The aging of civil infrastructure and aerospace structures has led to an increased need to
monitor the overall structural health. If growing damage not identified on time, it may has
serious consequences, both safety related and economic. However, the complexity of large
structures and the difficulty in accessing them makes the use of commonly existing
conventional Non Destructive Evaluation (NDE) methods such as visual inspection and
instrumental evaluation methods, impractical. An effective alternative in Structural Health
Monitoring (SHM) is the use of methods that depend on Vibration-Based Damage
Identification (VBDI) techniques. These methods use limited instrumentation to detect the
changes in the measured modal characteristics of the structure, that is, its frequencies and
mode shapes. These characteristics change with the physical properties of the structure
(stiffness, mass and damping matrices) and can be used to help find the location and extent of
damage. Optimal matrix update method is one of the VBDI algorithms that depends on finite
element modelling (FEM) of the structure and is therefore referred to as model-based damage
identification algorithm. The FRF differences method is also one of the VBDI techniques that
depends on the directly measured frequency response functions data and is therefore referred
to as non model-based or modal- based damage identification algorithm. However, VBDI
algorithms still faces a number of challenges that have not been fully resolved. Some of these
challenges are highlighted through modal tests designed to provide estimates of damage in a
3D eight-bay free-free frame. Details of tests on a healthy structure as well as on a structure
in which predetermined damage has been introduced are presented. A proposed algorithm
combining the aforementioned model-based and non-model based methods is introduced to
improve the reliability of damage detection. The algorithm is first tested through numerical
simulation to predicting damage on the basis of modal test data and the predictions are
compared with the known damage.