دانلود رایگان مقاله انگلیسی رفتار لرزه ای ستون های بتنی تقویت شده تحت میدان محدود شده - نشریه الزویر

ABSTRACT

The role of confining reinforcement in dissipating a large amount of energy in reinforced concrete (RC) elements at the time of seismic events has been well demonstrated in previous studies. In India, this practice of providing special confinement in RC elements began to be followed after the ductile detailing code came into being in the year 1993. Hence, the structures built before 1990s in seismicaly active areas had no ductile detailing, as the design recommendation of pre 90s era do not consider adequate confinement requirements. The Present experimental study is a part of an ongoing project of upgrading these old reinforced concrete structures, which had no special confinement and are located in seismically active regions. Three full-scale columns with different amount of transverse reinforcement (volumetric ratio of 1.31, 0.33, and 0.22) were tested in a specially made testing setup. One specimen was designed as per the current Indian guidelines for ductile detailing (IS 13920:2016), while the other two specimens were designed as per the guidelines which existed before the Code on ductile detailing was implemented. The response of the columns under quasi-static reverse lateral cyclic loading (at constant axial load) was recorded in terms of lateral load v/s deflection, moment v/s curvature, energy dissipation, stiffness reduction and different ductility parameters. As expected, the results of the study show inferior seismic performance of under-confined columns as indicated by the key strength and ductility parameters. The results have though quantified the strength and ductility deficit of these under confined RC columns, which shall be of great help in designing suitable retrofit for the existing structures constructed with no ductile detailing features.

CONCLUSIONS

The main aim of this study was to examine the seismic performance of under confined reinforced concrete columns designed before the ductile detailing codes were implemented. It is believed that this effort would enable designing suitable retrofit for these deficient columns. Towards this end, full scale stubbed square RC columns representing two under-confined columns and one reference well confined column were cast and tested. A large amount of response seismic data was collected to enable quantification of deficit in under-confined columns. Following conclusions can be drawn from this study:

 A maximum reduction (attained at 3.5% drift) in lateral load capacity by 13% and in moment capacity by 24% was recorded in under-confined columns as compared to reference well confined column designed according to the current code requirements. This significantly inferior response of under confined specimens despite having similar cross section indicated the adverse effects of higher tie spacing on the lateral load capacity. This observation also signifies that, inferior confinement due to larger spacing of tie allowed dilation of core concrete at higher drift levels (>2%) which ultimately resulted to inferior flexural stresses.

 A reduction in axial load carrying capacity in under-confined columns is 17% and 40% for specimens UC1 and UC2 respectively at 5% drift indicate incapability of these RC columns in maintaining the axial gravity loads during earthquake.

 Displacement ductility factor of 1.9 noted in the under confined columns was 52.5% lower than the desired value of 4 and Curvature ductility factors were 76.45% and 84.65% lower than the desired values of 20 in the deficient columns. Other key deformability and energy dissipation parameters also indicate poor performance of under confined concrete columns.

 This study highlights significantly lower values of structural performance indices in reinforced concrete columns designed according to the pre ductile detailing code era typical in many existing structures in seismically active areas of India and thus are in dire need of retrofitting to match current structural performance indices.