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The paper discusses the structural behaviour of concrete-filled steel tube columns (CFT) when applied to the top-down construction method as temporary internal supports for ceilings. Their ultimate capacity to take into account the actual boundary conditions of the column is essential for successful application in construction. The paper presents the full-scale in situ testing of four slender specimens with variable D/t ratios under concentric axial loading. The CFT columns were supported on the previously jacked concrete piles. In addition, detailed finite element numerical models in ABAQUS and PLAXIS computer programs were developed. The models include the nonlinear behaviour of materials and the nonlinear behaviour of soil. The soil–pile–column interaction and impact of the CFT column–pile connection stiffness on global column stability were considered. The numerical model was validated by comparison with the experimental results. In conclusion, the coefficient for the effective buckling length of the studied columns is proposed. Finally, the experimental results of the critical buckling forces were compared with widely used international design codes Eurocode 4-EC4, American standard-ACI and the Australian standard-AS.

Due to numerous advantages, concrete-filled steel tubular (CFST) columns have an increasingly important role in the civil engineering industry. Because of the expensive experimental testing of these members, it is beneficial to provide prognostic models. In this study, an artificial neural network (ANN) model for predicting the axial compressive strength of square CFST columns has been developed. A dataset of 1022 samples (685 stub columns and 337 slender columns) was collected from available literature in order to compare the accuracy of the fast predictive Levenberg-Marquardt algorithm (LM) and Eurocode 4 (EC4) design code. Analyses showed that the ANN model has better accuracy than EC4. Over a whole domain, the ANN model has higher coefficient of determination (R2), and lower root mean squared error (RMSE). The same conclusion is valid when two separate datasets are considered: one for stub columns and the other for slender columns. The benefit of the ANN model is its applicability in a broader range of column parameters. At the same time, EC4 puts several limitations on its use and gives satisfactory results only in limited circumstances. Empirical equations have also been proposed from the best ANN model, which is useful for engineering practice.

The composite concrete-filled steel tube columns are structural members with numerous advantages over the traditional reinforced concrete or the pure steel members. The behavior of these columns is highly nonlinear. This paper analyses the determination of the ultimate compressive capacity of circular concrete-filled tubes according to the Eurocode 4 design code and its accuracy in comparison with the experimentally available results from the literature. In order to improve the predictions of the column’s ultimate compressive capacity, two efficient machine learning algorithms are employed separately for the stub and the slender columns. The analyzed algorithms are the Decision tree and the Random forest. The research used an experimental dataset of 508 samples: 236 tests on the stub columns and 272 tests on the slender columns. It adapted the dataset to the provision of the Eurocode 4 design code. The predictions of the column’s ultimate axial capacity obtained by two ML algorithms and the Eurocode 4 are compared with the experimental test results on the validation dataset. The calculated R2 error measure has shown that the predictions obtained by the tree-based algorithms are superior compared to the design formulas offered by the Eurocode 4. The decision tree algorithm achieved the best accuracy measured with the highest value of R2 error measure between the two algorithms. The two analyzed algorithms are used to perform the sensitivity analysis on the considered problem. The sensitivity analysis used the feature importance, a technique that assigns a score to each of the input features based on their influence on the prediction of the output variable. As expected, the sensitivity analysis identified the outer diameter of the columns’ cross-section as the parameter with the most significant impact on the results.

The paper presents the numerical modelling of concrete-filled steel tube stub columns under axial compression. The nonlinear 3D finite element model for the analysis was developed using ABAQUS software. The modelling, boundary conditions, the constitutive models for steel and concrete and contact interaction between the steel tube and concrete surface are discussed in detail. Three numerical models are proposed and validated using collected experimental data results. Finally, the composite column axial capacity is compared with the Eurocode 4 calculations for composite columns.

