In ACI-435, a curve to determine the code multiplier (zeta) for calculating the long-term deflection is proposed. The curve starts with the first value of the factor (zeta) at the first month of loading. The question is why the first 30 days of loading are ignored?
The rate of increase of initial deflection(1st month loading) tend to be disproportionate to the long term deflection which is minimal and gradual and thus cant be parametric to be qualified as code multiplier on sustained study.
Long-term deflection are caused by shrinkage and creep of concrete (under the sustained load). The rate of increase of long-term deflection is decreasing with time. Usually the members are loaded (with dead load) within the first month as construction stages are continued. In most cases the members are fully loaded with dead loads after months of its construction and for this reason, the first month is ignored.
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