21.1 Analysis of Structure
Methods of analysis as in Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code shall be used. The material strength to be assumed shall be characteristic values in the determination of elastic properties of members, irrespective of the limit state being considered. Redistribution of the calculated moments may be made as given in 21.1.1.
21.1.1 Redistribution of Moments in Continuous Beams and Frames
The redistribution of moments may be carried out satisfying the following conditions:
a) Equilibrium between the internal forces and the external loads is maintained.
b) The ultimate moment of resistance provided at any section of a member is not less than 80 percent of the moment at that section obtained from an elastic maximum moment diagram covering all appropriate combinations of loads.
c) The elastic moment at any section in a member due to a particular combination of loads shall not be reduced by more than 20 percent of numerically largest moment given anywhere by the elastic maximum moment diagram for the particular member, covering all appropriate combination of loads.
d) At sections where the moment capacity after redistribution is less than that from the elastic maximum moment diagram, the following relationship shall be satisfied:
where
e) In structures in which the structural frame provides the lateral stability, the reduction in moment allowed by condition given in 21.1.1 (c) shall be restricted to 10 percent for structures over 4 storeys in height.
21.1.2 Analysis of Slabs Spanning in Two Directions at Right Angles
In general, the provisions of Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code shall apply.