Compressive Performance And Safety Evaluation Of Short Columns Made Of BFRP-Steel Tubes With Built-In OFBG Sensors

External confinement with fiber-reinforced polymer(FRP)composites has emerged as an effective means of improving the structural performance of existing reinforced concrete(RC)or circular hollow section(CHS)steel columns.However,existing research indicates that the sudden failure of FRP,can lead to a degradation of the confinement system,which contributes to the likely collapse of the structure afterwards.An increase in the cross-sectional design or thickness of the FRP may not be feasible due to the significant rise in weight and cost of the structure.Therefore,solutions that are affordable,practical,and efficient to design FRP confined columns with a self-sensing ability are highly recommended.Optical fiber sensors offer the possibilities to design FRP-based components and structures with self-sensing capabilities.The question remains whether the built-in sensors could provide necessary information on the health status of FRP confined columns.An ongoing research question that this dissertation attempts to explore is how to create structures with a balanced prior-and postpeak performance without adding significant load.Furthermore,existing load-strain models might not accurately predict the behavior of the new confined columns.Finally,the safety evaluation of FRP strengthened CHS steel columns is an important topic that needed to be investigated.This dissertation has addressed these issues as follows:1)The first objective is to develop and experimentally investigate some basalt fiberreinforced polymer(BFRP)-steel tubes with built-in fiber Bragg grating(FBG)sensors.The developed tubes are expected to overcome the challenges mentioned previously in confined concrete and externally strengthened steel columns.BFRP is chosen to provide a degree of confinement for delaying the buckling of both thin and thick section steel walls.Twenty-two strengthened and un-strengthened specimens were tested under axial compression.The effects of the BFRP strengthening and the Punched-in patterns on the CHS steel columns were investigated in terms of axial load capacity,failure mode,and load-displacement behavior.In addition,fifteen specimens were considered to investigate the sensing performance of the builtin FBG sensor,which revealed the possibility to capture strain information throughout the lifespan of the self-sensing tubes.Finally,the weight and cost analysis demonstrated the benefits of the proposed methods for steel confinement.2)A new concrete-filled double-skin tubular column(DSTC)with basalt fiber steel tubes(BFST)has been proposed as BFST-DSTC,to solve existing issues of confined concrete systems,such as the hybrid FRP-concrete-steel DSTC.Twenty-two specimens were tested under axial compression to investigate the effects of the design parameters on the behavior of the BFST-DSTC.The outcomes reveal that the BFST-DSTC exhibits the best performance in terms of load capacity,confinement ratio,failure and damage mechanisms,and ductility in prior and post peaks.The inner FRP jacket delays the buckling of the inner tube.The Punchedin patterns of the outer steel improve the confinement effectiveness of the FRP jacket.The BFST-DSTC displays a high post-peak performance with high-energy dissipation capacity that prevents the structure from collapse after the FRP jacket ruptured.Finally,a new confinement model is proposed to predict the ultimate point of the confined concrete.3)Existing analysis-oriented load-strain models are evaluated to predict the behavior of non-uniformly confined concrete under both FRP and steel confinement.In addition,the inner FRP strengthened CHS tube exhibits a unique behavior that differs from previous studies which used carbon steel or stainless steel tubes.New analysis-oriented models are established for describing the behavior of the BFST-DSTC columns and the BFRP strengthened CHS steel tubes under axial compression.The established models can predict the complete load-strain curves,and are suitable for engineering applications.The lack of experimental data has led the authors to evaluate the established models with other confinement systems by considering different parameters.4)To date,the safety evaluation and monitoring of FRP confined concrete columns remain as existing challenges to overcome due to their complex damage scenarios.Therefore,the newly proposed BFST-DSTC is further equipped with built-in FBG sensors to become a smart BFST-DSTC.The self-sensing performance of ten specimens under axial compression indicates a successful health evaluation of both,the FRP jackets and inner tube.Finally,the safety evaluation of the BFST-DSTC using the strain data gathered from the embedded FBG sensors is studied.5)Finally,the monitoring of buckling and safety evaluation of the BFRP strengthened CHS short steel columns with built-in FBG sensors are addressed.Early warnings issued through self-sensing may prevent damage of the BFRP strengthened CHS short columns.Tests on strengthened specimens were conducted under laboratory conditions.The buckling was controlled by thinning the middle section of the CHS steel wall.The test results indicated that the buckling strength can be indeed be monitored using the data gathered from the embedded FBG sensors.A thorough safety evaluation of the BFRP strengthened tubes is also discussed.