Zhong-Xian Li

Numerical Analysis of Lateral Inertial Confinement Effects on Impact Test of Concrete Compressive Material Properties

Dynamic material properties, in particular the dynamic strength, of concrete material are usually obtained by conducting laboratory tests such as drop-weight test and Split Hopkinson Pressure Bar (SHPB) test. It is commonly agreed that a few parameters associated with stress wave propagation will affect the test results, including the lateral and axial inertial effect, end friction confinement and stress wave reflection and refraction. Many different measures have been proposed to eliminate or limit the influences of these effects in dynamic tests of material properties. However, owing to the nature of dynamic loadings, especially those with high loading rates, it is very unlikely to completely eliminate these influences in physical testing. Moreover, it is also very difficult to quantify these influences from the laboratory testing data. In the present study, a refined mesoscale concrete material model is developed to simulate impact tests and to study the influences of lateral inertial confinement on concrete compressive strength increment at high strain rate. The commercial software AUTODYN is used to perform the numerical simulations. Numerical simulations of concrete specimens of different dimensions and under impact loads of different loading rates are carried out. The results are compared with those obtained from laboratory tests, with those specified in the code and simulated with homogeneous concrete material model. The reliability of the numerical simulation of impact tests is verified. It is found that the influences of lateral inertial confinement effect on Dynamic Increase Factor (DIF) is strain rate and specimen size dependent. Neglecting aggregates in concrete specimen in laboratory tests and numerical simulations lead to underestimation of DIF of concrete material.

A Multipoint Method for Spindle Error Motion Measurement

Summary A multi-point method for spindle error motion measurement is proposed in the paper. The radial error motion of the spindle is measured by 4 probes, where as the axial and tilt ones by 5 probes. Roundness error and flatness error of the specimens used for measuring the spindle error motions are separated and have little effect on the accuracy of spindle error motion measurement. For assuring high accuracy of measurements techniques for determining the actual angular positions of the probes and hardware of measuring system have been also developed. The proposed method can be used for real-time spindle error motion measurement with sub-micron accuracy.

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...

RC column failure probabilities to blast loads

Structural reliability analyses are commonly applied to estimation of probabilities of structural damage to static and dynamic loads such as earthquake, wind and wave loads. Although blast loadings acting on structures from accidental explosions or hostile bombings are very difficult to be accurately predicted owing to many uncertain parameters that influence explosion shock wave propagation and shock wave interaction with structures, reliability analyses of structural failure to blast loadings with consideration of uncertainties in blast loading and structural parameters are very limited. Instead, a large safety factor is usually used to account for uncertain variations in blast loading and structural parameters in blast-resistant design and analysis. This may lead to an inaccurate design of structures to resist blast loads, and an inaccurate assessment of structure performance in a given explosion scenario. In this study, reliability analyses of three example RC columns to randomly varying blast loads are carried out. The column dimensions, reinforcement ratios and material strengths are assumed to be normally distributed with the respective design parameters as the mean values. The mean value and standard deviation of the peak reflected pressure and duration of the blast load at various scaled distances are derived from available empirical formulae, and are used in this study to model the blast pressure variations. Failure probabilities of the example RC columns subjected to blast loads of different scaled distances are estimated. Numerical results are compared with those obtained with the deterministic blast loading or deterministic column property assumptions. The importance of considering the random variations of structural properties and blast loadings in assessing the blast load effects on RC columns is discussed.

Towards the Intelligent CMM

Abstract ‘Intelligent’ CMMs are likely to become the next generation of CMMs. In this paper the main features of an intelligent CMM are proposed. In operation, the geometric and measuring information of the part would be extracted from its CAD file. A measuring program including the selection of probe and accessories, determination of the features and parameters to be measured, number and positions of the measured points would then be generated automatically. The position and orientation of the part would be recognized by a CCD camera mounted on the CMM. Based on all this information an optimal measuring path is generated and possible collisions are taken into account. The feasibility and reliability of the system are evaluated by measuring several real workpieces.

Reliability Analysis of RC Columns and Frame with FRP Strengthening Subjected to Explosive Loads

