The evaluation of the capacity of buildings to withstand dynamic actions without collapses, and in case of a poor response, the planning of the retrofit intervention represents today one of the most pressing and common tasks for the engineers. A methodology largely adopted for the definition of the state of the existing buildings subject to seismic actions is based on the use of non-linear static analysis for increasing lateral actions and this procedure is nowadays considered a valid alternative to dynamic time-history methodology. On the basis of the performances required to the structure, the outcomes of the static non linear analysis allow the judgment of the structural safety and then the evaluation of the necessity of a seismic retrofitting. In this field, the adoption of FRP sheets is becoming more and more common, as widely documented by the huge amount of theoretical and experimental works and by applications on real structures. Nevertheless if it appears undeniable the effectiveness of the application of the FRP in order to guarantee increases of elements resistance in r.c. structures, previous studies developed by the authors have underlined as the influence of the linear elastic constitutive model of the composite materials can influence the global plastic capacity of simple structures. In the case of fiber arrangement parallel to the element axis very often brittle failure modes activate, associated with FRP debonding from the concrete, and anyway a not negligible reduction of plastic rotation in FRP strengthened elements occurs. Aim of the paper is the evaluation of the global ductility demand for a simple frame subjected to a peculiar FRP strengthening, and then the assessment of the composite material influence on the seismic vulnerability of the structure. At this purpose the seismic behaviour of a reference plane frame designed for vertical loads only is examined with non-linear static and dynamic analysis. The seismic behaviour of the frame is discussed on the basis of the capacity curve and spread of plastic hinges. Due to the expected inadequacy to withstand horizontal actions, a retrofitting intervention is designed, with the purpose of fulfilling the capacity design criterion according to the present seismic codes. The response of the reinforced scheme is then analysed again in static and dynamic fields. The outcomes of the analyses are then judged in the framework of a performance based approach, and the global ductility demand of the examined scheme is evaluated through the application of assessed methods and compared with the ductility capacity obtained by the performed analysed. Finally the vulnerability of the un-reinforced and reinforced schemes is evaluated and discussed, based on the ductility demand and capacity. The evaluation of the seismic vulnerability of the existing and reinforced frame is performed with the “Displacement Based” methodology in the framework of a “Performance Based” approach. In particular in three performance levels, related to three different seismic intensities are considered: - Damage Limitation (DL); - Severe Damage (DS); - Collapse Limit State (CO). In the framework of the displacement method the safety check of the structure is based on the comparison between the displacement demand and the capacity of a deformation index. The displacement demand is evaluated according to the N2 method. The vulnerability index is defined as the ratio between the displacement demand and capacity and when this value is higher than one the structure is not adequate for the analysed performance level. On the basis of the obtained results the following remarks can be addressed: - for the considered scheme, the main aim of the intervention, related to a variation of the failure mode, with the formation of plastic hinges in the beams instead of in the columns, is not completely achieved. The strengthening of the column leads to higher spread of the plastic hinges, respect to the un-reinforced frame, but the weak beam-strong column scheme is never reached. Anyway, a significant increase of strength is exhibited by the frame. - for low seismic levels (ag =0.15g) the increase of strength provided by the FRP is sufficient to grant an elastic behaviour of the frame for each performance level; - for intermediate seismic levels (ag= 0.25g) the reinforced scheme behaves again elastically in the DL state, while the un-reinforced frame exhibits a plastic behaviour; anyway the vulnerability index is less than one, as the displacement capacity is higher than the demand one. Only for the CO performance level, in the reinforced scheme the ductility capacity is lower than the demand one and the vulnerability index is higher than 1. - for high seismic levels (ag =0.35g) the intervention can be considered effective for reducing the seismic vulnerability only for the Damage Limitation performance level. These results are further confirmed by the values of the displacement ductility and behaviour factor evaluated for each analysed cases.
Coccia, S., Ianniruberto, U., Rinaldi, Z. (2006). Global ductility demand for FRP strengthened structures in seismic zone. In Condensed Papers (2). Sessions 10-20 (Proceedings of the 2nd fib Congress, June 5-8, 2006 – Naples).
