Use of check lines to check deflections (ACI)

Tekla Structural Designer
2021
Tekla Structural Designer

Use of check lines to check deflections (ACI)

Once the input parameters have been set as required to allow for creep and shrinkage, and the analysis has been performed, check lines can then be placed in order to check the deflections.

Download and open the tutorial model

  1. Download the tutorial models from here.
  2. Open the following tutorial model:
    • Slab deflection ACI - Check Lines.tsmd

Define Check Line Deflection Checks

Check lines have to initially be positioned using engineering judgment.

The deflection checks associated with each check line are selected from a predefined Deflection Check Catalogue. This is viewed by clicking Deflection Checks in the ribbon.

You can add new checks to the catalogue as required.

  1. From the Slab Deflection ribbon, click Deflection Checks

    Whilst four checks have been defined above, only three of these have been set to be used in new Check Lines. The more onerous cladding check which does not come from code requirements is not applied by default:

    • Sensitive finishes will check the differential deflections from when the sensitive finishes are applied to the final load event against a deflection limit of 1/480
    • Total will check the total deflections to the final load event against a deflection limit of 1/240
    • Live load only will check the instantaneous deflections for the live load only event against a deflection limit of 1/360
  2. Click OK to close the dialog.

Place Check Lines

We will define two check lines in this example:

  1. Open the St.1 (1) 2D plan view

    Note: When the 2D plan view is being viewed in 3D as above, the Create button on the Slab Deflection toolbar is grayed out.
  2. Click in the bottom right corner of the view to toggle the view until it is displayed in 2D.

  3. From the Slab Deflection toolbar, ensure the Results are set as above:
    1. Result - Deflections,
    2. Result Direction - Z,
    3. Result Type - Total,
    4. Event - Final load event
  4. Click Create
    Note: Check lines can only be created in a 2D view.
    Note: When you click Create, the Properties Window automatically includes the slab deflection checks from the catalogue for which “Use in new Check Lines” was checked.
  5. To place a check line running diagonally between columns in bottom right corner panel where the peak deflection occurs:
    1. Pick the start point as grid line intersection D/1.
    2. Pick the end point as grid line intersection E/2.

    Before creating a second check line, in the Properties Window add a check #3, by selecting Cladding from the droplist.
  6. Create the second check line along grid line 1.
    1. Pick the start point as grid line intersection A/1.
    2. Pick the end point as grid line intersection E/1.

  7. Create the third check line along grid line B.
    1. Pick the start point as grid line intersection B/1.
    2. Pick the end point as grid line intersection B/5.

  8. Create the final check line mid-way between grid lines 4-5 from grid line A to E.
    1. Pick the start point approximately as shown:

    2. Pick the end point approximately as shown:

  9. Press Esc to end the command.
    Note: In a real model you can add as many check lines to the model as you consider appropriate.
  10. Right click on the diagonal check line and from the context menu select Open deflections check view.


    The ribbon allows you to specify the total (as shown above), or differential or instantaneous results for the selected events. Tekla Structural Designer then draws average slopes between maximum and minimum points.

    A total deflection limit of Span / 240 is the same as saying that the average slope between points of maximum and minimum deflection = 1 / 120. In the view above the average slope between these points is 1 / 138, so the check passes.

    Doing the checks this way inherently deals with offset peak deflections (non-uniform load) and also cantilevers.

  11. Click within the Deflection load analysis view and change the Result type to Differential and check deflection and slopes between the Finishes added and Final load event. The resulting differential deflection is as shown:

    For the Sensitive Finishes check of differential deflection between finishes added and the final load limit had a limit of Span / 480, which is the same as saying that the average slope between points of maximum and minimum deflection = 1 / 240. In the view above the average slope between these points is 1 / 194, so this check fails.

Generate Check Line Reports

A tabulated report is available for each check line which itemizes each requested deflection check along the check line.

You can generate a check line report for an individual check line by right clicking the check line and selecting Member Report.

Return to the St.1 (1) 2D view, right click on the diagonal check line and select Report for Member

As previously stated, the Sensitive Finishes check fails since the slope for differential deflection is reported as 1:206 which is greater than the allowable slope limit of 1:240.

Review Check Line Status and Utilization

Every check line can have up to six different deflection checks assigned to it for different events. Every check line will therefore have a Pass/Fail status and a critical Utilization ratio.

  1. Click on the St.1 (1) 2D 2D view to make it active.
  2. To make it easier to see the check lines, change the Result droplist from Deflections to None.
  3. Click Status in the Check Lines group of the ribbon to see the pass/fail status graphically displayed for each check line.

