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Researches show that composite systems made by combining cold-formed steel (CFS) elements and concrete can produce excellent properties compared to traditional solutions. One of the solution which employs CFS elements and concrete is the subject of research within the LWT-FLOOR project. The LWT-FLOOR system consists of built-up CFS beams with corrugated web, connected by spot welds and concrete slab with an innovative shear connection. This paper presents a numerical investigation of two types of shear connections suitable for the proposed composite solution. One type of shear connection is completed using bolts which provide a direct shear connection between the flange profiles of the steel girder and the concrete slab. The second type of shear connection is achieved using a composite dowel rib connector realised through a steel beam corrugated web. In this case, the connection between the profiles of the steel beam and the concrete flange is realised indirectly, i.e., through spot welds between C profiles and corrugated web. The results of FE simulations indicate that a solution with bolts ensures higher shear, but reduces its ductility. These results provide the basis for the conduction of experimental push-out tests of the proposed shear connection types.

Composite structural systems have many benefits, such as a high degree of prefabrication, reusability and long span capability. The main advantage of such systems is manifested through the optimal utilisation of structural steel and concrete. Additionally, allowing for demountability and reusability, the system can also result in a much smaller carbon footprint at the end of life. These advantages are particularly emphasised with the implementation of cold-formed steel sections. This paper presents an innovative solution for a composite floor structure system consisting of built-up cold-formed profiles and concrete slabs made with profiled sheets. The influence of the corrugated web thickness, type of connection between steel beam elements, the degree of shear connection and steel beam height on the overall behaviour of the composite system is analysed. The performed FE analyses showed that the influence of connections between steel beam elements and the degree of shear connection had the most significant impact on system behaviour. In contrast, the steel beam height, the corrugated web thickness and spot weld density had less influence on the analysed composite system behaviour. The obtained results provide the basis for implementing laboratory research on the proposed system.

This paper investigates the bending resistance of an innovative lightweight composite floor system, LWT-FLOOR. The system consists of built-up cold- formed steel elements that are spot-welded and connected to the concrete slab using demountable shear connectors. As the system under investigation is new, the existing standards do not provide guidelines for calculating its bending resistance. This paper gives an overview of different analytical approaches and a comparison of calculated results using bending capacities from the numerical parametric study. Within the numerical parametric study, the influence of the height of the steel beam, the type and degree of shear connection, the connection between the steel elements, and the thickness of the corrugated web used for the bending capacity are investigated. Numerical results for the full shear connections resulted in lower bending capacities than the analytically calculated plastic bending resistances. However, numerically obtained bending capacities for partial shear connections were found to exceed the calculated characteristic non- linear bending resistances and bending resistances for partial shear connections. The obtained results will comprise the basis for further experimental tests, which will support the search for an optimal analytical approach for the bending resistance of the proposed composite system.

Presentation of LWT-FLOOR project activities related to implementation and analysis of the push- out test on shear connectors in composite beams cold-formed steel profiles-concrete at the 2nd LWT- FLOOR project Workshop.

Primjena spregnutih konstrukcija čelik-beton ima velike prednosti kao što su upotreba montažnih elemenata i svladavanje velikih raspona, te optimalna iskoristivost čelika i betona što se posebno očituje uporabom hladno oblikovanih čeličnih profila. Rad predstavlja numeričku parametarsku analizu inovativne spregnute međukatne konstrukcije sastavljene od hladno oblikovanih čeličnih profila i betonske ploče. Način spajanja čeličnih elemenata te stupanj posmične veze pokazali su znatan utjecaj na ponašanje spregnutog sustava LWT-FLOOR dok debljina valovitoga lima kao hrpta, broj zavara u poprečnom presjeku te razmatrane visine čeličnog presjeka nemaju značaj utjecaj za analizirani sustav.

