Transient response analysis

A transient response analysis calculates 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.

Capabilities

• Supported transient load types are "Applied loads" (general purpose time-dependent loads), "Initial displacements" (displacements that are enforced at time zero and then removed),  "Initial velocities" (velocities that are enforced at time zero and then removed), "Base acceleration (translation)" (support nodes accelerating in-phase in any direction), "Base acceleration (rotation)" (support nodes accelerating in-phase about any node), "Harmonic loads" (masses vibrating in a regular sinusoidal motion) or "Periodic loads" (arbitrary time-dependent loads that repeat periodically).
  
  
• A "Factor versus time table" can be defined for most load types that defines the time-dependent properties of the load source.
  
  
• Load sources of different types and with different time-dependent properties can be combined within a transient load case.
  
  
• Transient result load cases can be combined with static and other load case types using combination load cases.
  
  
• Damping can be taken into account and applied as either "Modal" or "Rayleigh" damping, or it can be disabled.
  
  
• You can choose the time range and the number of steps over that range at which calculations are done.
  
  
• After a transient response analysis, the model can be animated to show how it responds to the transient loads.
  
  
• Displacement, velocity, acceleration and phase diagrams can be displayed for any nodes in the structure. These are time-history curves that shows how the structure responds to the time-dependent load sources.
  
  
• Each time step (or just selected steps) can be converted to result load cases that contain displacements, forces, moments, stresses and reactions, just like you would get from a static analysis. You can show diagrams or get reports for these load cases or use them in the normal way in any of the design modules.
  
  
• Harmonic loads are supported in a transient response analysis, however if you are interested in the structure's frequency response (ie. displacement versus frequency) to harmonic loads then you should run a harmonic response analysis instead.
  

Important points

1. A transient response analysis is linear only and therefore cannot be performed if your model contains cable elements.
   
   
2. Because it is linear, a transient response analysis treats tension-only and compression-only members as normal members that can take tension or compression.
   
   
3. P-D and P-d effects are not taken into account during a transient response analysis.
   
   
4. A buckling analysis cannot be performed with transient load cases and therefore compression effective lengths from a buckling analysis are not available when doing a steel member design/check on transient load cases. If you are performing a steel member design/check on combination load cases that contain a mixture of static and transient load cases then the transient load cases will not contribute to the calculation of the compression effective lengths. This may not be correct and so you should consider specifying your compression effective lengths manually in those cases.
   
   
5. A transient response analysis requires a dynamic frequency analysis to be conducted first.
   
   
6. The transient response analysis must be repeated after a dynamic frequency analysis because its results will have been deleted.
   
   
7. The accuracy of the transient response analysis depends on the accuracy of the dynamic frequency analysis on which it is based. It is therefore important that you set up your model correctly to achieve accurate dynamic frequency analysis results. For example, if master-slave constraints are used then positioning of the master nodes is particularly important for correct mass distribution. For more information, refer to "Dynamic frequency analysis".

For a full step-by-step guide on how to prepare for and perform a transient analysis, refer to "Step by step guide to transient response analysis".