S. Remseth

Nonlinear static and dynamic analysis of framed structures

Abstract The present study is concerned with methods of nonlinear static and dynamic analysis of structures, particularly for application to geometrically nonlinear space frames. The displacement formulation of the finite element method is adopted. Large displacements are accounted for by using a material (Lagrangian) description of motion from a fixed reference frame, assuming small strains and moderately large rotations. Curved beam elements are considered by introduction of initial deflections in the element stiffness relations. Higher order axial interpolation polynomials are included in order to obtain an appropriate coupling between axial forces and bending for the frame members. Efficient methods for instability and postbuckling problems have been a main point for the study of nonlinear static behaviour. This also includes evaluation of nonlinear numerical solution methods, some of which being applicable also in dynamic analysis. The efficiency of these methods are discussed in connection with the nonlinear dynamic response analyses presented. For the integration of the incremental equations of motion, a method based on transformation to generalized coordinates on modal basis is proposed and studied as an alternative to direct integration. Numerical examples are presented for both static and dynamic analyses, and it has been considered important to show the applicability of the nonlinear analysis methods to practical structural problems. The advantages of post-processing and extensive use of computer graphics in presenting results for nonlinear space structure analyses are demonstrated.

DYNAMIC RESPONSE AND FLUID/STRUCTURE INTERACTION OF SUBMERGED FLOATING TUNNELS

Abstract Alternative approaches to stochastic dynamic response analysis of submerged floating tunnels subjected to wave loading are presented. For the purpose of establishing force, damping and mass coefficients for structural elements with three-dimensional flow conditions, fluid/structure interaction is modeled as finite element implementation of the Navier–Stokes equation. The numerical examples emphasize the effects of wave direction, shortcrestedness, damping, geometrical stiffness and frequency dependence in mass and damping coefficients.

Reliability-based analysis of a stretch-bending process for aluminium extrusions

Abstract The paper presents reliability analyses of a plastic forming process. Stretch-bending of extruded aluminium square hollow sections was studied through finite element analysis. The profiles were axially fixed and given a specified curvature by means of a bending die. Important response parameters in stretch-bending of extruded profiles are flange sagging and elastic springback. The sensitivity of the predicted response parameters was investigated with respect to variations in the material properties of the aluminium alloy, i.e. yield stress, strain hardening and plastic anisotropy. The material parameters were modelled as random variables with given statistical properties. The probability of the response parameters staying within specified limits could then be calculated by means of response surface methods and FORM/SORM. Finally, the consequences of the specified response limits on allowable variations of the material properties were established. The paper shows how reliability-based methods can be used to establish relations between the variation of model parameters (e.g. material properties, cross-sectional geometry and process parameters) and the resulting variation of characteristic response parameters for a given forming process.

An investigation of material properties and geometrical dimensions of aluminium extrusions

Abstract The increasing demand for aluminium extrusions calls for relevant statistical data to establish objective measures of uncertainties associated with the material description and cross-sectional geometry. A total of 392 tensile test specimens are machined from 49 rectangular hollow section extrusions in order to investigate how the material properties are distributed. The Kolmogorov–Smirnov hypothesis test indicates that the normal as well as lognormal distribution represents the observed data fairly well. Several model parameters are involved in the study, including two alloys, three angles with respect to the extrusion direction, two thicknesses and two tensile test coupon sizes. Regarding the geometrical dimensions, the thickness, width and height are measured, finding that the dimensions usually are somewhat smaller than the nominal values.

Titanium pipes subjected to bending moment and external pressure

Titanium is being considered as an alternative to steel for risers and flowlines, particularly for deep-water applications. However, steel capacity formulas for local buckling and collapse are inadequate for direct application to titanium pipes subjected to bending and high external pressure. If formulas developed for steel are applied directly, the model uncertainty is unknown. Ideally, extensive model testing of titanium pipes is required. This paper discusses the use of a supplementary numerical approach based on finite element analysis. The relationship between material model parameters being input to the analysis and the collapse/buckling capacity is investigated by performing a series of non-linear finite element analyses. The results from the non-linear response analyses are compared with experimental results for pipes with different D/t-ratios. Finally, the results are as a basis for proposing a formula for design check of titanium pipes subjected to high external pressure and bending moments.

Simulation and Monitoring of Floating Bridge Behaviour

Firstly, this paper presents a review of the main steps for simulating floating bridge behaviour. Both time-domain and frequency-domain approaches are presented. An exemplified model setup and simulation results from the selected case study are presented. Secondly, data obtained from extensive structural and environmental monitoring of the studied bridge are presented. Data analyses attempting to visualize the correlations between excitation sources and response quantities are discussed. Finally, an operational modal analysis is carried out, to attempt to identify the modal parameters from response measurements only.

Tube collapse analysis using finite elements

Abstract Oil pipelines in deep waters are subjected to extreme conditions of bending and external pressure during the laying process. The paper discusses a method for analysis of this problem using finite elements. The computational model is capable of accounting for very large displacements and gross yielding. Thus, nonlinear behaviour, instabilities and complete collapse mechanisms of the steel tube can be simulated. Important aspects of the solution algorithm for the nonlinear equations and the computer code design are discussed. Some examples in which the tube develops deep transverse and longitudinal buckles are presented.