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 details 77
horizontally cast column, except that at laps of reinforcement bars (in
both types of columns) the limit may be increased to 10%.
Comments
(a)
Tests have shown that where the steel ratio Ase/Ac is too low, eqn
(3.3-10) and hence eqn (3.4-2) are not applicable. AIso, reinforcement
is required to resist bending moments which may exist
irrespective of whether the design calculations show that they exist.
We have seen in Section 3.3 that shrinkage and creep cause a
redistribution of load from the concrete to the reinforcement. Unless
a lower limit is placed on the steel ratio, the steel stress may reach the
yield level (see Example 3.3-2) even when under service load. AIso,
the 0.4% lower limit helps to protect columns in structural frames
against failure in tension when, for example, the surrounding floors
ne ar the column are unloaded above but heavily loaded below, or
when the structural frame is subjected to unequal foundation
settlements.
(b) The upper limits on the steel ratios are to avoid congestion and hence
unsatisfactory compaction of the concrete. Indeed, the LStruct.E.
Manual [8] recommends an upper limit of only 4%.
(c)
Designers normally consider that a minimum of four longitudinal bars
should be used in a rectangular column and six bars in a circular
column. The LStruct.E. Manual [8] further recommends that
longitudinal bars should not be smaller than size 12 and their spacing
should not exceed 250 mm.
Lateral ties or Iinks (BS 8110: Clause 3.12.7)
(a) All longitudinal bars should be enclosed by Iinks (sometimes called
ties or stirrups), which should be so arranged that every corner and
alternate bar shall have lateral support provided by the corner of a
link having an included angle of not more than 135°. No bar shall be
further than 150 mm from a bar restrained by a link.
(b) For circular columns, where the longitudinal reinforcement is located
round the periphery of a circle, adequate lateral support is provided
by a circular link passing round the bars.
(c) Links should have a minimum diameter of at least one-quarter of that
of the largest longitudinal bar, and the maximum link spacing should
not exceed 12 times the diameter of the smallest longitudinal bar.
(The LStruct.E. Manual [8] further recommends that the link spacing
should not exceed the smallest cross-sectional dimension of the
column.)
Comments
The purpose of links is primarily to prevent the outward buckling of the
longitudinal bars, as illustrated in Fig. 3.3-2. The diameter and spacing of
the links are therefore related to the diameter of the longitudinal bars. The
minimum size is one-quarter of that of the largest longitudinal bar, but the
minimum size of any reinforcement bar in British practice is size 6. For
practical reasons, most designers regard size 8 bars as the minimum for use