Modeling and investigation of elasto-plastic behavior of steel–concrete composite frame systems

This paper presents a mixed finite-element model combining the fibered beam and layered shell elements using the general finite-element program MSC.MARC (2005r2) based on the discussion and comparison of the previous models. The proposed modeling procedure is intended for integrated elasto-plastic analysis of fully connected steel–concrete composite frames subjected to the combined action of gravity and monotonic lateral loads. The model is verified by extensive experiments and examples, and the behavior of composite frames is also investigated intensively. The slab space composite effect and the beam–column semi-rigidity are the two critical factors influencing the structural behavior. These two factors have not been considered simultaneously in some previous models but can be both included in the proposed model. Due to the complex slab space composite effect in composite frames, the previous models with a constant-width effective flange of slab can not trace the actual nonlinear slab behavior, but the proposed model can give accurate results. Since the slip effect between the steel beam and RC slab has negligible influence on the global calculation results of the structural system verified by the calculation examples, the slip effect is neglected in the present study to simplify the modeling procedure and enhance the calculating efficiency. The proposed model possesses good accuracy and broad applicability with simple modeling procedure and high calculation efficiency, and has a great advantage for the large-scale integrated analysis of multistory and high-rise composite frames.     

Fatigue performance and stiffness variation of stud connectors in steel–concrete–steel sandwich systems

In this study, a series of fatigue tests on six nominally identical push-shear specimens is conducted. The test specimens were subjected to an initial quasi-static test, up to a predefined maximum load, followed by a fatigue test to failure. For all the fatigue tests the mean applied load was the same while the load range varied to induce fatigue failure. The push-shear fatigue tests indicated that stiffness of the shear connections is gradually decreased during the test. Overall, the test results revealed that the lifetime of steel–concrete–steel sandwich systems under cycling loads could be predicted beforehand through the evaluation of the stiffness reduction in shear connections.     

Impact tests on steel–concrete–steel sandwich beams with lightweight concrete core

This paper studies the impact performance of Steel–Concrete–Steel (SCS) sandwich beams consisting of a lightweight concrete core sandwiched between two face plates that are connected by J-hook connectors. Impact tests were carried out by dropping free weights on to sandwich beams to investigate their structural response against impact loads. Test results revealed that the proposed J-hook connectors provide an effective means to interlock the top and bottom steel face plates, preventing them from separation during impact. The use of fibres in concrete core and J-hook connectors for composite action enhances the overall structural integrity of the sandwich beams when compared with those without such enhancement. An elastic–plastic analysis method is developed to predict the force-indentation relationship of sandwich sections subjected to local impact. Dynamic analysis based on the local force-indentation relationship is carried out to predict the impact force and global response behavior of the sandwich beams. The predicted results are compared with those obtained from the tests to validate their accuracy so that they can be used to evaluate the performance of sandwich beams under low velocity hard impact.     

Ultimate load behaviour of webs in steel–concrete composite plate girders

The paper is concerned with the tension field action in webs of steel–concrete composite plate girders. A three-dimensional finite element model has been used to carry out nonlinear analyses on composite plate girders. The results obtained from the finite element analyses are compared with those from experiments. It is observed from the comparative study that the proposed nonlinear finite element model is capable of predicting the ultimate load behaviour of steel–concrete composite plate girders to an acceptable accuracy. Results are presented to explain the development of the tension field action in the webs and to illustrate a measure of the contribution by the concrete slab acting compositely with the girder to the changes in tension field compared to a plain steel girder.     

Simplified nonlinear simulation of steel–concrete composite beams

A computationally efficient macro-model approach is proposed for investigating the nonlinear response of steel–concrete composite beams. The methodology accounts for material nonlinearity and interface slip between the concrete slab and the steel beam. The validity of the technique is evaluated through comparison of the macro-model-based simulations with results obtained from experimental testing of composite beams. Four full scale composite beams are tested under monotonic positive and negative bending. The results show that the proposed macro-element-model can capture the essential characteristics of the nonlinear load–deformation response of composite beams. Such an approach is a compromise between simplicity and accuracy and a viable alternative to detailed finite elements analysis. Additionally, a parametric study, including the compressive strength of slab concrete, the yield strength of the steel flanges and web, and the shear connection degree, of the steel–concrete composite beams subjected to positive moment is conducted utilizing the numerical macro-model proposed. The slips and their influences on the behaviors of composite beams during loading process have been analyzed.     

