PSI - Issue 64

Rosario Montuori et al. / Procedia Structural Integrity 64 (2024) 841–848 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Proposals of structural retrofitting 4.1. Structural Interventions for the RC cores

According to the results of the seismic vulnerability assessment, presented in Section 3.2, the main structural criticalities regard the shear behaviour of the concrete cores which could reach the crisis after severe seismic events. So, the retrofitting proposals should be devoted to improving the resistance of the concrete walls. This aim could be achieved by two main interventions: Thickness increasing of current concrete walls (I1); Steel jacketing of the concrete cores (I2). The intervention 1 is characterized by concrete jacketing of the RC cores as shown in Fig. 7a. The thickness of the new concrete jacket is equal to 100 mm. The longitudinal steel bars have been defined to improve the flexural resistance of the whole cores, while the horizontal bars increase the shear resistance of the walls. The diameter of the rebars is equal to 20 mm with a spacing ranged from 100 and 500 mm. The distribution of the steel bars changes from one wall to the other to obtain an optimized retrofitting. The retrofitting design has been performed by evaluating the reduced flexural resistance M̃ R of RC cores and the reduced shear resistance Ṽ R of the concrete walls according to Section C8.7.4.2.1 provided in Circular n.7 of 21.01.2019. In particular, the following relations have been applied to evaluate the ultimate behaviour of the RC cores: M̃ R =0.90 ∙ M Rd Ṽ R =0.90 ∙ V Rd (2) Where M Rd and V Rd are the flexural resistance of the cores and the shear resistance of the concrete walls according to DM 17.01.2018. The optimization of the intervention has been reached by fixing the specific reliability target, i.e. =1.00 , to determine the corresponding maximum resistance of the RC cores after the retrofitting. The intervention 2 is based on the application of steel jacketing by a combination of continuous plates and steel stripes to the concrete walls as shown in Fig. 7b. the thickness of the steel plates and stripes is equal to 10 mm with a variable distribution from one wall to the other. In this case, the design of the steel plates and stripes has been performed by considering the shear resistance V j according to Section C8.7.4.2.2 provided in Circular n.7 of 21.01.2019: V j =0.5 ∙ 2t s ∙ b ∙ f yk ∙ 0.9 ∙ d ∙ cot ϑ (3) where d and b are the height and the width of the steel elements, respectively; t is the thickness of the steel plate elements; s is the distance between the plate elements and finally ϑ is the slope of the crack for the shear failure in the concrete walls assumed equal to 45 o .The steel connectors consist of 8.8 class threaded M16 or M24 bars. At the base of the RC cores, the stiffening ribs with depth of 120 mm have been designed, while the anchor bolts, made of 8.8 class M20 or M24 bars, are characterized by variable lengths for each RC core. The steel connections have been defined according to the design rules of EN 1993-1-8. Table 4 shows a comparison of the seismic vulnerability indices before and after retrofitting with reference to the Life Safety Limit State and for both failure mechanisms, i.e., Flexural Failure of the RC cores (FF) and Shear Failure of the concrete walls (SF_w). It is immediate to recognize that both proposed solutions satisfy the condition ≥ 1.00 as requested by the current Italian codes. Table 4. Comparison of the vulnerability indices before and after retrofitting. Criterion [%] Current State I1 I2 FF 56 125 119 SF_w 9 100 180