Structural identification (St-ID) is an accepted paradigm for the assessment of constructed systems. The premise of St-ID is to obtain information about un-measurable responses, such as structural capacity, through correlation of experimental data with finite element models. This paper will focus on several limitations to St-ID, presented through two case studies; a typical steel-stringer highway bridge and a long-span, steel truss with a lift span. As part of an international research effort, the steel-stringer bridge was tested with numerous approaches over a span of two years. The bridge consisted of two sister bridges, each with four spans of varying lengths and geometries. The experimental efforts were focused on two spans. Testing methods included static truck testing, ambient and forced vibration testing, and in-service monitoring. Limitations of St-ID for this structure include correlation of local and global responses, structural nonlinearity and practical complications. The steel truss bridge is subject to an on-going, long-term monitoring project which includes calibrated live load and temperature strain measurements and instrumented bearings. The bridge was also tested on a shorter temporal scale through ambient vibration. Limitations of St-ID include difficulties with load rating given the predominant effects of intrinsic forces and mode coupling across spans.

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