The paper proposes an adaptive fiber section discretization scheme for inelastic frame elements. The scheme uses cubature rules for the efficient and accurate evaluation of the section response over the elastic portion of the section. As inelastic strains arise and penetrate into the section from the edges, the scheme converts the area under inelastic strains to a regular fiber discretization. This approach offers considerable advantages for the computational efficiency of large structural models with inelastic frame elements by minimizing the number of integration points in sections with limited inelastic response. The proposed scheme is presented for circular and rectangular cross sections, but the approach is applicable to other section shapes. Inelastic frame response examples demonstrate the benefits of the proposed discretization scheme for the nonlinear response history analysis of large structural models.

This paper proposes the use of artificial neural network (ANN) algorithms to estimate the ultimate compressive strength of slender circular concrete-filled steel tubular (CCFST) columns. A dataset of 1051 samples was applied to generate an appropriate ANN prognostic model. Empirical equations were also developed from the best neural network, and their results were compared with those obtained by Eurocode 4 (EC4) design code. Analyses show that the proposed ANN model has a better agreement with experimental results than those created with provisions of the EC4 design code.

The paper presents four different methods for viscoelastic analysis of steel-concrete composite structures: “exact” analysis method, simplified method, effective modulus method and method proposed by the European design code, Eurocode 4. The exact analysis method uses the mathematical theory of linear integral operators for presenting viscoelastic relations of the steel-concrete composite structures. The simplified analysis method is derived from the exact method. It adopts the assumption that generalized displacements change linearly with the concrete creep function. Due to this assumption, the problem equations transforms from nonhomogeneous integral equations to nonhomogeneous algebraic equations. The effective modulus method and the method proposed by the Eurocode 4 are algebraic methods where stress-strain relations for concrete are expressed through the effective modulus, which depend on time. On the numerical example of continuous steel-concrete beam, the results obtained using the mentioned approximate and the exact method are compared

Predmet disertacije je određivanje aksijalne nosivosti i stabilnosti kružnih šupljih čeličnihprofila ispunjenih betonom, u daljem CFT stubovi (eng. Concrete Filled Tubes).Problem istraživanja je određivanje stabilnosti CFT stubova, uzimajući u obzir krutostveze. Ovaj problem se javlja u ''top down'' metodi gradnje, kada se nakon izvođenja šipovau koje su ubetonirani CFT stubovi i tavanice na koti 0.00, vrši iskop ispod izvedenetavanice i izvođenje nadzemnog dela konstrukcije. U takvoj situaciji postavlja se pitanjenosivosti i stabilnosti CFT stubova koji se vezuju sa izvedenim šipovima na osnovu koga sedefiniše moguć iskop i izgradnja nadzemnog dela konstukcije...

In this paper, the methods for determination of the bending resistance of composite beams with partial shear connection and nonductile shear connectors, according to Eurocode 4, are studied. The Eurocode 4 predicts the use of two calculation methods: the method based on the nonlinear theory and the simplified method that assume linear relation between elastic and plastic bending moment resistance. The algorithm for the nonlinear section analysis is explained in detail. Using the own computer program for nonlinear section analysis, for one composite beam cross section, the differences in results obtained by these two proposed methods are discussed.

The paper presents the nonlinear section analysis for composite steel-concrete beams with different degrees of shear connection. The analysis is fiber based, i.c., integration over the cross section is performed numerically, and any uniaxial nonlinear material model can be assigned to the steel and concrete parts of the cross section or to the reinforcement bars. The analysis assumed full interaction between steel and concrete and therefore, is suitable for analysis of composite steel-concrete beam cross sections with nonductile shear connectors. Its accuracy is verified on few experimental results. The presented section analysis is used in the parameter study in order to evaluate different methods proposed by design codes for determining the bending moment resistance of composite cross sections with nonductile shear connectors and different degrees of shear connection. The following effects are considered: variation of concrete and steel material models, presence of slab reinforcement, and creep of concrete. Special attention is paid on two different constructional methods: propped and unpropped. The weaknesses of the simplified design method in determining bending moment resistance are identified and recommendations for practical design analysis are formulated.

