Structural Concrete - Hassoun

16.14 Moment Redistribution of Maximum Negative or Positive Moments in Continuous Beams 597
f y
Percentage change in moment, q'
20
7.5
60 80
Available
318–99
318–05
q' = 1000ε 1
f y
60
80
(All f y and f pu )
0 0.0075
0.020 0.025
Net tensile strain, ε 1
Figure 16.31 Permissible moment redistribution for minimum rotation capacity [22].
Courtesy of ACI-PCA
decrease in the negative moments at supports and positive moments between supports of continuous
beam, q ′ , calculated by the elastic theory, must not exceed (1000 ε t )%, with maximum of 20%.
Moment redistribution is allowed only when ε t ≥ 0.0075, indicating adequate ductility is available
at the section at which moment is reduced. When ε t < 0.0075, no moment redistribution is allowed.
The modified negative moments must be used to calculate the modified positive moments within
the span, ACI Code, Section 6.6.5.1. Moment redistribution does not apply to members designed
by the direct design method for slab systems. (Refer to Chapter 17.)
In summary, the percentage of decrease in maximum negative or positive moments in continuous
beams is as follows:
1. When ε t ≥ 0.0075, moment redistribution is allowed (ρ∕ρ b > 0.476).
2. When ε t = 0.0075, the percentage of moment redistribution is 75%(ρ∕ρ b = 0.476).
3. When ε t ≥ 0.020, the percentage of moment redistribution is 20%(ρ∕ρ b = 0.217).
4. When 0.0075