Rigorous slab deflection analysis example (Eurocode)

Tekla Structural Designer
2022
Tekla Structural Designer

Rigorous slab deflection analysis example (Eurocode)

There are many input parameters that will have an impact on the rigorous deflection estimates and would therefore need to be considered. For details see: Factors that affect rigorous slab deflection estimates

For this exercise, initially it will be assumed that the default settings have already been reviewed and set as required.

Download and open the tutorial model

  1. Download the tutorial models from here.
  2. Open the following tutorial model:
    • Slab deflection EC.tsmd

Establish some slab reinforcement

Prior to running a Slab Deflection Analysis, a reasonable level of slab reinforcement should already be provided. This can be achieved by designing all slabs and patches as follows:

  1. From the Analyze toolbar, click Analyze All (Static)
  2. From the Design toolbar, click Design Slabs
  3. From the Design toolbar, click Design Patches

Review the Model Event Sequence

To review the model event sequence:

  1. From the Slab Deflection toolbar, click Event Sequences
  2. Click Model Event Sequence
    This has already been setup for this example as shown below:
    Deflections to the end of each of the above event periods will be calculated by the analysis.
  3. Click OK to close the dialog.

Perform Iterative Slab Deflection Analysis

To establish some initial results (with all parameters left as default values) you could just click Analyse All from the Slab Deflection toolbar, however, in a real model rather than analysing all levels at once, it can be more efficient to work on just the current level, or a selected level. Obviously considering just a single level reduces the time necessary to undertake the iterative slab deflection analysis.

  1. Open a 2D plan view of the level Typical floor
  2. From the Slab Deflection toolbar, click Analyze Current

    After analysis the current view automatically switches into the Slab Deflections View regime.

Review Deflections for Events

Deflections can be reviewed for each event by making selections from the Event droplist in the ribbon.

You are able to 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).
The image below shows total deflection contours for the final load event.
Note: To see the image as shown, you may need to adjust some of the selections in Scene Content, for example Slab Patches should be de-selected.

As a comparison with the simple approach earlier (39.7mm), the Total deflection at the final load event for the chosen location is 35.2mm.

Note: In the above contour plot the deflections are not exactly symmetrical - this is because reinforcement is in the outer layer in the Y direction making the slab stiffer in that direction.

Total deflections to the end of each of the event periods in the Event Sequence are available and could also be displayed as required.

In the Event Sequence there is an event for “Sensitive Finishes added” - we shall now show differential deflection between this and the final event.

  1. From the Slab Deflection toolbar, change the Result Type to Differential
  2. Select the Start Event as Sensitive Finishes added
  3. Select the Event as Final load event
    The 2D view now displays the differential deflections between “Sensitive Finishes added” and the final event.

Review Other Results

In addition to the deflections you can display the extent of cracking at any load event.

You can also review the relative stiffness in a particular result direction for any specified event.

You can also review the area of effective reinforcement for a chosen result direction for each FE element.

  1. Select the Result as Extent of Cracking, and then:
    1. Select the Result Direction as X,
    2. Select the Result Type as Total,
    3. Select the Event as Propping removed,
    The extent of cracking for the Propping removed load event is displayed.
  2. Select the Result as Relative Stiffness, and then:
    1. Select the Result Direction as X - Creep + Cracking + Shrinkage,
    The relative stiffness for the Propping removed load event is displayed.
  3. Select the Result as Effective Reinforcement, and then:
    1. Select the Result Direction as X Top,
    The effective reinforcement for the X Top direction is displayed.
    Note: When you hover over any FE element in the slab deflection view regime - values are provided within the tooltip.

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 toolbar, click Deflection Checks

    Whilst three checks have been defined above, only two of these have been set to be used in new Check Lines:

    • Sensitive finishes will check the differential deflections from when the sensitive finishes are applied to the final load event against a deflection limit of 1/500
    • Total will check the total deflections to the final load event against a deflection limit of 1/250
  2. Click OK to close the dialog.

Place Check Lines

Note: Check lines can only be created in a 2D view.

We will define several check lines in this example:

  1. Ensure you are in the Typical floor 2D plan view and if necessary change:
    1. the Result back to Deflections,
    2. the Result Type to Total,
    3. the Event to Final load event
  2. Click Create
    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.
  3. To place the check line at approximately mid-span between grid lines 4-5 from grid line A to E:
    1. Pick the start point to the left of grid line A as shown:

    2. Pick the end point to the right of grid line E as shown:

    3. Press Esc to end the command.
  4. Right click on the check line and choose Open deflections check view from the context menu.

    The deflection results along the length of the check line are displayed.

    The Slab Deflection toolbar 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.

