PSI - Issue 64

Saim Raza et al. / Procedia Structural Integrity 64 (2024) 1200–1207 Raza / Structural Integrity Procedia 00 (2024) 000 – 000

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3. Design details In the proposed concept, the formwork is supposed to carry its own self-weight, the self-weight of the cast concrete topping, especially when it is wet, and the live loads from the construction machinery. It should be noted that the dead and live loads are expected to be carried by the cast concrete once it has hardened, therefore the critical phase from the design point of view of the formwork is before cast concrete has hardened and is in the wet state. 3.1. Geometric details and design loads The feasibility of a 30 mm thick formwork with a ribbed topology was evaluated for a span length of 5m in this study. The geometric details of the formwork are shown in Fig. 2. The distance of the neutral axis from the top edges of the cross-section was 105 mm. The design was performed assuming a total slab depth (including cast concrete topping) of 210 mm for span lengths of 4 to 6 m. The ratio of the mass of formwork to the mass of cast concrete topping was 0.6 in the design. The live loads from the construction machinery were assumed to be 2.4 kPa for design based on Jha (2012). The design was performed assuming a concrete compressive strength of 100 MPa and tensile strength of 3 MPa, which is typical for the 3D printing mortar to be used for printing in this study. The tensile strength of 3 MPa for a mortar with a compressive strength of 100 MPa is lower than typical values observed for conventional mortars. This is because of the corrugated layer structure of 3DPC specimens, which lead to stress concentration in the interlayer region, resulting in tensile failure at lower strengths

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Fig. 2. Geometrical details of the 3DPC formwork designed for 4 to 6m span length

3.2. Cracking moment demand and post-tensioning loads For the given design loads, the moment demand on the formwork was determined assuming simply supported conditions. As such, with a factor of safety of 20%, the design bending moment was estimated to be 12 kNm, 19.2 kNm, and 28.8 kNm for the span lengths of 4m, 5m, and 6 m, respectively. With the design goal of avoiding the cracking of formwork under the design loads, the post-tensioning loads were calculated for various eccentricities of the tendon, as shown in Fig. 3. Note that the maximum possible eccentricity for ø15.7 tendon in this topology was 67 mm. Since the formwork geometry is very thin, therefore, the upper limit of post-tensioning loads is controlled by the buckling load capacity. The elastic buckling load was estimated to be about 2300 kN, 1500 kN, and 1000 kN for span lengths of 4m, 5m, and 6m, respectively, whereas the nonlinear buckling load via Riks analysis in Abaqus was found to be about 30% of the elastic buckling loads. Considering the limitation of the nonlinear buckling, the permissible post-tensioning loads for the span lengths of 4m, 5m, and 6 m were estimated to be 690 kN, 450 kN, and 300 kN (i.e. 30% of the elastic buckling load). This means that for a span length of 4m, no eccentricity is required for the tendon, whereas for a span length of 5m, eccentricity needs to be greater than zero, and similarly eccentricity for a span length of 6m needs to be greater than 45mm.