The creep coefficient calculation is done according to Annex B.1 of EN1992-1-1.
The creep coefficient ϕ (t,t_{0}) may be calculated from: ϕ (t,t_{0}) = ϕ_{0}×β_{c} (t,t_{0})
where ϕ_{0} is the notional creep coefficient and may be estimated from: ϕ_{0} = ϕ_{RH}× β (f_{cm}) × β (t_{0})
where ϕ_{RH }is a factor to allow for the effect of relative humidity on the notional creep coefficient:
where RH is the relative humidity of the ambient environment in %.
This parameter can be defined in the General Design dialog, Creep section:
Design Assumptions – General Design dialog, Relative Humidity
RH = 50%
f_{cm} is the mean compressive strength of concrete in MPa at the age of 28 days
Design Assumptions – Reinforced Concrete dialog, f_{ck}
f_{ck} = 25 mPa
f_{cm} = f_{ck} + 8 = 25 + 8 = 33 MPa
h0 is the notional size of the member in mm, where:
A_{c} is the cross-sectional area
u is the perimeter of the member in contact with the atmosphere
Geometry – Geometry dialog, Width
β(f_{cm}) is a factor to allow for the effect of concrete strength on the notional creep coefficient:
β(t_{0}) is a factor to allow for the effect of concrete age at loading on the notional creep coefficient:
The effect of the type of cement on the concrete creep coefficient may be taken into account by modifying the age of loading, t_{0}, in the expression above, according to the following expression:
This parameter can be defined in the Design Assumptions dialog, Creep section.
Design Assumptions – General Design dialog, Concrete loading age (t0)
t_{0 }= 28 days
α is a power that depends on the type of cement:
α = -1 for slowly hardening cement, S:
α = 0 for normal or rapid hardening cement, N
α = 1 for rapid hardening high strength cement, R
=> α = 0
β_{c}(t, t_{0}) is a coefficient describing the development of creep overtime after loading, and maybe estimated considering the following situations:
1. if the creep ϕ(∞, t_{0}) option is checked in the Design Assumptions dialog, the age of concrete at the considered instance (t) is infinite and β_{c}(t, t_{0}) = 1 (not calculated)
2. if the creep ϕ(t, t_{0}) option is checked in the Design Assumptions dialog, the age of concrete at the considered instance (t) is imposed by the user and β_{c}(t, t_{0}) is calculated as follows:
when the High strength concrete option is selected in the Reinforced Concrete dialog:
Design Assumptions – Reinforced Concrete dialog, High Strength concrete
where
a = 0.37 and b = 2.8, if the Concrete with silica fume option is selected
a = 0.4 and b = 3.1, if the Concrete with silica fume option is not selected
when the High strength concrete option is not selected in the Reinforced concrete dialog (as in our example):
where t is the age of concrete in days at the moment considered
This parameter can be defined in the Design Assumptions dialog, Creep section:
Design Assumptions – General Design dialog, Concrete age (t)
t = 365 days
t - t_{0 }is the non-adjusted duration of loading in days
β_{H }is a coefficient depending on the relative humidity (RH in %) and the notional member size (h_{0} in mm). It may be estimated from: