عملکرد لرزه ای قاب های بتن مسلح پس از آتش سوزی
Abstract
1- Introduction
2- Experimental program
3- Results and discussion of the fire experiment
4- Results and discussions of the quasi-static cyclic loading test
5- Conclusions
References
Abstract
To investigate the post-fire seismic behaviour of reinforced concrete (RC) frames with different column-to-beam bending capacity ratios, four specimens were fabricated, which included a strong-beam-weak-column frame and a strong-column-weak-beam frame either under room temperature or after being exposed to fire. The fire test was conducted in a furnace chamber, followed by quasi-static tests under a low-frequency cyclic load. The crack patterns, hysteretic loops, plastic hinges, and failure modes were investigated in the loading process. The influence of two factors, i.e., the fire exposure and the column-to-beam bending capacity ratio, on the mechanical performance, ductility, stiffness degradation, and energy dissipation was compared and analysed. The experimental results indicated that the ultimate bearing capacity, the stiffness, the ductility factor, and the energy dissipation capacity of the RC frames decreased after fire exposure. The bearing capacity of the strong-beam-weak-column frame decreased even more seriously. Although the yielding displacements of the post-fire frames increased, their ultimate displacements decreased. In addition, the strong-column-weak-beam frame under room temperature failed in the form of beam-end plastic hinging, while after being exposed to fire, the failure mode changed to shear-bond failure in column.
Introduction
Reinforced concrete (RC) frame structures constructed prior to 1980s generally do not meet the requirements of seismic design code in mainland China [1] due to the deficient design which only took gravity loads into consideration and ignored the lateral actions such as earthquake and winds. For instance, some old buildings were designed as sub-standard RC frames with strong beams but weak columns, which might result in harmful brittle failures such as joint shearing and column hinging. In addition, building fire is one of the most frequent disasters that will happen in RC buildings. Although the post-fire RC frames can generally be retrofitted using proper strengthening techniques such as enlarging of joint area with newly cast concrete [2] and fibre reinforced cementitious composites [3–5], adhesive bonding or near surface mouting [6] of fibre reinforced polymers (FRP), bonding or anchoring of steel plates [7–9], etc. they may encounter an earthquake attack in their subsequent post-fire service life thus fail and even collapse. Therefore, for these sub-standard RC frames, not only the researches on their residual bearing capacity is necessary, but also those focusing on their post-fire seismic performance is of great significance. Most existing studies mainly focused on the post-fire performance of concrete/steel materials and RC members rather than the behaviour of RC structures. Numerous investigations were conducted to study the residual mechanical properties of concrete and reinforcing steel, as well as the bond-slip of steel-concrete interface after exposure to fire. It was found that the compressive strength and elasticity modulus of concrete [10,11], the deformability, yield strength and ultimate strength of reinforcing steel [12,13], as well as the steel-concrete bond strength [14,15] would be affected significantly by high temperatures. Similarly, as regarding to studies on the residual post-fire mechanical performance of RC beams [16–18], columns [19,20] and slabs [21,22], researchers found that the bearing capacity of these RC members were deteriorated by elevated temperatures, and the deterioration increased with fire exposure time.