The paper presents the simplified matrix stiffness method for analysis of composite and prestressed beams. The method is based on the previously developed "exact" analysis method that uses the mathematical theory of linear integral operators to derive all relations without any mathematical simplifications besides inevitable idealizations related to the material rheological properties. However, the method is limited since the closed-form solution can be found only for specific forms of the concrete creep function. In this paper, the authors proposed the simplified analysis method by introducing the assumption that the unknown deformations change linearly with the concrete creep function. Adopting this assumption, the nonhomogeneous integral system of equations of the "exact" method simplifies to the system of algebraic equations that can be easily solved. Therefore, the proposed method is more suitable for practical applications. Its high level of accuracy in comparison to the "exact" method is preserved, which is illustrated on the numerical example. Also, it is more accurate than the well-known EM method.

The paper presents nonlinear analysis of square CFT columns using distributed plasticity fiber elements. Behaviour of CFT columns is nonlinear and it is necessary to include different nonlinear effects into the numerical model in order to simulate their behaviour properly. Model proposed in this work considers: nonlinear behaviour of concrete and steel using nonlinear stress-strain models, confinement effect and geometrical nonlinearities. Tests exposed to static loading with different slenderness, different D/t ratio (where D is the total dimension of a cross section and t is the thickness of steel tube) and different loading conditions are analyzed. Stress-strain models that best approximate the behaviour of CFT columns are determined from a detailed parametric study. The proposed numerical model is validated by comparing numerical with experimental results available in the literature., U radu je prikazana nelinearna analiza kvadratnih CFT stubova, pomoću fiber konačnog elemenata raspodeljene plastičnosti. CFT stubove karakteriše nelinearno ponašanje, što je potrebno imati u vidu u numeričkom modelu. Model predložen u ovom radu uzima u obzir sledeće nelinearne uticaje: nelinearno ponašanje betona i čelika primenom odgovarajućih konstitutivnih relacija, efekat utezanja betona i geometrijske nelinearnosti. Analizirani su uzorci izloženi delovanju statičkog opterećenja koji imaju različitu vitkost, odnos D/t (odnos ukupne dimenzije čeličnog profila [D] i debljine čeličnog profila [t]), kao i uzorci izloženi različitim tipovima naprezanja. Na osnovu parametarske analize, određeni su modeli materijala za čelik i beton, kao i njihovi parametri koji najbolje aproksimiraju ponašanje CFT stubova. Tačnost modela proverena je zatim i upoređivanjem sa eksperimentalnim podacima dostupnim u literaturi.

The paper discusses two formulations for the fiber based distributed plasticity beam/column element. For force based and displacement based formulations, basic details and main differences are emphasized. The differences in results of analysis when elements with these two formulations are used are demonstrated on two numerical examples. The first example involves the nonlinear static pushover analysis of low-rise frame with distributed element loading. The second example is nonlinear time history analysis of two-story frame subjected to earthquake loading. The differences in global and local response depending on the used finite element discretization are shown. In conclusion, the advantages of force based formulation and directions for the use of the two elements are summarized., U radu su prikazane dve formulacije fiber konačnog elementa raspodeljene plastičnosti. Za metodu sila i metodu deformacija, ukazano je na osnovne razlike dve metode. Razlike u rezultatima analize koje se dobijaju primenom fiber elementa prema ove dve formulacije prikazane su putem dva numerička primera. Prvi primer obuhvata statičku pushover analizu rama s raspodeljenim opterećenjem duž greda. Drugi primer je nelinearna dinamička analiza dvospratnog rama izloženog zemljotresnom opterećenju. Prikazane su razlike kod dobijenog globalnog i lokalnog odgovora u zavisnosti od formulacije i diskretizacije rama. U zaključku, rezimirane su primene dve formulacije fiber elementa i date napomene u vezi s njihovim korišćenjem.