T-stub Component Research Articles

Assessment of the T-stub joint component at ambient and elevated temperatures

Abstract The collapse of the World Trade Centre revealed problems in the design of some structural elements. It was recognized that details relating to the structural joints play a very significant role in the behaviour of structures under accidentals loads such as, impact followed by fire [1] . A very high performance of the connections is demanded to prevent fracture and a way to achieve this is to enhance the ductility and rotation capacity of the connection. In accordance with Eurocode 3, EN1993-1-8 [2] , the T-stub model is used to evaluate the behaviour of the joint’s tensile components, which are responsible for assuring the ductility of the joint due to their high deformation capacity. This paper presents a study with the aim of evaluating the behaviour of a welded T-stub component subject to static loading, at ambient and elevated temperatures, and carried out at the University of Coimbra [3] . The key variables include the thickness of the flange, the type and diameter of the bolts and the temperature of the T-stub. The experimental results are used to validate a finite element model that provides insight into the behaviour of the T-stub, in terms of resistance, stiffness, deformation capacity and failure modes at different temperatures. Good agreement between the numerical results and experimental tests was reached. The results show that the T-stub fire resistance and stiffness’s (initial and post-limit) reduce with the increase of temperature and the T-stub’s ductility and failure modes are also dependent on the temperature. Using the numerical results ductility indices are defined and calculated; the evolution of these ductility indices, as the temperature rises, is more pronounced for less ductile plastic failure modes. Finally, the numerical results are compared with those reached by simplified analytical models available in the literature.

Influence of the flexural rigidity of the bolt on the behavior of the T-stub steel connection

T-stubs are generally used to represent the tension zone of bolted moment resisting connections. The bolts are considered at present as components loaded only in tension. This paper investigates the influence of the bolt bending on the behavior of the T-stubs. A 3D finite element model was first developed and validated by comparison with experimental results found in the literature. The numerical model was then used to analyse the behavior of various T-stubs regarding mainly the values of the axial tensile forces and the bending moments in the bolts. The results show the influence of the bolt bending moment on the failure mode of some T-stubs. The numerical model was also used to evaluate the accuracy of the analytical interaction formula determining the resistance of isolated bolts subjected to bending moment and axial tensile force. Finally, an analytical model is proposed to evaluate the bending moments and the axial tensile forces in the bolts considered as T-stub components. The comparisons with the numerical model results show that the analytical model represents well the behavior of T-stubs considering the bending moments in the bolts.

Assessment of the T-stub joint component at ambient and elevated temperatures

The collapse of the World Trade Centre revealed problems in the design of some structural elements. It was recognized that details relating to the structural joints play a very significant role in the behaviour of structures under accidentals loads such as, impact followed by fire [1]. A very high performance of the connections is demanded to prevent fracture and a way to achieve this is to enhance the ductility and rotation capacity of the connection. In accordance with Eurocode 3, EN1993-1-8 [2], the T-stub model is used to evaluate the behaviour of the joint’s tensile components, which are responsible for assuring the ductility of the joint due to their high deformation capacity.This paper presents a study with the aim of evaluating the behaviour of a welded T-stub component subject to static loading, at ambient and elevated temperatures, and carried out at the University of Coimbra [3]. The key variables include the thickness of the flange, the type and diameter of the bolts and the temperature of the T-stub. The experimental results are used to validate a finite element model that provides insight into the behaviour of the T-stub, in terms of resistance, stiffness, deformation capacity and failure modes at different temperatures. Good agreement between the numerical results and experimental tests was reached. The results show that the T-stub fire resistance and stiffness’s (initial and post-limit) reduce with the increase of temperature and the T-stub’s ductility and failure modes are also dependent on the temperature. Using the numerical results ductility indices are defined and calculated; the evolution of these ductility indices, as the temperature rises, is more pronounced for less ductile plastic failure modes. Finally, the numerical results are compared with those reached by simplified analytical models available in the literature.

A numerical-informational approach for characterising the ductile behaviour of the T-stub component. Part 2: Parsimonious soft-computing-based metamodel

The accuracy of the component-based method relies heavily on the characteristic response of their constitutive elements. To properly assess the deformation capacity of the whole connection, modelling the complete force–displacement curves of the components, from the initial stiffness to fracture, is necessary. This paper presents a numerical-informational method for calculating the ductile response of the T-stub component. In order to reduce the intensive computation of the finite element (FE) method, the results of numerical simulations are used to train a set of metamodels based on soft-computing (SC) techniques. These metamodels are capable of predicting, with a high degree of accuracy, the key parameters that define the force–displacement curve of the T-stub. In addition, a feature selection (FS) scheme based on genetic algorithms (GAs) is included in the training process to select the most influential input variables. This scheme leads to overall and parsimonious metamodels that improve the method’s generalisation capacity.The mean absolute error (MAE) in the prediction of each key parameter reports values below 5% for both validation and test results. This demonstrates the strong performance of the SC-based metamodels when comparing them with the FE simulations. Finally, this hybrid method constitutes a suitable tool to be implemented in non-linear steel connections software.

