The Fire Resistant Performance of RC Column with Confined Lateral Reinforcement According to Fire Exposure Condition

횡방향 철근으로 구속된 철근콘크리트 기둥의 화재 노출조건에 따른 내화성능

Abstract

When reinforced concrete structures are exposed to fire, their mechanical properties such as compressive strength, elasticity coefficient and rebar yield strength, are degraded. Therefore, the structure's damage assessment is essential in determining whether to dismantle or augment the structure after a fire. In this study, the confinement effect of lateral reinforcement of RC column according to the numbers of fire exposure face and stirrup was verified by fire resistant test with the heating temperatures of $400^{\circ}C$, $600^{\circ}C$ and $800^{\circ}C$. The test results showed that the peak stress decreases and peak strain increases as the temperature is getting higher, also transverse ties are helpful in improving the compressive resistance of concrete subjected to high temperature. Based on the results of this study, the residual stress of confined concrete under thermal damage is higher at the condition of more lateral reinforcement ratio and less fire exposure faces. The decreasing ratio of elastic modulus of more confined and less exposure faces from the relationship of load and displacement was also smaller than that of opposite conditions.

이 연구에서는, 기둥 횡방향 철근의 화재 후 잔존 역학적 특성 규명 연구의 일환으로, 횡철근비와 기둥의 고온 노출면 수 차이에 따른 횡방향 철근의 손실강도 보상효과를 잔존 압축강도, 변형률 및 탄성계수와 하중-변위 곡선의 상대적 비교분석을 통해 규명하였다. 실험변수로 띠철근의 간격과 고온 노출면 수를 변수로 한 실험체를 제작하여 가열실험을 수행하였다. 이때 전기로 온도를 $400^{\circ}C$, $600^{\circ}C$ 및 $800^{\circ}C$로 설정하여 $13.33^{\circ}C$/분의 속도로 가열하고 2시간동안 그 온도를 유지시켰다. 냉각된 실험체에 대해 응력-변형률 곡선을 구하기 위한 압축실험을 수행하고, 이로부터 탄성계수, 잔존 내력 및 변형률 등의 잔존 역학적 특성을 분석하였다. 실험결과, 고온 노출 면이 많은 기둥이 수열온도 증가에 따라 탄성계수 감소율이 크게 나타나고, 횡철근비가 크면 수열온도 증가에 따라 탄성계수 감소율이 작게 나타났는데, 이로부터 기둥위치와 횡철근비 등이 내화설계나 화재 안전진단 시 고려되어 합리적인 내화성능 평가가 이루어져야 할 것으로 여겨진다.

Fig. 1. Confinement of concrete by lateral reinforcement

Fig. 2. Behavior of column with lateral reinforcement

Fig. 3. Geometry of test specimens(unit : mm)

Fig. 4. Temperature Cycle

Fig. 5. The furnace and test setup

Fig. 6. Compression test

Fig. 7. Temperatures of specimen at 800℃

Fig. 8. Normalized peak stress versus temperature(ST5 type)

Fig. 9. Normalized peak stress versus temperature(ST9 type)

Fig. 10. Comparison of residual stress between ST5 and ST9 type specimen

Fig. 11. Comparison of strains between ST5 and ST9 type specimen

Fig. 12. Elastic modulus of high temperatures compared with E of 20℃(ST 5 TYPE)

Fig. 13. Elastic modulus of high temperatures compared with E of20℃(ST 9 TYPE)

Fig. 14. Elastic modulus due to heating faces(ST5)

Fig. 15. Elastic modulus due to heating faces(ST9)

Fig. 16. Load-displacement curves depending on different heating faces with ST5 type specimen

Fig. 17. Load-displacement curves depending on different heating faces with ST9 type specimen

Table 1. The concrete mixture proportions

Table 2. Test results on specimens

Table 3. Percent residual peak stress after thermal cycle

Table 4. Difference of residual stresses between ST9 and ST5 type(%)

Table 5. Strains by difference of heating faces and stirrups at 10MPa of stress

Table 6. Remaining elastic modulus resulting from a thermal cycle

Table 7. Variation in elastic modulus due to heating faces