T.V.S.R.Appa Rao

Structural integrity assessment of offshore tubular joints based on reliability analysis

Abstract The paper presents a reliability-based approach and the investigations conducted based on it to assess the structural integrity of offshore tubular joints. The reliability analysis is carried out using Monte Carlo simulation technique and first order reliability method (FORM). The crack in addition to the geometry, material, and loading of the tubular joint are modeled as random variables to compute the reliability of the joint. Linear elastic fracture mechanics principles are employed to evaluate the crack growth. Degradation in the reliability of the tubular member due to crack propagation is estimated. Structural integrity assessment is carried out for joints having two adjacent discrete cracks to study the influence of the relative sizes of cracks on the reliability of the member. The significance of randomness in the variables modeled and the sensitivity of the reliability index on those variables are studied using example problems. The application of the results of the present study to schedule inspections with optimal time interval is highlighted. Finally, a method based on Bayesian approach is applied to update the reliability of the member based on the outcome of in-service inspections. The paper contributes to the knowledge of the application of reliability-based techniques for integrity assessment of offshore tubular joints.

Studies on performance of curved beam finite elements for analysis of thin arches

Abstract The finite element model with cubic polynomial shape functions to analyse thin arches has been modified to relieve membrane locking effects and thus to achieve better performance. Two different ways of improving the element stiffness have been studied, one by adopting the reduced integration technique and the other by adopting a least square fit to achieve field consistency for membrane strains. The curved cantilever beam of Kikuchi and two fixed circular arches of Dawe having different ratios of radius to thickness (one deep, the other shallow) were studied by using the improved models. The results obtained by the improved models are compared with those of others. The model with three point integration for membrane energy gives, as expected, identical results in comparison with field consistency. The model with two point reduced integration in membrane and four point integration for bending has been found to be superior in convergence.

A new parallel overlapped domain decomposition method for nonlinear dynamic finite element analysis

Abstract In this paper a new parallel algorithm for nonlinear transient dynamic analysis of large structures has been presented. An unconditionally stable Newmark-β method (constant average acceleration technique) has been employed for time integration. The proposed parallel algorithm has been devised within the broad framework of domain decomposition techniques. However, unlike most of the existing parallel algorithms (devised for structural dynamic applications) which are basically derived using nonoverlapped domains, the proposed algorithm uses overlapped domains. The parallel overlapped domain decomposition algorithm proposed in this paper has been formulated by splitting the mass, damping and stiffness matrices arises out of finite element discretisation of a given structure. A predictor–corrector scheme has been formulated for iteratively improving the solution in each step. A computer program based on the proposed algorithm has been developed and implemented with message passing interface as software development environment. PARAM-10000 MIMD parallel computer has been used to evaluate the performances. Numerical experiments have been conducted to validate as well as to evaluate the performance of the proposed parallel algorithm. Comparisons have been made with the conventional nonoverlapped domain decomposition algorithms. Numerical studies indicate that the proposed algorithm is superior in performance to the conventional domain decomposition algorithms.

Studies on stress concentration at shell-column junctions of hyperboloid cooling towers

Abstract Only a few investigations have been conducted so far to study stress concentration at shell-column junctions of column-supported hyperboloid cooling towers. These have been confined to the use of axisymmetric shell finite element models. A critical look at the procedures employed and results obtained by these studies indicates that there is a need for verification by adopting other numerical procedures and also that there is scope for improvement. This paper presents stress concentration studies conducted by using a 33 degrees-of-freedom doubly curved triangular shell element and a 3D column element specifically developed for this application. A method of analysis is presented in the paper for determining forces in columns and also stress concentrations at shell-column junctions of a hyperboloid cooling tower. Case studies conducted on typical cooling towers are presented. The results obtained in this investigation are compared with those obtained earlier. The paper also discusses the relative merits of the methods of analysis used by the authors and by other researchers.

Nonlinear analysis of reinforced concrete hyperboloid cooling towers—I. Material model, finite element model and validation

Abstract This is the first part of a two-part series of papers in which the constitutive material modelling of reinforced concrete, in shell structures, which resist applied loads predominantly through membrane action, is presented. The material model includes the effects of tensile cracking, tension stiffening, compression softening, interface shear transfer, and change in material stiffness due to crack rotation. A four-noded isoparametric curved shell element has been used in the nonlinear finite element analysis. The results obtained by using the model for analysis of a shear wall panel subjected to in-plane loading have been compared with those from experimental investigation.

