. The use of the strain gage to determine stresses in steel structures . tions, however, are twofold. fl) Two men areneeded as observers, as against one for the shorter instrument.(2) It can only be used to determine stresses in comparativelylong members and only over comparatively long lengths on it would be useless in such a test as the one on the IJorthAvenue Bridge, where it was desired to investigate for high localstresses in a bent bar. In preparing for a test, the members to be tested areselected and the position of the gage lines upon them carefullydetermined. Gage holes .05
. The use of the strain gage to determine stresses in steel structures . tions, however, are twofold. fl) Two men areneeded as observers, as against one for the shorter instrument.(2) It can only be used to determine stresses in comparativelylong members and only over comparatively long lengths on it would be useless in such a test as the one on the IJorthAvenue Bridge, where it was desired to investigate for high localstresses in a bent bar. In preparing for a test, the members to be tested areselected and the position of the gage lines upon them carefullydetermined. Gage holes .055 in. in diameter are then drilled toa depth about equal to the diameter. The drill used in thesetests was a hand drill and considerable care was exercised inkeeping it as nearly as possible normal to the surface to bedrilled. The holes were then reamed to make them smooth. Thereading of the Ames gage head for each hole was next taken whilethe member was unstressed. No records were made of these readingsuntil five consecutive readings were obtained which varied by not. 8 more than two divisions on the Ames gage head, and the number thenrecorded was the average of the five. This average number willhe known as an observation. This precaution was necessary, forit was found that a variation in the direction or intensity ofthe pressure used in entering the legs of the strain gpge intothe holes seriously affected the readings obtained. Becausedifferent observers applied the strain gage to the holes differ-ently, the same observer was always used in obtaining observations under zero load and under applied load. While observations on the established gage lines were beingtaken, reference observations were taken on gage lines establishedupon separate bars of steel. These observations were taken veryfrequently. One was usually obtained for every four or fiveobservations upon the member being tested. The observations onthese separate bars are known as standard observations and thebars them
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Keywords: ., bookcentury1900, bookdecade1910, booksubjecttheses, bookyear1914
