Time affect sets

This page has been superseded by Time affect sets

Time affect sets

The information in the Time Affect Sets is not used by any of the computation facilities under Load-cases on the main menu nor by the Transformed Section Properties facility.

When the user applies a Time Affect Set (using the Apply a set item under Time affect sets on the main menu) the Other Distortion of the various components are affected and that in turn affects the main computations.

The manual keying of shrinkage and creep directly into the Other Distortion number boxes in the component edit dialogs might seem to be a trivial mater and that there is no need for time affect sets.

It is envisaged that the application of the time affects will be part of a sequence of computation and that sequence will be repeated many times as the design of a structure progresses from an initial concept to a final perfected design. The repeated use of a Time Affect Set makes it worthwhile.

The Time Affect Set edit dialog

To get a Time affect set edit dialog from a blank new cross-section description:

  1. Click on the “Add” item under Time Affect Sets on the main menu. This will add a Time Affect Set to the Time Affect Set list.
  2. Click on this new Time Affect Set in the list to select it.
  3. Click the “Edit” item under Time Affect Sets on the main menu. This will open the Time Affect Set edit dialog with a Time Affect Set description that is all default values.

The Time Affect Set edit dialog is a table edit dialog although unlike the other table edit dialogs it has a fixed number of lines; there being one line for each component. The table has four columns:

  1. A line number
  2. The component name
  3. A creep coefficient. That is the ratio of the creep strain to the elastic strain of the loading causing the creep.
  4. A shrinkage; a strain that is constant over the area of the component. This is to be expressed as a positive number although when the set is applied it is a negative contribution to the Other Distortion strain.

Near the top of this dialog is a combination list box labeled “Creep load =” and displaying the selection “No affect”. The other possible selections from this list are the Load-cases in the main Load-case list.

Applying a Time Affect Set

A Time Affect Set pertains to a specific period of time in the life a structural member. The envisaged circumstances during that period and the duration length of the period would be important in determining the various coefficients and shrinkage values. Also important is the loading during that period.

In the physical model that is the basis of this facility the loading is assumed to be constant during the period. A load case describing that loading is required and that implies objective values for all three loading variables.

If there is a significant bending moment the Given Bending Moment should work. If however, the bending moment is small it may be necessary to resort to the Utility: Curvatures and Axial Load method and use manual trial and adjustment of the curvatures to find the distortion corresponding to the loading.

The procedure is:

  1. Add a Load-case, give it a name indicating its purpose and set it up with the envisaged loading that would cause the creep.
  2. Add a Time Affect Set, give it a name indicating its purpose and fill in the appropriate values for the creep coefficients and shrinkage.
  3. In the Time Affect Set select the load case set up in step 1 as the creep load.
  4. Compute the Load-case set up in step 1.
  5. Apply the Time Affect Set using the Apply Set item under Time Affect Sets on the main menu.

Note that only steps 4 and 5 would be required in every cycle of a design process.

Note also that the application of a Time Affect Set makes a change in the cross-section description and thus makes all the Load-case results invalid.

In many circumstances creep and shrinkage affects the loading on a cross-section so that the assumption of constant loading is not true. This procedure is a compromise; an approximation.

Time affects not included in the automatic facilities

The Apply Set item under Time Affect Sets on the main menu automatically affects the “other” distortion variables on the component description dialogs. With the use of these facilities there should never be a need to edit these variable directly on those dialogs.

Nevertheless those variables on those dialogs are editable.

Any affect can be included in a point component and any affect that obeys plane-sections-remain-plane within a shape component can be included in a shape component. For example concrete drying shrinkage that varies through the depth of a horizontal slab can be included as a curvature. Also thermal expansion and contraction can be included. For example the day to night change in the temperature of a bridge deck slab can be more at the top surface than the bottom which implies curvatures.