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An experimental study is performed on the elasto-plastic hysteretic behaviors of steel cantilever columns of which box cross-section changes abruptly. The purpose is to check the instability phenomenon which was found in the numerical study. Eight columns are tested under the loading patterns of constant vertical and alternating horizontal loads. Test results show that the collapse behaviors of such columns can be classified into three type modes Y, V and S. These modes depend on the location of plastic zone every half cycle, accumulated plastic deformation and P-delta effects. If the difference between critical loads such as initial yield or rigid plastic loads of each cross-section is canceled due to P-delta effects, the mode S appears and the corresponding load-displacement curves become unstable compared with those of modes Y and V.

JPEG PICTURES:

The following pictures demonstrate the change of collapse mechanism of column with variable cross-sections subjected to horizontally cyclic load.

1. Setup of test column with variable cross-section [16kB]
2. Step 1 Plastic deformation at the bottom of lower cross-section [15kB]
3. Step 2 Plastic deformation at the bottom of upper cross-section [16kB]
4. Step m Plastic deformation at the column base and residual deformation at the bottom of upper cross-section [15kB]
5. Step n Plastic deformation in the upper cross-section and residual curvature at the column base [15kB]
6. Step n+1 S shaped mode appears due to the combined action of P-delta effect and residual deformation [15kB]

It should be noted that the location of plastic deformation at Step n is different from that at Step n+1. Then, a numerical study gives corresponding horizontal load H - displacement delta curve and sway modes of column as follows:

Fig. Cyclic instability of column with variable cross-section