Study On Long-Term Performance Of The Joints Of FRP Profile And Analysis Of The Truss Structure

Fibre reinforced polymer (FRP) has been widely applied in engineering structures owing to its advantages such as light weight, high strength and corrosion resistance. Apart from the widely used FRP sheet and bar in strengthening and repairing, FRP pultruded profile, which can be applied in newly built structures, has also attracted the attention of engineers. Although FRP pultruded profile performs apparent anisotropy, its strength in the direction of fiber is extremely high, and its compressive strength can generally satisfy the structure requirements, making it suitable for the members in truss structures, which only bear axial load. The truss structure, fully constructed by FRP pultruded profiles, can make the best of their light weight and high strength, and substantially reduce self-weight of structure, upon satisfying the requirement of load capacity and stiffness. Thus, light weight and high performance of truss structure can be realized. Meanwhile, corrosion problem of steel in traditional truss structures can also be resolved by FRP pultruded profiles. The structural safety can be enhanced and long-term maintenance cost can be saved. Durability and joint connection are the key factors affecting the long-term behavior when applying FRP pultruded profiles to truss structures. However, researches on the long-term behavior of FRP pultruded profiles and their joints are very limited at home and abroad.A constructed truss bridge using FRP pultruded profiles was adopted as engineering background in this paper. Considering the complicated service environment and joint connection, the long-term behaviors of FRP pultruded profiles and their joints were theoretically and experimentally researched. Numerical analysis and comparison of truss bridge constructed by FRP pultruded profiles were conducted through ANSYS. The main research contents are shown as follows:(1) Considering the complicated service environment of FRP pultruded profiles, static and fatigue test and durability test under the environment of acid rain, salt mist and ultraviolet (UV) ray were conducted on BFRP specimens. The S-N curves under fatigue load and degradation curves of mechanical properties under different environments were obtained. The degradation mechanisms of FRP pultruded profiles under different environments were revealed through SEM analysis. The results show that the failure under fatigue load begins from the cracks in the middle of specimens, which propagate and lead to the ultimate failure with the mode of fiber rupture. Due to the addition of fillings, the fatigue behaviour of FRP profile performs poorer than that of FRP bar. The main damage caused by the corrosion of acid rain takes place on the fiber-matrix interface. BFRP profile possesses advantageous acid-resistance. The strength retentions of 2mm and 5mm thick specimens under pH=2/55? with 9 weeks aging time are up to 81.5% and 89.5%, respectively. According to life-time prediction, at the temperature of 30? and service life of 300 years, strength retentions of 5mm thick specimens under the environments of pH=4 and pH=2 reach to 78.37% and 77.28%, respectively. Unlike acid rain, the damage caused by UV ray and salt mist mainly focus on the corrosion of resin. BFRP profile possesses satisfactory UV- and salt-resistance. Under the environment of UV ray coupling with exposed to rain for 9 weeks, the strength reductions of 2mm and 5mm thick specimens are only 5.91% and 3.63%, respectively. After exposed to salt mist for 9 weeks, the strength retentions of 2mm and 5mm thick specimens are respectively up to 93.49% and 96.04%.(2) Apart from the traditional steel cover plate/steel bolt connection form, a lighter FRP cover plate/FRP bolt was proposed in this paper to reduce the self-weight and enhance the durability of joint. The joints with different connection materials were researched experimentally under static and fatigue loads. The results show that no significant difference in strength can be observed among the specimens with group-bolt joints regardless of the connection material. The joints using FRP bolts behave less scatter of fatigue lives and higher fatigue strength compared with those using steel bolts subjected to the same stress level.(3) Numerical analysis was conducted on the truss bridge of engineering background though ANSYS. The hybrid design plan, using BFRP profile as primary members in truss structure and using GFRP, with lower cost, as minor members, was proposed to replace the original structure fully constructed with GFRP. The main purposes are to reduce structural deflection and control cost. The static analysis shows that the structural deflection can be effectively controlled when BFRP profiles were fully or partly applied to the two main trusses in the middle span. Model analysis shows that the structure using BFRP profiles performs higher natural frequency and better seismic behaviour than that using GFRP profiles. Furthermore, the vibration mode maintains unchanged regardless of the type of FRP profile. The first two models are mainly transversal deformation, explaining the sensitivity of truss structure to horizontal earthquake. The main deformation concentrates on the upper part of structure under dynamic load due to the smaller stiffness of upper chords than lower chords. Thus, upper chords need to be emphasized in the design of FRP truss structures.