۱٫ Introduction

۲٫ Modelling of steel-jacketing action and application to RC fiber-section columns

۳٫ Design optimization framework

۴٫ Case study structure and specific design optimization assumptions

۵٫ Results of the optimization

۶٫ Effectiveness of the optimization framework in case of structural irregularities

۷٫ Conclusions

References

Abstract

Retrofitting of existing reinforced concrete (RC) frame structures by steel angles and battens (steel-jacketing) is a commonly employed technique used to retrofit beams and columns against gravity and seismic loads. Steeljacketing (SJ) effectively provides additional deformation and strength capacity to RC members but its application is associated with noticeable downtime of the building and non-negligible costs, depending on the amount of structural and non-structural manufacturing and materials. This paper presents an optimization framework aimed at the minimization of seismic retrofitting-related costs by an optimal placement (topological optimization) and amount of steel-jacketing reinforcement. In the proposed framework a 3D RC frame fiber-section model implemented in OpenSees is handled by a genetic algorithm routine that iterates reinforcement configurations to match the optimal solution. The feasibility of each solution is controlled by the outcomes of a static pushover analysis in the framework of N2 method. Results will provide optimized location and amount of steel-jacketing reinforcement, showing how effective and sustainable reduction of retrofitting costs is achievable maintaining a specified safety level.

Introduction

Retrofitting of reinforced concrete columns with cages arranged by steel angles and battens (steel jacketing) is a widely employed technique to improve strength and deformation capacity of beams and columns of existing buildings presenting critical conditions with respect to seismic and gravity loads. Steel jacketing of columns can be generally arranged in two ways. The first provides a moment resisting connection between the steel cages and the slabs (Fig. 1a). In this case, besides the confinement action exerted by the cage, additional flexural strength is provided. Since moment resisting connection are not always easy to realize, steel jacketing is often arranged by simply applying the cages (Fig. 1b). Even in this case, a certain additional flexural resistance is observed because of friction forces transfer between the steel angles and the concrete column (Campione et al. 2017 [1]), but the most significant contribution is related to the increase of deformation capacity as consequence of the strong confinement action. Experimental and numerical investigation have been carried out in the last years both for the first [2–۵] and the second [1,6–۱۰] typology of arrangement.

Despite its effectiveness in providing additional strength and deformation capacity to RC members, it should be said that steel jacketing is an invasive strengthening technique. In fact, the reinforcement of columns provides also the demolition and reconstitution of eventual portions of masonry infills and plaster. This is associated with significant direct costs and noticeable downtime for the building. A second issue regards the design of the intervention in terms of individuation of the columns to retrofit and the choice of the battens area and spacing. In fact, when approaching by non-linear static analysis (pushover), as a method to assess the performance before and after retrofitting intervention, a significant number of attempt iterations are needed to individuate the most suitable retrofitting configuration, especially when the number of columns is large and the building has irregular configuration. In the absence of a specific optimization process this generally brings the designer to adopt overall compromise solutions which allow obtaining effective seismic performance without optimization of the costs. Structural optimization is widely recognized as a valuable computational tool allowing engineers to obtain cost-effective designs.