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)

Deflection control in design (BS 8110) 169
troubles [1, 2]. BS 8110 states that the final deflection, including the effects
of creep and shrinkage, should not exceed either of the following limits:
(a) span/250;
(b) span/500 or 20 rom, whichever is the lesser, after the construction of
the partitions or the application of finishes (BS 8110: Amendment
No. 1, 1986).
These deflection limits are given as being reasonable values for use in
practical design. The limit (a) of span/250 is considered to be that beyond
which the deflection will be noticed by the user of the structure. The limit
(b) is to prevent damage to partitions and finishes. Both limits are intended
for general guidance only; where, for example, a special type of partition is
used, the manufacturer's advice should be sought.
In design, it is usual to comply with the above deflection limits by a
straight-forward procedure of limiting the ratio of the span to the effective
depth [7 -10]; it is only in exceptional cases that deflections are actually
calculated (see Section 5.5) and compared with the limiting values. The
practical procedure recommended by BS 8110: Clause 3.4.6 may conveniently
be summarized as follows (see also the Comments at the end).
Step 1 Basic span/depth ratios
Select the basic span/effective depth ratios (usually referred to as the
basic span/depth ratios) in Table 5 .3-1. For flanged sections with bwl b >
0.3, obtain the span/depth ratio by linear interpolation between the
values given in Table 5.3-1 for rectangular sections and for flanged
sections with bwlb = 0.3. (Note: For a flanged section, b is the effective
flange width.)
Step 2 Long spans
For spans exceeding 10m, there are three cases to consider, depending
on whether it is necessary to limit the increase in deflection (to span/500
or 20 rom as stated above) after the construction of the partitions or
finishes:
(a) If it is not necessary to limit such an increase in deflection, then the
basic span/depth ratio obtained (in Step 1) from Table 5.3-1
remains valid.
(b) If it is necessary to limit such an increase, and the structural
member is not a cantilever, then the basic span/depth ratio
obtained from Table 5.3-1 should be multiplied by a modification
factor equal to 10/span.
Table 5.3-1 Basic span/effective depth ratios (BS 8110: Clause 3.4.6.3)
Support condition
Rectangular sections
Flanged sections bbw :S 0.3
Cantilever
Simply supported
Continuous
7
20
26
5.6
16.0
20.8