# Stability - Stable & Unstable Structures & Members

## 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 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 Structural Members

## STABILITY OF BEAMS

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

- The number of unknown reactions must be greater or equal to available equations of equilibrium
- All the reactions should not be parallel to each other.
- 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.