F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

434 Practical design and detailing
Case I!
Case n : - - - - - -
Casem: ---·
Redistributed:-----
Envelope:
mR
l236lkNm
Redistribution
Case I: No redistribution
Case II: Increase support M to 520 k Nm
i.e. 30% increase
Casem: Reduce support M to 520 kNm
i.e. pb =0·83 (Eqn.4.9-3: x/d s 0·43)
Fig. ll.S-6 Bending moment envelope
span BC, the design bending moment M is governed by the condition
imposed by eqn (4.9-4), which states that Mat any section must not be
taken as less than 0.7Mc, where Me is the maximum elastic moment at
that section. With reference to Fig. 11.5-6, the moment redistribution
ratio pb for Case III is 520/629 = 0.83. Equation (4.9-3) then imposes
the following condition on the xld ratio of the beam as finally designed:
X
d :5 (pb - 0.4)
i.e. xld must not exceed 0.43 for Pb = 0.83.
Step 4 Shear forces
Figure 11.5-7 shows the calculations for shear forces (see Comments on
Step 4 at the end for a sample calculation). Figure 11.5-8 shows the
shear force envelope.