Explanation:


When a truss is said to be internally unstable, it means that the internal forces within the truss members cannot be determined uniquely. This occurs when there are more unknown forces (internal forces) than the available equations of equilibrium. In other words, the truss is statically indeterminate.

Statically Determinate:


A statically determinate truss is one in which the internal forces can be determined using the equations of static equilibrium. This occurs when there are the same number of unknown forces as the available equations of equilibrium. In a statically determinate truss, the internal forces can be uniquely determined and the truss is internally stable.

Statically Indeterminate:


A statically indeterminate truss is one in which the internal forces cannot be determined using the equations of static equilibrium alone. This occurs when there are more unknown forces than the available equations of equilibrium. In a statically indeterminate truss, the internal forces cannot be uniquely determined and the truss is internally unstable.

Reason for not using internally unstable truss:


Internally unstable trusses should not be used because they are prone to excessive deformations and failure. The internal forces cannot be accurately determined, which means that the truss may experience unpredictable and uncontrolled load distributions. This can result in unexpected stresses and deformations, leading to structural failures.

Importance of structural stability:


Structural stability is crucial for the safety and reliability of any structure. In the case of trusses, internal stability ensures that the members are subjected to the intended forces and load distributions. It allows for accurate analysis and design, ensuring that the truss can safely carry the anticipated loads without excessive deflections or failures.

Conclusion:


In summary, when a truss is internally unstable (statically indeterminate), it should not be used as it can lead to unpredictable load distributions, excessive deformations, and structural failure. Statically determinate trusses are preferred as they allow for accurate determination of internal forces and ensure structural stability.