| Structural Monitoring |  | In-situ Stress Measurement | | Bridges |
Both the ASTM E837 test method and hole drilling strain gauge method are used to determine in-situ stress in metallic bridge components. A further development of the technique by Strainstall for concrete bridges, uses special strain gauge targets and drilling equipment to enable measurements to be made on prestressing wires as small as 4mm diameter. This technique has been widely used during programmes of Post-Tensioning Bridge Inspection, where the opportunity has been taken to measure the post-tensioning forces in tendons, whilst exposed for inspection.
One further variation is pressure compensation slotting, which provides a direct measurement of uniaxial stress in the structure. Cable stays and tie rods can also be tested using vibration monitoring equipment, to measure natural frequency, from which 
the tension in the cable or rod can be calculated.
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