Abstract: Fundamental principles of mechanics have recently beenbrought to bear on problems concerning very largestructures. Fields of study include tectonic platemotion, nuclear waste repository vault closuremechanisms, the flow of glacier and sea ice, andhighway bridge damage assessment and residual lifeprediction. Quantitative observations, appropriate forformulating and verifying models, are still scarcehowever, so the need to adapt new methods ofexperimental mechanics is clear. Large dynamic systemsoften exist in environments subject to rapid change.Therefore, a simple field technique that incorporatesshort time scales and short gage lengths is required.Further, the measuring methods must yield displacementsreliably, and under oft-times adverse field conditions.Fortunately, the advantages conferred by anexperimental mechanics technique known as specklephotography nicely fulfill this rather stringent set ofperformance requirements. Speckle seemed to lend itselfnicely to the application since it is robust andrelatively inexpensive. Experiment requirements areminimal - a camera, high resolution film, illumination,and an optically rough surface. Perhaps most importantis speckle's distinct advantage over point-by-pointmethods: It maps the two dimensional displacementvectors of the whole field of interest. And finally,given the method's high spatial resolution, relativelyshort observation times are necessary. In this paper wediscuss speckle, two variations of which were used togage the deformation of a reinforced concrete bridgestructure subjected to bending loads. The measurementtechnique proved to be easily applied, and yielded thelocation of the neutral axis self consistently. Theresearch demonstrates the feasibility of using wholefield techniques to detect and quantify surface strainsof large structures under load. !11
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