Lightweight steel–concrete–steel sandwich system with J-hook connectors

This paper investigates a new concept for designing composite structures comprising a lightweight concrete core sandwiched in between two steel plates which are interconnected by J-hook connectors. Specifically, lightweight concrete (density less than 1450 kg/m³) and novel J-hook connectors have been developed for this purpose. The hook connectors are capable of resisting tension and shear, and their uses are not restricted by the core thickness. Push-out tests confirms that the shear transfer capability of J-hook connector is superior to the conventional headed stud connector in achieving composite action between steel plate and concrete core. Twelve sandwich beam specimens have been tested to evaluate the flexural and shear performance subjected to static point load. Parameters investigated include degree of partial composite, concrete with and without fibres and concrete strength. Using Eurocodes as a basis of design, theoretical model is developed to predict the flexural and shear capacity considering partial composite and enable construction of sandwich structures with J-hook connectors. Compared with test results, the predicted capacity is generally conservative if brittle failure of connectors can be avoided. Test evidence also shows that inclusion of 1% volume fraction of fibres in the concrete core significantly increases the beam flexural capacity as well as its post-peak ductility.     

Structural fire performance of earthquake-resistant composite steel–concrete frames

Seismic and fire design of a building structure may be two very demanding tasks, especially if included in a performance based design philosophy. For the time being, the necessary harmonization on the regulations concerning these two design fields is almost missing, thus preventing the effective possibility of an integrated design. Besides, while many countries have already moved towards the use of performance-based codes for seismic design, the application of such methodologies for the fire design of structures is still limited in scope. Within this framework, the development of suitable procedures introducing structural fire performance issues for a comprehensive design methodology is needed.In this paper, a numerical investigation for the assessment of the structural fire performance of earthquake resistant composite steel–concrete frames is presented. With reference to a case study defined in the framework of a European Research Project, a great effort was devoted to the identification of the key structural parameters allowing for a possible correlation between the predictable performances under seismic and fire loadings, when these two are considered as independent actions.At the conceptual design level, the most suitable structural solution with respect to both design actions was chosen, including composite beams and circular steel concrete-filled columns. The frame was designed in order to resist severe seismic action according to the ductile design approach provided by Eurocode 8; the parameters affecting members’ sizing were outlined in this phase. Afterwards, the seismic performance of the designed frame was investigated by means of non-linear static analyses; once the seismic performance objectives were met, in order to evaluate the structural fire performance of the whole frame a set of criteria was defined. To this purpose, thermo-mechanical analyses under different boundary conditions were developed and in order to identify the possible mechanisms leading to structural failure, the state of stress at the critical cross-sections at different times of fire exposure was investigated. Another point of main concern was represented by the assessment of the influence of different restraining conditions on the achieved fire resistance rating and kind of structural failure.Moreover, the proposed methodology allowed making an estimate of the amount of axial restraint provided to the heated beams by the surrounding structure; in this view, the importance of choosing column elements in function of their flexural stiffness was revealed, in order to correlate it with the predictable performances under both seismic and fire loadings.     

Flexural strength of steel–concrete composite beams reinforced with a prestressed CFRP plate

Experimental studies have reported that externally-bonded carbon fibre reinforced polymer (CFRP) plate can effectively improve the stiffness and strength of steel–concrete composite beams. This paper presents an analytical solution developed to calculate the flexural strength of strengthened composite beams. The solution assumes certain failure modes and varies the locations of the neutral axis. Non-linear finite element (FE) method was also used to calculate the flexural strength of the strengthened composite beams. Experimental results from literature were employed to validate both the analytical and the FE results. The findings show that the FE analyses are in good agreement with the test data in load–deformation curves. The flexural capacity obtained from the closed-form solution and the FE analyses have a reasonably overall agreement with the experimental results, which demonstrates the present closed-form solution is simple yet accurate. The analyses also show the flexural strength is not influenced by the permanent load and the prestressing force when failure results from rupture of the CFRP plate, but the flexural strength reduces with the permanent load and increases with the prestressing force when failure results from crushing of concrete.     

Tests on cyclic performance of FRP–concrete–steel double-skin tubular columns

Eight FRP–concrete–steel double-skin tubular columns were tested under constant axial load and cyclically increasing flexural loading. The main parameters in the tests are axial load level and number of fibre reinforced polymer (FRP) layers. The influence of those parameters on the strength, ductility, stiffness, and energy dissipation was investigated. It was found that, in general, FRP–concrete–steel double-skin tubular columns exhibit high levels of energy dissipation prior to the rupture of the longitudinal FRP, but experience a sudden drop in the lateral load capacity after that. The ductility of the specimens can be improved to some extent due to the existence of the axial compressive load in current tests.     