A numerical-informational approach for characterising the ductile behaviour of the T-stub component. Part 1: Refined finite element model and test validation

In the component-based method, the ductility characterisation of the tension component plays a primary role in accurately predicting the rotation capacity of the whole connection. In this paper, a refined finite element (FE) model is presented to assess the complete force–displacement response of the T-stub component, from the initial stiffness up to the fracture point. The refined FE model includes a non-linear continuum damage mechanics model that considers the onset of the damage, the damage evolution law, and the component failure. A parametric study was conducted to evaluate the influence of the damage parameters in the overall response of the T-stub component. The numerical results obtained from the refined FE model strongly agree with the experimental results of 18 test specimens. The comparison reveals the proposed FE model’s satisfactory accuracy, with an average fitting error below 9%. This paper constitutes the first part of a comprehensive methodology based on combining FE analysis and soft computing techniques. The ultimate goal of this methodology is to predict the key parameters that define the force–displacement response of the T-stub component.

Case Study for Bolted T-Stub Connection Design

This paper is mainly performed to investigate T-stub connection that is described on the basis of ideal strength limit states. The determination of T-stub based on the full plastic strength of the steel beam in accordance with 2005 AISC Seismic Provisions. The T-stub connections considered herein were performed to include the T-stub component of bolted moment connection frames. Therefore, the proposed T-stub models will be evaluated by comparing the required factored bar strength. T-stub components using ten high strength bolts with wider gages are demonstrated in this design. In addition, equations for connection design will be described in this paper. Finally, new design methodology is applied to T-stub connections suggested in this study.

Mechanical models for the analysis of bolted T-stub connections under cyclic loads

The mechanical models used to simulate the complete behavior of full-scale bolted T-stub connections under cyclic loads are mainly treated in this paper. These mechanical models are composed of individual T-stub components modeled as nonlinear spring elements in order to reliably reproduce their various response mechanisms interacting with one another in the connection. The hysteresis behaviors of the T-stub components including bolt/flange uplift, stem elongation, and relative slip deformation combined with bolt bearing are simulated by the multi-linear cyclic stiffness models characterized from their actual force–deformation response mechanisms each. The nonlinear component springs, which contain these idealized stiffness properties, are implemented into the user joint element produced based on the mechanical model so as to numerically generate the complete behavior of the full-scale connections with considerable accuracy. The analytical predictions performed on the joint element are evaluated against the experimental tests with respect to stiffness, strength, and deformation. Thus, the adequacy of the proposed modeling approach is verified through comparisons between analytical predictions and experimental test results. Finally, it can be shown that the mechanical model proposed in this study has the satisfactory potential to predict the response of the T-stub components as well as the behavior of the T-stub connections through analytical studies.

Mechanical modeling of bolted T-stub connections under cyclic loads Part I: Stiffness Modeling

This paper deals with mechanical models which make it possible to reliably simulate the complete moment–rotation curves in the full-scale T-stub connections subjected to cyclic loads. The behavior of these bolted connections becomes complex because the various response mechanisms of individual connection components interact with one another and have influence on the overall rotational stiffness of the connection. Accordingly, the mechanical joint models are made up of individual T-stub components modeled as nonlinear springs. The behaviors of component members including tension bolt uplift, bending of the T-stub flange, elongation of the T-stem, relative slip deformation, and bearing deformation are reproduced by the multi-nonlinear stiffness models obtained from their force-deformation response mechanisms. These stiffness properties should be assigned into the component springs implemented into the joint element so as to numerically generate the behavior of full-scale connections with considerable accuracy. Thus, this part (Part I) intends to focus on describing the stiffness models, which are based on the basic component spring theory, in an effort to provide insight into the behavior, failure modes, and ductility of T-stub components in the connection.

Stiffness Modeling of Bolted T-Stub Connection Components

The results of 48 T-stub component tests and eight full-scale T-stub connection tests are used to develop and calibrate a comprehensive T-stub connection stiffness model. The model, based on basic component spring theory, incorporates deformations from tension bolt elongation, bending of the T-stub flange, elongation of the T-stem, slip of the T-stub relative to the beam flange, bearing deformation of the T-stem, and bearing deformation of the beam flange. Nonlinear material models, for both the tension bolts and the T-stub base material, are included. A sophisticated slip/bearing routine accounts for the misalignment of shear bolt holes and variable slip conditions. Comparisons of multilinear model predictions with experimental data show that the model accurately predicts connection stiffness response and deformation at failure.

Bolted Steel Connections: Tests on T-Stub Components

The results of tests on 48 T-stub specimens are presented and discussed. These tests were carried out as part of a SAC Phase Ii project in order to provide insight into the behavior, failure modes, and ductility of these components. The main variables tested include the size of the T-stub, the gauges of the bolts, and the type and diameter of the bolts. The primary intent was to develop a large database to calibrate simplified models suitable for design. In order to develop these models, a comprehensive instrumentation system that could identify the different components of deformation in the individual T-stubs was utilized. Most of the T-stubs failed by net section fracture through the stem and by tension fracture of the bolts, but generally the failure was after significant plastification had occurred. Other failure modes observed included bolt shear and block shear. The results indicate that current design equations provide conservative estimates of the ultimate strength of the T-stubs but that they are not necessarily good predictors of the governing failure modes.