New Partially Cracked Concrete Walls & Members

Tekla Structural Designer
Tekla Structural Designer

New Partially Cracked Concrete Walls & Members

This new feature in Tekla Structural Designer 2021 is an addition to existing features 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 - commonly composed 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. All previous releases have had 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?

This release adds a further refinement to modeling the effect of cracking on stiffness with a new partially cracked option for the intermediate condition between (fully) cracked and uncracked 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. For more see this Help topic:Cracked, partially cracked, and uncracked concrete members

  • Key Aspects of the new feature are:
    • Scope - the feature is applicable to:

      • Wall type: Meshed shear wall (not mid-pier), Material = Concrete, Fabrication = Cast in place & precast

      • Beams - Material = Concrete, Fabrication = Cast in place, Post tensioned & precast

      • Columns - Material = Concrete, Fabrication = Cast in place & precast

    • New Property Settings - the properties for walls, beams and columns are adjusted for the new feature; the “Assume Cracked” setting is now listed in the “Analysis & design control“ group (previously “Design Control” for beams and columns) and a further option “Partially” is added. When this is selected you can enter the degree of cracking as a % from zero (fully uncracked) up to 100% (fully cracked).
    • Partially cracked modification factor - the partially cracked Modification Factor (ModFactorPartially Cracked ) is automatically calculated by linear interpolation for the specified percentage (x%) between the factors set for uncracked (ModFactorunCracked) and cracked (ModFactorCracked) as follows:

      • ModFactorPartially Cracked = ModFactorunCracked - x% * (ModFactorunCracked - ModFactorCracked)

      • 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.

      • Member stiffnesses are then automatically adjusted by the interpolated partially cracked modification factor values, these being calculated and applied for every property pair for which different values are specified for cracked and uncracked (specific to the factor values currently set in the model), thus:

        • Meshed Walls - the interpolated factor applies to properties E, G and/ or t as appropriate in the same manner as the cracked/ uncracked factors.

        • Beams & Columns - the interpolated factor applies to the section properties as appropriate in the same manner as the cracked/ uncracked factors.

    • Review View > Show/Alter State - the controls for the “Assume cracked” attribute are enhanced to allow review, application and editing of the new partially cracked setting as illustrated below. While the pictures show the review/ settings for walls, the functionality for concrete beams and columns is the same.

      • Mode options are adjusted - a new “Review” mode is added (in addition to Review wall stresses) and the editing options are now “Toggle” and “Set On”.

      • Review Mode - In Review Mode the color-coding and legend are enhanced to show the distribution of applied partially cracked values. With this mode active, the cursor tooltip also includes the Partially cracked % value and calculated EI values for major and minor axis inertia*.

        • *Note that the EI values are for illustrative purposes only, should you wish to compare them with your own calculated values - the analytical adjustment of stiffness is made by application of the partially cracked modification factors as discussed above.

      • Set On Mode - when enabled a new “Assume cracked” option is displayed with the edit options of “Yes”, “Partially” and “No”. With “Partially” selected, you can check on the “Set Percentage” option and enter your desired % value. This can then be applied graphically to the model using mouse operations. As shown below, the color-coding and legend adjust in relation to the current % value to show you where it is currently applied, the distribution of other % values and Uncracked/ Cracked.
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