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)

Slender columns (BS 8110) 279
heights for bending about the major axis and about the minor axis. If the
effective height/depth ratio in respect of either axis is 15 or more, the
column should be designed as a slender column.
Step 2 Bending about a minor axis
For bending about a minor axis, the column should be designed for its
ultimate axial load N together with the total moment Mt which is the
greatest of those given by the equations below:
Mt = Mi + Madd
Mt = Mz
(7.5-1)
(7.5-2)
(7.5-3)
(7.5-4)
where Mi = the initial moment as given by eqn (7.4-8);
Madd = the additional moment as given by eqn (7.4-6);
M 2 and M 1 = the initial end moments as defined under eqn
(7.4-8);
emin = the design minimum eccentricity, to be taken as
0.05h or 20 mm, whichever is the smaller, where h
is the dimension in the plane of bending.
Comments on Step 2
(a) Equations (7.5-1) to (7.5-4) were derived in Section 7.4.
(b) Madd is calculated from eqn (7.4-6):
Madd = NfJaKh
where fJa may be calculated from eqn (7.4-5) or obtained from Table
7.5-1; K is an optional reduction factor (K :s; 1) which may be taken
as unity in the first instance (see Step 5 below); his the depth of the
column in the plane of bending (i.e. h is the smaller dimension for
minor axis bending, and is the larger dimension for major axis
bending).
Table 7.5-1 Values of Pa (see eqns 7.4-5 and 7.4-6)
30 35 40 45 50 ~5 60
/Ja 0.07 0.11 0.20 0.31 0.45 0.61 0.80 1.01 1.25 1.51 1.80
• Here, b' is the smaller dimension of the column section, as defined immediately above eqn
(7.4-5).
Step 3 Bending about a major axis
For bending about a major axis there are two conditions to consider:
Condition 1: The ratio of the length of the longer side to that of
the shorter side is less than 3.
Condition 2: lelh :5 20.
Case 1: If both Conditions (1) and (2) are satisfied, then the column is