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The virtual load method is studied in this paper for damage assessment of beam type structures. The main advantage of this technique is that damage assessment can be accomplished by using only the vibration modes of the current structure. A series of virtual load vectors are constructed to obtain the same pure bending states for different segments of the beam. And then the curvatures of these segments loaded in pure bending are compared to identify the damage locations and extents. It was found that the proposed method is very suitable for damage assessment of those existing beam type structures.

The intersection of neutral layer and the transverse section is called neutral axis. The position of neutral-axis will be changed when the corresponding beam structure is damaged. In this work, the static loading test of recycled concrete beam is carried out to verify the damage detection method based on neutral axis position. Experimental results show that structural damage can be detected by the change of the neutral-axis position.

The flexibility-based damage identification methods have attracted much attention in structural engineering field in recent years. In this paper,three derived sensitivity methods based on structural flexibility are presented for damage identification,namely,the diagonal sensitivity,the principal eigenvector sensitivity and the unfavorable unit load distribution sensitivity. Compared with the original flexibility sensitivity technique,the effect of truncating higher-order modes can be partly reduced in the derived sensitivity methods. A three-span continuous beam was used to illustrate the feasibility and superiority of the derived sensitivity methods for structural damage detection. Results obtained by the first four modes and only the first mode are both given for comparison. When the first four modes are used,it is found that the principal eigenvector sensitivity and the flexibility sensitivity methods can achieve more accurate results than those obtained by the diagonal sensitivity and the unfavorable unit load distribution sensitivity techniques. When only the first mode is used,one can see that the principal eigenvector sensitivity and the unfavorable unit load distribution sensitivity techniques are more effective than the flexibility sensitivity and the diagonal sensitivity methods. Consequently,it is necessary to chose suitable sensitivity technique for the specific engineering problem to get satisfactory detection results.