Strain Gage is intended mainly for long-term or short-term strain measurements on structural steel members such as tunnel linings, arches, struts, piles, sheet piling, etc. The primary means of the attachment is by conservative arc welding, but they may similarly be used to monitor strain fluctuations on concrete or rock surfaces using anchors grouted into boreholes. Strains are measured using the vibrating wire principle: a length of steel wire is tensioned between two mounting blocks that are welded to the steel surface being tested.