Cracked, partially cracked, and uncracked concrete members

Assuming concrete sections (walls, columns and beams) 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 cracked/uncracked status. It can be set for individual concrete beam spans, column stacks and wall panels as follows:

Yes - the section is fully (100%) cracked
No - the section is uncracked (0%)
Partially - the section is partially cracked (anywhere between 0 - 100%)

While it can be set directly in the member properties, Assume cracked 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.

Usage of cracked, partially cracked, and uncracked

This feature is of especial use in the design of tall buildings, for which the engineer needs to particularly assess and modify the stiffness of the lateral force resisting system (usually composed primarily of concrete (shear) walls) to control lateral displacements. The stiffness of concrete members is determined in large part by their state of cracking - which can vary between the ‘extreme’ conditions of entirely uncracked and fully cracked.

You have the option to specify the (fully) cracked and uncracked conditions for concrete members (beams, columns and walls) and control their stiffness accordingly and separately via Modification Factors.

Note: The default values of these for the Building Analysis assume the loading is long term and should be modified when considering short term loading - for more see this TUA article Where do the default values for cracked and uncracked properties of concrete come from?.

You also have the option to specify an intermediate state, partially cracked i.e. neither uncracked nor fully cracked, which is applicable to beams, columns and walls (meshed). The partially cracked state is set by a simple degree of cracking % for which the stiffness modification factor is automatically calculated and applied.

Tall Building Companion Features - as mentioned, this feature adds to and can be considered in the context of a number of other Tekla Structural Designer features targeted particularly at the design of tall buildings. These are:

Dynamic Analysis - Wind Tunnel Data Report Generation
Design for high strength concrete grades (Eurocode) - for more on this see the Tekla Structural Designer 2019i Release Notes
Stresses in 2D elements
2D In-Plane Stress Contouring useful for assessing extent of cracking of Concrete Walls
Diaphragm loads and diaphragm load tables

Partially cracked modification factor

Assume cracked can be set to:

Yes - the section is fully (100%) cracked
No - the section is uncracked (0%)
Partially - the section is partially cracked
- Percentage - anywhere between 0 - 100%

The partially cracked modification factor (ModFactor_{Partially Cracked} ) is automatically calculated by linear interpolation for the specified percentage (x%) between the factors set for uncracked (ModFactor_unCracked) and cracked (ModFactor_Cracked) as follows:

ModFactor_{Partially
Cracked} = ModFactor_unCracked - x% * (ModFactor_unCracked - ModFactor_Cracked)
- Thus for example for the default short term (Modal Analysis & RSA) concrete meshed wall modification factors of cracked = 0.5 and uncracked = 1.0, a Partially cracked value of 50% gives a modification factor = 1.0 - 0.5*(1.0 - 0.5) = 0.75.
- The member stiffnesses are then adjusted accordingly using the calculated partially cracked modification factor values:
  - Meshed Walls - the factor is applied to properties E, G and/ or t as appropriate in the same manner as the cracked/ uncracked factors (specific to the factors currently set in the model)
  - Beams & Columns - the factor is applied to section properties in the same manner as the cracked/ uncracked factors (specific to the factors currently set in the model).

Determining 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:

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

Analyze the model to determine some initial stresses.
Open a Review View and click Show/Alter State.
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 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.
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 the reported stresses in more detail you might choose to open a Results View in order to display stresses for specific envelopes or combinations.
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.
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.
Reanalyze the model to take into account the revised cracked/uncracked properties.
Review the results once more and make further adjustments if required.