Behavior of portal frames of steel hollow sections exposed to fire

This paper presents a numerical study concerning the behavior of hollow sections steel portal frames exposed to fire. A model is developed to employ both thermal and structural responses incorporating material and geometric non-linearities. To establish the failure mechanism of a frame under fire conditions, a failure criterion is proposed and validated against available experimental data. The failure temperatures predicted through the suggested failure criterion show good agreement compared to the experimental results. A parametric study is then conducted using the calibrated model to focus on failure mechanisms and associated failure temperatures. Variables considered are fire condition and rafter’s inclination angle. The assessment of frame performance is based on the generated failure mechanism and enhancement of failure temperature due to the chosen parameters. Results indicate that the studied variables strongly affect the failure mechanisms of portal frames. Contradictory, their effects on the failure temperature are minimal. Finally, the study presents vital outlines for the designer to find out and hence trace the failure mechanism prior to the completion of the final design stage. Only at this point, the optimum fire protection or adequate section capacity can be accomplished and may seriously be implemented in the field of industrial steel constructions

On the performance of circular concrete-filled high strength steel columns under axial loading

This work presents a numerical study to investigate the performance of circular high-strength steel tubes filled with concrete (CFT) under monotonic axial loading. A model is developed to implement the material constitutive relationships and non-linearity. Calibration against previous experimental data shows good agreement. A parametric study is then conducted using the model and compared with codes provisions. Strength and ductility of confined concrete are of primary concern. Variables considered are yield stress of steel tube and column diameter. The assessment of column performance is based on axial load carrying capacities and enhancements of both strength and ductility due to confinement. Two parameters namely strength enhancement factor (Kf) and ductility index (μ) are clearly defined and introduced for assessment. Results indicate that both concrete strength and ductility of CFT columns are enhanced but to different extents. The ductile behaviors are significantly evident. The increase in yield stress of steel tube has a minimal effect on concrete strength but pronounced effect on concrete ductility. However, reduction in ductility is associated with using high-tensile steel of Grade 70. The overall findings indicate that the use of high-strength tube in CFT columns is not promising. This finding may seriously be considered in seismic design

A new beam-column model for seismic analysis of RC frames – Part II: Model Verification

In this investigation, the performance of the simplified Flexibility-Based Fiber Model (FBFM), proposed in Part I of this study, is evaluated. The proposed model relies on calculating the inelastic lengths at the ends of the Reinforced Concrete (RC) beam-column member in every load increment and using preset flexibility distribution functions along the inelastic lengths to integrate the overall element response. The model eliminates the need for monitoring the responses of many segments distributed along the member length which results in a significant reduction in computations. The model performance is evaluated in this study on a one element level of a beam-column element and on a structure level of a 3-story frame. The selected structures are subjected to static pushover, static cyclic and earthquake loading conditions. The results of the proposed model are compared with the outcomes of the conventional FBFMs. The comparison is achieved using global performance parameters such as the maximum drift ratios and local performance parameters such as the maximum strains in steel and concrete at the plastic hinge regions. The analysis conducted indicates that the proposed model is capable of describing with satisfactory accuracy and computational efficiency the response of RC frame structures

Behavior of stub girder floor system with partial shear connection

A stub-girder floor system is a composite system constructed from a continuous steel beam and a reinforced concrete slab separated by a series of short, typically wide, flange sections called stubs. The finite element method has been used in the analysis of this composite system where it is capable to represent the constituent parts, adopt adequate elements and use appropriate solution techniques. As the behavior of stub-girders presents significant nonlinear effects, it is fundamental that the interaction of all different components should be properly modeled as well as the interface behavior. The present work focuses on the modeling of stub-girders with full and partial shear connection in two and three dimensions. The proposed model contains all the main structural parameters and their associated nonlinearities (concrete slab, steel beam, stubs, and shear connectors). In this model, the shear connectors are modeled as springs to consider the geometry of studs in addition to the nonlinearity due to the interaction between the shear connector and the concrete slab. Tests and numerical results available in the literature are used to validate the models. Based on the proposed finite element model, an extensive parametric study of stub-girders is performed, considering the material properties, relative dimensions and shear connector characteristics, where valuable recommendations and conclusions are achieved

Finite element analysis of beam-to-column joints in steel frames under cyclic loading

