Input
Portion of LL acting long-term (typ. 0.10–0.30).
A's / (b·d) at midspan (positive moment).
Results
λΔ multiplier—
Sustained Δ_i (DL + sus.LL)—
Long-term Δ_lt = λΔ · Δ_i,sus—
Δ_lt + sustained Δ—
Total Δ (incl. full LL)—
Allowable—
Method
ACI 318 §24.2.4.1 — λΔ = ξ / (1 + 50ρ′) where ρ′ is the compression-reinforcement ratio at midspan (or at support for cantilevers). The multiplier applies only to the sustained portion of the instantaneous deflection. Sustained = full DL + the long-term fraction of LL.