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Concrete In Australia : March 2008
651MPa compared to a design yield value of 500MPa. The section capacity at the support was calculated to be 15.18kNm when taking into account the actual concrete strength and ultimate steel strength of 651MPa, less than a 5% difference from that recorded. The failure mode observed in the test slab was very similar to the fi rst test slab. The failure occurred at the face of the intermediate support with sudden failure of the top reinforcement. Also there was no sign of the concrete crushing anywhere in the slab. Figures 4a and 4b show the load at failure and the failure section at the intermediate support respectively. A defl ection curve of the test slab at selected load stages is shown in Figure 5. The displaced shapes are shown with reference to the displaced position after the settlement was imposed. The curve shows that the slab was able to deflect signifi cantly at mid span before failure. The span to deflection ratio Table 2. Moment redistribution over the span. Serviceability (DL+0.7LL) Ultimate Load (1.2DL+1.5LL) Penultimate Mexpt. (kNm) Figure 6. Moment vs Rotation for (half of) the plastic hinge region. Table 2. Moment Redistribution over the span. Load Stage Melastic (kNm) 5.40 6.93 6.32 8.62 9.66 12.55 just before the failure was 97 (average). Moment redistribution From the measured support reaction over the intermediate support and known imposed loads, it was possible to calculate the moment in the spans as well as at the face of the central support occurring during the experiment for different load cases. With the recorded value of the central support reaction, experimental end support reactions were determined by assuming symmetry. The symmetry of the imposed loading was also confi rmed from the comparable deflection values ß ? (%) 22.08 26.68 23.03 where for both spans obtained by the photogrammetry survey. The degree of moment redistribution is defi ned according to the CEB Task Group 2.2  as: ß d = (1-d) x 100 = M Mexpt elas was calculated using elastic theory and also taking into account the moment due to the support settlement. The experimental hogging moment also includes the effects of support settlement and is found using the end reaction and the known values and positions of the applied loads. Table 2 shows the values of ß for various load conditions. It is important to note that this moment redistribution can mainly be attributed to cracking, since yielding of the reinforcement at the critical section did not occur until the applied load had reached approximately 24kN. The elastic hogging moment at the central support, Melas Moment vs rotation curve Figure 5. Vertical displaced position of slab at various load stages. The moment rotation curve was determined for the plastic hinge region close to the central support. The rotation was determined over the plastic hinge length, taken as equal to the depth of the slab, and based on the transducer data for different load cases. However, in the test slab 2 due to the malfunctioning of a few transducers, it was not possible to determine the rotation over the whole plastic hinge length assumed. Therefore the moment and rotation values (see Table 3) have been calculated based on consideration of only half of the plastic hinge length (ie: over 60mm adjacent to the intermediate support face). Figure 6 shows the relation between the experimental moment and rotation. The moments and rotations are those occurring after the support settlement has been imposed. Concrete in Australia Vol 34 No 1 41 , (1) (2)