Seismic Deficient RC Structure Collapse Safety Margin And FRP Retrofit

Recently, strong earthquake makes its frequent appearances around the world, which could impose severe destruction on building structures and induce massive causalities and property loss. In China, a country consistently visited by earthquakes, the large quantity of reinforced concrete (RC) frame structures in its urban areas are the main earthquake sufferers. Therein, low ductile RC structures, either designed in accordance with obsolete codes issued before 1970s or resulted from the corrosion of the seaboard environment with Chlorides, tend to experience a severe damage, sometimes even a collapse. Therefore, an urgency pops up to retrofit such building structures so as to elevate their seismic capacity. On the other hand, with its favorable characteristics, e.g. high specific strength and specific modulus, excellent corrosion-resistance and durability, and ease to apply, fiber reinforced polymer (FRP) won its wide application in the field of structural seismic retrofit. As a light associated with structural anti-collapse capacity, that is, the principle goal of performance based seismic design, collapse margin is herein taken an instructive index to guide FRP seismic retrofit and guarantee its reasonability, whereby unnecessary economic expense is obviated while structural anti-collapse capacity is kept sufficient. The main research works done are as following:(1) The beam-column joints of low ductile RC frame were numerically simulated and subsequent FRP retrofit was excuted. From the scratch, on basis of existing theories about three parameter identification, a discussion on the applicability of pre-and post-FRP-retrofitted low ductile beam-column joints was made. A viable diagonal-compression-strut (DCS) model was then selected to implement improvements on the constraint assumption and the strength reduction assumption for pre-and post-FRP-retrofitted low ductile beam-column joints, whereafter proposing a new parameter identification method. Finally, the software framework OpenSees was adopted to simulate the low ductile beam-column joint assembly, either before or after FRP seismic retrofit. The outcome was compared with test results to check the proposed parameter identification method. The refined modelling of beam-column joint accomplished in this stage paved a good foundation for the analysis on low ductile structure.(2) The initial damage of a low ductile structure and its global damage development under pushover was depicted, and the deficient low ductile structures were numerically retrofitted with FRP material so that they carry the targeted collapse margin ratio (CMR). Firstly, multi-mode damage model was used to depict the initial damage of a low ductile RC structure compared to corresponding newly-built intact structure as well as its global damage development under pushover, where-after seismic collapse safety margin evaluation was implemented on basis of pushover anslysis. Then, with the consideration of structural continual service life, a targeted CMR was put forth to instruct the FRP seismic retrofit on the low ductile RC structure. It is more economical if the retrofit is done with a premise of a sufficient seismic collapse capacity of the retrofitted structure in its continual service life. Succeeding case studies verified the reasonability of above-mentioned damage depiction and the feasibility of the instruction of the targeted CMR on seismic retrofit.(3) The initial damage of corroded RC structures and their global damage development under strong earthquake were discussed, and CMR-based FRP seismic retrofit was achieved. The time dependence of the CMR of the pre-and post-retrofitted corroded structures was thus illuminated. OpenSees was again taken advantage of, and the multi-mode damage model was adopted to depict the initial damage. The impact of the rust of steel bars on the initial damage of the corroded structural and the evolution trend of the associated global damage under strong earthquake were then scrutinized. Thereafter, with the consideration of different remaining service life of the corroded RC structures, the seismic safety margin of the corroded structure was evaluated using incremental dynamic analysis (IDA). FRP seismic retrofit was then implemented on the corroded RC structures with inadequate seismic anti-collapse capacity. Three representative retrofitting time and the post-retrofitted development of rust of steel bars were specially taken into account. Case studies again were employed to check the reasonability of the depiction of the initial damage and the global damage of the corroded RC structures, where the time-dependence of the CMR of the pre-and post-retrofitted corroded structures were demonstrated. At last, analyses on the structures involved in the case study was implement through pushover method for collapse safety margin. Compared to the results of IDA, overestimation of structural anticollapse capacity does exist herein, though the difference has an acceptable magnitude.(4) Computation outcome of the collapse safety margin of the low ductile RC structures and the corroded RC structures were compared. Thus verified is the appliability of collapse criterion in the multi-mode damage model of concern and the feasibility of the intact structure selected when numerically modelling the RC structures, either low ductile or corroded.