S.S. Ajeesh

Bio: S.S. Ajeesh is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Buckling & Cold-formed steel. The author has an hindex of 3, co-authored 4 publications receiving 24 citations.

Identification of buckling modes in generalized spline finite strip analysis of cold-formed steel members

Abstract: In this paper, a mode identification technique in the context of spline finite strip method (SFSM) is presented to compute the contribution of primary (global, distortional and local) and secondary (shear/transverse extension) buckling modes. The base vectors corresponding to individual buckling modes are developed based on the principles of generalized beam theory. The buckling mode shape in SFSM is approximated as a linear combination of these orthonormal base vectors to evaluate the participation of individual buckling mode. The proposed mode identification technique is able to successfully quantify the participation of different buckling modes and the mode participation is comparable with mode identification using finite strip method (FSM) and generalized beam theory (GBT). Illustrative examples are presented to calculate the participation of individual modes in cold-formed steel sections under different loading and boundary conditions. Also the specific application of mode identification in SFSM is demonstrated.

A constrained spline finite strip method for the mode decomposition of cold-formed steel sections using GBT principles

Abstract: In this paper, an analysis procedure has been presented for cold-formed steel sections, for decomposing the buckling modes obtained using spline finite strip method (SFSM) into their primary and independent buckling modes such as local, distortional and global buckling. This procedure utilizes principles of generalized beam theory (GBT) to evaluate the restraint matrices corresponding to different modes. The restraint matrices are integrated in to spline stiffness matrices using transformation technique to extract pure modes corresponding to local, distortional and global modes. The proposed analysis technique termed as constrained spline finite strip method (cSFSM) has been validated for cold-formed steel open cross-sections subjected to axial compression and bending under various boundary conditions. The results are in agreement with buckling stresses evaluated using constrained finite strip method (cFSM) and GBT.

Simplified semi-analytical model for elastic distortional buckling prediction of cold-formed steel flexural members

Abstract: The expressions for elastic distortional buckling stress predictions available in literature are used with reasonable accuracy, but are either iterative or involve increased calculation effort for design. It is an accepted fact among researchers that these expressions are not evaluated enough for the distortional stress prediction of sections with complex lip stiffeners. In this paper, a candidate model for distortional buckling stress predictions is presented which is semi analytical in nature and is simple to incorporate in the direct strength method (DSM) of cold-formed steel design. The proposed expression incorporates the effect of complex lip stiffeners on the elastic distortional buckling capacities of cold formed steel flexural members. In the proposed model, the (i) translational stiffness at lip-flange junction and (ii) rotational stiffness at the flange-web junction, are derived from regression analysis of wide range of cross sectional dimensions. The proposed method is calibrated with semi-analytical finite strip method presented in the literature and this formulation has been demonstrated to be good in comparison with recently published numerical based distortional buckling predictions.

TWLIGHT-IITM—A Computational Utility for Elastic Buckling Stress Predictions of Cold-Formed Steel Elements

Abstract: A new design method, Direct Strength Method (DSM), has been introduced by researchers in recent years for calculating the ultimate design strength of cold-formed steel sections. DSM is basically a stress-based approach, wherein the strength of the section is reduced to take care of buckling effects. The ultimate strengths corresponding to three buckling modes, local, distortional, and global modes are distinguished in DSM, in which the elastic buckling stresses are determined using analytical expressions or by using numerical methods like finite element, finite strip, or spline finite strip method. A finite strip computational procedure for evaluating the critical elastic buckling stresses has been proposed in the present study. Also, decomposition of buckling modes into pure modes can be performed using the formulation. This formulation has been validated for a range of cross-sectional dimensions under uniaxial compression and flexure and can be used as a design utility for IS 801 based on DSM.

An energy-based approach to buckling modal decomposition of thin-walled members with arbitrary cross sections, Part 1: Derivation

Abstract: This paper presents the generalisation of an energy-based method for the modal decomposition of buckled shapes of thin-walled members. This comprehensive method is derived and validated for fully decomposing the elastic buckling solution of a thin-walled member into the pure buckling mode classes of global, distortional, local, shear and transverse extension. The first three modes are de-facto prerequisites for buckling capacity predictions found in current design standards for thin-walled structures. In the literature, two main methods, namely the generalised beam theory (GBT) and the constrained finite strip method (cFSM), are widely employed for modal decomposition. Recently, an alternative energy-based approach has been presented for the decomposition of buckling modes into the classical local, distortional and global modes. This method is generalised in the present study to achieve a complete decomposition that also accounts for shear and transverse extensional modes in addition to global, distortional and local modes. In this method, each of the buckling classes is separated by imposing constraints that are defined by enforcing specific criteria on the total strain energy of the member. The adopted criteria are based on the fundamental mechanical assumptions of the GBT which were also implemented in the conventional cFSM and later were further detailed for modal classification in the generalised cFSM. This paper is accompanied by a paper in which derivation of a modified global torsion mode for sections with closed loops is presented and the applicability of the proposed method is demonstrated using a series of numerical examples.

A finite strip for the vibration analysis of rotating cylindrical shells

Abstract: In this article, a two-node finite strip with eight degrees of freedom for the free vibration analysis of pre-stressed rotating cylindrical shells is formulated. The circumferential mode shape profiles are described exactly using trigonometric functions. The axial mode shape profiles are approximated by bar and beam shape functions for membrane and bending displacements, respectively. In this way, a semi-analytical formulation is facilitated so that discretisation is required only in the axial direction. The accuracy and convergence of the developed finite strip are confirmed by comparisons with the analytical results. Excellent agreement is observed both for stationary and rotating shells.

Identification of buckling modes in generalized spline finite strip analysis of cold-formed steel members

Abstract: In this paper, a mode identification technique in the context of spline finite strip method (SFSM) is presented to compute the contribution of primary (global, distortional and local) and secondary (shear/transverse extension) buckling modes. The base vectors corresponding to individual buckling modes are developed based on the principles of generalized beam theory. The buckling mode shape in SFSM is approximated as a linear combination of these orthonormal base vectors to evaluate the participation of individual buckling mode. The proposed mode identification technique is able to successfully quantify the participation of different buckling modes and the mode participation is comparable with mode identification using finite strip method (FSM) and generalized beam theory (GBT). Illustrative examples are presented to calculate the participation of individual modes in cold-formed steel sections under different loading and boundary conditions. Also the specific application of mode identification in SFSM is demonstrated.