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4 Kinematically unstable structures could not be analyzed by methods of structural mechanics. They represent mechanisms and are studied by engineering mechanics. Before starting the force analysis, one should check if the structure kinematically stable or not. The reason of instability could be internal or external. internally deficientexternally deficient CLASSIFICATION OF STRUCTURAL ANALYSIS PROBLEMS

5 Instability could be instantaneous and permanent. Usually, structures which are unstable instantaneously, could be analyzed as geometrically nonlinear problems, but this is a special part of structural mechanics science. CLASSIFICATION OF STRUCTURAL ANALYSIS PROBLEMS

6 The structure is statically determinate if internal forces in all members and all constraint forces could be determined using equations of equilibrium only. CLASSIFICATION OF STRUCTURAL ANALYSIS PROBLEMS statically determinate statically indeterminate

7 EXAMPLES OF TRUSSES USED IN BRIDGES

Statically determinateStatically indeterminate Equilibrium equations could be directly solved, and thus forces could be calculated in an easy way Equilibrium equations could be solved only when coupled with physical law and compatibility equations Stress state depends only on geometry & loading Stress state depends on rigidities Not survivable, moderately used in modern aviation (due to damage tolerance requirement) Survivable, widely used in modern aviation (due to damage tolerance property) Easy to manufactureHard to manufacture 8 CLASSIFICATION OF STRUCTURAL ANALYSIS PROBLEMS