Some structures, both military and civilian, might experience explosive loads during their service life. Owing to high uncertainties in blast load predictions and structural parameters, accurate assessment of the performances of structures under explosion loads is a challenging task. For example, a number of experimental studies using fiber-reinforced plastic (FRP) strengthening of RC structures have been reported in the literature. Most of these studies demonstrate that FRP strengthening is effective in increasing the blast load–carrying capacities of RC structures. However, significant variations in the effectiveness of FRP strengthening have also been observed owing to the large uncertainties in blast loading, RC and FRP material properties, and workmanship in preparing the test specimens and conducting experimental tests. Very few studies that take into consideration these uncertainties in analyzing the effectiveness of FRP strengthening of RC structures on blast loading resistance can be found in the literature. This study performs a reliability analysis to assess the performance of RC columns with or without FRP strengthening in resisting blast loads. Statistical variations of blast loading predictions derived in a previous study are adopted in this study. To define structural performance, pressure-impulse (P-I) curves with a damage criterion on the basis of axial load-carrying capacity that were developed in previous studies for RC columns without strengthening or with FRP strip, FRP wrap, or both FRP strip and wrap strengthening are used. Considering the uncertainties in blast loading predictions and RC column and FRP material properties and dimensions, limit-state functions corresponding to different damage levels of RC columns with or without FRP strengthening are formulated. The statistical variations of blast loading, RC column dimension, longitudinal and transverse reinforcement ratio, and concrete, steel, and FRP material strength and FRP thickness are considered. The failure probabilities of RC columns corresponding to different damage levels with or without FRP strengthening are calculated. The effectiveness of FRP strengthening on the RC column’s blast loading resistance capacities is discussed. The importance of considering the random fluctuations on blast loading and RC column parameters in assessing the blast loading effect on RC columns is demonstrated. A structural system reliability analysis is also carried out to examine the probability of structural collapse as a result of blast loading applied to the front of an example two-span multistory RC frame. The results obtained in this study demonstrate the effectiveness of FRP strengthening on structure protection and can also be used to assess RC structure performance under blast loadings.

Failure Analysis and Damage Assessment of RC Columns under Close-In Explosions

AbstractBlast loads acting on structural components under close-in explosions are nonuniform; therefore, the damage to structural members under such blast loads is local and the failure mechanism is more complex as compared with that of distance explosions. Because of the lack of an accurate blast load model, a precise damage analysis method, an efficient damage assessment method, and practical protective measures, the blast-resistant design of structural members under close-in explosions faces great challenges. In this paper, a fluid-solid coupling numerical method is proposed and then used to investigate the failure mechanism of reinforced concrete (RC) columns under close-in explosions. The results show that local failure, such as crushing, cratering, and spall of concrete, is more likely to happen when the RC column is subjected to close-in blast loads. Parametric studies are carried out to investigate the influences of column dimensions and the reinforcement ratio on the damage degree of RC columns. ...

Numerical Analysis of Foam-Protected RC Members under Blast Loads

Due to the threat of terrorist activities worldwide, research on the protection of building structures from the effects of explosions is critical in order to avoid catastrophic damage to buildings. Protecting our infrastructures means protecting lives. Metallic foam is an economical, light-weight and recyclable material used as a sacrificial cladding to protect structures. Its efficient energy absorption enables metallic foam to mitigate the blast energy acting on the protected structure. This paper describes our numerical investigation of the protective performance of metallic foam cladding on reinforced concrete (RC) structural members using LS-DYNA. In the numerical model, Modified Honeycomb (Material 126) from the LS-DYNA material library was used to represent the aluminium foam while Continuous Surface Cap Model (Material 159) was selected to model the behaviour of concrete. The numerical model was validated by field blast testing results. Using the validated numerical model, parametric studies were conducted to assess the influence of different foam properties on the pressure-impulse (P-I) diagrams of the foam-protected RC slabs. The influence of the thickness of the RC members was also investigated. The derived P-I diagrams will prove useful in the preliminary design of the foam cladding on RC members.

Investigation of Blast Effects on Double-Skinned Composite Steel Tubular Columns:

In recent years, concrete filled double skin steel tube (CFDST) members have gained interest due to its attractive properties such as ease of construction, light weight, high strength and good seismic resistance, and thus it is expected that these members have the potential of being used in construction of buildings. However, there is lack of understanding about the inelastic behaviors of CFDST members under blast loads. In this paper, based on the ConWep airblast loading model, the blast resistance of typical circular CFDST columns used in engineering field is investigated and the multiple failure modes of CFDST columns under blast loading are analyzed. The influence of explosive charge weights and column axial loading condition on the response of CFDST columns are investigated through parametric study. Finally, the direct shear and flexural failure modes of CFDST columns are analyzed, and uncoupled P-I (pressure-impulse) diagrams are obtained based on an equivalent single degree of freedom (SDOF) system. This study helps to understand the non-linear behaviors of CFDST columns subjected to blast loading.

Optimized Design of Foam Cladding for Protection of Reinforced Concrete Members under Blast Loading

AbstractA load-cladding-structure (LCS) model was used to study the mitigating effect provided by metallic foam cladding against blast loading on reinforced concrete (RC) structural members. The model considered the interactions between an external blast load, a protecting foam cladding, and a target RC structural member. The effectiveness of the LCS model was validated by field blast tests conducted in 2009. The validated model was then used to derive pressure impulse diagrams of the foam-protected RC members. Afterwards, two nondimensional parameters representing the relationship between the foam cladding and the target RC member were characterized. Using the suggested nondimensional parameters, normalized pressure-impulse (p−i) diagrams for the foam-protected RC members were generated. The effects of the two nondimensional parameters on the p−i diagrams were investigated by comparing the corresponding asymptotes. Based on the predicted results, an optimized design of the foam cladding for RC structural...