CHAPTER 5 BEHAVIOUR OF BEAM-COLUMN JOINT UNDER CYCLIC LOADING

Transcription

1 72 CHAPTER 5 BEHAVIOUR OF BEAM-COLUMN JOINT UNDER CYCLIC LOADING 5.1 GENERAL In this chapter the behaviour of beam-column joints cast using M20 and M25 concrete under cyclic loading have been enumerated. Here the parameters like load carrying capacity, ductility factor energy dissipating capacity and stiffness are discussed for specimen I F21 cast using M20 concrete with 1.5% of steel fibre and 0.2% of polypropylene fibre subjected to cyclic loading. Similarly the above parameters were determined for all the beam- column joint specimens cast using M20 and M25 concrete under cyclic loading and they are discussed in Chapter LOADING AND LOAD DEFLECTION BEHAVIOUR Seven numbers of beam column joint each from M20 and M25 concrete were subjected to static cyclic loading. The history of load sequence followed for the test was presented in Figure 5.1.

2 73 Figure 5.1 History of Cyclic Load Sequence Load was applied by using hydraulic jack at a distance of 50 mm from the free end of the beam. Totally four cycles were imposed. The load versus deflection diagram for specimen I F 21 is shown in Figure 5.2. The deflection steps selected are 5mm, 10mm, 15mm, 20mm, 25mm and 30mm. The load was applied gradually and measured for every 5mm deflection. Figure 5.2 Load Deflection Curve for Specimen I F21

3 LOAD CARRYING CAPACITY The first crack load was witnessed during the first cycle of load deflection curve in O series specimens and in the second cycle in the S and F series specimens. As the load level was increased further cracks were developed in other portions. The ultimate load carrying capacity of I F 21 was 9.1 kn. 5.4 DUCTILITY FACTOR Ductility of a structure is its ability to undergo deformation beyond the initial yield deformation while still sustaining load. This is illustrated in the load versus deflection diagram as shown in Figure 5.3 (Load deflection curve for specimen I F 21). The first yield deflection is assumed as bilinear behaviour of the beam column joint which is obtained as 10 mm (Thirugnanam 2010). It is obtained by the horizontal distance between the origin and point of intersection of tangents drawn from load deflection curves of the first cycle and last cycle. y shown in the figure 5.3 is the yield deflection for the same specimen. Ductility factor = Ultimate displacement Yield displacement y

4 75 Figure 5.3 Determination of Yield Deflection from the Load Deflection Curve for I F21 The ductility factor values for various load cycle of the I F21 were worked out and presented in Table 5.1. Figure 5.4 shows the variation of ductility factor for the specimen I F21. When a structure is subjected to cyclic loading, cumulative ductility up to any load point is defined as the sum of the ductility at maximum load level attained in each cycle up to the cycle considered. This is the important parameter to be considered for earthquake resistance feature of a structure. The cumulative ductility factor values were found for various cycles and are presented in Table 5.1. The cumulative ductility factor was found to increase from 0.5 during first cycle of loading to 5 during 4 th cycle of loading. Figure 5.4 Variation of Ductility Factor for specimen I F 21

5 76 Figure 5.5 Variation of Cumulative Ductility Factor for Specimen I F 21 Figure 5.5 shows the variation of cumulative ductility factor for specimen I F21 subjected to cyclic loading in each cycle. 5.5 RELATIVE AND CUMULATIVE ENERGY DISSIPATING CAPACITY When the beam-column joint is subjected to cyclic loading such as those experienced during heavy wind or earthquake, some energy is absorbed in each cycle. It is equal to the work in straining or deforming the structure to the limit of deflection. The relative energy absorption capacities during various load cycles were calculated as the area of the load versus deflection diagram. Relative energy absorbed during the first cycle of loading was calculated as 2.52 knmm and during 4 th cycle 85 knmm. The relative energy absorption capacity values for all the cycles were given in Table 5.1. Figure 5.6 shows the variation of relative energy dissipation capacity with respect to the number of cycles.

6 77 Figure 5.6 Variation of Relative Energy Dissipation Capacity Figure 5.7 Cumulative Energy Dissipation Capacity The cumulative energy dissipation capacity of the beam-column specimen was obtained by adding the energy dissipation capacity of the joint during each cycle considered and the values are presented in Table 5.1. Figure 5.7 shows the cumulative energy dissipation capacity of the specimen I F21 subjected to cyclic loading in each cycle.

7 STIFFNESS Stiffness is defined as the load required to cause unit deflection of the beam-column joint. The following procedure is used to calculate stiffness of the joint. The recorded loads and corresponding displacements at the end of each half cycle were used to calculate stiffness of each specimen during each cycle of test (Ganesan et al. 2007b). The stiffness in each cycle was calculated using a line drawn between the maximum positive displacement point in one half of the cycle and the maximum negative displacement point in the other half of the cycle. Although approximate, the stiffness was used to provide a qualitative measure of the stiffness degradation in the specimens. A line 0-1 joining the origin and the peak load of the first cycle, as shown in Figure 5.8, is drawn. The slope of this line is known as the secant stiffness (Shannag et al. 2005). Similarly, from the slope of the line joining the points 0 to 2, 3 and 4 gives the secant stiffness of second, third and fourth cycle respectively. The values of secant stiffness obtained for each cycle are plotted for specimen I F 21 and the plot is shown in Figure Figure 5.8 Procedure Adopted for Determining Stiffness

8 79 In general with the increase in the load there is degradation of stiffness occur. Figure 5.9 Stiffness Degradation of Specimen I F 21 Table 5.1 Experimental Results of Specimen I F12 Cast using M 20 Concrete Subjected to Cyclic Loading Relative Max Max Cum Cum Energy Cycle Ductility Energy Stiffness Load in Deflection Ductility Dissipation No Factor Dissipation Factor kn in mm Factor Capacity Capacity SUMMARY Out of 14 beam-column joint specimens cast by using M20 and M25 concretes, one sample specimen cast using 1.5% of steel fibre and 0.2% of polypropylene fibre reinforced M20 concrete subjected to cyclic load was discussed. The load deflection curve for the sample specimen and its load carrying capacity were also discussed. The sample calculations for calculating yield deflection, ductility factor, relative energy dissipating capacity, cumulative energy dissipating capacity and stiffness were worked out for each cycle for the sample specimens subjected to cyclic loading.