The non-destructive structural health monitoring procedure presented here is used for large-area monitoring of complex steel structures. Piezoceramic transducers are applied as a cluster to the structure under observation with the principle of beam-forming. The transducers are used both as sensors and as actuators and initiate guided Lamb wave propagation in the structure. Damage is detected by changed structural response. In the application presented here the method is applied to 10 mm thick steel plates under cyclic fatigue loading, and cracks as small as 1 mm long are identified at the anticipated critical regions at the notches. The distinct results show the capability of the developed procedure to detect and locate real damage in complex steel structures.