Behavior of steel fiber–reinforced high-strength concrete at medium strain rate

Impact compression experiments for the steel fiber–reinforced high-strength concrete (SFRHSC) at medium strain rate were conducted using the split Hopkinson press bar (SHPB) testing method. The volume fractions of steel fibers of SFRHSC were between 0 and 3%. The experimental results showed that, when the strain rate increased from threshold value to 90 s^-1, the maximum stress of SFRHSC increased about 30%, the elastic modulus of SFRHSC increased about 50%, and the increase in the peak strain of SFRHSC was 2-3 times of that in the matrix specimen. The strength and toughness of the matrix were improved remarkably because of the superposition effect of the aggregate high-strength matrix and steel fiber high-strength matrix. As a result, under impact loading, cracks developed in the SFRHSC specimen, but the overall shape of the specimen remained virtually unchanged. However, under similar impact loading, the matrix specimens were almost broken into small pieces.     

Practical nonlinear analysis of steel–concrete composite frames using fiber–hinge method

A fiber–hinge beam–column element considering geometric and material nonlinearities is proposed for modeling steel–concrete composite structures. The second-order effects are taken into account in deriving the formulation of the element by the use of the stability functions. To simulate the inelastic behavior based on the concentrated plasticity approximation, the proposed element is divided into two end fiber–hinge segments and an interior elastic segment. The static condensation method is applied so that the element comprising of three segments is treated as one general element with twelve degrees of freedom. The mid-length cross-section of the end fiber segment is divided into many fibers of which the uniaxial material stress–strain relationship is monitored during analysis process. The proposed procedure is verified for accuracy and efficiency through comparisons to the results obtained by the ABAQUS structural analysis program and established results available from the literature and tests through a variety of numerical examples. The proposed procedure proves to be a reliable and efficient tool for daily use in engineering design of steel and steel–concrete composite structures.     

Dynamical identification and modelling of steel–concrete composite high-speed railway bridges

In the last few years, the construction of new high-speed (HS) railways across Europe, as well as in many other countries, has required many different bridges and viaducts. Together with classical concrete solutions, new steel–concrete composite typologies have been developed, giving light and cheap structures. Despite these studies and applications, some concerns still remain about the definition of reliable models for the evaluation of their actual dynamical behaviour under HS train passage. In particular, the influence of many structural and non-structural components, such as cross-girders and ballast, are still not well recognised. In this paper, open problems related to the dynamical assessment and modelling of new steel–concrete four-parallel-girder and box-girder solutions are exposed and analysed. A suitable procedure, based on operational modal analysis, model updating and train–bridge interaction analysis is applied to two bridges, recently built in the new Italian HS network, in order to assess and verify their dynamic behaviour under operative conditions.     

An isogeometric approach for the analysis of composite steel–concrete beams

An isogeometric approach based on non-uniform rational B-spline (NURBS) basis functions is presented for the analysis of composite steel–concrete beams. A refined high-order theory is considered in deriving the governing equations using the principle of virtual work. The employed theory satisfies all the kinematic and stress continuity conditions at the layer interfaces and considers effects of the transverse normal stress and transverse flexibility. The global displacement components, described by polynomial or combinations of polynomial and exponential expressions, are superposed on local ones chosen based on the layerwise concepts. The present isogeometric formulation does not need incorporating any shear correction factor. Moreover, in the present isogeometric formulation, the number of unknowns is independent of the number of layers. The proposed isogeometric formulation is validated by comparing the present results with the available published and the three-dimensional (3D) finite element results. In addition to correctly predicting the distribution of all stress components of the composite steel–concrete beams, the proposed formulation is computationally very economic.     

A multidisciplinary approach for fatigue assessment of a steel–concrete high-speed railway bridge on Sesia river

Steel–concrete composite bridge solutions have been more and more exploited in the new high-speed (HS) lines of European railway networks. New design solutions, introduced during a period of quick expansion for railway networks, amplified open problems related to dynamic effects, train–bridge interaction phenomena, fatigue loadings, structural modelling, fatigue life and comfort. In this article, results obtained by long-term dynamic monitoring of Sesia viaduct, a medium span double-box composite bridge of the new Italian HS network, are described and analysed. Structural modal properties were determined in order to evaluate the real-time dynamic behaviour and its correlation with environmental conditions. A suitable numerical procedure was then implemented in order to identify typology, length and velocity of trains crossing the bridge, to evaluate the intensity of deck vertical accelerations as a function of train speed and to obtain a reliable evaluation of real traffic spectra. A final fatigue assessment on welded connections was executed evaluating fatigue spectra by the aforementioned real traffic spectra and assuming S–N curves obtained by suitably executed experimental tests.     

