Report - PEER - University of California, Berkeley

3.6 Failure ModeBased on the calculated hierarchy of the strengths of J-mode and B-mode, type of failuremode of J, or B is determined by applying rules given in Table 1.Table 1. Prediction of failure mode of knee jointFailure type Condition ExplanationJ M j < M b Joint shear failureB M j > M b Beam yielding4. CORRELATION STUDYFigure 5 shows the result of calculating for four specimens from references whichshow typical types of failure, B: beam yielding, J: joint shear failure or T: prematureanchorage failure. To apply the theory the following rules were used. If the column haslarger capacity than beam, column reinforcing detail is neglected in estimating thestrength of knee joint. If the amount of joint shear reinforcement in vertical and horizontaldirection is different, average value was used to estimate the T 5 . The columnlongitudinal bars in mid-layer is included as vertical joint shear reinforcement. Iflength of column and beam is different, averaged length is used.Specimen L345-30-3 (Inoue et al.2003) shown in Fig. 5(a) exhibited joint beamyielding failure (B). The predicted maximum moment is very closed to the observedvalue. The specimen L-8 (Nakamura et al. 2003) in Fig. 5(b) showed yielding of longitudinalbar as well as joint shear failure accompanied with strength degradation. Thecalculated and observed strength show good agreement.Figure 5(c) shows specimen L-BH2 (Tabata 2001) which exhibited joint shearfailure (J). The J-mode strength at beam bar yielding is smaller than that of B-mode,hence the predicted failure mode was J. Figure 5(d) shows the specimen L-180-L(Okano 2003) in which premature anchorage failure (T) of beam bars in outside faceoccurred under closing loading. The observed strength is much smaller than predicted.This is because the analyses here assume perfect anchorage. So it is necessary to incorporatethe failure criteria of anchorage failure to enhance the accuracy of the modeltaking into account the anchorage detailing in beam-column joint.4.1 Correlation Study with Inventory of Knee Joint TestsTable 2 compares the calculated strength, failure mode with observed ones. All thespecimens in the list have aspect ratio of unity. The observed failure modes were basedon description in reference. If the information was not reported, the failure mode wasdetermined by considering the failure pattern and hysteresis loops.465