GFRP 보강 콘크리트 보의 한계상태별 지배 메커니즘에 관한 정량적 평가

초록

This study systematically evaluates the sectional efficiency and limit-state governing mechanisms of glass fiber reinforced polymer (GFRP)-reinforced concrete beams in accordance with the Korean structural design code KDS 14 20 68. To quantitatively assess the serviceability limit state (SLS) constraints, specifically deflection and crack control, associated with the low elastic modulus of GFRP bars, a comprehensive parametric analysis was conducted involving 243,750 design combinations. Sectional design and forward-search optimization were performed across various material, geometric, and reinforcement parameters. Based on the Design Governance Factor (DGF = nSLS/nULS), defined as the ratio of the number of bars required to satisfy the SLS criteria to that required for the ultimate limit state (ULS), the overall design feasibility rate was 72.3% (176,245 cases), comprising 95,557 single-layer (39.2%) and 80,688 double-layer (33.1%) placement cases. At the minimum code-required elastic modulus (Ef = 45 GPa), the mean DGF was 1.3, and 43.1% of all combinations (48.9% of feasible cases) were governed by the SLS criteria. This result indicates that current design practices require an average of 30% more reinforcement than that solely required to satisfy ULS flexural strength requirements. Increasing the GFRP bar diameter from G13 to G25 reduced the required number of bars from 8.9 to 5.0, increased the mean clear spacing from 24.6 to 59.5 mm, and increased the single-layer placement rate from 7.1% to 89.4%, thereby effectively resolving reinforcement congestion. Furthermore, increasing Ef from 45 GPa to 60–65 GPa reduced the proportion of designs governed by SLS criteria to 9.0%, yielded reinforcement volume savings of up to 11.7%, and rendered the mean DGF to ideally converge to 1.0. Increasing the concrete compressive strength further reduced long-term deflection by 15.1% and lowered the mean DGF to 1.19. These findings provide quantitative engineering evidence to support the development of elastic modulus grading and structural optimization guidelines for GFRP-reinforced concrete beams within the Korean structural design code framework.

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