Concrete member cracked or uncracked status

Tekla Structural Designer
Tekla Structural Designer

Concrete member cracked or uncracked status

Assuming concrete sections are cracked has a direct affect on analysis - smaller Modification Factors are applied to cracked sections, (typically a cracked member is assumed to have half the stiffness of an uncracked member), causing an increase in deflection. Indirectly the design can also be affected because the sway/drift sensitivity calculations are also influenced by this assumption.

The Assume cracked setting in the member properties is used to specify the cracked/uncracked status of individual concrete beam spans, column stacks and wall panels. While it can be set directly in the member properties, it can also be set or toggled graphically in a Show/Alter State Review View. For meshed walls a Review Wall Stress feature is provided to speed up this process.

Wall cracked properties

For engineers investigating the sway characteristics of taller buildings the cracked/uncracked status of walls is a complex topic of critical importance.

A couple of challenges faced by engineers are:

  1. If using general stiffness adjustments - what are the realistic adjustment values for cracked and uncracked panels?

  2. How can the cracked/uncracked status of wall panels be easily determined?

Tekla Structural Designer provides a Review Wall Stress feature in Show/Alter State to significantly assist with the latter task for meshed panels. This graphically displays the current cracked/uncracked status of each panel and indicates in which panels the stress threshold has and has not been exceeded. It then allows you to update the cracked status to be compatible with the threshold.

Use of the Review Wall Stress feature requires a degree of engineering judgement - you will need to make your own choices with regard to:

  • strength or service level stress?

  • basing the stress threshold on max instead of in-plane stress contours?

  • whether the panel cracks for any level of stress, or could a higher threshold be considered?

You will also need to be familiar with the limitations and assumptions that apply.

Workflow for reviewing wall cracked properties

To determine an appropriate cracked status for wall panels, you could adjust the following basic workflow to suit your needs:

  1. Analyze the model to determine some initial stresses.

  2. Open a Review View and click Show/Alter State.

  3. In the Show/Alter State Properties window:
    • Set the Attribute to Assume cracked

    • Set the [M]ode to Review Wall Stress

    • Set the Result type to Strength or Service as you deem appropriate.

    • Set the Stress type to In-plane tension - Y or Max tension - Y as you deem appropriate.

      Note: If using In-plane tension - Y contours:
      • these give an average stress through the thickness of the wall and therefore exclude local stress concentrations that exist as a result of out of plane bending

        • (engineers often choose to ignore out of plane bending when considering the overall cracked status of panels)

    • Enter the Stress threshold that you want to work to.

    In the below example, the most likely likely scenario where everything was initially set as cracked is shown; in lower panels this is a correct setting, in the higher ones they could be changed to uncracked.

  4. By hovering the cursor over an individual panel a tooltip is displayed. This reports the maximum stress value from all nodes in the panel and the case/combination in which it occurs. This value is used to determine if the stress threshold has been exceeded.

    Note: To investigate reported stresses in more detail you might choose to open a Results View to Display 2D results of tension stress contours for specific envelopes or combinations.
  5. At this point you could choose to manually set or toggle the cracked/uncracked status of the individual panels, however Auto update provides a quicker way to make all panels compatible.

  6. To make all panels automatically compatible:
    • Click to expose the Auto update button.

    • Click the Auto update button

    Each panel is updated as follows:
    • uncracked panels in which the stress threshold has been not exceeded are unaffected,

    • uncracked panels in which the stress threshold has been exceeded are set to cracked,

    • cracked panels in which the stress threshold has been exceeded are unaffected,

    • cracked panels in which the stress threshold has not been exceeded are set to uncracked.

  7. Reanalyze the model to take into account the revised cracked/uncracked properties.

  8. Review the results once more and make further adjustments if required.

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