Sectional Analysis And Seismic Damage Performance Design For FRP-retrofitted Reinforced Concrete Columns

Since performance-based seismic design (PBSD) methodology was put forward by Northern American researchers in 1995, such methodology has been widely accepted and developed in the field of earthquake engineering. Under strong ground motions, building structures would undergo severe damage caused by significant inelastic deformation and cumulative hysteretic energy dissipation. Damage-based seismic design can provide a reasonable performance evaluation of structures under severe earthquakes. Fiber reinforced polymer (FRP) laminates have been applied more and more frequently in seismic retrofit for deficient building structures. Research activities were focused on the seismic performance of individual structural components, e.g. concrete columns, bridge piers, etc, with FRP retrofit. However, investigations on the performance of FRP-retrofitted concrete structures are still very limited. The damage-based performance design method is also at starting point.In this paper, nonlinear sectional analysis of FRP-retro fitted reinforced concrete columns are carried out and corresponding program in Matlab language is made. The accuracy of the program is validated through a comparison between computational results and test data. Moreover, a large amount of calculations on the complete moment vs. curvature relation curves are conducted and the N-M interaction curves are obtained from those calculations. From these curves, it can be seen that unidirectional and bidirectional FRP laminates can significantly improve the ductility capacity of reinforced concrete columns. Beside this, bidirectional FRP laminates can also increase the flexural resistance of retrofitted columns.The simplified moment vs. curvature formulations corresponding to concrete cracking, steel tension yielding, steel compressive yielding, concrete crushing and FRP longitudinal rupture of FRP retrofitted columns are derived based on some assumptions. Through a comparison with the predictions from nonlinear sectional analysis program, the accuracy of those simplified expressions is also verified. As a result of the comparison, for unidirectional FRP laminates, the moment vs. curvature relationship of a FRP-confined column with large eccentricities can be characterized by a trilinear simplified relationship. A bilinear or trilinear simplified relationship can be used for a FRP-confined column with small eccentricities. As to the application of bidirectional FRP laminates, a trilinear and bilinear simplified relationship can be applied fo a FRP-confined column with large and small eccentricities, respectively. Note that all the relationships, including the simplified and complete, considered in the paper, are derived based on the assumption of strong confinement provided by FRP laminates with columns.A shear-type five-story reinforced concrete frame is designed according to current codes. Then, the seismic performance of this structure for pre-retrofitting and post-retrofitting under strong ground motions corresponding to the fortification intensity of 8 degree is evaluated through by combining the improved capacity spectrum method and damage-based design method. Accompanied with this evaluation, the procedures of damage-performance-based retrofit design with FRP laminates are also addressed, the global ductility capacity of the weak floor is enhanced greatly through FRP retrofitting, and the global damage of the structure is therefore substantially reduced.