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Steel Designers' Manual - 6th Edition (2003)
610 Composite beams
permits a relaxation of bending moment. A simplified approach is to redistribute
the support moment based on gross (uncracked) section properties by the amounts
given in Table 21.1. Alternatively, moments can be determined using the appropriate
cracked and uncracked stiffnesses in a frame analysis. In this case, the permitted
redistribution of moment is less.
Table 21.1 Maximum redistribution of support moment based on elastic design of continuous
beams at the ultimate limit state
Classification of compression flange at supports
Plastic
Assumed section
properties at Slender Semi-compact Compact Generally Beams (with nominal
supports slab reinforcement)
Gross uncracked 10% 20% 30% 40% 50%
Cracked 0% 10% 20% 30% 30%
The section classification is expressed in terms of the proportions of the compression
(lower) flange at internal supports. This determines the permitted redistribution
of moment. A special category of plastic section is introduced where the
section is of uniform shape throughout and nominal reinforcement is placed in the
slab which does not contribute to the bending resistance of the beam. In this case
the maximum redistribution of moment under uniform loading is increased to 50%.
A simplified elastic approach is to use the design moments in Table 21.2 assuming
that:
Table 21.2 Moment coefficients (multiplied by free moment of WL/8) for elastic design of
continuous beams
Classification of compression flange at supports
Plastic
Location Slender Semi-compact Compact Generally Beams (with
nominal slab
reinforcement)
Middle 2 spans 0.71 0.71 0.71 0.75 0.79
of end 3 or 0.80 0.80 0.80 0.80 0.82
span more
spans
First 2 spans 0.91 0.81 0.71 0.61 0.50
internal 3 or 0.86 0.76 0.67 0.57 0.48
support more
spans
Middle of 0.65 0.65 0.65 0.65 0.67
internal spans
Internal supports
(except first)
0.75 0.67 0.58 0.50 0.42
Redistribution 10% 20% 30% 40% 50%