Continuous Surface Cap Research Articles

Verification Analysis of the Degradation of Frequency Parameters Under the Action of Seismic Load

The article discusses the results of a numerical study of the effect of local damage to bearing structures on the degradation of frequency parameters during seismic effects. The studies were carried out in the LS-DYNA software package which implemented the nonlinear concrete model — Continuous Surface Cap Model (CSCM). Design models of a building are modelled both with regard to direct reinforcement using volumetric FE (finite elements) and flat FE. The object of modelling is a 3-storey building of cellular framing. The analysis of the results of the conducted studies suggests that the effect of degradation of natural vibration frequencies cannot be estimated in the building modelled by means of bar FE. The degradation is insignificant; the frequency reduction occurs within the limits of errors. At the moment, it is not possible to identify the effect of degradation in the bar model in this formulation. To do this, it is necessary to conduct additional researches, including those with other models of materials. Taking into account certain assumptions, the LS-DYNA software package allows computational studies of the effect of local damage to bearing structures on dynamic characteristics, including those resulting from an earthquake.

Degradation of Dynamic Parameters of Reinforced Concrete Buildings during an Earthquake

The article presents the results of a numerical study of the effect of local damage to load-bearing structures on the dynamic characteristics during an earthquake. The studies were carried out in the LS-DYNA software package, which implemented the nonlinear concrete model, Continuous Surface Cap Model (CSCM). The computational model of the building uses solid finite elements for concrete and bar elements for reinforcing bars. As the object of modelling, a 1-storey building with a frame structural layout was chosen. Studies have shown that during an earthquake with an intensity of 7 points, specified by the corresponding accelerogram, there is no change in natural frequencies, that is, the damage resulting from this earthquake is insignificant. During an earthquake with an intensity of 8 points, a significant (up to 55 %) reduction in natural frequencies occurs. This suggests that the structure is damaged, and the overall stiffness of the building is reduced. Analysis of the results shows that the most intense decrease in natural frequencies occurs in the active phase of an earthquake. After the end of the active phase, the frequencies stabilize and stop falling. The LS-DYNA software package enables to perform studies of the effect of local damage of the load-bearing structures on the dynamic characteristics during an earthquake with the direct reinforcement of concrete with reinforcing bars and can be used in the calculations and the design of buildings and structures.

An experimental and numerical study of reinforced conventional concrete and ultra-high performance concrete columns under lateral impact loads

This paper presents an experimental and numerical study on the dynamic behaviour of axially-loaded reinforced conventional concrete (RC) and ultra-high performance concrete (UHPC) columns against low-velocity impact loading. The test specimens were divided into two groups with square and circular cross-section shapes, and each group includes both RC and UHPC columns. The impact scenario was modelled with a drop weight falling freely on the column mid-span. Brittle failure with shear plug formation was observed in RC columns while UHPC columns remained a flexure response with minimal damage under severe impact loads. To further interpret the experimental data, detailed finite element (FE) models were developed for RC and UHPC columns. A Continuous Surface Cap Model (CSCM) which accounts for the triaxial material strength, post peak softening and strain rate effect was adopted for UHPC material. After validating the material and structural model based on the testing data, extensive numerical simulations were performed to predict the UHPC column residual loading capacity after lateral impacts. Impact mass-velocity (M-V) diagrams were derived for the UHPC column damage assessment, and analytical formulae which could be easily applied to generate M-V diagrams were derived based on parametric studies.

Concrete constitutive models for low velocity impact simulations

Concrete structures are commonly exposed to low velocity impact loads originating from windborne/waterborne debris, vehicle/vessel collision, and rock fall. For the performance assessment of concrete structures under such loads, several constitutive models have been developed to date. To compare the accuracy of the available models for practical applications, the current study evaluates four constitutive models, i.e., continuous surface cap model (CSCM), elasto-plastic damage cap (EPDC) model, Karagozian and Case concrete (KCC) model, and Winfrith concrete model. For this purpose, the constitutive models are first examined at the material level through single element simulations under basic stress paths, such as uniaxial compression and tension, as well as triaxial compression. A range of measures, such as post-peak softening, shear dilation, and confinement effect, are extracted and compared. The investigation is then extended to understand how the concrete constitutive models perform at the structure level. This is achieved by replicating drop hammer tests on reinforced concrete (RC) and concrete filled steel tube (CFST) beams. Investigation of these two structural categories provides a unique opportunity to further evaluate the accuracy of the concrete constitutive models in interaction with the most common reinforcement details. To achieve this goal, the impact responses of RC and CFST beams are compared with full-scale experimental test data. Upon understanding the capabilities of each constitutive model in predicting the structural behavior and damage, the most important modeling parameters are examined. The outcome of this study facilitates the selection and use of concrete constitutive models for the design and assessment of concrete structures subjected to various low velocity impact loads.

