Total, differential, and instantaneous deflection types

Tekla Structural Designer
Tekla Structural Designer

Total, differential, and instantaneous deflection types

Once the slab deflection analysis has been run, three deflection types are available for review:

  • Total deflection at the end of any event.
  • Differential deflection between any two events (Start of Event and End of Event).
  • Instantaneous deflection (not actually needed for TR 58).
    • This is the deflection when the entire event loading is applied to a version of the model using the established extent of cracking along with short term analysis properties.
    • US codes require an assessment of the instantaneous deflection associated with the imposed load only. This is achieved by adding extra events at the same time as the final event where only the required imposed load is applied.

Understanding differential deflections

When looking at differential deflections it is important to appreciate that an event is not a single point in time, it is a time period which has different deflections at the start and the end - so it is logical that there can be a differential deflection for a single event.

To clarify the selections requested in the ribbon:

  • Start Event defines the start of the start event
  • Event defines the end of the event

Therefore, if you specify the same event as the start and the end event, you will still see a differential deflection.

To clarify further, consider the scenario below:

  • Event 1 - Construction loading
  • Event 2 - Application of Sensitive Finishes
  • Event 3 - Application of Additional Finishes
  • Event 4 - Occupation (all loading with live load set at long term factor)
  • Event 5 - Final loading event (as above with live load set at 100%)

To determine the maximum differential deflection relevant to the application of sensitive finishes you would define this as the deflection between Events 2 and 5 (Start of Event 2 to End of Event 5). If you opted to pick Events 1 and 5 you would get an un-conservative deflection reported since the effect of construction loading is also considered.