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)

14 Limit state design concepts
there is an element of judgement and experience related to the relative
values a community places on human life, permanent injury and property
damage as compared with a possible increase in initial investment.
Similarly, in the design calculations the design strength for a given
material and limit state is obtained by dividing the characteristic strength
by the partial safety factor for strength Ym appropriate to that material and
that limit state:
design strength = _l_ x characteristic strength
Ym
(1.5-2)
Although it has little physical meaning, the global factor of safety, or
overall factor of safety, has been defined as the product YmYf· The value
assigned to this factor (indirectly, through the values assigned to Ym and,
particularly, Yr) depends on the social and economic consequences of the
limit state being reached. For example, the ultimate limit state of collapse
would require a higher factor than a serviceability limit state such as
excessive deflection. Table 1.5-1 shows the Yr factors specified by BS 8110
for the ultimate limit state. These Yr values have been so chosen as to ensure
also that the serviceability requirements can usually be met by simple rules
(see Chapter 5). In assessing the effects of loads on a structure, the choice
of the Yr values should be such as to cause the most severe stresses. For
example, in calculating the maximum midspan moment for the ultimate
limit state of a simple beam under load combination (I), Yr would be 1.4
and 1.6 for dead and live load, respectively. However, in calculating the
maximum midspan moment in the centre span of a three-span continuous
beam, the loading would be 1.4Gk + 1.6Qk on the centre span plus the
minimum design dead load of l.OGk on the exterior spans.
Table 1.5-2 shows the Ym values specified by BS 8110. These values take
account of (1) the importance of the limit state being considered and (2)
the differences between the strengths of the materials as tested and those
of the materials in the structure.
It is worth noting that the selection of Yr values in BS 8110 is largely
empirical; the Yr values have been so chosen that, in the design of common
structures, much the same degree of safety is achieved as for similar
structures designed in accordance with those earlier codes of practice
(CP 114, CP 115 and CP 116). The authors wish to quote Heyman [11]:
Table 1.5-1 Partial safety factors for loads Yr: ultimate limit state
(BS 8110:Clause 2.4.3.1)
Dead Imposed
Combination
when effect of load is
of loads Adverse Beneficial Adverse Beneficial
(I) Dead + imposed 1.4 1.0 1.6 0
(II) Dead+ wind 1.4 1.0
(III) Dead+ wind
+imposed 1.2 1.2 1.2 1.2
Wind
1.4
1.2