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)

BS 8110 design charts-their construction and use 103
determine the minimum required values of b and d. What is the neutral
axis depth factor x/d for such a section?
SOLUTION
The dimensions will be a minimum if Q and e' have the maximum values of
3 and 1.5% respectively. From the design chart (Fig. 4.5-2):
Therefore
say
b~2 = 10.1, i.e. b(2~bf = 10.1
b3 = (1600)(l0 6 ) = 25 35 X 106
(2.52)(10.1) .
b = 293.7 mm;
300 mm by 750 mm
d = 2.5b = 735 mm
From Fig. 4.5-2: x/d = 0.4 approximately.
Example 4.5-6 (NOT to be attempted until after reading Chapter 7)
BS 8110: Clause 3.4.4.1 states that, for a beam section subjected to
combined bending and axial force, the effect of the axial thrust may be
ignored if it does not exceed 0.1fcu times the cross-sectional area.
Comment.
SOLUTION
Reference to Fig. 7.1-7 makes it clear that for small values of
a(= Nlfcubh). an increase in the axial thrust leads to an increase in
{3 ( = M I feu bh ) . For the particular column section illustrated in Fig. 7.1-7,
{3 increases with a up to a * 0.2. In design, it is fairly safe to say that, for a
up to 0.1, {3 always increases with a. Hence where the axial thrust on a
beam section does not exceed O.lfcubh, ignoring its presence will err on the
safe side.
Limit on lever arm
BS 8110: Clause 3.4.4.1 states that the lever-arm factor zld should not be
taken as greater than 0.95. This restriction applies irrespective of whether
the parabolic-rectangular stress block (Fig. 4.4-3) or the simplified
rectangular stress block (Fig. 4.4-5) is used.
Effective span
It is convenient at this point to quote BS 8110's definitions of effective span
and effective length:
(a) The effective span of a simply supported beam is the smaller of:
(1) the distance between centres of supports; or
(2) the clear distance between supports plus the effective depth.
(b) The effective span of a continuous beam is the distance between
centres of supports.
(c) The effective length of a cantilever is: