behavioural factors that could not be included in the mathematical modelling because of difficulties in their quantification. The theoretical evaluation of the bridge requires accurate information about material properties, support behaviour, contribution of non-structural members and other factors. Many a time, simplified assumptions are made to account for these parameters in analysis. It makes one believe that even highly rigorous methods of analysis cannot be relied upon unquestionably to predict the actual response of a bridge. There is no better way to understand the shortcomings of the mathematical models used for design (or) evaluation of bridges than to investigate the behaviour of bridges through load testing. Bridge testing is rapidly becoming a major tool in evaluation and may be seen as an acceptance proof test. Load testing provides several benefits from a safety point of view. Load testing helps to assess the behaviour of the bridge under specified loading condition and also to evaluate the load carrying capacity of particular structural configuration. This involves instrumenting the bridge at various critical locations and measurement of strain and deflection responses under the test loads. Selection of response locations, sensor types, installation techniques and data acquisition system are all very important and challenging in actual load testing of bridges. Load testing can have several benefits in evaluation, provided the tests are simple to perform and of reasonable cost. The results must also be able to be interpreted in a manner to realistically assess the behaviour and load carrying capacity of the bridge. Two types of bridge testing are available: (i) proof load testing, and (ii) diagnostic/behaviour tests. In the proof load test, a large load level is used, which assures that there is sufficient strength capacity for the bridge. Several problems exist with this type of test. The test is costly and a high level of risk is involved if the test programme is not properly planned. Hence, many bridge owners are reluctant to undertake such tests. The diagnostic/behaviour test is the most common type of bridge test in which some load, often much below the service load, is used and the response measured for the tested load level. The bridge response for higher classes of loading can be obtained by extrapolation. Behaviour tests are carried out either to study the mechanics of bridge behaviour or to verify other methods of analysis, the objective in the latter case being that after verification, the methods can be used with confidence in the design and evaluation of bridges. One double-lane Bailey bridge, 24.38 m (80 ft) span, was instrumented with 50 strain gauges and 14 dial gauges at various critical locations to obtain the structural responses under static loading. This paper describes the instrumentation details, loading arrangement, testing procedure and measurement of bridge response during the load test. The maximum load permitted on the bridge as per Indian Road Congress (IRC) guidelines was estimated from this investigation.