Stability – Stable & Unstable Structures & Members

Stability

Definition

The resistance offered by a structure to undesirable movement like sliding, collapsing and over turning etc is called stability.

  • Stability depends upon the supports conditions and arrangements of members.
  • Stability does not depend upon loading.

STABLE STRUCTURES

A stricter is said to be stable if it can resist the applied load without moving OR A structure is said to be stable if it has sufficient number of reactions to resist the load without moving.

UNSTABLE STRUCTURE

A structure which has not sufficient number of reactions to resists the load without moving is called unstable structures.

Stability of different structures

STABILITY OF TRUSS

A truss is said to be stable if it is externally and internally stable

EXTERNAL STABILITY OF TRUSS:

Externally a truss is said to be stable if

  • All the reactions are not parallel to each other.
  • All the reactions are not concurrent i.e. passing through same point

INTERNAL STABILITY OF TRUSS:

Internal stability of truss depends upon the arrangements of members and joints as

*  If  m + r = 2j internally stable 
*  If  m + r < 2j internally unstable 
*  If  m + r > 2j indeterminate

Where m = number of members, J = number of joints, R = number of unknown reactions.

For complete stability the should be both internally and externally stable

STABILITY OF BEAMS

A beam is said to be stable if it satisfy the following conditions.

  1. The number of unknown reactions must be greater or equal to available equations of equilibrium
  2. All the reactions should not be parallel to each other.
  3. There should be no concurrent force system i.e. unknown reactions should not pass through the same point or line.

STABILITY OF FRAME

A frame is said to be stable if it satisfy the following condition.

The number of unknown reactions must is greater to equal to available equations of equilibrium.

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