Collapse loads of reinforced concrete cylindrical water tanks using limit analysis approach

Abstract Evaluation of the margin of safety against collapse of reinforced concrete cylindrical water tanks requires knowledge about the load carrying capacity of the tanks. The limit analysis procedure can be used to estimate the collapse loads of cylindrical tanks. In this paper, determination of the collapse loads of reinforced concrete circular cylindrical water tanks, having varying reinforcement along the height, by applying the limit analysis approach is discussed. The relevant mathematical formulations and the details of the computer program TANK developed for estimating the collapse loads of short, medium height and long cylindrical tanks are presented. The formulations presented are general in nature and different distributions of hoop reinforcement along the height of the tank wall can be considered. The results obtained using this program are found to be in good agreement with those reported in the literature. Numerical results for example problems with different hoop reinforcement distributions are given.

Nonlinear analysis of reinforced concrete hyperboloid cooling towers—II. Parametric study and results

Abstract In this second and final part of this series of papers the details of parametric studies conducted to assess the influence of various geometric and material parameters on the load-displacement response of three reinforced concrete hyperboloid cooling towers are presented. The material model adopted for the nonlinear finite element analysis is described in part I of this paper.

Comparative efficiencies of three parallel algorithms for nonlinear implicit transient dynamic analysis

The work reported in this paper is motivated by the need to develop portable parallel processing algorithms and codes which can run on a variety of hardware platforms without any modifications. The prime aim of the research work reported here is to test the portability of the parallel algorithms and also to study and understand the comparative efficiencies of three parallel algorithms developed for implicit time integration technique. The standard message passing interface (MPI) is used to develop parallel algorithms for computing nonlinear dynamic response of large structures employing implicit time-marching scheme. The parallel algorithms presented in this paper are developed under the broad framework of non-overlapped domain decomposition technique. Numerical studies indicate that the parallel algorithm devised employing the conventional form of Newmark time integration algorithm is faster than the predictor-corrector form. It is also accurate and highly adaptive to fine grain computations. The group implicit algorithm is found to be extremely superior in performance when compared to the other two parallel algorithms. This algorithm is better suited for large size problems on coarse grain environment as the resulting submeshes will obviously be large and thus permit larger time steps without losing accuracy.

A procedure to assemble only non-zero coefficients of global matrix in finite element analysis

Abstract A procedure to assemble only non-zero coefficients of a global matrix of finite element analysis in a one-dimensional array is proposed. The procedure uses a stencil matrix and a template matrix at intermediate stages to finally arrive at the one-dimensional global array. The intermediate operations are done with matrices consisting of only integers. The element matrices are handled only during the final step of the assembly. The procedure is simple and modular. The procedure has been implemented in a computer program and demonstrated using an example problem in the context of h-adaptive refined finite element meshes.

Stress resultants in hyperboloid cooling tower shells subjected to foundation settlement

Abstract The effect of differential settlement of columns supporting a natural draught hyperboloid cooling tower on the stress resultants in the tower shell is analysed using discrete finite element modelling of the shell and the supporting base. The quadrilateral facet shell elements and 3-D beam elements are used to model the shell and the columns respectively. The stress resultants obtained due to an imposed settlement are compared with those reported earlier by other investigators. In the existing literature, the stress resultants are found using the Boussinesq equation to calculate the approximate edge loading (column forces) on the cooling tower shell due to differential settlement. It is shown in the present study that using the Boussinesq solution for the calculation of edge loading may lead to an over-estimation of the column forces and hence the stress resultants. The magnitude of stress resultants is also found to be dependent on the column flexibility. The paper also presents an investigation in which the local amplification of stress resultants in the vicinity of the shell-column junction of the cooling towers (stress concentration) due to dead load and foundation settlement is studied. It is shown that the stress concentration can be severe, i.e., up to seven times the average stress resultants for dead load, and upto five times the average stress resultants for the foundation settlement.