# Flexural Strength and Stiffness of Donut-Type Voided Slab
## 5 Conclusions  
In this study, voided slabs with donut-type void shapers were developed based on an analytical study in order to effectively reduce the self-weight of slabs. In addition, the flexural capacities of the donut-type voided slabs were investigated by the hole of the void shaper and the fixing method. Finally, the feasibility of applying the current design code [14] to donut-type voided slabs was examined.  
1. In terms of strength, donut-type voided slabs demonstrated higher ultimate flexural strength than the theoretical value calculated, in accordance with the current design code [14]. The test results showed the ultimate strength of 'OF-V-S-D' and 'OF-V-D-M' were 98% and 105% compared with 'Solid', and it is superior to the present voided slab systems. Therefore, donut-type voided slabs can be designed based on the nominal flexural strength prescribed in the current design code [14].
2. In terms of ductility, the ductility of the donut-type voided slab was similar to that of the solid and superior to the present voided slabs. In addition, donut-type voided specimens were secure enough, with a displacement-ductility ratio of more than four, to perform as a general flexural member. Therefore, the donut-type voided slab can be applied as a flexural member in place of the conventional heavy solid slab.
3. In terms of stiffness, the flexural stiffness of 'OF-V-S-D' and 'OF-V-M-D' showed 8.7% and 7.7% deterioration in flexural stiffness compared to that of 'Solid' between the initial cracking and yield point, respectively. However, donut-type voided slab specimens are similar or superior to 'OF-V-S-R' and present voided slabs. The difference in stiffness was caused by the reduction of the cross-section area by voids and the center hole of the void shaper. To take into consideration these factors, it is suggested that the minimum section should be used for the calculation of the effective moment of inertia.
4. The test results showed the effective moment of inertia at the yield point was lower than the theoretical value. According to previous research [16], the current design code overestimates the flexural stiffness of the members having low reinforcement ratios, such as slabs. In addition, [PERSON] et al. [9] argued that the reduction in the flexural stiffness of voided slabs could be attributed to the decrease in the bond strength of rebar due to voids. Therefore, more studies should be conducted to evaluate the serviceability of the donut-type voided slabs and estimate their deflection.  
Figure 15: Comparing between theoretical and experimental stiffness based on section properties: (**a**) Solid; (**b**) OF-V-S-D; (**c**) OF-V-M-D; (**d**) OF-V-S-R.  
**Author Contributions:** Original draft preparation and editing, J.-H.C. and H.-K.C.; planning the test program, H.-S.J. and H.-K.C.; performing the tests and investigation, H.-S.J.; analyzing the results and reviewing the article, H.-K.C. and J.-H.C.; and supervision and review writing, H.-K.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant founded by the Ministry of LAND, Infrastructure and Transport (22 CTAP-C164325-02).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data are contained within the article.  
**Conflicts of Interest:** The authors declare no conflict of interest.