Approximated Creep Coefficient

Approximate creep coefficient at time t as the sum of the creep coefficient components of the elements

φ_{a p p} (t) = \sum_{k = 1}^{n} φ_{k} \cdot (1 - e^{- (t - t_{0}) / τ_{k}})

φ(app)	Approximated creep coefficient at time t
φk	Creep coefficient of the element k
k	Current element
n	Number of elements
e	Euler's number
ti	Current time
t0	Time of loading start
τk=ηk/Ek	Delay time of the current element

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