Arul Jayachandran S

The present paper deals with the nonlinear finite element formulations for the computational post buckling and large deformation analysis of reticulated shell systems using the corotated-updated Lagrangian (CR-UL) formulation. A concept of advanced/total analysis of skeletal metal structures is introduced. The purpose of the integrated formulation is to detect the progressive loss of stability and strength in the overall system. In reticulated systems, the rigid body displacements are predominant and hence their kinematics is handled through transformation matrices, which separate the strain producing (natural) deformations. The natural deformations are handled using the natural tangent stiffness. The inelastic and buckling behavior of the axial members in a reticulated system have considerable bearing on the snap-through limit points and ultimate strengths as well as control the cause of stability driven or ultimate strength driven collapse. To incorporate the inelastic behavior of the members, a mixed hardening model was adopted to account for the shift and changes in yielding parameters upon stress reversal after snap-through and possible future extension to cyclic loading. The flow rules and modified yield criterions are used in a state determination procedure inside a double loop iterative process that computes for both the geometric nonlinearity by CR-UL and the member inelasticity. The formulation is verified by the analysis a single layer dome for the roof of a petroleum tank. The analysis of some practical structures showed that area for improvement in optimizing designs without compromising safety. The examples explain how inherent safeguards against instability are present in reticulated shell structures in the real world conditions.

The present study deals with the shear lag phenomenon in cold formed angles under tension, which are connected on one leg. A new expression for shear lag factor which represents the net section reduction coefficient has been suggested in the present paper. The proposed expression based on the regression analysis of 108 experimental results reported in the literature is validated by experiments involving net section failure in angles under tension. Totally 18 experiments were carried out on single angles fastened with bolts to the gusset plates under tension loaded upto net section rupture mode of failure. The experimental test parameters considered are number of bolts, pitch and shear lag distances and ratio of connected leg length to unconnected leg length. The tensile capacities are evaluated by various specifications such as AS/NZS:4600:2005, NAS:2001, AISC:2005, BS:5950-Part5:1998, IS:800-2007 and the proposed equation. A comparative study of tensile capacities predicted based on various codes and the experiment results is presented in this paper. For the tested range of specimens, both NAS:2001 and AISC:2005 standards over-predicted the capacities for all the specimens. The IS:800-2007 and AS/NZS 4600:2005 predictions are good for the specimens with three bolted connections and unconservative in the case of specimen with two bolts. Both BS:5950-Part-5:1998 and the proposed equation for IS:801 predict good estimate of the tensile capacity of cold formed angle members. The proposed equation for cold formed steel tension members, which is in the same format of IS:800 (2007) (Indian code for Hot rolled steel design), has been demonstrated to be good. © 2011 Elsevier Ltd. All rights reserved.

Space structures are common in covering large area with few or no intermittent supports. The catastrophic failures of space structures are frequently reported and it urges to develop suitable techniques to prevent such failures. The composite space structures are found to be an efficient solution, in which the forces in the critical compression members are redistributed and safe guarded with concrete slabs. In this study, numerical studies were conducted using a standard finite element analysis package. Effect of shear connectors with varying heights on the behaviour of the space structure is studied. Results are compared with the steel space structure and composite space structure with full shear interaction. The maximum deflection observed in the steel space structure for the service load is less than span/300 and the maximum deflection observed in the steel concrete composite space structure for the service load is less than span/360, which ensures the serviceability conditions. The results show about 20 to 30 percent of reduction in deflection with about 250 percent increase in stiffness due to the composite action for the service load. The results substantiate that the length of shear connector should be equal to the concrete depth for more efficient composite interaction. The composite action is more effective and efficient in composite space structure for larger spans compared to shorter spans.

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 experimental study of the effect of plate slenderness ratio and column slenderness ratio on the collapse load of simply supported stiffened plates with initial imperfections, loaded in compression, is presented. A generalized computer program for the semiempirical solutions based on the strut approach and the orthotropic plate approach is developed. A finite element analysis program based on the orthotropic plate approach is developed and a new collapse criterion is introduced. The analytical calculations are compared with the experimental results and uncertainty parameters are calculated. The effect of initial geometric imperfections, plate slenderness ratios, and column slenderness ratios on the collapse load of stiffened plates is studied. A set of conclusions is drawn based on the experimental and analytical studies carried out.

