Method 2: Simplified Event Sequence + Rigorous Creep and Shrinkage Allowance

Tekla Structural Designer
Tekla Structural Designer

Method 2: Simplified Event Sequence + Rigorous Creep and Shrinkage Allowance

In Method 1 we used a simplified event sequence (without addressing the possibility of early age loading), to see how a single multiplier can be applied to allow for the combined effects of creep and shrinkage.

In Method 2 we will re-use the same model, but edit the previous simplified event sequence so that creep is considered in the cracked section analysis. The multiplier will also be edited so that it only considers shrinkage effects.

Download and open the tutorial model (if required)

Note: If you are reusing the model from the Method 1 exercise you can skip this step.
  1. Download the tutorial models from here.
  2. Open the following tutorial model:
    • Slab deflection ACI - Simplified Event Sequence.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

Set up the rigorous creep and shrinkage allowance

In this method the Allowance for Shrinkage Effects multiplier is set to allow for shrinkage only.

  1. From the Slab Deflection toolbar, click Settings
  2. In the dialog, click Aging, Creep & Shrinkage
  3. Ensure the Aging Coefficient is set to User defined and the Allowance for shrinkage effects in total deflection is set to 0.25.

    The above factor allows for shrinkage only.

    Refer to Shrinkage allowance for an explanation of where this value comes from.

  4. Click OK to close the dialog.

Edit the Event Sequence to ensure that creep is accounted for in the analysis

  1. From the Slab Deflection toolbar, click Event Sequences
  2. Click Model Event Sequence
  3. Ensure that the Number of Exposed Faces is set to 2 for each event.

    The creep coefficient Ct will be calculated based on this, thus affecting the modulus of elasticity used in the analysis. For further details of this calculation, see: Aging Coefficient - User Defined or Automatic?

Review the Restraint Constant

  1. Open the Structure 3D view.
  2. Select all the slabs in the model and via the Properties Window, ensure the Restraint Constant is set to 4.0

Perform Iterative Slab Deflection Analysis

  1. Open a St.1 (1) 2D view.
  2. From the Slab Deflection toolbar, click Analyze Current

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

  3. Review the deflections.
    The predicted deflection estimate is 1.5” (using a Restraint Constant of 4.0).

Adjust the Restraint Constant and Re-analyze

Initially the Restraint Constant was set assuming significant restraint; we will now investigate the effect of assuming insignificant restraint.

  1. Open the Structure 3D view.
  2. Select all the slabs in the model and via the Properties Window, change the Restraint Constant to 7.5

  3. Return to the St.1 (1) 2D view.
  4. From the Slab Deflection toolbar, click Analyze Current
    With these settings the total deflection predicted at 5 years reduces to 1”

Generate Composite Modulus Report

An effective composite modulus report can be obtained by right clicking a slab and choosing Export Eff. modulus report to Excel. The report details the composite modulus Ec determined for each event.

To display the Composite Modulus Report:

  1. Set the Result to None to display the slabs.
  2. Right click the slab between gridline D-E/1-2 and Export Eff Modulus report to Excel>For the current slab item

    Comparing this report to the report obtained when using the simplified creep and shrinkage allowance, we can see a difference in the value of the Composite Modulus Ec used. In this method, the age of concrete is taken into account.

    To verify the result, consider Event 2, 3 and 4 where the event ends at 1825 days:

    • Start event t0 = 10 days, Event being considered, ti = 1825 days
    • Time between events (ti-t0) = 1825-10 = 1815 days
    • Modulus of Elasticity, E for 4500 psi concrete grade = 4000 ksi (from Material database)
    • Assumed Ultimate Creep Coefficient, Cu = 2.35
    • Assumed Aging coefficient χ = 0.8. This is typically in the range 0.7 to 0.9
    • Slab depth, d = 11 ¼ in
    • Number of exposed faces, h = 2.

      Ct = [(ti-t0) / ( 26 e(0.36 x d/h) ) + (ti-t0)] x Cui = [1815 / (26 e(0.36 x 11 ¼ / 2) + 1815] x 2.35 = 2.1199


      Ēc = (t, t0) = Ec (t, t0) / (1 + χ Ct)

      For event 2, 3 and 4, Composite Modulus, Ec = 4000 / (1 + 0.8 x 2.1199) = 1484 ksi

Summary of Results

Using the Simplified event sequence plus rigorous creep plus shrinkage allowance method, the results can be tabulated below:

Approach Restraint Constant for Modulus of Rupture Creep approach Assumed Shrinkage %

Total deflection

(Final load event)

ACI 435 (creep included in analysis) 4.0

Cu = 2.35,

h = 2

and χ = 0.8

25% 1.5”
ACI 435 (creep included in analysis) 7.5

Cu = 2.35,

h = 2

and χ = 0.8

25% 1.0”
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