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)

146 Reinforced concrete beams-the ultimate limit state
not drawn for members of secondary importance, and the theoretical cutoff
points are not then known without further calculation. BS 8110 permits
the following simplified rules to be applied, where the beam supports
substantially uniformly distributed loads. In these rules, l refers to the
effective span length, and ifJ the bar size.
(a) Simply supported beams: All the tension bars should extend to within
0.08/ of the centres of supports. At least 50% of these bars should
further extend for at least 12¢ (or its equivalent in hooks or bends)
beyond centres of supports.
(b) Cantilevers: All the tension bars at the support should extend a
distance of l/2 or 45¢, whichever is greater. At least 50% of these
bars should extend to the end of the cantilever.
(c) Continuous beams of approximately equal spans
(1) All the tension bars at the support should extend 0.15/ or 45¢
from the face of support, whichever is greater. At least 60% of
these bars should extend 0.25/ and at least 20% should continue
through the spans.
(2) All the tension bars at midspan should extend to within 0.15/ of
interior supports and 0.1/ of exterior supports. At least 30% of
these bars should extend to the centre of supports.
(3) At a simply supported end, the detailing should be as given in
(a) for a simply supported beam.
Concrete cover for durability (BS 8110: Clause 3.3)
Table 2.5-7 in Section 2.5(e) gives the concrete covers to meet the
durability requirements for beams and other structural members. Note
that in Table 2.5-7, the meaning of nominal cover is as defined in
BS 8110: Clause 3.3.1.1, namely: the nominal cover is the design depth of
the concrete cover to all steel reinforcement, including links. It is the
dimension used in the design and indicated on the drawings.
Fire resistance (BS 8110: Clause 3.3.6)
The fire resistance of a beam depends on its width and the concrete cover,
as shown in Table 4.10-3. Note that, in practice, the fire resistance
requirements may dictate the size of the beam and the concrete cover.
Table 4.10-3 Fire resistance requirements for beams
(BS 8110: Part 2: Clause 4.3.1)
Fire
rating
(hours)
Minimum width (mm)
Simply
supported
Continuous
Concrete cover to MAIN
reinforcement (mm)
Simply
supported
Continuous
1
2
3
4
120
200
240
280
80
150
200
240
30
50
70 3
80"
20
50
60"
70 3
" See BS 8110: Part 2 : Clause 4 for protection against spalling.