3.1 Failure Observations
The maximum diameter over thickness value for local buckling provided by EC4[3]indicates the local buckling would not occur in the steel tube.However,it should be mentioned that the EC4 maximum value corresponds to the value of classification for Class 3 section in EC3[4].This means the EC4 can only tell that the steel tube would not have local buckling in the elastic stage.Whether the local buckling would happen in the plastic stage should be based on the classification according to EC3.It is then found that the steel tube falls into Class 2 but very close to Class 1 according to EC3,implying that the local buckling would possibly happen in the plastic stage but may experience a large plastic deformation firstly.Figure 2 shows the failure modes of the steel tubes under axial compression.The local buckling could be found in the CFST and DSTC specimens,outwardly and inwardly respectively,and it is severer for the CFST specimens since the outward buckling is more likely to happen than the inward buckling.The local buckling was not noticeable for the DTCC specimens as the steel tubes were restrained from both sides.
Figure 3 shows the failure modes of GFRP tubes.No significant difference was found when the replacement ratios of recycled coarse aggregates varied.All DSTCs and DTCCs specimens failed in rupture of the GFRP tubes.
Figure 2 Failure observations on steel tubes
Figure 3 Failure observations on GFRP tubes
Figure 4 Axial load-axial strain curves:effect of replacement ratio