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)

Partial safety factors 13
proof stress of reinforcement, or the ultimate load of a prestressing tendon,
below which not more than a prescribed percentage of the test results
should fall. Specifically, BS 8110 defines the characteristic strength of
concrete as that value of the cube strength below which not more than 5%
of the test results may be expected to fall. Similarly, BS 4449 and 4461
define the characteristic strength of steel reinforcement as that value of the
yield stress below which not more than 5% of the test material may be
expected to fall. Similarly, the characteristic strength of a prestressing
tendon is that value of the ultimate strength below which not more than 5%
of the test results may be expected to fall.
Current limit state design philosophy assumes that the strengths of
concrete and steel are normally distributed. Hence, from Examples 1.3-2
and 1.3-4(c), it is clear that in British practice the characteristic strength A
is the mean strength fm less 1.64 times the standard deviation a:
A = fm - 1.64a *(1.4-1)
Therefore, for the same specified value of the characteristic strength, the
higher the value of the standard deviation, the higher will be the necessary
value of the mean strength. In the production of concrete and steel,
producton costs are related to the mean strength; a higher value of the
mean strength will necessitate the use of a more expensive material. On the
other hand, to reduce the standard deviation requires a higher degree of
quality control and hence a higher cost. In practice, a compromise is struck
between these conflicting demands, in order to achieve overall economy.
In the general context of limit state design, the characteristic load is that
value of the load which has an accepted probability of not being exceeded
during the life span of the structure. Ideally, such a value should be
determined from the mean load and its standard deviation. However,
because of a lack of statistical data, it is not yet possible to express loads in
statistical terms, and in current practice the so-called characteristic loads
are simply loads which have been arrived at by a consensus that makes
them characteristic loads. For example, in Great Britain the load values
quoted in BS 6399 : Part 1 and CP3 : Chapter V : Part 2 are accepted by BS
8110 as being the characteristic loads.
1.5 Partial safety factors
In limit state design, the load actually used for each limit state is called the
design load for that limit state and is the product of the characteristic load
and the relevant partial safety factor for loads Yt=
design load = Yf X characteristic load (1.5-1)
The partial safety factor Yf is intended to cover those variations in loading,
in design or in construction which are likely to occur after the designer and
the constructor have each used carefully their skill and knowledge. It also
takes into account the nature of the limit state in question; in this respect
* Readers should pay special attention to equations with bold numbers_