Global ductility demand for FRP strengthened structures in seismic zone
COCCIA, SIMONA;IANNIRUBERTO, UGO;RINALDI, ZILA
2006-01-01
Abstract
The evaluation of the capacity of buildings to withstand dynamic actions without collapses, and in case of a poor response, the planning of the retrofit intervention represents today one of the most pressing and common tasks for the engineers. A methodology largely adopted for the definition of the state of the existing buildings subject to seismic actions is based on the use of non-linear static analysis for increasing lateral actions and this procedure is nowadays considered a valid alternative to dynamic time-history methodology. On the basis of the performances required to the structure, the outcomes of the static non linear analysis allow the judgment of the structural safety and then the evaluation of the necessity of a seismic retrofitting. In this field, the adoption of FRP sheets is becoming more and more common, as widely documented by the huge amount of theoretical and experimental works and by applications on real structures. Nevertheless if it appears undeniable the effectiveness of the application of the FRP in order to guarantee increases of elements resistance in r.c. structures, previous studies developed by the authors have underlined as the influence of the linear elastic constitutive model of the composite materials can influence the global plastic capacity of simple structures. In the case of fiber arrangement parallel to the element axis very often brittle failure modes activate, associated with FRP debonding from the concrete, and anyway a not negligible reduction of plastic rotation in FRP strengthened elements occurs. Aim of the paper is the evaluation of the global ductility demand for a simple frame subjected to a peculiar FRP strengthening, and then the assessment of the composite material influence on the seismic vulnerability of the structure. At this purpose the seismic behaviour of a reference plane frame designed for vertical loads only is examined with non-linear static and dynamic analysis. The seismic behaviour of the frame is discussed on the basis of the capacity curve and spread of plastic hinges. Due to the expected inadequacy to withstand horizontal actions, a retrofitting intervention is designed, with the purpose of fulfilling the capacity design criterion according to the present seismic codes. The response of the reinforced scheme is then analysed again in static and dynamic fields. The outcomes of the analyses are then judged in the framework of a performance based approach, and the global ductility demand of the examined scheme is evaluated through the application of assessed methods and compared with the ductility capacity obtained by the performed analysed. Finally the vulnerability of the un-reinforced and reinforced schemes is evaluated and discussed, based on the ductility demand and capacity. The evaluation of the seismic vulnerability of the existing and reinforced frame is performed with the “Displacement Based” methodology in the framework of a “Performance Based” approach. In particular in three performance levels, related to three different seismic intensities are considered: - Damage Limitation (DL); - Severe Damage (DS); - Collapse Limit State (CO). In the framework of the displacement method the safety check of the structure is based on the comparison between the displacement demand and the capacity of a deformation index. The displacement demand is evaluated according to the N2 method. The vulnerability index is defined as the ratio between the displacement demand and capacity and when this value is higher than one the structure is not adequate for the analysed performance level. On the basis of the obtained results the following remarks can be addressed: - for the considered scheme, the main aim of the intervention, related to a variation of the failure mode, with the formation of plastic hinges in the beams instead of in the columns, is not completely achieved. The strengthening of the column leads to higher spread of the plastic hinges, respect to the un-reinforced frame, but the weak beam-strong column scheme is never reached. Anyway, a significant increase of strength is exhibited by the frame. - for low seismic levels (ag =0.15g) the increase of strength provided by the FRP is sufficient to grant an elastic behaviour of the frame for each performance level; - for intermediate seismic levels (ag= 0.25g) the reinforced scheme behaves again elastically in the DL state, while the un-reinforced frame exhibits a plastic behaviour; anyway the vulnerability index is less than one, as the displacement capacity is higher than the demand one. Only for the CO performance level, in the reinforced scheme the ductility capacity is lower than the demand one and the vulnerability index is higher than 1. - for high seismic levels (ag =0.35g) the intervention can be considered effective for reducing the seismic vulnerability only for the Damage Limitation performance level. These results are further confirmed by the values of the displacement ductility and behaviour factor evaluated for each analysed cases.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