Nowadays, composite steel-concrete systems are among the most cost-effective construction systems for application in multi-storey buildings as they combine structural efficiency and allow accelerated construction speed. Additional optimisation of these systems is possible through the application of cold- formed steel elements and innovative solutions for shear connections. Such optimisation of composite systems is the topic of the LWT-FLOOR project ongoing at the Faculty of Civil Engineering in Zagreb. This paper presents a numerical study of two various cold- formed steel beam types connected to a concrete slab placed within profiled steel sheeting and interconnected using the bolted shear connection. The first cold- formed steel beam type consists of built-up sections using back-to-back cold-formed steel C profiles, while the second type additionally uses a corrugated web inserted between the webs of the cold-formed steel C profiles. To evaluate shear connection behaviour, simulations of the push-out test are performed where either solution with two bolts per rib placed symmetrically or the solution with one bolt per rib placed in a staggered manner is used. Compared to the shear connection with two bolts per rib it is expected that staggered bolt placement would have much less influence on the shear connection strength in the case of cold- formed steel profiles without corrugated web than in the case when the corrugated web is used. Furthermore, a previous study on the behaviour of bolted shear connection in composite systems using cold-formed steel profiles indicates that the proposed values of the reduction factor, when profiled steel sheeting ribs are placed transverse to the steel beam may be too conservative or, in some cases, even completely unnecessary. Therefore, a numerical study on the composite system using cold-formed steel profiles and the corrugated web was further expanded to evaluate the influence of different parameters on the value of the reduction factor.

With the intention of increasing the advantages of composite steel-concrete systems, the cold-formed steel (CFS) profiles have been used. In this way, the weight of the whole system has been reduced. Additionally, combining two different materials, such as CFS and concrete, can avoid their inherent disadvantages. The main advantages of composite beams are: - weight savings of steel between 30-50%, compared to non-composite beams, - increased stiffness can result in the reduction of beam height, - reduction of cladding costs, - allowing more space for mechanical services. The research project Innovative lightweight cold- formed steel-concrete composite floor system - LWT-FLOOR, ongoing at the University of Zagreb, Faculty of Civil Engineering, Croatia, investigates a composite floor system that offers benefits in terms of reusability and a high degree of prefabrication. The LWT-FLOOR system is formed from built-up steel beams and cast-in-place concrete slab connected by innovative shear connection. Innovative demountable shear connection is one of the main advantages of the proposed system and is used to decrease the impact on the environment. A built-up CFS beam is composed of four cold-formed C sections connected by spot welds with corrugated web (CW) and shear plates. Due to the relatively small thickness of CW, weight reduction is allowed compared to hot-rolled profiles or welded I sections. The advantage of this type of built-up beam is that CW increases stability against local and lateral-torsional buckling and web crippling. Experimental research within the LWT-FLOOR project is divided into five phases: - experimental research of base material, - experimental research of spot welds between CFS sections, - experimental tests of shear connection, - experimental tests of full-scale LWT-FLOOR system without openings, - experimental tests of full-scale LWT-FLOOR system with openings. As an extension of experimental research, the proposed system will be evaluated through extensive numerical and probabilistic parametric studies and life-cycle analyses. This report presents results of preliminary numerical research on the LWT-FLOOR system with a different arrangement of spot welds in CFS cross- section (two or three spot welds) and variable degrees of shear connection. Numerical research was conducted using ABAQUS/CAE software. To support numerical results, analytical results were also provided. Considering that CFS sections are usually class 3 and 4, their plastic resistance is possible to achieve only in the cases with the plastic neutral axis in the concrete slab or on the contact between the concrete slab and steel section. In that case, the steel cross-section will be completely or almost completely in tension. Because of the mentioned situation, local instability will be avoided, and bending resistance can be calculated. Because of a large number of nonlinearities in the model (geometry, material, contacts), dynamic explicit analysis is used. Initial imperfections were created through static analyses of the system with fully tied elements. In this way, the imperfection shape has been achieved with required displacement. The load was set as a vertical displacement in multipoint constraints (MPC) that transfer the load onto four loading points. Used characteristics for spot welds were used from results of tensile shear test given in available literature, while wire elements were used to model the shear connectors. The presented LWT-FLOOR project integrates state- of-the-art knowledge in a new and productive spot welding technology and innovative cold-formed steel–concrete composite solutions to develop a cost-effective and competitive floor system. The LWT-FLOOR system can give vital benefits in terms of a high degree of prefabrication, reusability and suitability for larger spans to maximise the values of building components and materials. The proposed analytical solution for bending resistance of composite cross-section is based on the elastic bending resistance of steel beams. The numerical parametric FE analyses show that the bending capacity of the LWT-FLOOR beams made of cold-formed steel and the cast-in-place concrete slab can be compared with analytical results. Results of the parametric FE study show that the bending capacity of the LWT-FLOOR beams made of cold-formed steel and the cast-in-place concrete slab is affected by the resistance of the cold- formed steel components and by the degree of shear connection. In the case of a fixed beam solution, the resistance of the LWT-FLOOR beam is governed by the resistance of the steel beam, i.e., spot welds. In the cases with pinned beam solution, the results show that the resistance of the LWT-FLOOR beam is governed by the degree of shear connection. At the same time, the increased number of spot welds from two to three between channel flanges and corrugated web does not influence the beam resistance. This means that the beam resistance in the pinned solution is rather more affected by the stability of the cold-formed steel components than the number of spot welds, with a significant influence on the type and degree of shear connection. Funding: This work has been supported in part by Croatian Science Foundation under the project UIP-2020-02- 2964.