Identification of Concrete Material Model Parameters Using Optimisation Algorithms

The application of nonlinear material models of concrete within numerical simulations focused on the design of safer and more economical protective concrete structures is currently the subject of investigation of many scientific researchers. However, one basic problem related to the nonlinear modelling of concrete is that very often there is a lack of knowledge about the material model parameters whose values must be defined. The solution to this problem can be in what is termed as inverse parameter identification, an approach which is presented in this paper. Specifically, the material parameters of the Continuous Surface Cap Model for concrete are identified within this paper using optimisation algorithms. The subsequent comparison of parameter identification results with experimental data shows the efficiency of the presented approach.

Verification of the reinforced concrete column bar model based on the test results

Modern software packages for calculating buildings and structures for various types of action make it possible to simulate the structure and its elements in sufficient detail and to reflect adequately the behavior of this structure. However, detailed simulation with the use of solid finite elements requires a large amount of computing time to perform calculations. This problem can be solved by moving from solid finite elements to bar elements. At the same time, it is necessary to verify the bar calculation models. The article compares the calculation results of a reinforced concrete column simulated in two ways: with the use of solid finite elements based on the actual reinforcement and with the use of bar finite elements. The concrete material for the reference model is specified using a Continuous Surface Cap Model nonlinear model implemented in the LS-DYNA software package. This model reflects the non-linear behavior of concrete and enables to consider the joint behavior of concrete and reinforcing bars. The diagrams of concrete behavior in the bar model are adopted in accordance with SP 63.13330.2012 “Concrete and reinforced concrete structures. Revised edition of SP 52-101-2003”. The study compares the results obtained by the breaking load value and the fracture pattern of the column under consideration.

Verification of the eccentrically compressed reinforced concrete column calculation model based on the results of a full-scale experimental study

The article presents the results of a numerical experiment consisting in a test of an eccentrically compressed reinforced concrete column and comparison of the results obtained with the results of experimental studies. With the development of numerical methods and software packages, methods (techniques) of modeling structural elements using more detailed calculation models with solid finite elements that allow direct consideration of the joint behavior of concrete and reinforcing bars become relevant. The use of such methods requires verification of individual load-bearing structural elements, such as columns, beams, slabs. The article refers to a nonlinear concrete model – Continuous Surface Cap Model (CSCM). This model is implemented in the LS-DYNA software package and enables to consider the joint behavior of reinforcing bars and concrete, using bar (for reinforcing bars) and solid (for concrete) finite elements. An eccentrically compressed reinforced concrete column of square section with dimensions of 150х150х1150 (h) mm is chosen as an object of modeling. The studies have shown that the ultimate breaking load on the column based on the results of numerical modeling is consistent with the experimental values (discrepancy does not exceed 3.4%). The pattern of development of cracks and fractures, obtained from the results of modeling in the LS-DYNA software package corresponds to the pattern of fractures obtained as a result of experimental studies. According to comparison of the results obtained, it can be argued that a numerical test showed good agreement with the results obtained during full-scale experiments.

Verification of the reinforced concrete beam model based on the results of a full-scale experimental study

The article presents the results of a numerical experiment consisting in a bending test of a reinforced concrete beam and comparison of the results obtained with the results of full-scale experiments. In most cases, it is not possible to adequately consider all types of nonlinearities when using simplified bar and plate elements. The problem can be solved by using more detailed computational models with solid finite elements, allowing to consider directly the joint behavior of reinforcing bars and concrete. The studies were carried out in the LS-DYNA software package, which implemented the nonlinear concrete model – Continuous Surface Cap Model (CSCM). This model allows to consider the joint behavior of reinforcing bars and concrete, using bar (for reinforcing bars) and solid (for concrete) finite elements, thereby helping to overcome existing shortcomings in the diagrams of concrete behavior. As an object of modeling, a reinforced concrete statically determinable beam of rectangular section with dimensions of 1,000 х 50 х 100 (h) mm is considered. The conducted studies showed that the ultimate load on the beam based on the results of numerical modeling is quite consistent with the experimental value (8.5% discrepancy). The arrangement of cracks and the fracture pattern obtained from the modeling results in the LS-DYNA software package are in good agreement with the results of the tests. The LS-DYNA software package will allow correct solid modeling of bending reinforced concrete elements with specification of nonlinear diagrams of concrete and reinforcing bars deformation and can be used for research, calculation and design of reinforced concrete elements of buildings and structures.