The aim of this paper is to present a simple and accurate three-dimensional (3D) finite element model (FE) capable of predicting the actual behavior of beam-to-column joints in steel frames subjected to lateral loads. The software package ANSYS is used to model the joint. The bolted extended-end-plate connection was chosen as an important type of beam–column joints. The extended-end-plate connection is chosen for its complexity in the analysis and behavior due to the number of connection components and their inheritable non-linear behavior. Two experimental tests in the literature were chosen to verify the finite element model. The results of both the experimental and the proposed finite element were compared. One of these tests was monotonically loaded, whereas the second was cyclically loaded. The finite element model is improved to enhance the defects of the finite element model used. These defects are; the long time need for the analysis and the inability of the contact element type to follow the behavior of moment–rotation curve under cyclic loading. As a contact element, the surface-to-surface element is used instead of node-to-node element to enhance the model. The FE results show good correlation with the experimental one. An attempt to improve a new technique for modeling bolts is conducted. The results show that this technique is supposed to avoid the defects above, give much less elements number and less solution time than the other modeling techniques

A new beam-column model for seismic analysis of RC frames – Part I: Model derivation

In this study, a reliable and computationally efficient beam-column model is proposed for seismic analysis of Reinforced Concrete (RC) frames. The model is a simplified version of the Flexibility-Based Fiber Models (FBFMs), which rely on dividing the element length into small segments and dividing the cross section of each segment into concrete and steel fibers. In the proposed model, only the two end sections are subdivided into fibers and uniaxial material models that consider the various behavioral characteristics of steel and concrete under cyclic loading conditions are assigned for the cross section fibers. The proposed model is simpler than the FBFMs as it does not require monitoring the responses of many segments along the element length, which results in a significant reduction in computations. The inelastic lengths at the ends of the proposed model are divided into two inelastic zones; cracking and yielding. The inelastic lengths vary according to the loading history and are calculated in every load increment. The overall response of the RC member is estimated using preset flexibility distribution functions along the element length. A flexibility factor η is utilized to facilitate selecting the proper flexibility distribution shape. The proposed model is implemented into the computer program DRAIN-2DX

Numerical simulation of buffeting longitudinal wind forces on buildings

A developed numerical simulation technique for buffeting longitudinal wind forces on rectangular buildings is presented in this paper. This technique mainly aims to generate time histories with prescribed probabilistic characteristics for the longitudinal wind load turbulent component which is considered as a zero mean stationary multivariate one-dimensional stochastic process, then the instantaneous wind load time histories are obtained by adding each mean wind load component to the corresponding generated time history of the turbulent component. A simplified procedure, which takes the advantage of aerodynamic admittance function, is proposed to estimate the longitudinal wind load on buildings in the frequency domain. A very efficient simulation algorithm based on spectral representation method which depends on superposition of trigonometric functions with random phase angles is used to perform the transformation from the frequency domain to the time domain. A MATLAB function is coded to implement the proposed simulation technique. By testing the proposed technique statistically, it has been noted that there are good agreements between the temporal and target auto/cross-correlation functions of the simulated wind forces, and by testing it structurally, the generated instantaneous wind load time histories show a good ability in giving a reasonable dynamic response for the studied building. Keywords: Wind, Simulation, Spectral representation, Correlation, Buffetin

Behavior of four-bolt extended end-plate connection subjected to lateral loading

A considerable number of literatures have been published on the behavior of end-plate connections in ordinary moment-resisting frames. It was found, experimentally, that this type of connection might act as either a fully-rigid or a semi-rigid connection depending mainly on the thickness of the end plate and the diameter of bolts. In recent years, due to their good ductility and their good ability of energy dissipation, extended end-plate connections are recommended to be widely used in special moment-resisting frames subjected to lateral loads. The purpose of this study is to investigate the effect of both the material and geometric properties of four-bolt extended end-plate connections upon their behavior when subjected to lateral loading. This is done through a parametric study upon a finite element model using the multi-purpose software package ANSYS. The parametric study takes into account 12 parameters which are expected to be effective on the behavior of the studied connection. The results are presented by the relation between the storey drift, which represents the rotation of the connection and the applied lateral load, which simulates the moment on the connection. The results verify that the chosen parameters are considered effective depending on the energy dissipation of the connection