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Structural Engineering Software


Transient Response Analysis

Dynamic Response to Transient Loads

The SPACE GASS transient response module analyses the effect of time-dependent load sources that act in any regular or arbitrary manner on a structure. Typical transient load sources include wind gusts, seismic events, vibrating machines, blast loads, pulse loads, footfall excitation, moving traffic, etc.

  • Problem types are:
    • Applied loads
    • Initial displacements
    • Initial velocities
    • Base translation acceleration
    • Base rotation acceleration
    • Harmonic loads
    • Periodic loads
  • Load sources of different types and with different time-dependent properties can be combined within a transient load case.
  • User-defined factor vs time tables can be defined for most load types that define the time-dependent properties of the load source.
  • Transient, harmonic, spectral and static analysis results can be combined.
  • Damping can be Modal or Rayleigh.
  • You can specify the number of calculation steps and the time range.
  • The structure can be animated to see how it responds to the transient loads.
  • Graphs of displacements, velocities and accelerations vs time are available.
  • Displacements, forces, moments, stresses and reactions are available for each time step.

Operating the module simply involves you defining your time-dependent load sources, creating some transient load cases and then running the analysis. Once analysed, you can animate the model to see how it responds to the transient loads or you can generate graphs of displacements, velocities or accelerations vs time. From there you can convert any of the time steps to load cases that contain displacements, forces, moments, stresses and reactions, just like you would get from a static analysis.

Transient Load Cases

Each transient load case is defined by a source load case, problem type, mass case, base acceleration (for base acceleration problem types), factor, frequency (for harmonic problem types), period (for periodic problem types) and phase (for harmonic or periodic problem types). You can combine loads with different time-dependent properties by defining multiple lines in the datasheet that share the same transient load case number.

Problem Types

Transient response problem types are:

  • Applied loads (general purpose time-dependent loads).
  • Initial displacements (displacements that are present at time zero and then removed).
  • Initial velocities (velocities that are present at time zero and then removed).
  • Base translation acceleration (support nodes accelerating in-phase in any direction).
  • Base rotation acceleration (support nodes accelerating in-phase about any axis).
  • Harmonic loads (masses vibrating in a regular sinusoidal motion).
  • Periodic loads (arbitrary time-dependent loads that repeat periodically).


A transient analysis lets you choose the list of load cases to be analysed, the modes to be considered, the damping settings, the time range and the number of time steps.

Each time step represents a snapshot of the structure's response to the transient loads. Put together, they show the complete structural response and can be represented as an animation or as time-based graphs.


The process by which free vibration steadily diminishes in amplitude is called damping. In actual structures, the damping mechanisms include friction at steel connections, opening and closing of micro-cracks in concrete, fireproofing, and friction between the structure itself and non-structural elements such as partition walls.

SPACE GASS allows you to disable damping or specify it as "Modal" or "Rayleigh". Modal damping uses a constant damping ratio for each dynamic mode, whereas Rayleigh damping uses a combination of mass-proportional and stiffness-proportional damping.

Factor vs Time Table

Each transient load case also contains a factor vs time table that defines the time-dependent properties of the applied load, base acceleration and periodic load sources. If no table is defined then the load source is just assumed to be suddenly applied at time zero and held constant. Harmonic loads generally don't require a factor versus time table, however a table can be defined to further vary the time-dependent effect of a harmonic load source if required.


The transient animation shows a realistic representation of how the structure responds to the transient loads. You can change the speed and amplitude of the animation on-the-fly by using the controller at the bottom.


Reports that summarize the transient response results for each load case list the maximum translations, velocities and accelerations together with the time step at which they occur.

Creating Result Load Cases

You can convert any of the transient time steps into load cases that contain displacements, forces, moments, stresses and reactions, just like you would get from a static analysis. They can then be used to generate bending moment diagrams, shear force diagrams, stress diagrams, etc. or used in any of the SPACE GASS steel or RC concrete design modules.