Constitutive model of ultra-high-performance fiber-reinforced concrete for low-velocity impact simulations

Ultra-high performance fiber reinforced concrete (UHPFRC) is regarded as a promising material to resist impact and shock loadings. Many physical experiments have been performed for UHPFRC members under static and dynamic loads. However, less emphasis has been placed on the rationality of the constitutive model of UHPFRC in finite element (FE) simulations. Hence, this paper aims to develop an adequate constitutive model of UHPFRC for low-velocity impact simulations. The Karagozian & Case concrete (KCC) model used for blast analysis is first proved to have low accuracy in predicting the impact-induced responses. Therefore, the continuous surface cap model (CSCM) that behaves well in concrete modeling are modified to model UHPFRC by using the existing experimental data. The failure surface functions of UHPFRC including triaxial compression (TXC), triaxial extension (TXE) and torsion (TOR) are completely calibrated. The strain rate parameters in tension and compression are modified based on the current experimental data of UHPFRC. Numerical results show that the proposed material model is capable of adequately exhibiting the tensile strain-hardening behavior and predicting the uniaxial and triaxial compression strengths of UHPFRC. The impact-induced responses obtained using the proposed constitutive model agree very well with the experimental results.

Optimization-Based Inverse Identification of the Parameters of a Concrete Cap Material Model

Issues concerning the advanced numerical analysis of concrete building structures in sophisticated computing systems currently require the involvement of nonlinear mechanics tools. The efforts to design safer, more durable and mainly more economically efficient concrete structures are supported via the use of advanced nonlinear concrete material models and the geometrically nonlinear approach. The application of nonlinear mechanics tools undoubtedly presents another step towards the approximation of the real behaviour of concrete building structures within the framework of computer numerical simulations. However, the success rate of this application depends on having a perfect understanding of the behaviour of the concrete material models used and having a perfect understanding of the used material model parameters meaning. The effective application of nonlinear concrete material models within computer simulations often becomes very problematic because these material models very often contain parameters (material constants) whose values are difficult to obtain. However, getting of the correct values of material parameters is very important to ensure proper function of a concrete material model used. Today, one possibility, which permits successful solution of the mentioned problem, is the use of optimization algorithms for the purpose of the optimization-based inverse material parameter identification. Parameter identification goes hand in hand with experimental investigation while it trying to find parameter values of the used material model so that the resulting data obtained from the computer simulation will best approximate the experimental data. This paper is focused on the optimization-based inverse identification of the parameters of a concrete cap material model which is known under the name the Continuous Surface Cap Model. Within this paper, material parameters of the model are identified on the basis of interaction between nonlinear computer simulations, gradient based and nature inspired optimization algorithms and experimental data, the latter of which take the form of a load-extension curve obtained from the evaluation of uniaxial tensile test results. The aim of this research was to obtain material model parameters corresponding to the quasi-static tensile loading which may be further used for the research involving dynamic and high-speed tensile loading. Based on the obtained results it can be concluded that the set goal has been reached.

Verification of the spar model of a reinforced concrete beam

The paper presents the outcomes of the research aimed at the verification of the computational model simulating a reinforced concrete beam employing spar and 3D finite elements, with account taken of physical nonlinearity. Structural calculations involved in R&D and design effort must factor in non-linear structural behavior both of the material and of structural elements. In particular, this is valid for the analysis of special types of impact (seismic, accidental etc.). However, design models under development may incorporate significantly varying models of nonlinear materials as well as different kinds of finite elements, and both have to be verified. This article considers two models of a hinged (pinned) beam. In the first case, the modeling was based on 3D elements for concrete and on spar elements for reinforcement using Euler-Lagrange coupling of finite elements belonging to concrete and reinforcement (three-dimensional model). The second case the simulation based on spar finite elements (spar model). Both the spar model and 3D model allow for non-linear nature of concrete and reinforcement. In case of concrete the material was set using the Continuous Surface Cap Model, while the reinforcement was modeled involving a bilinear diagram of material behavior.

Comparative analysis of results from experimental and numerical studies on concrete strength

Some results of numerical experiments of testing concrete cubes and prisms on unconfined compression, and the comparison of results obtained with experimental and specified data, are presented in the article. When performing calculations of structures in a nonlinear setting, it is very important to choose adequate deformation diagrams or material models. Because of the fact that there are no instructions how to use the diagrams of concrete and armature deformation in collaboration of steel and concrete, the simulation of reinforced concrete structures by finite elements of the same type without any assumptions is impossible. Numerical experiments have been performed in the LS-DYNA software package. This software package allows simulating the collaboration of concrete and steeling with the help of three-dimensional (for concrete) and rod (for the reinforcement) finite elements. As samples, a cube and a prism with dimensions of 150×150×150 mm and 150×150×600 mm, respectively, have been taken. The samples are simulated by solid finite elements. For the simulation of concrete, the non-linear CSCM (Continuous Surface Cap Model) material is used. The tests were carried out with samples of the following classes of concrete as for cylinder compressive strength: C12, C16, C20, C25, C30, C35. This corresponds to the following classes of cube compression strength: B15, B20, B25, B30, B37, B45. The tests have been carried out considering the friction coefficients between the plates of a testing machine, and a sample. The performed researches have shown that the destruction nature of the samples in a numerical experiment corresponds to the failure nature in real tests. The investigated model of CSCM concrete can be used in the calculation of concrete and reinforced concrete structures with acceptable accuracy for main classes of concrete.

