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)

Design and detailing-illustrative examples 79
In practice, Ac is usually taken for simplicity as the nominal cross-sectional
are a without deduction for the are a of the steel. In this case,
N = (0.4) (40) (300 2 ) + (0.75) (460) (1885) N
= 2090 kN
Example 3.6-2
Design a short, braced reinforced concrete column for an ultimate axial
load of 2000 kN. Given: fcu = 40 N/mm 2 , fy = 460 N/mm 2 .
SOLUTION
From eqn (3.4-2),
N = O.4fcuAc + 0.75fyA sc
Assume a 2% steel ratio, i.e. Ase = 0.02Ac, say,
(2000) (10 3 ) = (0.4) (40)Ac + (0.75) (460) (O.02A c)
Ac = 87340 mm 2 , say 300 mm square
If a 300 mm square section satisfies the fire-resistance and architectural
requirements, it is only necessary to calculate Ase from eqn (3.4-2).
(2000) (10 3 ) = (0.4) (40) (300 2 ) + (0.75) (460)A sc
Ase = 1623 mm 2
The are a of six size 20 bars is 1885 mm 2 from Table A2-1. Therefore adopt
the following values:
column section
main bars
links
nominal cover
300 mm square
6--size 20
size 8 at 200 mm centres
30 mm to links (see Tables 2.5-7 and 3.5-1)
Figure 3.6-1 shows the reinforcement arrangement for a typicallength of
column between ftoors. (See Comment (b) below on standard method of
detailing. See also Fig. 3.6-2.)
Comments
(a) The reinforcement details in Fig. 3.6-1 satisfy BS 8110's requirements
as listed in Section 3.5.
Main steel ratio = ~:; = 2.1% > 0.4% and < 6%
Link size
Link spacing
Nominal cover
8 mm > 1f4 of 20 = 5 mm
200 mm < 12 times 20 = 240 mm
30 mm to links, which is suitable for moderate
exposure (Table 2.5-7) and adequate for a 4-
hour fire resistance (Table 3.5-1)
(b) Figure 3.6-1 serves the purpose of showing the beginner a fairly clear
picture of how the reinforcement bars are arranged. However, the
way in which the reinforcement has been drawn and described is not
suitable for use in detail drawings for actual construction purpose.