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)

The actual span/depth ratio = (11 m)/(700 mm)
= 15.71 < 16.33
Crack control in design (BS 8110) 173
Hence BS SUO's deflection limits are unlikely to be exceeded.
Example 5.3-2
If the allowable span/depth ratio determined in accordance with the steps
above turns out to be smaller than the actual span/depth ratios, what
remedial actions may be taken?
SOLUTION
Possible remedial actions include:
(a)
(b)
(c)
The effective depth d may be increased to bring the actual span/depth
ratio down to the allowable value.
Additional tension steel can be provided over and above that
required by the Mlbd 2 value. This will reduce the service stressfs (see
eqn 5.3-1(b)) and increase the modification factor (see eqn 5.3-1(a)).
A third possibility is to carry out a full deflection calculation, using
the BS 8110 procedure as explained in Section 5.5. It will usually
show that the span/depth ratio procedure is conservative.
5.4 Crack control in design (BS 8110)
BS 8110 : Clause 3.12.11.2.1 states that surface crack widths should not, in
general, exceed 0.3 mm. Excessively wide cracks are objectionable mainly
because they affect the appearance of the structure. The corrosion of the
reinforcement depends mainly on the concrete cover and the porosity of
the concrete; research on the effects of crack width on corrosion has not
been conclusive [12-14], but it is prudent to limit crack widths in an
aggressive environment, even when the structural member cannot be seen.
In practical design, it is usual to comply with the 0.3 mm crack-width
limit by a straightforward procedure of limiting the maximum distance
between bars in tension, as recommended by BS 8110: Clause 3.4.7.
BS 8110's detailing rules for crack control are conveniently summarized in
Fig. 5.4-1.
Comments on Fig. 5.4-1
(a) In measuring the clear distances ab between tension bars, ignore any
bar with a size smaller than 0.45 times that of the largest bar. (Note:
0.45, and not 0.5, is used so that, say size 12 bars may be used with
size 25 bars).
(b) Similarly, in measuring the clear distance ac to the corner, ignore any
bar having a size less than 0.45 times that of the largest bar.
(c) The side bars should have a size not less than ~(sbbl/y) where sb is
the centre-to-centre bar spacing and b the beam width. (Note: b need
not be taken as greater than 500 mm.) For sb = 250 mm, which is the
maximum permissible spacing, the minimum size:; of the side bars are
0.75~b (for /y = 460 N/mm 2 )
l.OO~b (for /y = 250 N/mm 2 )