Robust design of steel and concrete composite structures

Accidental events, such as impact loading, are rare events with a very low probability of occurrence but their effects often leads to very high human losses and economical consequences. An adequate design should not only reduce the risk for the life of the occupancy, but should also minimize the disastrous results and enable a quick rebuilding and reuse. A robust design prevents the complete collapse of the structure when only a limited part is damaged or destroyed. Design against disproportionate collapse is usually based on the residual strength or the alternate load path methods. Identification of an alternate path may lead to an effective and cost efficient design for progressive collapse mitigation by redistributing the loads within the structure. The continuity of the frame and of the floor represent essential factors contributing to a robust structural response. They in fact enable development of 3D membrane action. A European project focusing on robustness of steel and steel and concrete composite structures subjected to accidental loads is still ongoing. In the framework of the project the authors concentrated their studies on the redundancy of the structure through slab-beam floor systems as well as through ductile joint design. At this aim, two 3D full scale substructures were extracted from a reference building and experimentally investigated with the purpose to get an insight into the mechanisms allowing the activation of the alternate load paths resources, when a column collapse. The paper illustrates the main features of both the specimens tested and the experimental campaign. The preliminary results of the tests are presented and discussed

Numerical models for the analysis of shear walls in light steel residential buildings

AbstractIn recent years, the use of steel light housing structural solutions made of cold‐formed thin‐walled profiles (CFS) is becoming increasingly popular. Lightness, high structural efficiency, durability, rapidity and simplicity of erection of the building and its finishes are some of the main advantages of these systems, which make them attractive and competitive with respect to more traditional constructional solutions. In these buildings, which do have a skeleton made of cold‐formed steel profiles completed by sheathings made of various materials, the key role of transmission of both vertical and horizontal loads from the floors to the foundation, is played by the shear walls. Recently, the University of Trento carried out a project aimed to develop an industrialized housing system made of CFS members. In this framework, experimental and numerical studies of the in‐plane lateral response of shear walls were performed. In particular, this paper summarizes first the experimental program and then it focuses on the main features of numerical models and on their validation in both monotonic and cyclic regime. The critical parameters governing the response of these complex systems are finally identified and discussed

Geometric Nonlinear Isoparametric Spline Finite Strip Analysis of Perforated Thin-Walled Structures (No. R880)

In previous reports [1, 2], the isoparametric spline finite strip method was successfully applied to the in-plane stress and bending linear elastic analysis of perforated plates. The method has been successfully applied to the elastic linear analysis and buckling analysis of folded plate structures [3, 4]. In the present report the application of the isoparametric spline finite strip method is further extended to the geometric nonlinear analysis of perforated folded plate structures. The general theory of the isoparametric spline finite strip method is briefly introduced. Kinematics, strain-displacements and constitutive assumptions are described and applied to the spline finite strip method. The derivation of the tangential and secant stiffness matrices is presented by applying the equilibrium condition and its incremental form. A large part of report is reserved to a review of the available nonlinear solution techniques, notably the cylindrical arc-length method. Classical nonlinear complex plate and shell problems are analysed and compared with exact solutions or with well established numerical results in order to demonstrate the reliability of the method. Furthermore, examples of the geometric nonlinear analysis of perforated flat and stiffened plates are included to highlight the effect of perforations on the behaviour of thin plate elements

Shear Locking in Isoparametric Spline Finite Strips (No. R876)

The Mindlin plate bending theory includes shear deformations in the formulation to better reproduce the behaviour of thick plates and to avoid difficulties in satisfying continuity requirements along strip boundaries. Unfortunately, in many thin plate applications over-stiff solutions are encountered due to the development of spurious stiffness contributions in the numerical solution. This phenomenon is usually referred to as “locking” and if associated with an insufficient representation of bending and shear deformations is called “shear locking”. A method for testing the performance of finite elements with regard to shear locking is described. The method is called “the monomial test” and has been introduced by Briassoulis (1988) who applied it to a series of beam and plate elements. The monomial test is applied for the first time to the isoparametric spline finite strip method. The effect of shear locking on the performance of the isoparametric spline finite strip is investigated in detail as is the ameliorating influence of selective reduced integration. The influence of the order and magnitude of the distortion of the strip, the integration scheme and the number of longitudinal sections of the isoparametric spline finite strip are analysed. Numerical examples are included to illustrate the concepts described