Osim povoljnog iskorištenja upotrijebljenih materijala u spregnutim sustavima, spregnuti sustavi čelik-beton poznati su po montažnoj ugradnji i sposobnosti svladavanja velikih raspona. S obzirom na navedene prednosti, mogućnost uporabe ovog tipa sustava je raznolika. Ovaj rad predstavlja numeričku analizu spregnutog nosača izvedenog od hladno oblikovanog čeličnog profila koji je povezan inovativnim demontažnim sredstvima s betonskom pojasnicom. Karakteristike osnovnog materijala, točkastih zavara te sredstava demontažnog posmičnog spoja dobivene su laboratorijskim ispitivanjima provedenim u sklopu LWT-FLOOR projekta na Građevinskom fakultetu Sveučilišta u Zagrebu. Istražen je utjecaj broja točkastih zavara u sustavu čeličnog nosača, utjecaj vrste betona te stupnja sprezanja posmičnog spoja na nosivost spregnutog nosača.

Presentation of design and fabrication of material, spot welded and push-out specimens for related activities for LWT-FLOOR project at the 2nd LWT- FLOOR project Workshop.

In this paper laboratory testing of base material and spot welds is explained. Also, results of testing are shown and will be used for further research for numerical models and parametric analyses.

Except for the main advantage of the composite systems, which is the efficient utilisation of used materials, these structural systems are known for reusability, high degree of prefabrication and long- span capability. Because of these advantages, the range of applications of composite structures is continuously expanded. Since cold- formed steel sections have many benefits compared to hot-rolled sections, their implementation in composite systems has become more attractive to researchers in recent years. This paper represents numerical research on the bending behaviour of composite cold-formed steel- concrete beams. Spot- welding technology for connecting cold-formed steel elements and innovative demountable shear connections with bolts were used as a part of the LWT-FLOOR project at the Faculty of Civil Engineering, University of Zagreb, Croatia. Cold- formed steel elements are built-up beams with back-to-back profiles as flanges and corrugated webs. The obtained results provide the basis for implementing laboratory research on the proposed system. Experimental results of tensile base materials and spot welds behaviour were used to support numerical research. Numerical analyses were conducted in Abaqus/CAE, where the influence of the degree of shear connection, spot weld density, concrete type, steel cross-section thickness and the diameter of the shear connector were analysed. The performed analyses showed that the influence of the degree of shear connection and spot weld density have a significant impact on system behaviour. The influence of concrete type is negligible when comparing the models with the same spot weld density and full shear connection. The same influence becomes greater for models with the partial shear connection. In addition, results show that the thicknesses of the corrugated web and C profile highly influence the bending resistance of the analysed system. Corrugated web thickness results in the highest flexural stiffness and bending resistance in the case of tied steel elements. Furthermore, for steel beam elements connected by spot welds, the corrugated web thickness had a more significant influence on the beam resistance. Regarding the diameter of the shear connector, it is concluded that it has a negligible influence on the bending resistance of the analysed system due to the crushing of concrete for both diameters.