Cracked deflection over time

To calculate the deformation over time:

• Click or use the menu ‘Analysis – Deformation over time’.
  The following dialogue appears:

• Reinforcement type
  This option allows you to choose with which reinforcement the stiffness of the cross-sections should be determined. You can choose between:
  • only the theoretical reinforcement (incl. minimum reinforcement)
  • only the practical defined reinforcement
    This option is often used to verify the cracked deformation in already existing plates because you can enter the applied reinforcement as practical reinforcement.
  • both the theoretical (incl. minimum reinforcement) and practical reinforcement
    This is the default setting.
• Factor β
  This parameter takes the nature of the load into account.
  • β = 1.0 for single short term load
  • β = 0.5 for long-term and/or repeated loads.

In practice, it is suggested to always use 0.5, because neither fixed loads, nor the service loads are single and short-term loads.

Note: In the former standard ENV 1992-1-1 the factor β was divided into two factors β₁ and β₂ where β₁ takes into account the adhesion between concrete and the bars. For bars with better adhesion, we take β₁ equal to 1.

• Extend cracking theory to axial forces
  Flexural stress will cause tension. Tension will cause cracks. And the cracks will decrease the flexural stiffness of the element.
  But axial stresses can also cause tension and cracks. They will not affect the flexural stiffness, but the axial stiffness!
  If you check this option, both the flexural and axial stiffness will be reduced so the cracking in for example a wall loaded with axial tension, can also be evaluated.

Background on the calculation

To limit the damage caused to partition walls, a restriction is often imposed on the additional deflection. After all, by the time the lighter walls are installed, the supporting structure has already undergone part of its final deformation. Any damage that may still occur is due to the additional deflection that occurs after the walls have been installed. The additional deflection is the result of variable loads occurring later, but also includes the delayed (time-dependent) deformation of loads that have already occurred.

The sequence and timing of the various loads are important for estimating the harmful deflection. However, the increase in cracking over time cannot be taken into account exactly. Diamonds uses the following method:

• For each load group, Diamonds determines:
  • the instantaneous cracked deflection
  • and delayed deflection due to creep.
• The cracked deflection at different times is then calculated by superposing the previously mentioned deflections. More specifically, at a given time, the instantaneous deflection due to all loads that have already occurred will be taken into account in full. The delayed deflection, on the other hand, will only be taken into account for a certain percentage, depending on the time considered.