Free vibration and dynamic response of sandwich composite plates with auxetic honeycomb core

This paper deals with the free vibration and dynamic responses of composite sandwich plates. The sandwich plate has three layers in which two face sheets are made of isotropic material, and the core layer is made of auxetic honeycomb structures with a negative Poisson's ratio. A smoothed finite element model based on the first-order shear deformation theory is established for the analysis purpose. In the model, only the linear approximation is necessary, and the discrete shear gap method for triangular plate elements is used to avoid the shear locking. The Newmark direct integration technique is used to capture the dynamic responses of the sandwich plates. The convergence study is made, and the accuracy of present results is validated by comparison with available data in the literature. The influence of geometrical parameters, material properties, and boundary conditions are explored and discussed. Numerical results show that auxetic materials have several different responses compared to conventional materials, and these behaviors are strongly influenced by the internal structure of the auxetic material.

Evaluation of modulus of elasticity for eco-friendly concrete made with seawater and marine sand

The ultimate aim of this study is to use experimental work for evaluating the modulus of elasticity (MOE) of Geopolymer concrete (GPC) using marine sand as fine aggregate and seawater for the mix. Four different groups of concrete mixtures, namely CP1a, CP1b, CP2a, CP2b were identified. While the CP1a mix was prepared using GPC with marine sand and seawater, the CP1b was made by adding sodium sulfate (Na2SO4) into the CP1a mix. The same procedure was applied for CP2a and CP2b mixtures; however, instead of using GPC, Portland Cement was used as the binder for the CP2 group (OPC). A total of 12 test samples were cast and tested to determine the development of MOE of GPC and OPC over time. The MOE of concrete was measured at 3, 7, 28, 60, and 120 days. Experimental results were then compared to the MOE obtained using the empirical equation from ACI 318 - 2008. It was found that the experimental MOE of both OPC and GPC specimens was higher than the estimated MOE values from ACI standards. The added sodium sulfate yielded a significant effect on the MOE of OPC but produced a minimal influence on the MOE of GPC.

A new type of hollow-shallow steel and concrete composite floor beam

Recently in Vietnam, steel-concrete composite structures especially composite beams are widely constructed in high-rise buildings. To apply broader in construction field mainly in secondary beam systems, the new type of slim-floor composite beam is proposed to aim at reducing the cost, saving the raw material, and decreasing the overall floor depth for sustainable development orientation. This type of floor beam structure consists of built-up hollow-shallow steel beam mandatory connected with cast in situ concrete slab through the openings at both side of web along the beam. The shear connection level of composite beam is depended on not only the friction at the connected surface between hollow steel section and concrete but also the shear resistance of concrete dowels, which go through the openings. The paper deals with an innovative shape of cross-section and design philosophy of composite beam according to EN 1994-1-1.

Investigation of the effects of opening size and location on punching shear resistance of flat slabs using Abaqus

The paper presents a numerical study on the effects of opening size and location on punching shear resistance of flat slabs without drop panels and shear reinforcement using ABAQUS. The study proposes an ABAQUS model that is enable to predict the punching shear resistance of flat slabs with openings. The model is validated well with the experimental data in literature. Using the validated numerical model, the effects of opening size and location on the punching shear resistance of flat slabs are then investigated, and the numerical results are compared with those predicted by ACI 318-19 and TCVN 5574:2018. The comparison between experimental and numerical results shows that the ABAQUS model is reliable. The punching shear resistances calculated by ACI 318-19 and TCVN 5574:2018 with different opening sizes and locations are agreed well to each other, since the design principles between two codes now are similar.

Coupled finite-discrete element modeling and potential applications in civil engineering

Since its appearance at the last of seventy decades, the Discrete Element Modeling (DEM) has been widely used in the modeling of geomaterials but regrettably limited to small scales problems by considering grains interactions. Recently, a new trend has emerged in combining DEM with other methods. The coupled approach allows extending the methods toward a wide range of civil engineering applications. Among them, FEM×DEM coupling has been the topic of research over the past decade. The FEM×DEM coupling has been mainly developed in two categories: direct interaction and multi-scale coupled models. In the first regard, this paper summarizes the basic principle of FEM and DEM, then reviews a number of possible direct coupling strategies between FEM and DEM together with potential applications in civil engineering. The second objective is to develop a model that combines these two above mentioned methods in a multi-scale approach. The results obtained by the developed model have been proved to efficiently tackle the complicated problem in engineering applications by assessing both macro and micro features and establishing the linking information between them.

