الفهرس | Only 14 pages are availabe for public view |
Abstract Dynamic properties of a hydraulic structure in terms of; frequency/time period, mode shape and damping properties, play the major role in seismic design of hydraulic structures or assessment of existing hydraulic structures. The seismic design depends mainly on the value of ground acceleration which is a function in the natural period of the structure. Therefore, the problem of choosing the value of ground acceleration in design or assessment reflects on the cost needed for establishing or habitation of existing hydraulic structure. Most of current codes do not involve the seismic design of this kind of structures. Also, most of the existing hydraulic structures in Egypt, such as barrages were built using bricks or plain concrete which has low resistance against tension stresses induced in the piers due to seismic excitation. These structures are social, strategic and sensitive for seismic loads. Besides that, the major benefits of hydraulic structures are improving the situation of irrigation, improving river navigation as well as eliminating the water shortage problems. Therefore, this current research aims to estimate the real dynamic properties of a hydraulic structure as the first step to find its behavior under the seismic loads. Also, investigating which parameters should be involved in and their effect on results. To reach this aim, the Old Rayah Menoufi Barrage was selected as a case study, that was simulated in three-dimensional Finite Element (FE) models using ANSYS release 15 FE Software. The dynamic properties of the Old Rayah Menoufi Barrage in terms of mode shapes, natural frequencies, and damping ratio were investigated experimentally and numerically. The Old Rayah Menoufi Barrage is an old existing hydraulic structure in Egypt that was permitted to be tested by the Construction Research Institute (CRI) - National Water Research Center (NWRC) - Egypt. After the estimation of the dynamic properties of the barrage, a time history analysis is mounted for the most realistic FE model, investigating its dynamic behavior under an Earthquake (EQ) time history record. For the numerical modal analysis, five various models are established to simulate the studied barrage, that are varied in the base condition (stiffness of the soil). However, the Experimental Modal Analysis (EMA) gives the realistic behavior of the tested structure, it requires some control to get accurate results. This control is represented in several factors; the method of excitation, the sampling rate of the recorded data, and the locations grid of the measurement sensors. Therefore, a prefinite element model is required to adjust these factors. This model is called Initial Model (IM). In this model, all structural elements of the barrage were simulated using solid 3D elements with its real corresponding material properties. Water through the vents and the lock chamber of the barrage was modelled using Housners` simulation [1]. The fixed base condition of the barrage was adopted in this model, neglecting the Soil Structure Interaction (SSI). Great significant differences were observed between them, so four Updated Models (UM1, 2, 3, and 4) were mounted to decrease this variation and achieve more realistic results close to the Experimental Work (EW). In the updated models, the SSI was considered through this model, changing the base condition from fixed to soil subgrade reactions. The vertical stiffness of the subgrade reactions was chosen as an average value of the Bowles` subgrade reactions [2], according to the soil properties under the base of the used barrage. Since, the horizontal subgrade reactions were considered as a percentage of the vertical reaction (0%, 10%, 30% and 50% for UM1, UM2, UM3 and UM4, respectively). The updated models are compared with the EW to obtain the most realistic results. These comparisons showed that UM3 with the ratio of 30% of the vertical subgrade reaction is the most matching model that is close to the EW. |