    If we return to the original deemed-to-satisfy check - this was performed diagonally between columns in bottom right corner panel of the slab - we will now revisit this using a check line.

  5. Create a check line running diagonally between columns in bottom right corner panel (from D/1 to E/2) where the peak deflection occurs.

  6. Right click to open the deflections check view for the new check line.

    A total deflection limit of Span / 250 is the same as saying that the average slope between points of maximum and minimum deflection = 1 / 125. In the view above the average slope between these points is 1 / 155, i.e. less than the limit.

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

Generate Check Line Reports

A tabulated report is available for each check line which itemises 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.

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

    As previously noted, the slope above is reported as 1:155 which is not less than the allowable slope limit of 1:125 and hence a Pass.

    If you click within the Load Analysis Deflection view, the ribbon changes to allow you to display deflection results and slopes for the Result Type - Total or Instantaneous for a chosen event, or Differential between chosen events.

  2. Switch to the Load Analysis Deflection view for the diagonal check line.
  3. From the Slab Deflection toolbar, change the Result type to Differential and check deflection and slopes between theSensitive finishes added and Final load event.

    We can add as many check lines to the model as we consider appropriate.

  4. Return to the Typical floor 2D view.
  5. Add three further check lines using the default deflection checks in the catalogue as follows:
    1. along grid line 1, from A/1 to E/1
    2. along grid line B, from B/1 to B/5
    3. half way between grid line A and B, starting at grid line 1 and finishing at grid line 5

  6. Press Esc to end the command.

    If you select each Check Line in turn you are able to edit the deflection checks associated with it in the Properties Window.

  7. Select the check along grid line that runs along grid line 1 and ensure that it also has a Check #3 defined as Cladding.

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 Typical floor 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 slab items between C-D / 2-3 and C-E / 3-4 so the slab reports Unknown as no checks have been performed.
    • One passing check line crosses the slabs between C-E / 4-5 so the slab reports a Pass.
    • 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 slab items Fail where the check lines runs along A/1-E/1.
  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. Highlight the check line running along grid line B and make a note of its utilization.
    Tip: Press the tab key if necessary to highlight the check line when it is directly under the mouse cursor.
  2. Click Slab Reinforcement in the Show/Alter State group to show the existing reinforcement.
  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 the Reinforcement Direction to BarsY to see the bars in that direction

  4. Click on one of the corner slab panels to select H16 150 as the reinforcement to be applied.

  5. Click on the eight slab panels currently showing H16 bars at 175 spacings to change them to the 150 spacings.

  6. Change the Reinforcement Direction to BarsX to see the bars in that direction.
  7. Edit the same 8 panels in exactly the same way, so that the bars in X match the bars in Y.
  8. Click Analyse Current to update the results
  9. Click Utilization in the Check Lines group to show the critical utilization for each check line once again.
  10. Investigate the tooltip results

    You should find that although some utilizations have reduced, the line along grid line B is still failing. At this point you could begin to look at the impact of the various other input parameters. For now, we will adjust the concrete grade of the typical floor slab group from C30/37 to C35/45.
  11. Right click slab SI 4 between D-E/1-2 on the typical floor level and choose Edit.... from the context menu.
    Note: Editing a slab item via the right click context menu updates the parent slab properties, not just the individual slab item. Hence any changes will be applied to all the slab items in S 1.
  12. On the Slab concrete page of the dialog, change the Concrete Strength to C35/45 and click OK

    To update the check line results we need to re-run the analysis. A chasedown analysis is automatically performed as part of the slab deflection analysis, however, it should be borne in mind that some edits could affect the element design i.e. reducing the concrete slab thickness would result in an increase to the required reinforcement and hence a Design Concrete (Static), Slab or patch design may be required again.
  13. Click Analyse Current again to update the results.
  14. Review the Check Lines Status.

    The check line with the more onerous Cladding deflection limit is still failing, but we can clearly see improvements in the results. It has reduced from a Utilization ratio of 1.538 to 1.262.

    In fact, to obtain a Pass we would have needed to increase the concrete grade to C45/55.

    Note: For the purpose of this exercise, as the cladding check is not a code requirement we will simply disable it before proceeding to generate the model report.
  15. Select the check line along grid 1 and then in the Properties window, reset Check #3 to None.

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 customised 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 Chapters and Options, drag Concrete>Slab/Mat Design per Plane>Slab Deflection Check Lines to the Report Structure area

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

  8. In the Filter dialog, click Add and select Selected levels and ensure a check against Typical floor

  9. Click OK to return to the Report Contents dialog.
  10. In the Report Structure, right click View, then choose Settings...
  11. Select the Slab Deflection view you created earlier

  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 the Show Report command to open the report.

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