Recycling of the product of thermal inertization of cement–asbestos for the production of concrete

A novel field of research in materials science is the recycling of secondary raw materials for construction and building materials such as concrete. This paper describes the successful recycling of as much as 20 wt% of the product of thermal transformation of cement–asbestos for the formulation of concrete. The main mineralogical phases present in the product of transformation of cement–asbestos are C2S, ferrite, and Al-, Ca-, Mg-rich silicates such as akermanite (ideally Ca₂MgSi₂O₇) and merwinite (ideally Ca₃Mg₂Si₂O₈). The behavior of this secondary raw material, termed KRY·AS, in commercial concrete was investigated using five different mixtures in which various portions (0, 5, 10, 15 and 20 wt%) of cement were substituted by KRY·AS. The results of preliminary technological tests (slump test, compressive strength, flexural strength after 28 days, and depth of penetration of water under pressure after 28 days) were discussed and interpreted with the aid of chemical, mineralogical and SEM analyses.One of the major results is that after 28 days, although all the concrete samples are invariably classified as “ordinary concrete” according to the UNI 6132 tests, those diluted with KRY·AS display a lower resistance to compression with respect to the standard. On the other hand, they recover compressive strength and display values identical to that of the standard after 90 days. The addition of the secondary raw material has the effect to slow down the kinetics of setting/hardening because the main cement phase present in KRY·AS is C2S which has a slower rate of hydration with respect to C3S.     

Structural performance of lightweight steel-foamed concrete–steel composite walling system under compression

This paper presents the results of an experimental and analytical investigation on the structural behaviour of a composite panel system consisting of two outer skins of profiled thin-walled steel plates with lightweight foamed concrete (LFC) core under axial compression. The gross dimensions of the test specimens were 400 mm×400 mm×100 mm. A total of 12 tests were carried out, composed of two duplicates of 6 variants which were distinguished by two steel sheeting thicknesses (0.4 mm and 0.8 mm) and three edge conditions of the sheeting. The density of LFC was 1000 kg/m³. Experimental results include failure modes, maximum loads and load-vertical strain responses. In analysis, full bond between the steel sheets and the concrete core was assumed and the LFC was considered effective in restraining inward buckling of the steel sheets. Using the effective width method for the steel sheets, the load carrying capacities of the test specimens were calculated and compared with the experimental results. It was found that a combination of the Uy and Bradford plate local buckling coefficients with the Liang and Uy effective width formulation produced calculation results in good agreement with the experimental results. Finally, a feasibility study was undertaken to demonstrate the applicability and limit of this new composite walling system in low rise construction.     

Flexural behavior of strengthened steel–concrete composite beams by various plating methods

The effect of pre-intermediate separation on the flexural behavior of strengthened steel–concrete composite beams by either adhesively bonded carbon fiber reinforced polymers (CFRP) sheet or welded/bonded steel plate was studied. In the case of strengthened by CFRP sheet, two different attachment patterns, namely, CFRP sheet wrapped around the flange of the I-beam and CFRP sheet wrapped around the flange along with a part of the web, were examined by testing four different strengthened steel–concrete composite beams under four point bending (4PB). Two of these beams were strengthened by fully bonded CFRP sheet with the two different patterns, while, the others are similar but have pre-intermediate debonding area of 50 mm length × flange width at the bottom surface of the lower flange. In the case of strengthened by steel plate, three different attachment patterns of steel plate to the soffit of the beams, namely, discontinuously welded, end welded, and bonded/welded steel plates, were also tested under 4PB.The experimental results showed that, there is no growth of the intermediate debonding before the yield of the lower flange occurred for all strengthened beams by CFRP sheet. After yielding, the beams with pre-debonding area showed lower flexural capacity than those with fully bonding due to the rapid growth of the intermediate debonding. On the other hand, there is a difference in the yield load between the three different patterns of the welded steel plates with a marginal difference in the elastic stiffness.     

Numerical investigation of inelastic buckling of steel–concrete composite beams prestressed with external tendons

The ultimate resistance of a continuous composite beam is governed by either distortional lateral buckling or local buckling, or an interactive mode of the two which is sharply different from the torsional buckling mode in a bare steel beam. A finite element model is developed and based on the proposed FE model, inelastic finite element analysis of composite beams in negative bending is investigated, considering the initial geometric imperfection and the residual stress patterns and the FE results are found agree well with the test results. Parametrical analysis is carried out on the prestressed composite beams with external tendons in negative bending. Factors that influence load carrying performance and buckling moment resistance of prestressed composite beams are analyzed, such as initial geometric imperfection, residual stress in steel beams, force ratio, which is defined as the extent of prestressing force and negative reinforcement in the beams, as well as the slenderness ratios of web, flange, and beams. By varying cross-section parameters, 25 groups of composite beams under negative uniform bending with initial geometric imperfection, residual stress as well as different force ratios, 200 beams in total are studied by means of the FE method. The computed buckling moment ratios are drawn against the modified slenderness proposed by the authors and compared with the Chinese Codified steel column design curve. It is demonstrated that the tentative design method based on the Chinese Codified design curve can be used in assessment of buckling strength of composite beams in a term of the modified slenderness defined.