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)

402 Practical design and detailing
Dead loads
Examples of dead loads are those due to finishes, linings, waterproofing
asphalt, isulation, partitions, brickwork and, most importantly, self-mass.
Typical values can be found in References 4 and 6 and some examples are
given in Table 11.2-2 (seep. 405). The unit mass of reinforced concrete is
considerable and a typical value is 2400 kg/m 3 (normally taken as 24 kN/m 3
in design calculations) and it is here that the student meets his first
problem. To design even a simply supported beam, he needs to guess the
beam size before he can include its self-weight in the analysis.
The Manual for the Design of Reinforced Concrete Structures (Institution
of Structural Engineers, 1985) gives some help in this respect for practising
engineers. Undergraduates, however, are mainly concerned with simple
structural members such as slabs, beams and columns; for them the following
simplified procedure may be adequate for preliminary design and
member sizing purposes.
Step 1 Fire resistance
Fire resistance requirements [7, 8] may at times dictate the minimum size
of a structural member, even though the loading may be comparatively
low. Hence a useful starting point would be the fire resistance tables:
(a) Slabs: Table 8.8-1;
(b) Beams: Table 4.10-3;
(c) Columns: Table 3.5-1.
Step 2 Concrete cover
The concrete cover to be used depends both on the fire resistance
requirement and the durability requirement (Table 2.5-7). The concrete
cover to be used should be the larger of that required by Table 2.5-7 and
that by the relevant fire resistance table in Step 1.
Step 3 Span/ depth ratio of beams and slabs
Table 5.3-1 gives the span/effective depth ratios for beams and slabs.
These are the minimum values which should not normally be exceeded.
In particular, in the preliminary sizing of beams, it is advisable to assume
a span/depth ratio of about 12 for simply supported beams, 6 for
cantilevers and, say, 15 for continuous beams.
Step 4 Resistance moment of beams and slabs
The adequacy of the assumed member size can be checked with
Fig. 11.2-1, where(! = Aslbd and e' = A~/bd. The figure has been
prepared using the beam design chart in Fig. 4.5-2. 'Rules of thumb'
suggest that the width b of a rectangular beam should be between 113 and
2/3 of the effective depth d, and Fig. 11.2-1 can be used as a guide in
selecting b.
Step 5 Shear resistance of beams
Guidance on the shear resistance of beams is given in Fig. 11.2-2, where
(! = As! bvd-see Step 4 of the shear-design procedure in Section 6.4.
Shear is not normally a problem in slabs supported on beams.
Step 6 Effective height of columns
The ratio of the effective height to the smaller lateral dimension should