## 3 Results and Discussion  
### Failure Modes and Load-Displacement Curves  
Figure 11 shows an overview of the failure modes of the specimens. The slipping between concrete core and steel layers, the buckling of above steel layers, the cracking of concrete core, and separation of the steel layer from the concrete core because of the removal of CSC shear connectors, especially from the place of plug welds, were the most crucial failure modes of the SCS system. Generally, as shown in Figure 12, the trend of the curves includes 3 stages. The first is elastic behaviour, the second is a flexural failure and finally, despite the core crushing, the load increases due to membrane action. An important point in these slabs with CSC connectors is that at the end of the flexural failure and before the membrane activity begins, the diagram drop of punching failure does not occur. This may be because of the excellent distribution of the connectors, so even after flexural failure, there is a good bonding between the core and the steel face plates.  
Figure 10: SCS sandwich slab under quasi-static load.  
One of the main disadvantages of SCS slabs with NWC core is the impossibility of using them in marine structures such as ship-hull due to high weight. So, for this reason, this study was focused on reducing the weight of the SCS slab by using LWC. In Figure 12, the load-displacement curve of the different specimens was shown.  
Given the upward trend of the curve even after the onset of membrane activity, it is assumed that the starting point of membrane activity is the ultimate strength of the specimens. According to Figure 12, the typical behaviour of the SCS with NWC core in terms of both stiffness and resistance is better than those produced with plain LWC core and LWC core reinforced by steel fibres. Additionally, the ultimate strength of the specimen with the NWC core is about 670 kN while the ultimate strength of the specimen with LWC core gets roughly two-thirds to around 410 kN; Also, the ultimate flexural strength improves to about 550 kN with using steel fibres. However, the NWC core is about 35% heavier than the LWC and SLWC cores that can be a fundamental problem for specific applications. According to Figure 12, the highest stiffness is for SCS with NWC core, and the least stiffness is for SCS with LWC core. This indicates that the presence of fibres increases both the stiffness and the maximum flexural strength compared to LWC core without steel fibres. In addition, in LWC and SLWC slabs, the connectors acted in less load because of the less compressive strength of LWC than NWC. Figure 13c shows, despite the higher bearing capacity of the SCS with SLWC core, more control over punching has been established than the LWC core. This indicates that the fibres prevent large cracks in the punching area.  
Figure 11: Deformation of the bottom and top plate.  
Figure 12: Load-displacement curves of specimens.  
Besides, the cracks were propagated by increasing the load (Figure 14). As Figure 13(a), some of the connectors in the centre of the specimens were detached due to plate slipping and the buckling of the steel face plates. Moreover, the plug weld rupture of the second end of the shear connectors and as a result, the detached CSC connectors was one of the major failure modes of the proposed system. The experiments outcomes showed that in areas where stress concentration was more considerable, the failure of the shear connectors from the plug welds was inevitable; however, this partial joint could significantly improve the shear behaviour of the CSCs compared with the one end welded CSCs studied by [PERSON] and [PERSON] [30; 31]. On the other hand, in the specimen with steel fibres deformation less occurred and the buckling was less in the bottom plate than SCSLWC system (Figure 13(b),c) while the buckling occurred in the top plate by increasing load in all specimens.