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)

278 Eccentrically loaded columns and slender columns
[a\
(b)
Fig. 7.4-4
M2 = the larger initial end moment, which is always taken as
positive.
Referring to eqn (7.4-7), the total moment M 1 might work out to be less
than M2 ; this could occur when Madd is small, particularly for a column
bent in double curvature. In such a case, the critical section is clearly where
M 2 acts; hence the condition has to be imposed that
Mt ~ M2
BS 8110: Clause 3.8.3.2 imposes the further condition that
(7.4-9(a))
Mt ~ Mt + !Madd
(7.4-9(b))
Earlier, it was stated under eqn (7.3-2) that a column should be in any
case designed for a moment of at least Nemin. Hence we have the further
condition that
(7.4-9(c))
where emin is the design minimum eccentricity, to be taken as 0.05h or 20
mm, whichever is less.
Summary
For a slender column, the total moment M 1 to be used in design is
calculated from eqn (7.4-7), subject to the further conditions imposed by
eqns (7.4-9(a), (b), (c)).
7.5 Slender columns (BS 8110)
The technical background to BS 8110's design recommendations was
explained in Section 7 .4; BS 8110's design procedure for braced slender
columns may be summarized as follows.
Step 1 The effective height le
Using eqn (7.2-2) and Table 7.2-1, determine the effective column