The in-situ evaluation of the tensile load in cables and tie-rods plays a primary role in the continuous monitoring and health assessment of strategic as well as historic structures. The main advantage of the indirect determination of cable forces from dynamic tests in operational conditions is the opportunity to achieve accurate, cheap and fast quality checks in the construction phase (after pre-stressing), and safety checks and structural maintenance over the structure lifespan. Based on the most recent developments in the field of operational modal analysis, an automated system for continuous monitoring of axial loads based on dynamic measurements has been developed. The system consists of a distributed measurement system based on programmable hardware and wireless modules, and of a centralized data processing server for the estimation of the tensile loads in the monitored cable from records of its dynamic response to ambient vibrations. The experimental identification of the flexural modes in terms of natural frequencies and mode shapes is used to solve an inverse problem for the identification of the axial loads. A prototype of the SHM system has been installed to monitor the tensile load in one of the cables of a sample steel arch. Preliminary experimental results from continuous monitoring of the cable are discussed pointing out the effect of environmental factors.