Local buckling of thin-walled circular hollow section under uniform bending

This article presents a semi-analytical finite strip method based on Marguerre’s shallow shell theory and Kirchhoff’s assumption. The formulated finite strip is used to study the buckling behavior of thin-walled circular hollow sections (CHS) subjected to uniform bending. The shallow finite strip program of the present study is compared to the plate strip implemented in CUFSM4.05 program for demonstrating the accuracy and better convergence of the former. By varying the length of the CHS, the signature curve relating buckling stresses to half-wave lengths is established. The minimum local buckling point with critical stress and corresponding critical length can be found from the curve. Parametric studies are performed to propose approximative expressions for calculating the local critical stress and local critical length of steel and aluminum CHS.

Finite element modelling of rectangular concrete-filled steel tube stub columns incorporating high strength and ultra-high strength materials under concentric axial compression

This study presents a unified approach to simulate the behavior of rectangular concrete-filled steel stub columns incorporating high strength and ultra-high strength materials subjected to concentric axial compression. The finite element model is developed based on Abaqus software, which is capable of accounting for geometrical nonlinearity, material plasticity, and interaction between multi-physics. The proposed model incorporates the influences of residual stress for welded-box steel sections and initial imperfection. A novel stress-strain relation of confined concrete is proposed to account for the composite action, which might increase the strength and ductility of infilled concrete under multi-axial compressive conditions. Various verification examples are conducted with wide ranges of geometrical and material properties. The simulation results show that the proposed model can accurately predict the ultimate strength, load-deformation relations, and failure mode of the experimental specimens.

A simplified variant of the time finite element methods based on the shape functions of an axial finite bar

In the field of structural dynamics, the structural responses in the time domain are of major concern. There already exist many methods proposed previously including widely used direct time integration methods such as ones in the β-Newmark family, Houbolt’s method, and Runge-Kutta method. The time finite element methods (TFEM) that followed the well-posed variational statement for structural dynamics are found to bring about a superior accuracy even with large time steps (element sizes), when compared with the results from methods mentioned above. Some high-order time finite elements were derived with the procedure analogous to the conventional finite element methods. In the formulation of these time finite elements, the shape functions are like the ones for a (spatial) 2-order finite beam. In this article, a simplified variant for the TFEM is proposed where the shape functions similar to the ones for a (spatial) axial bar are used. The accuracy in the obtained results of some numerical examples is found to be comparable with the accuracy in the previous results.

Simplified calculation of flexural strength deterioration of reinforced concrete T-beams exposed to ISO 834 standard fire

Reinforced concrete (RC) T-shaped cross-section beam (so-called T-beam) is a common structural member in buildings where beams and slabs are monolithically cast together. In this paper, a simplified calculation method based on Russian design standard SP 468.1325800.2019 is introduced to determine the flexural strength of RC T-beams when exposed to ISO 834 standard fire. The idea of 500oC isotherm method, which is stipulated in both Eurocodes (EC2-1.2) and SP 468, is applied associated with specifications of temperature distribution on T-beams’ cross sections and the temperature-dependent mechanical properties of concrete and reinforcing steel. A case study is conducted to explicitly calculate the flexural strength deterioration (FSD) of T-beams compared to that at ambient temperature. A calculation sheet is established for parametric studies, from which the results show that the FSD factor of RC T-beams is adversely proportional to the dimensions of the beam’s web and flange. However, the effect of these components of T-beams is not significant.

Mix design of high-volume fly ash ultra high performance concrete

The addition of supplementary cementitious materials (SCMs) to replace cement, especially with a high volume (> 50%), is an effective way to reduce the environmental impact due to the CO2 emissions generated in the production of ultra-high performance concrete (UHPC). Unfortunately, no official guidelines of UHPC using a high volume of SCMs have been published up to now. This paper proposes a new method of mix design for UHPC using high volume fly ash (HVFA), that is referred to the particle packing optimization of the Compressive Packing Model proposed by F. de Larrard. This proposed method also considers the heat treatment curing duration to maximize the compressive strength of HVFA UHPC. The experimental results using this proposed mix design method show that the optimum fly ash content of 50 wt.% of binder can be used to produce HVFA UHPC with a compressive strength of over 120 MPa and 150 MPa under standard curing and heat treatment, respectively. Moreover, the embodied CO2 emissions of UHPC reduces 56.4% with addition of 50% FA.