Linear Elastic Isoparametric Spline Finite Strip Analysis of Perforated Thin-Walled Structures (No. R878)

In previous reports [1, 2], the isoparametric spline finite strip method was successfully applied to the in-plane stress and bending linear elastic analyses of perforated plates. In this report the application of the isoparametric spline finite strip method is further extended to the linear elastic analysis of tri-dimensional perforated folded plate structures. The general theory of the isoparametric spline finite strip method is briefly introduced. Kinematics assumptions and the procedure to combine in-plane (membrane) and bending characteristics are set out. Particular attention is paid to the procedure for rotating the stiffness matrix and load vector from local to global coordinates. The reliability of the method is demonstrated by comparisons with finely meshed finite element analyses. Square stiffened plates in compression and bending containing openings of different shapes are analysed

Isoparametric Spline Finite Strip Method for the Bending of Perforated Plates (No. R877)

In a previous report [1], the isoparametric spline finite strip method was successfully applied to the in-plane stress linear elastic analysis of perforated thin-walled structures. In the present report the application of the isoparametric spline finite strip method is further extended to the bending of perforated plates. The general theory of the isoparametric spline finite strip method is briefly introduced while the development of the Mindlin bending theory and its matrices are discussed in detail. The reliability of the method is demonstrated by comparisons with finely meshed finite element analyses. Square plates in bending containing openings of different shapes are analysed

Elastic Buckling Analysis of Perforated Thin-Walled Structures by the Isoparametric Spline Finite Strip Method (No. R879)

In previous reports [1, 2], the isoparametric spline finite strip method was successfully applied to the in-plane stress and bending linear elastic analysis of perforated plates, and to the linear elastic analysis of folded plate structures [3]. In the present report the application of the isoparametric spline finite strip method is further extended to the elastic buckling analysis of perforated folded plate structures. The general theory of the isoparametric spline finite strip method is briefly introduced. The kinematics assumptions, strain-displacement and constitutive relations of the Mindlin plate theory are described and applied to the spline finite strip method. The corresponding matrix formulation is utilised in the equilibrium and stability equations to derive the stiffness and stability matrices. A number of numerical examples of flat and folded perforated plate structures illustrate the applicability and accuracy of the proposed method

Experimental assessment of an asymmetric steel–concrete frame under a column loss scenario

Several noteworthy accidents clearly pointed out the risk of disproportioned collapse of framed structures. Design codes recently recognized it by adding a new requirement: the structural robustness. Among the different approaches to check robustness, the most popular is associated with the column loss scenario: the analysis should verify that, in case of a column loss, an alternative load path does exist, limiting the portion of structure affected by collapse. Consequently, numerous experimental and numerical studies of 2D and 3D structures were carried out in recent years to identify the mechanism of load transfer from the damaged to the undamaged part of the structure. This knowledge becomes an essential and fundamental key for assuring adequate resistance against progressive collapse by the development of catenary action in the beams and membrane action in the floor slab. Studies of reinforced concrete systems and of bare steel sub-assemblies are numerous. More recent is the focus on the response of steel–concrete composite structures subjected to accidental events. Furthermore, most of these studies focused on the characterization of 2D sub-assemblies or 3D in-scale framed structures. This paper presents an experimental assessment of the structural response of a 3D full-scale steel and concrete composite frame under the column loss scenario. The results are finally compared with the response of a frame with the same overall geometry but different columns’ layout, tested by the Authors within the same research programme

Isoparametric Spline Finite Strip Method for In-plane Stress Analysis (No. R848)

The finite strip method has proved to be an accurate and efficient tool for the analysis of structures having regular cross-section and mechanical proprieties along the longitudinal axis. The spline finite strip method has furthermore proved to be a more flexible tool for the analysis of structures with general support conditions and, utilising the isoparametric mapping, structures with a geometry varying along the longitudinal direction, such as curved slab bridges. In this report, the isoparametric spline finite strip method is applied to the analysis of plate containing cut-outs of different shape and subjected to in-plane stresses. The mapping technique and the theory for the general in-plane stress condition are outlined, as is a novel method for assembling the strips in order to model the particular case of a cut-out. To prove the reliability of the isoparametric spline finite strip method, three different shapes perforation in rectangular plates in traction are analysed. The shapes of the cut-outs presented are a circular, a rectangular and a key shaped hole. The result are compared with exact solutions and finite element analyses