    Tip: You can also hover over a check line and the tooltip displays the utilization and pass/fall status.
  4. Click Utilization in the Check Lines group to show the critical utilization for each check line and investigate the tooltip results.

    Once check lines are set up and any re-analysis is undertaken i.e. due to optimization of a slab design, adding reinforcement etc. then the check lines are automatically re-checked so you will obtain instant feedback on status and utilization

Review Slab Status and Utilization

Every check line is associated with at least one slab item. If all checks lines associated with the slab item pass then the slab item passes.

Both the Status and the Utilization can be reviewed.

  1. Click Status in the Slab Deflections group of the ribbon to see the pass/fail status of each slab.

    • No check lines cross the majority of the slab items so they currently have a status of Unknown.
    • A fail at any point in a check line causes the status of every slab item crossed by the check line to report as a Fail. Hence, the 4 slab items crossed by the horizontal check line Fail.
    • One slab item is being crossed (only just, in one corner) by the passing diagonal check line, this is the only slab to have a pass status.
  2. Click Utilization in the Slab Deflections group to show the Utilization of each slab item.

    This is the worst utilization from all associated check lines.


Optimization

If you find that a slab either fails deflection checks (or passes with a low utilization) then changes will need to be made. There are many areas where adjustments can be made.

  • Adjust slab/panel depths?
  • Adjust reinforcement?
  • Re-consider the Event timings and loadings?
  • Adjust other settings?

Any or all changes are possible.

When changes are necessary we would recommend that it will be more efficient to work on one level at a time. The automatic re-checking of check lines will help considerably with this optimization.

The analysis is extremely quick and since everything is contained within one model file, it allows “What If” scenarios to be considered to find the optimum solution.

In this exercise we will start by looking at the impact of adjusting the reinforcement.

  1. Click Slab Reinforcement in the Show/Alter State group to show the existing reinforcement.

  2. In the Properties Window:
    1. Leave the Slab Reinforcement to modify as Panel
    2. Leave the Slab Layer to modify as Bottom
    3. Change the Reinforcement Direction to BarsY to see the bars in that direction

  3. In the Properties Window:
    1. Leave the Slab Reinforcement to modify as Panel
    2. Leave the Slab Layer to modify as Bottom
    3. Change Apply to User Bars
    4. Change Bar size to 7
    5. Change Bar spacing to 6in

  4. Click on each panel to apply the new reinforcement.

  5. Repeat the above process to apply the same bars in the X reinforcement direction also.
  6. Click Analyze Current to update the results
  7. Click Utilization in the Check Lines group to show the critical utilization for each check line once again.
  8. Investigate the tooltip results

    We can clearly see improvements in the results, however the check line along grid 1 with the more onerous Cladding deflection limit is still failing.

    At this point in a real model you could begin to look at the impact of the various other input parameters, such as changing the concrete grade.

    For the purpose of this exercise, and as the cladding check is not a code requirement, we will simply disable it, as follows:

  9. Select the check line along grid 1
  10. In the Properties window, reset Check #4 to None.

  11. Click Analyze Current to update the results
  12. Click Status in the Check Lines group.
    All of the check lines should now pass.

Generate Model report

A Slab Deflection Check Lines model report can be created for the selected Model Filter (entire structure, level, plane or sub structure). This lists all the check lines for the chosen model filter. To help identify the check lines in the report it is sensible to include a saved picture of the scene view displaying check lines and their associated reference within the report.

  1. In Scene Content, switch on the Text display for the Slab Deflection Checks.

    Tip: The check line references can be customized using the Name property for each individual check line.
  2. Right click in the Typical floor 2D view and choose Save View Configuration... from the context menu, then specify a name.

    This saved view can be included in the Check line report.

  3. On the Report ribbon, click Model Report...
  4. Click Add and provide a Name “Check Line Report” for the report.
  5. In Chapters and Options, drag View to the Report Structure area
  6. In the Report Structure, right click View, then choose Settings...
  7. Select the Slab Deflection view you created earlier

  8. Click OK to return to the Report Contents dialog
  9. In Chapters and Options, drag Concrete>Slab/Mat Design per Plane>Slab Deflection Check Lines to the Report Structure area

  10. In the Report Structure, expand Concrete> Slab/Mat Design per Plane> Slab Deflection Check Lines and right click, Model Filter> Edit/New

  11. In the Filter dialog, click Add and select Selected levels and ensure a check against St. 1 (1)

  12. Click OK to return to the Report Contents dialog.
  13. Click OK to exit and save the report.

    A report structure called Check Line Report has now been saved that contains a view and the check lines.

  14. To display the report.
    1. Use the Select drop list in the ribbon to select Check Line Report
    2. Click Show Report to open the report.

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