An improved criterion to minimize FE mesh-dependency in concrete structures under high strain rate conditions

In the context of rising demand for reliability and safety in concrete structures subjected to blast or impact loading, the behavior of concrete under high strain rate conditions is an important issue. Since concrete subjected to impact loading associated with high strain rates shows quite different material behavior from that under monotonically increasing quasi-static loads, several material models have been proposed and used to describe the high strain rate behavior of concrete under blast and impact loadings. In the process of modeling the high strain rate condition with these material models, mesh dependency in the used finite element (FE) is the key problem because simulation results under high strain-rate conditions are quite sensitive to the applied FE mesh size. This means that the accuracy of simulation results for concrete structures may be strongly dependent on the FE mesh size. This paper introduces an improved criterion that can minimize the mesh-dependency of simulation results on the basis of the fracture energy concept, and the HJC (Holmquist Johnson Cook), CSC (Continuous Surface Cap), and K&C (Karagozian & Case) models are examined to trace their relative sensitivity to the employed FE mesh size. Consistent with the purpose of a perforation test for a concrete plate under a projectile (bullet) impact, the residual velocities of a projectile after perforation are compared. Correlation studies between analytical results and associated parametric studies show that the variation of residual velocity with the used FE mesh size is substantially reduced and the accuracy of simulation results is improved by applying a unique failure strain value determined according to the proposed criterion.

Calibration of the continuous surface cap model for concrete

The continuous surface cap (MAT 145) model in LS-DYNA is known by its elegant and robust theoretical basis and can well capture many important mechanical behaviors of concrete. However, it appears to be less popular than many other constitutive models in engineering application due to many material parameters involved in the model formulation which are difficult to calibrate. This study presents an effective calibration method to determine the material parameters for this model as functions of uniaxial compression strength and the maximum aggregate size of concrete according to formulas from CEB-FIP code and concrete test data from other published literatures. The obtained parameters can be conveniently used for occasional users with little or no information on concrete in hand. We further compare the predictions of stress–strain relationship in tension and compression under different confining pressures as well as hydrostatic compression by the model, and validate the model based on impact test of RC beams. Besides, the model is further compared against a similar model-MAT 159 in terms of model performance. The results demonstrate that the model based on the calibrated parameters is capable of offering reasonable and robust predictions.

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.

Local impact effects on concrete target due to missile: An empirical and numerical approach

Concrete containment walls and internal concrete barrier walls of a Nuclear Power Plant safety related structures are often required to be designed for externally and internally generated missiles. Potential missiles include external extreme wind generated missiles, aircraft crash and internal accident generated missiles such as impact due to turbine blade failure and steel pipe missiles resulting from pipe break. The objective of the present paper is to compare local missile impact effects on reinforced concrete target using available empirical formulations with those obtained using LS-DYNA numerical simulation. The use of numerical simulations for capturing the transient structural response has become increasingly used for structural design against impact loads. They overcome the limits of applicability of the empirical formulae and also provide information on stress and deformation fields, which may be used to improve the resistance of the concrete. Finite element (FE) analyses of an experimental impact problem reported by Kojima (1991) are carried out that are able to capture the missile impact effects; in terms of local and global damage. The continuous surface cap model has been used for modelling concrete behaviour. A range of missile velocity has been considered to simulate local missile impact phenomenon and modes of failure and to capture the concrete response from elastic to plastic fracture. A comparison is then made between the empirical formulations, numerical simulation results, and available experimental results of slab impact tests. While the numerical simulation is able to capture the experimental trend and results, a comparison of penetration depth and scabbing and perforation limits as per different empirical formulation shows substantial divergence.

Rock failure induced by dynamic unloading under 3D stress state

A commercial finite element program, LS-DYNA, was employed to simulate the unloading process of rocks under three dimensional (3D) stresses. The continuous surface cap model (CSCM), was used to model rock behaviour. Using this model, the unloading failure mechanisms of hard rock in a confined state were investigated during the unloading process. The results indicated that when rocks under 3D stress state experience unloading, the process is dominated by strain energy density (SED) rate. The effects of different unloading paths and different confining stresses can be characterised by the SED rate. A significant finding of this study is that the SED rate can quantify the unloading process. Based on the findings, rock failure can be induced by rapid unload of initial stress. In the practical underground excavation engineering, dynamically controlling the SED rate can increase the excavation potential of rocks, minimising the required external excavation energy by using the energy of the stressed rock.