Slab Deflection Property Choices

Tekla Structural Designer
2021
Tekla Structural Designer

Slab Deflection Property Choices

Aging Coefficient - User Defined or Automatic?

Tekla Structural Designer calculates a modulus of elasticity for each load event which accommodates both creep and aging.

The Aging, Creep and Shrinkage page on the Slab Deflection Settings dialog allows you to choose the method for this.
  • Automatic - the modulus of elasticity used in the analysis (termed the composite modulus) is calculated rigorously according to the procedure defined in the Concrete Society Technical Report 58. This requires an early age event history to be defined by way of a detailed event sequence.
  • User defined - the modulus of elasticity used in the analysis is calculated in accordance with ACI 435 Equation 3.33c:

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

    Where:

    • The aging coefficient χ must be specified in the event sequence. The recommended value is 0.8. This is a user specified value.
    • The creep coefficient Ct required in the above equation is automatically determined by Tekla Structural Designer using equation A-18 from ACI 209 taking account of shape and size effects.

    The equation for Ct is [(ti-t0) / (26 e(0.36 x V/S)) + (ti-t0)] x Cui

    Where:

    • For a slab the volume to exposed surface ratio V/S is taken as h/n where h is the slab thickness and n is the number of exposed surfaces. If n is set to zero then the creep coefficient is taken as zero thus eliminating creep from the analysis properties.
    • Start of first event, t0
    • End time of event under consideration, ti
    • The Ultimate Creep coefficient, Cu must be specified in the event sequence. In normal conditions the recommended value from ACI 435R page 55 is Cu = 2.35. A different value can be specified according to guidance in ACI 209 if desired.

Shrinkage Allowance, or Combined Creep and Shrinkage Allowance?

When creep effects are included in the analysis, the Allowance for Shrinkage Effects multiplier can be set at a value which caters for shrinkage only. Alternatively you can increase the multiplier to allow for both creep and shrinkage, provided that you ensure that creep is not also accounted for in the analysis.

When using the latter approach it is important to ensure that the Aging coefficient is set to User defined.

Restraint Constant - Significant or Insignificant Restraint?

For each of the three methods considered in this example the Restraint constant (ACI) will be varied in order to model different restraint assumptions.

Restraint constant values from ACI 435 are:

  • For situations with significant restraint - 4.0
  • For insignificant restraint - 7.5

This will allow us to compare deflections estimations based on the assumption of significant restraint against those assuming insignificant restraint.

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