Experimental and Finite Element Investigations of FRP Stirrups Effects on Cyclic Behavior of Concrete Joints

1. In recent decades, the use of FRP fibres in concrete structures has been considered by researchers due to their high corrosion resistance. The use of FRP composites in strengthening members of reinforced concrete structures such as beams and joints has been of great interest for civil engineers in recent years [1]. Various designing codes for using FRP material in concrete structures were presented. Many existing reinforced concrete buildings were already designed for gravity loads and lateral forces that may be much smaller than those prescribed by existing building codes [2]. Berg and others have studied on cost analysis of an FRP reinforced concrete bridge deck [3]. Their conclusions showed that construction of an FRP reinforced concrete bridge deck using conventional construction technology and labor was accomplished with a 57% savings in construction labor over nominally identical steel rebar reinforced deck2. Methodology2.1. Experimental study: In order to examine the effect of FRP stirrups distance on the cyclic behavior of concrete joints, two half-scale specimens were constructed with the same steel longitudinal bars (in columns and totally at the top and bottom of the beams) and same dimensions, 250 mm square columns and 200 by 250 mm beams (height by width). The first joint had closed space FRP stirrups considering design code ductility regulations and was introduced as FDJ (FRP Ductile Joints), and the other had wider double spaced FRP stirrups and was introduced as FNJ (FRP Non-ductile Joints). Characteristics of the joints have been shown in Fig. 1 and given in Table 1.2.2. FE modeling: ANSYS finite element program was used to examine the behaviour of these two connections (with different distances of stirrups) and the calibration or prepared models against laboratory test results. The finite element models were generated at two different groups, each group with two different stirrup spacing.Table 1. Details of experimental joint specimens with CFRP materialsSpecimen Number of rods (14 mm) Number of rods(12 mm) Stirrups Distance in the ductile region (mm) Stirrups Distance in the non-ductile region (mm) Number of stirrups in joint Bottom Top Beam Column Beam Column FDJ 8 3 3 50 50 100 150 2FNJ 8 3 3 100 100 100 150 13. The distance between stirrups has an important effect on the cyclic behaviour of connections and the amount of energy absorption and dissipation. The distance between stirrups is much influential on the behaviour of RC connections. To design the FRP stirrups cross section, equivalent cross section of steel stirrup was adopted by assuming 6 mm diameter steel stirrups and 28.3 mm2 cross section and 260 MPa yield strength. According to design code regulations of ACI 318, shear capacity of steel stirrups was calculated by Eq. (1).Where Asv, Fy, d and Ss are the sum of the areas of two steel stirrups legs, steel bar yield strength, effective height and spacing of steel stirrups, respectively. If FRP fibers are used, shear capacity of FRP stirrups is calculated by Eq. (2) based on ACI 440 and Canadian design code CSA.Where Vf, Afv, s and ffv are shear capacity of FRP stirrups, the sum of the areas of two FRP stirrups legs, spacing of stirrups, and effective tensile strength of FRP stirrups. According to ACI-440-R1, effective tensile strength in FRP stirrups is assumed to be 0.004 Ef when Ef is the modulus of elasticity of FRP stirrups. Required equivalent cross section of FRP stirrups was calculated according to Eq. (3) by assuming 0.4% strain for FRP stirrups and equality of shear capacity of FRP and steel stirrups. 4. The experimental results showed that the additional capacity of the specimen with closely spacing stirrups was 12% more than that of the companion specimen with widely spacing stirrups, and also the ductility coefficient of the former specimen was 26% higher than of that of the later specimen. The results also showed that the maximum strains gained at the connection panel stirrups was about 50% more that the maximum allowable amount given at the design code provisions. Numerical analysis showed that all specimens reinforced with FRP stirrups designed based on design code ductility regulations at closed space had averagely 8% higher loading capacity and joints with closed ductile stirrups had higher ductility coefficients up to 20% compared to joints with wide stirrup spacing.