Manual for Design and Detailings of Reinforced Concrete to Code of ...

Version 2.3 May 2008From Figure 2.1 it can be seen that stress strain curve by BS8110 envelops thatof the Code, indicating that design based on the Code will be slightly lesseconomical. Design formulae for beams and columns based on these stressstrain curves by BS8110, strictly speaking, become inapplicable. A fullderivation of design formulae and charts for beams, columns and walls aregiven in Sections 3, 5 and 7, together with Appendices C, F and G of thisManual.Table 4.2 of the Code tabulated nominal covers to reinforcements underdifferent exposure conditions. However, reference should also be made to the“Code of Practice for Fire Resisting Construction 1996”.To cater for the “rigorous concrete stress strain relation” as indicated in Figure2.1 for design purpose, a “simplified stress approach” by assuming arectangular stress block of length 0.9 times the neutral axis depth has beenwidely adopted, as similar to BS8110. However, the Amendment No. 1 of theCode has restricted the 0.9 factor to concrete grades not exceeding 45. For 45< f cu ≤ 70 and 70 < f cu , the factors are further reduced to 0.8 and 0.72respectively as shown in Figure 2.20.0035 for f cu ≤ 600.0035 – 0.0006(f cu – 60) 1/2 for f cu > 600.67f cu /γ m0.9x for f cu ≤ 45;0.8x for 45 < f cu ≤ 70;0.72x for 70 < f custrainstressFigure 2.2 – Simplified stress block for ultimate reinforced concrete design2.4 Ductility Requirements (for beams and columns contributing to lateral loadresisting system)As discussed in para. 1.3, an important feature of the Code is the incorporationof ductility requirements which directly affects r.c. detailings. By ductility werefer to the ability of a structure to undergo “plastic deformation”, which is5