# Force: Spring Damper

Command ElementModifies a spring-damper force element.

## Format

<Force_SpringDamper
id               = "integer"
i_marker_id      = "integer"
j_marker_id      = "integer"
stiffness        = "real"
damping          = "real"
length           = "real"
/>

## Attributes

id
Element identification number (integer > 0). This number is unique among all Force_SpringDamper elements.
i_marker_id
Modifies the Reference_Marker at which the force or torque is applied. This is designated as the point of application of the force.
j_marker_id
Modifies the Reference_Marker at which the reaction force or torque is applied. This is designated as the point of application of the reaction force.
length
Modifies the free-length or the installed (reference) length of the spring. It has to be strictly positive. A zero or negative value is not allowed.
stiffness
Modifies the spring stiffness. For translational spring-dampers, this has units of force/unit translational deflection. For rotational spring-dampers, this has units of torque/radian. Note that stiffness > 0. A negative value is not allowed.
damping
Modifies the damping coefficient. For translational spring-dampers, this has units of force/unit translational deflection velocity. For rotational spring-dampers, this has units of torque/unit angular deflection velocity.

Note that damping > 0. A negative value is not allowed.

Modifies any assembly or pre-load in the spring-damper. This is the force or torque when the distance or angle between the I and J markers is equal to the value specified by length.
A positive value implies a force that tends to increase the distance or a torque that tends to increase the angle (repulsive), and vice versa.

## Example

<Force_SpringDamper
id        = "105001"
length    = "334.12"
stiffness = "138.444"
damping   = "0."
/>