Method 3: Detailed Event Sequence + Rigorous Creep and Shrinkage Allowance

Tekla Structural Designer
2021
Tekla Structural Designer

Method 3: Detailed Event Sequence + Rigorous Creep and Shrinkage Allowance

In this approach, a detailed event sequence has been defined which includes propping loads at early events to create an overall load history. Creep is considered rigorously alongside this load history by setting the Aging coefficient to Automatic. This means that an effective composite modulus value to the end of each event is calculated in accordance with the Concrete Society Technical Report 58.

Note: The aging coefficient method should only be set to Automatic when defining a detailed event sequence that includes a realistic assessment of early propping load events.

Download and open the tutorial model

  1. Download the tutorial models from here.
  2. Open the following tutorial model:
    • Slab deflection ACI - Detailed 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

Review the Detailed Event Sequence

To display the Event Sequence

  1. From the Slab Deflection toolbar, click Event Sequences
  2. Click Model Event Sequence
    A detailed event sequence has already been created:

    You will note that:

    • Ultimate Creep Coefficient, Cu set at 2.35 for this example
    • Number of exposed faces, h set at 2 for this example
    • Construction load assumed set at 10.4 psf in this example
    • Propping load % assumed during early age propping events 160% and 140% assumed.

Set up Rigorous Creep

This method requires that the Aging coefficient is set to Automatic. Creep is then considered automatically as part of the analysis for each event sequence.

A user defined allowance for Shrinkage effects in the total deflection is also specified.

To specify the multiplier for shrinkage effects only but also consider creep in the analysis

  1. From the Slab Deflection toolbar, click Settings
  2. In the dialog, click Aging, Creep & Shrinkage
  3. Ensure the Allowance for shrinkage effects in total deflection is set to 0.25, and the Aging Coefficient is set to 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 for the more simplified ACI “rigorous” approach to creep undertaken earlier.

    • ACI composite modulus calculation for the Final Load Event = 1484 ksi (also verified by hand calculation earlier)
    • TR58 composite modulus calculation for the Final Load Event = 1531 ksi
    • These values compare favorably.

    Note, however, that if we compared event 5 above where the composite modulus, Ec = 1284 ksi using the ACI method, we can establish the aging coefficient as:

    Ec = E / (1 + X * Ct)

    1284 = 4000 / (1 + χ x 2.1199)

    χ = [(4000 / 1284) - 1] / 2.1199 = 0.9978

    This increased aging coefficient occurs due to the significant propping loads at early events, which creates an overall load history that is almost equivalent to constant loading.

Summary of Results

Using the Detailed event sequence plus rigorous creep and 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 = automatic based on TR58 25% 1.5”
ACI 435 (creep included in analysis) 7.5 Cu = 2.35, h = 2 and χ = automatic based on TR58 25% 1.0”
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