Experimentally Determined Continuous Displacement Influence Lines for Bridges

The Influence line (IL) is an important concept in bridge design and load-capacity evaluation and can be used to calculate live load effects and to position loads to have their maximum effect. Generally, ILs serve two purposes: (1) ILs can be used qualitatively to determine the position of the live loads that will lead to maximum effects, and (2) they can be used quantitatively to calculate the maximum response value directly by making use of ILs. A theoretical IL can be derived using equilibrium equations or the virtual work method and are available for common structures in the literature. However, theoretical IL's do not account for the in-situ performance of bridges, which is important for the calculation of the true load carrying capacity. So it is necessary to obtain an experimental IL. The usefulness of ILs in determining the load capacity of the bridge has long been realized by engineers. Previous studies on experimental IL studies have focused mainly on strain (or stress) experimental Ils. This can be attributed to the difficulty in measuring small displacements of a bridge using conventional displacement sensors. However, with the development of innovative methods for measuring displacement, such as GPS and laser, the limitations of conventional displacement measurement techniques have been, or will soon be, largely overcome. The displacement measurement by using these new technologies in field test is becoming more and more convenient, fast, and easy. In order to utilize the benefits of these new displacement instruments, one way is through the experimental displacement ILs. In this paper, one methodology, which is referred as the Continuous Experimental Displacement Influence Line (CDIL) method, has been developed to experimentally determine a displacement influence line of a bridge from a semi-static diagnostic load test. The procedure to determine the experimental displacement influence line is presented and verified on one bridge.