Amin Heidarpour

Numerical Modeling of Concrete-Filled Double-Skin Steel Square Tubular Columns under Blast Loading

AbstractConcrete-filled double-skin tubes (CFDST) have been widely used in constructing high-rise buildings, arch bridges, and factories over the past years. Although a number of researches have been conducted to study the behavior of CFDST columns under a variety of loading conditions, their performance when subjected to lateral impact load is still lacking. In this paper, numerical models of CFDST columns with two different cross sections are developed: one is with a CHS (circular hollow section) outer and CHS (circular hollow section) inner, and the other one is with SHS (square hollow section) outer and SHS (square hollow section) inner. Conventional concrete is filled in double-skin steel tubes. Different blast loads are applied to the surface of these columns for dynamic analysis. In addition, different axial loads are also applied to simulate combined load conditions. The displacement-time history obtained from each simulation is recorded and then compared. The key parameters that affect the perfor...

Performance of Unstiffened Welded Steel I-Beam to Hollow Tubular Column Connections Under Seismic Loading

Earthquake causes wide and severe damage to building structures, due to not just the great ground motion but also secondary actions, such as impact, blast or fire, occurring after earthquake. The extreme combined loading scenario should be considered for safety of buildings and lives. Taking fire for example, the combined load can be considered as an event in which the structures are first partially damaged under an earthquake and then attacked by fire. In order to investigate the post-earthquake loading scenario, it is important to assess the partial damage caused by earthquake on different components of structures. The behavior of welded steel I-beam to hollow square tubular columns is investigated herein. A detailed experimental study is presented in which two groups of unstiffened welded steel connections, with the same configurations, subjected to static and cyclic loading are considered. The flexibility and strength of the connections are measured, while the damage phenomena and failure modes are explored during the tests. The connection damage is found to be a cumulative fracture developing process which leads to significant gradual degradation of the mechanical properties of the connection. The quantificational evaluations of the cyclic loading induced damage are also carried out to investigate the connection damage level according to different loading intensities. A finite element modeling numerical study is also carried out to validate the experimental results and a good agreement is achieved. The test results and FE modeling provide a benchmark data for the unstiffened welded connections and can be used for further investigations of the connections subjected to combined actions such as post-earthquake fire.

Performance of Double-Angle Bolted Steel I-Beam to Hollow Square Column Connections Under Static and Cyclic Loadings

Earthquake causes severe damage to buildings and infrastructure, due to not only the ground motion but also secondary actions, such as impact, blast and fire which would occur after an earthquake. In order to investigate the post-earthquake loading scenarios, it is important to assess the partial damage of structures caused by earthquake. This paper presents the behavior of double-angle bolted steel I-beam to hollow square tubular column connections under static and cyclic loading. A detailed experimental study is presented in which two groups of bolted steel connections with different column wall thickness are considered. The flexibility and strength of the connections are measured, while the damage phenomena and failure modes are explored in the tests. The connection damage under cyclic loading is found to be an accumulative developing process of fracture which leads to significant gradual degradation of the mechanical properties of the connections. Quantitative evaluations of the cyclic loading induced damage are carried out to investigate the damage level of connections according to different loading scenarios. The test results herein provide a detailed understanding of the behavior of the double-angle bolted connections under seismic loading, which would be useful for further investigations under post-earthquake actions.

A Fully Discretized Nonlinear Finite Strip Formulation for Pre-Buckling and Buckling Analyses of Viscoelastic Plates Subjected to Time-Dependent Loading.

A semi-analytical fully discretized finite strip method is developed to investigate the pre-buckling and local buckling of viscoelastic plates with different boundary conditions subjected to time-dependent loading. The mechanical properties of the material are considered to be linear viscoelastic by expressing the relaxation modulus in terms of Prony series. The fully discretized finite strip equations are developed using a two-point recurrence formulation, which leads to a computationally superior formulation. Time history of maximum deflection of plates with different end conditions is calculated. The effects of thickness, length of plate, and transverse loading on critical buckling load are also studied.

Local Buckling of Rectangular Viscoelastic Composite Plates Using Finite Strip Method

This article presents the stability analysis of reinforced viscoelastic composite plates subjected to in-plane loading using a finite strip method. The equations governing the stiffness and geometry matrices of moderately thick rectangular plates in the time domain are presented using higher-order shear deformation theory and effective moduli method. The critical buckling loads of viscoelastic plates are determined through solving the eigenvalue problems related to the global stiffness and geometry matrices. The accuracy of the developed model is verified against the results reported elsewhere while a comprehensive parametric study is presented to show the effects of different variables on local buckling coefficients.

