This page has been superseded by Chronological sequences

Chronological Sequence

A sequence of computations that follows the chronological sequence of events in the life of a structural member is envisaged. This provides a framework for the description of some of the facilities in this application. It is also envisaged that such a sequence will be repeated many times as a design progresses from an initial concept to a final perfected design.

Load cases with all three loading variable as objectives

Both the adoption of a stage load and also the computation of creep require a load case where all three loading variables are objectives. This is one of the purposes of the “Given bending moment” method. This method will not work however in situations where the bending moment is small. An example to illustrate this is a pretensioned member immediately after the tendons are released. The method “Utility: Curvatures and axial load” is provided for this situation. In the case of the pretensioned member immediately after the tendons are released the axial load is zero. With the curvatures also set to zero this method will compute the bending moment if the member stays flat on the pretensioning bed. That bending moment can be compared with the bending moment from the self weight if the member were to lift itself off the pretensioning bed. A bending moment with zero curvature that is greater indicates the member would lift off the bed. In this case the curvature can be adjusted manually to find the curvature where the bending moments are equal.

Time affects

For the steps to apply a Time Affect Set that includes creep computation see Time Affect Set – Applying a Time Affect Set.

Adopting a stage load

The initial state and stage distortion facilities have a purpose illustrated by two parallel structural members in close association but structurally independent; each having a loading and corresponding distortions. At a particular instant in the construction process these two members are joined so they act as one structural member. Immediately after that instant the resulting member has a loading that is the combined affect of the two loadings before and each component has the same distortion as before that instant.

The facilities use three XSF files; one for each of the contributing members cross-sections, and one for the resulting member cross-section.

The procedure is:

1. For each of the contributing members set up the XSF file including load cases and Time Affect Sets for the chronological sequence before the members are joined and save the file.

2. With each of the contributing member XSF files add a Load-case, give it a name indicating its purpose and set it up with the envisaged loading at the instant members are joined and save the file.

3. With each of the contributing members XSF files:

1) Do the computation for the chronological sequence before the members are joined

2) Compute the load case for the instant the members are joined

3) In the Load-case list on the main dialog make sure the load case for the instant the members are joined is selected.

4) Click on main menu Load Cases “Adopt stage load” item.

5) save the file.

4. Open the file for the resulting member cross-section.

5. Delete all the components and all the materials that remain from a previous cycle of the design process. The Load-cases and Time Affect Sets from the previous cycle should remain and can be used again.

6. For each of the contributing members (with the resulting member file open):

1) Click the main menu “Components” menu item “Import from another XSF”. This should bring up the “Other XSF files” dialog.

2) Click on the “Open other XSF” button. This should bring up the standard “Open file” dialog.

3) Navigate to, and open the file for the contributing member cross-section. This should populate the various list boxes in the “Other XSF files” dialog.

4) In the “Components to import” list select each component and then click on the “Import” button. This should copy the components descriptions into the resulting member and also the materials they reference (unless there is a material of the same name already there).

5) Click on the close button to close the “Other XSF files” dialog.

6) Save the file.

This procedure sets up the initial state of the resulting member.

A further step is a check and also illustrates the meaning of this. Use a load case to do a computation with the “Utility: Curvatures and one strain” method with all three distortion variables set to zero. This should result in a loading that is the combined affect of the loadings on the contributing members.

Note the setting up of the Load-cases and Time Affect Sets in the resulting XSF file for the chronological sequence after the members are joined need only be done in the first cycle of the design process. Likewise steps 1 and 2 should be required only once.

This description of this procedure has been devised to cope with the most complicated design. In practice often the procedure is less complicated. For example the setting of wet concrete can be perceived as the instant of joining; the newly set concrete and any mild steel embedded in it being one of the contributing members. That contributing member has no loading and no distortion. In these cases the XSF file for the other contributing member can be used for the resulting member. In effect the new concrete is added.

At the beginning of the next cycle in the design process that concrete and the steel embedded in it can be deleted.

If the chronological sequence starts before any concrete is cast or pretension tendons are tensioned all the stage distortions and other distortions should be set to zero at the beginning of each cycle. The main menu Load Cases “Zero all stage distortions” item and main menu Time Affect Sets “Set all components to zero” item are intended for this.

The aging of concrete

The stress-strain relationship of concrete changes as the concrete ages. A sequence of xsf files could be set up that each have a set of material descriptions appropriate to different points in the chronology. “Materials” on the main menu has an item “Import from another xsf file”. This leads to a dialog where the other file can be stipulated and it includes a button labeled “Overwrite all” intended for this purpose.