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The calculation of deflection is done according to chapter 7.4.3 from EN 1992-1-1. The verification of deflections should be performed due to the following reasons:
The behaviour of the reinforced cross-section for deflection needs is the same as used for the stiffness calculation and can be also expressed in term of moment and strain (deformation) diagram. The final value of stiffness is calculated using interpolation formula between state (I) deformation for uncracked concrete section (ξ = 0) and state (II) deformation for fully cracked concrete section (no tension carries) (ξ = 1) dependently on the ratio of stress in reinforcement from cracking load and acting load. The dependency of cracking moment on strain in concrete is visible from the following figure. The value of deformation is then recalculated from the stiffness and acting load.
Generally, there are three main effects which affect the values of deformation
In concrete structures, deflections increase with time under sustained load. The greater part of the deflection normally occurs under sustained loads. Therefore, long-term deflections are calculated under a best estimate of the sustained load during the lifetime of the structure. The design load for calculating long-term deflections is the permanent load
Effect of concrete cracking is irreversible process. Therefore, it is necessary to calculate long-term deflections using a effective tensile concrete strength which corresponds to the worst cracking during the lifetime of the structure.
In fact creep is the continuous deformation of a member under sustained load. As it is mentioned in "Assumptions", creep effect is covered in calculation via effective modulus of elasticity which is calculated using creep coefficient.