Nonlinear Analysis of Composite Beams with Partial Interaction in Steel Frame Structures at Elevated Temperature

Flooring systems containing steel-concrete composite beams are common in steel frame structures and it is widely recognized that their behavior under fire loading is profoundly different to that of simply supported composite beams under fire loading. This difference in behavior is due to the presence of restraints provided by cooler members in a compartment fire in a composite frame structure and there is extensive evidence from fire tests that composite beams with unprotected steel components in steel frames perform much better than simply supported composite beams with unprotected steel components. In order to model the structural response of a composite beam restrained in this way at elevated temperature, recourse is needed to a geometric nonlinear formulation, since the transverse beam deflections are large and interact with the substantial axial compressive force in the member at the early stages of the fire. This paper presents such a formulation, which incorporates partial interaction between the concrete slab and steel component, as well as the degradation of the stiffnesses of the components of the composite beam prior to yield at elevated temperature. The generic technique that is developed is shown to agree with solutions reported elsewhere and provides a structural model for the response of a composite beam in fire with the potential for inclusion in prescriptive code rules for rational fire engineering based designs.

Influence of elevated strain rate on the mechanical properties of hollow structural sections

As protective design engineering becomes more prevalent, cold-formed steel hollow structural sections are often desired design components. As such, it is necessary to understand the behavior of hollow structural sections subject to air-blast loading, including the material response under elevated strain rates. Dynamic tensile tests have hence been performed on subsize tensile coupons taken from the flats and corners of cold-formed rectangular hollow section members. Dynamic yield stresses were obtained at strain rates from 0.1 to 18 s−1, which encompasses and exceeds the range recorded during far-field blast arena testing. The dynamic increase factor was calculated for each data point and synthesized with previous cold-formed rectangular hollow section tests at even higher strain rates (100–1000 s−1). The data set was used to determine Cowper–Symonds and Johnson–Cook parameters. The resulting material models can now be used to determine the strength increase of cold-formed rectangular hollow sections subject to a wide range of impulsive, elevated strain rate loads.

Effect of Post-Fire Curing on the Residual Mechanical Properties of Fire-Damaged Self-Compacting Concrete

Concrete is recognized for being a fire-resistant construction material. At elevated temperatures concrete can, however, undergo considerable damage such as strength degradation, cracking, and explosive spalling. In recent decades, reuse of fire-damaged concrete structures by means of developing techniques to repair the degraded material has gained interest amongst researchers. Autogenic self-healing methods such as re-curing in water has proven to partly restore the strength of concrete. The extent of restoration is dependent upon various parameters such as concrete type, exposure temperature, and post-fire curing conditions for example. The use of selfcompacting/ consolidating concrete (SCC) has become common in the construction industry due to its high workability and low permeability. This paper presents the results of an experimental study aimed at investigating the improved mechanical properties of high temperature exposed SCC concrete by the autogenic self-healing phenomenon resulting from water re-curing. The residual mechanical properties including strength, modulus of elasticity and ultimate strain of the material upon application of different post-fire curing regimes are presented herein with special emphasis on the effect of thermal profile including exposure time, temperature and cooling rate. The experimental results confirm that the recovery of material properties in fire-damaged SCC concrete is contingent on the post-fire water curing conditions.

Mechanical Properties of Structural Steel under Post-Impact Fire

Accurate prediction of material properties under combined high strain rate and elevated temperature are essential for safe design of structures to withstand post-impact fire situations such as collision by heavy vehicles followed by fire. Numerous material tests performed in recent years do not address the influence of such sequential loading on the mechanical properties of mild steel. An inclusive test program is carried out in the Civil Engineering Lab at Monash University to investigate the post-impact fire properties of Grade 350 structural steel and the results are presented here. Specimens have undergone interrupting high strain rate tensile loading, controlled locally at defined levels of elongation, to account for different deformation states. Different damage levels are introduced for each rate of strain with respect to the displacement corresponding to the ultimate stress (f sub u). Subsequently, the partly damaged specimens are subjected to static tensile loading to failure at high temperature conditions. Material behaviour of pre-damaged steel is compared to those of each individual loading scenario and to design code expressions. The test results demonstrate that the combined actions are profoundly different from that in which the structure is subjected to either high strain rate or thermal loading and notably vary from those predicted in different codes. Moreover, it is shown that the strength and ductility of mild steel are significantly dependent on the rate of loading, the pre-deformation history and the temperature it is subsequently exposed to. The experimental results can be used by researchers and structural engineers as benchmark data for calibrating current material model constants and/or developing new material models which take into account the coupled effect of high strain rate and temperature for rational fire analysis and design of steel structures.

GEOMETRIC NON-LINEAR MODELLING OF PARTIAL INTERACTION IN COMPOSITE T-BEAMS IN FIRE

The formulation for partial shear interaction in composite beams was developed over 50 years ago in closed form. This formulation is suitable for many cases encountered in practice, but it does not account for the geometric non-linearity produced by the “P-delta” effect. This effect is important in a compartment in a steel or composite framed building in a fire, where the initially large compressive forces which develop because of restraint against thermal expansion interact non-linearly with the large deflections of the composite floor slab, and a geometrically non-linear formulation must necessarily be invoked to capture the partial interaction behaviour of the composite floor system during a fire. This paper provides an elastic analysis of partial interaction at moderately low levels of temperature, so as to shed light on the significance of the parameters that affect the structural behaviour as well as to provide a precursor for an inelastic analysis to treat catenary action. The differential equations which quantify the problem are derived and solved numerically, and an illustrative example is worked.