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.
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.
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.
- 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.