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 (BS 81 10) 143
(d) Transverse reinforcement in flanged beams: Transverse reinforcement
shall be provided near the top surface of the flange, over the full
effective width b. The area As 1 of such reinforcement should not be
less than 0.15% of htl, where ht is the flange thickness and I the beam
span.
Comments
(a) The minimum requirement for As will come into operation for those
beams which, for architectural or other reasons, are made substantially
larger than required by strength calculations. If the steel ratio is too
low, the ultimate moment of resistance may become less than the
cracking moment of the plain concrete section computed on the basis
of its modulus of rupture. As soon as the load is sufficient to crack the
beam, sudden collapse occurs without warning.
(b) As regards the transverse reinforcement in flanged beams, it can be
shown from the principles of mechanics, and from laboratory experiments,
that the compression force in the flange tends to induce
longitudinal cracks at the flange/web junctions. The above-specified
transverse reinforcement controls such longitudinal cracking as well
as shrinkage and thermal cracking.
Minimum areas of compression reinforcement (BS 8110: Clause 3.12.5)
Where compression reinforcement is required for the ultimate limit state,
A~ should not be less than 0.2% of bh for a rectangular beam or 0.2% of
bwh for a flanged-beam web in compression. Use size 16 or larger bars [14].
Maximum areas of main reinforcement (BS 8110: Clause 3.12.6)
Neither As nor A~ should exceed 4% of bh (or 4% bwh for flange beams).
Links or stirrups (BS 8110: Clauses 3.4.5 and 3.12.7)
Links or stirrups are required either to resist shear (see Chapter 6) or
to contain the compression reinforcement against outward buckling. The
minimum practical link size is size 8. The following requirements must
be met:
(a) Where compression reinforcement is used in a beam, links of at least
one-quarter of the diameter of the largest compression bar must be
provided, at a spacing not exceeding 12 times the diameter of the
smallest compression bar. These links should be so arranged that
every corner and alternate bar in an outer layer of reinforcement is
supported.
(b) Links for shear resistance, where required, must satisfy eqns (6.4-2)
and (6.4-3).
(c) Except in beams of minor structural importance, such as lintels,
minimum links are required along the entire span; see eqn (6.4-2).
Comments
Links provided to restrain the compression reinforcement can be considered
fully effective in resisting shear, and vice versa. For anchorage of
links, see Section 6.4, Step 6.