The nonlinear behavior of space structures has been researched by various authors using the total and updated Lagrangian formulations. In this paper, a corotated-updated Lagrangian (CR-UL) formulation is used for studying the postbuckling behavior of steel reticulated systems. This formulation is used since space truss elements contain large components of rigid body displacements. The global tangent stiffness is derived from the corotation (CR) principle and the local (natural) tangent stiffness is derived using the updated Lagrangian (UL) finite element formulation. A state determination procedure is used for including the inelastic behavior with mixed hardening. Here members are assumed to reach the material strength limit state, which is reasonable in large span space structures based on serviceability governed design, with no consideration made for member buckling. The effect of bolt slip in the member joints is included by using a sudden slip model under the service loads. The global nodal displacements and coordinates are updated based on the analytical model of the bolt slip displacement using a simple affine transformation. The effect of bolt slip on the global displacements and member forces has been studied and compared with the results of previous authors.

The postbuckling behaviour of thin shells has fascinated researchers because the theoretical prediction and their experimental verification are often different. In reality, shell panels possess small imperfections and these can cause large reduction in static buckling strength. This is more relevant in thin laminated composite shells. To study the postbuckling behaviour of thin, imperfect laminated composite shells using finite elements, explicit incremental or secant matrices have been presented in this paper. These incremental matrices which are derived using Marguerre's shallow shell theory can be used in combination with any thin plate/shell finite element (Classical Laminated Plate Theory - CLPT) and can be easily extended to the First Order Shear deformation Theory (POST). The advantage of the present formulation is that it involves no numerical approximation in forming total potential energy of the shell during large deformations as opposed to earlier approximate formulations published in the literature. The initial imperfection in shells could be modeled by simply adjusting the ordinale of the shell forms, The present formulation is very easy to implement in any existing finite element codes. The secant matrices presented in this paper are shown to be very accurate in tracing the postbuckling behaviour of thin isotropic and laminated composite shells with general initial imperfections.

Uprights act as a column member in the rack system. In pallet racking systems, all upright sections are perforated to accept the hooks of the beam end connector and are the heart of the structural system. The perforations not only play an important role in the rack industry (for holding component in place), but are prone to more problems compared to any other form of structural member without perforation. The present study focuses on the behavior of perforated uprights under axial concentric loading. For better understanding, experiments were conducted on four specimens of steel rack uprights, by varying the thickness of sections as 1.6 and 1.8 mm with limitation on the slenderness ratio 10 and 18. Experimental investigations on the cold formed upright sections were studied for different type of configuration and their results are presented. Stress distribution across the specimen is also presented.

In this paper, studies on global behaviour of cold formed steel pallet rack systems using a second-order frame analysis formulation are reported. Calibration studies on the developed second-order beam formulation have been carried out. The different global analysis procedures stipulated in the popular code of practice for the design of cold formed steel storage racks (EN 15512 and RMI (2012)) are briefly reviewed. Using the formulation, studies are carried out on tall racks, especially focusing on semi-rigidity of connections and stability. Later, a shake table test is conducted on a two-level pallet rack to investigate the dynamic behaviour of cold formed steel racks subjected to base excitation. A typical earthquake input of El Centro (1940) North-South component is applied, and the behaviour of the cold formed steel racks is studied. The connection flexibility of the upright - connector tab has been taken from an earlier publication by the author. All joints in the numerical model and experimental work are treated as semi rigid. The paper then describes the effect of connection rigidity on the dynamic performance of the racks. Although the racks could not be subjected to its maximum design load per level, the dynamic characterization that is observed during the test is reported in this paper. It is identified that, a combined dynamic analysis calibrated with numerical solutions is the correct way to assess the seismic performance of cold formed storage racks.

There is a steady rise in the usage of cold formed sheets made as standing seam panels in the roofs of pre-engineered buildings. Nevertheless, there are also reports of failure of these roof panels while in service. The behaviour of these panels cannot be captured analytically. A literature study is carried out on the testing and design methods of the cold formed steel (CFS) metal roof system. It is seen from literature survey that none of the international codes of practice specifies the procedure for the calculation of structural capacity of a standing seam roof system unless supported by experimental evidence. In view of that, the structural performance of a standing seam CFS metal roof system under wind uplift is tested in a vacuum chamber as per ASTM E1592. The objective of the presented work is the study of the behaviour and failure modes of the rigid purlin-sheeting system with standing seam CFS panel under uplift. The paper presents the details of an experimental investigation to achieve this objective. This study and the experimental infrastructure is first of its kind in India. The paper also presents various limit states of sheeting - purlin interaction failure based on experimental evidences. It is concluded that in order to treat the sheet-purlin dual load resisting behaviour, the disengagement of the sheet with purlin through the halter must be avoided. One of the main conclusions of the work is the concept of adjustable halter length in regions of high pressure in the roofs.