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The creep coefficient calculation is done according to Annex B.1 of EN1992-1-1.
The creep coefficient  φ(t,t0) may be calculated from: φ(t,t0) = φ0 x β C(t,t0) where φ is the notional creep coefficient and may be estimated from:

φ0 = φRH x β(fcm) x β(t0)

where φRH  is a factor to allow for the effect of relative humidity on the notional creep coefficient:

formula 1

where RH is the relative humidity of the ambient environment in %.
This parameter can be defined in the “Standard Assumptions” dialog, “Creep” section: where RH is the relative humidity of the ambient environment in %.
This parameter can be defined in the “Standard Assumptions” dialog, “Creep” section: RH=50%

fcm  is the mean compressive strength of concrete in MPa at the age of 28 days fck  = 25MPa

fcm = fck  + 8 = 25 + 8 = 33 MPa

h0 is the notional size of the member in mm where: Ac is the cross-sectional area

μ is the perimeter of the member in contact with the atmosphere  β(fcm)  is a factor to allow for the effect of concrete strength on the notional creep coefficient: β(t0) is a factor to allow for the effect of concrete age at loading on the notional creep coefficient: The effect of type of cement on the creep coefficient of concrete may be taken into account by
modifying the age of loading t0 in the expression above according to the following expression: t0 is the age of concrete at loading in days

This term can be defined in the “Standard Assumptions“ dialog, “Creep“ section: t0 = 28 days

α is a power which depends on type of cement:

= -1 for slowly hardening cements, S
= 0 for normal or rapid hardening cements, N
= 1 for rapid hardening high strength cements, R

α = 0 β(t, t0) is a coefficient to describe the development of creep with time after loading, and may be estimated considering the following situations:

“Creep  φ(∞ ,t0)” option is checked in the “Standard assumptions” dialog => the age of concrete at the considered instance (t) is infinite and βc(t, t0) = 1 (not calculated)
“Creep  φ(∞ ,t0)” option is checked in the “Standard assumptions” dialog => the age of concrete at the considered instance (t) is imposed by the user and βc(t, t0 ) is calculated as follows:
When “High strength concrete” option is checked in the “Reinforced concrete” dialog: Where:

• a = 0.37 & b = 2.8, if the “Concrete with silica fume” option is checked
• a = 0.4 & b = 3.1, if the “Concrete with silica fume” option is unchecked

When “High strength concrete” option is unchecked in Reinforced concrete dialog (the situation in our example) t is the age of concrete in days at the moment considered.

This parameter can be defined in the “Standard Assumptions” dialog, “Creep” section: t = 365 days

t - t is the non-adjusted duration of loading in days

βH is a coefficient depending on the relative humidity (RH in %) and the notional member size (h0 in mm). It may be estimated from: Related Products Graitec BIM Designers - Concrete Series - Rebar Design & Detailing