# Numerical Study on the Bending Performance of Steel-Ribbed Composite Slabs for Substations
## 2 Design Scheme of Steel-Ribbed Composite Slabs
### Design Parameters of Steel-Ribbed Composite Slabs  
The steel-ribbed composite slabs feature higher stiffness and bearing capacity compared with other slab systems such as composite slabs with steel-bar trusses. The composite slab consists of the precast base slab with inverted T-shaped ribs and post-cast concrete, as shown in Figure 2. The inverted T-shaped rib has a concrete upper flange and a thin steel web. The design parameters of the steel-ribbed composite slab are described as follows. Figure 1: Daocheng (Huangbuling) 500 kV substation. (**a**) Aerial view; (**b**) Layout plan for the 8.800 m floor (mm). Figure 2: Illustration of the steel-ribbed composite slab (mm). According to the Technical Specification for Prestressed Concrete Composite Slab with Steel Ribs (T/CECS 1289-2023) [22], the specified strength grades of the concrete for the precast base slab and the post-cast concrete layer should not be lower than C40 and C30, respectively. For the steel, the specified yield strength should not be lower than 235 MPa. Given that the design load for substations is high, the precast base slab and post-cast concrete layer both utilize C40 concrete, which has a cubic compressive strength of 40 MPa for a specimen measuring 150 mm \(\times\) 150 mm \(\times\) 150 mm. The steel ribs in the precast base slab are Z-shaped, with a thickness of 1 mm and a height of 67 mm, as depicted in Figure 2. The yield strength of the steel ribs is 235 MPa. Circular holes, spaced 200 mm apart, are arranged in the middle of the steel ribs. The upper end of each steel rib is embedded within a concrete flange that has a width of 150 mm and a thickness of 40 mm, while the lower end is anchored in the precast concrete slab. Prestressing steel (labeled as 1 in Figure 3), composed of A5H 1570-grade stress-relieved spiral ribbed steel, is placed longitudinally in the precast concrete slab, aligned with the direction of the steel ribs. The controlling stress of the prestressing steel is 1100 MPa, 70 percent of the ultimate strength 1570 MPa. Steel bars of diameter C8, spaced at 150 mm (labeled as 3 in Figure 3), are oriented perpendicular to the direction of prestressing, possessing a yield strength of 400 MPa. At the ends of the slab, the arrangement of steel bars is intensified, labeled as 2 in Figure 3. Longitudinal steel rebars of 2C6 (labeled as 4 in Figure 3) and transvers steel rebars of C6 spaced at 375 mm (labeled as 5 in Figure 3) are placed in the concrete flange of steel ribs. The slanted cut at the edge of the precast base slab, aligned with the direction of the parallel prestressing, measures 20 mm \(\times\) 20 mm. The thickness of the precast base slab is 40 mm, increasing to 110 mm when including the ribs. The total thickness of the slab after composition is 130 mm. To investigate the impact of joint positioning following dense splicing under bidirectional loading, two sizes of precast base slabs--designated as A and B--have been developed, as shown in Figure 3. The size of slab A is 1650 mm \(\times\) 2500 mm, with three steel ribs and 22 prestressing steels (ratio of 0.201%), and slab B measures 1100 mm \(\times\) 2500 mm, with two steel ribs and 14 prestressing steels (ratio of 0.192%). The center distance between adjacent steel ribs in both slabs is 550 mm. The manufacture cost of the steel-ribbed composite slab is slightly higher than that of other slab systems due to the consumption of more steel. However, the scheme is still cost-effective, benefitted by dense splicing in the prefabricated construction, which reduces the costs of formwork, support, and labor for wet joints on site. For bidirectional slabs after dense splicing, the position and number of splicing joints are critical. Two classical splicing configurations, i.e., double-spliced and triple-spliced, are studied here and compared with the cast-in-place monolithic slab, as shown in Figure 4. The dimensions of the bidirectional composite slabs are 3300 mm \(\times\) 2500 mm.