Bar buckling in ductile reinforced concrete walls : causes, consequences and control. (2020)
Type of ContentElectronic Thesis or Dissertation
Thesis DisciplineEarthquake Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsTripathi, Mayankshow all
Flexurally-dominated reinforced concrete (RC) walls are widely used as an effective lateral load- resisting system for building systems located in seismically active regions. Observations from past earthquakes in Chile (2010) and New Zealand (2010-11) have demonstrated that slender RC walls can be susceptible to brittle compression-dominated failure, rather than the desired ductile tension- governed response. Reconnaissance studies following these earthquakes reported that buckling of longitudinal reinforcing bars was a commonly observed failure mode that restricted the deformation capacity of RC walls. In addition to buckling, fracture of buckled reinforcing bars was also observed. In the current methodology for the design of anti-buckling transverse reinforcement, international design standards emphasise only on the spacing of transverse reinforcement as a critical parameter to restrict bar buckling in RC structures, presuming that the area of transverse reinforcement determined for the concrete confinement is adequate to restrict buckling. Furthermore, the probability of reinforcing bars to buckle while unloading from a large inelastic tensile strain is not incorporated in the design framework. Therefore, this thesis aims to address these key issues pertaining to bar buckling in ductile RC walls.
Bar buckling causes reduction in the compressive stress carried by reinforcing bars. Therefore, to better understand and quantify the ill-effects of buckling on the hysteretic behaviour of reinforcing bars, monotonic and cyclic tests on reinforcing bars with different yield strengths and slenderness ratios are conducted in this study. In addition to bar buckling, fracturing of buckled reinforcing bars also adversely affects the seismic performance of RC structures. Therefore, the effect of buckling on the low-cycle fatigue life of reinforcing bars is also quantified by conducting fatigue tests on bars with different yield strengths and slenderness ratios. The fatigue test data is utilised to develop fatigue life models relating the total-strain amplitude and total-dissipated energy with the fatigue life of reinforcing bars as a function of buckling parameter. Additionally, the experimental test data from monotonic, cyclic and fatigue tests on reinforcing bars are used to develop a path-dependent cyclic stress-strain model that incorporates the effect of bar buckling and low-cycle fatigue.
Next, the inadequacy of current design procedures to restrict bar buckling in the boundary elements of slender RC walls is evaluated by conducting tests on three large-scale code-compliant rectangular walls with different boundary zone transverse reinforcement detailing. Two of the test specimens satisfied the anti-buckling transverse reinforcement requirement prescribed in NZS3101:2006, while in the third test specimen the arrangement of transverse reinforcement was modified to limit the buckling of boundary zone longitudinal reinforcing bars to single tie spacing. The test results reaffirm the susceptibility of RC walls designed according to the current New Zealand Concrete Standard to failure associated with bar buckling. In addition, the results from the test on the wall specimen with improved detailing indicate that bar buckling can be limited to single tie spacing by designing transverse reinforcement using a mechanics-based approach. The overall hysteretic response of RC walls can be improved by enhancing the local-compression response of wall boundaries. Therefore, in this research experimental and numerical studies are also carried out to scrutinise the effect of concrete compressive strength on the seismic response of slender RC walls.
The performance of slender RC walls under seismic excitation depends on the efficacy of the wall boundary elements to resist axial strain demands. Therefore, axial tests on rectangular prisms idealised as the boundary elements of slender RC walls are carried out to investigate the effect of key parameters on performance of wall boundaries. The key parameters identified in this study include: (i) loading history; (ii) transverse reinforcement detailing (arrangement, spacing and diameter); (iii) longitudinal reinforcement detailing (yield strength and area); and (iv) cover concrete. A comparative evaluation of the test results is carried out and the effect of the identified parameters on the performance of prism specimens is scrutinised. Additionally, the reliability of testing RC prisms to simulate the failure mechanisms in wall boundaries is also assessed by testing prism specimens identical to the boundary elements of the tested RC walls under axial strain histories measured at the wall boundaries. The wall and prism responses are then compared to evaluate the limitations of testing prism specimens to simulate failure mechanisms in wall boundaries.
Finally, the results from the tests on reinforcing bars, RC walls and RC prisms are consolidated and guidelines are proposed for the design of anti-buckling transverse reinforcement. The proposed design procedure recognises the importance of explicitly considering the tie leg stiffness on the buckling-resisting capacity of transverse reinforcement. Additionally, recommendations are proposed to limit the spacing of transverse reinforcement such that the compressive stress deterioration in longitudinal reinforcing bars is controlled until the limiting curvature prescribed in the NZS3101:2006 is achieved.