Case Study

Research work on the Koyna Dam, India.

In 1967, the 103m high Koyna mass concrete dam in India was subjected to a magnitude 6.5 earthquake. Accelerometers on the site recorded the time histories of the event and a later survey showed that cracking had occurred in parts of the dam. Due to the availability of field data the dam has been the subject of a number of studies in recent years. Currently, the University of Wales in Cardiff, UK are modelling the dam using LUSAS Civil & Structural to investigate its response to changes in numerical concrete cracking models.

The aim of the study was to derive a concrete cracking model to allow for dynamic cyclic loadings and produce results that closely matched those of the experimental data. LUSAS was ideal for this because it contains a material model interface which allows users to develop and research constitutive models for their own use.

During a seismic event a variety of factors can affect the predicted response of a dam. Of particular interest is the unloading curve because when cracks open and close it is well established that the crack stress-strain curve does not return to zero but to some finite tensile strain indicating that particles have 'wedged' in the open crack.

Using the derived concrete model, 3 analyses were undertaken with the same finite element mesh, time step, and material data. The only difference between the analyses being the shape of the softening and unloading curves. Bilinear softening assuming no wedged particles, and exponential softening both with and without wedged particles were investigated. For each analysis, gravity and hydrostatic loads were applied to the structure and a dynamic analysis was run applying the combined vertical and horizontal ground motions.

The concrete model with exponential softening and wedged particles gave the lowest response overall both in terms of response spectra and peak accelerations. For cyclic dynamic applications it appears to be important to incorporate both a realistic softening curve and crack wedging behaviour into a numerical concrete model. Work on this study is continuing to see if other parameters also have a strong influence on dynamic structural response.