How is the mobility of a statically indeterminate structure characterized?

• A. lesser than or equal to -1
• B. 0
• C. 1
• D. greater than/equal to 2

Answer: (A)

The mobility of a statically indeterminate structure refers to the number of independent displacements or degrees of freedom that it possesses due to the existence of internal stresses that cannot be resolved through equilibrium equations alone.

For statically indeterminate structures, the mobility is characterized by being lesser than or equal to -1. This is derived from the relationships between the number of reactions, the number of supports, and the number of joints or components involved in the structure. Essentially, a structure is classified as statically indeterminate when the number of expected constraints (equilibrium equations) is insufficient to solve for the corresponding unknown reactions, leading to a negative mobility value.

In simpler terms, a statically indeterminate structure is "over-constrained," meaning it has more constraints than can be satisfied by equilibrium equations alone. This condition creates additional complexities, such as the potential for internal stress redistribution and deformation patterns that would not occur in statically determinate structures, which typically have a mobility of zero or positive values. Thus, the correct characterization of mobility for statically indeterminate structures aligns with the criteria of being less than or equal to -1.