Material COV
Construction Steel 0.01
Concrete 0.04
0.12Timber- sawn beam or strut
- laminated beam, planed 0.10
Table 7.1: Coefficients of variation of self-weight for different materials JCSS (2001).
The dead load consists of the self weight of non structural components which might be made
of a material different to the structural components. Hence, the uncertainty associated with the
permanent load of steel structures could increase due to possible dead load contributions and
visa versa in the case of concrete and timber structural components. Due to these different
effects and in consistency with Melchers (1987) a coefficient of variation for the total
permanent load equal to 0.10 can be assumed for the total permanent load.
Due to an apparent systematic underestimation of permanent loads by the engineer, it has been
suggested (see e.g. Melchers (1987), Schneider (1994)) to assume that the mean value of the
permanent load is 5% larger than the nominal value. However, there is limited evidence of
this effect and therefore the mean value is usually set equal to the nominal value.
As the physical dimensions as well as the density of a structural (or non-structural) component
can be assumed to be Lognormal distributed, the uncertainty associated with the weight of one
such component can, as a consequence of the central limit theorem, appropriately be modelled
by a Lognormal distribution.
In case the permanent load is considered resulting from the contribution of several
components the resulting permanent load will tend to become Normal distributed (due to the
central limit theorem). With good approximation it may in general be assumed that the
permanent load is Normal distributed.
Live Floor Loads
The live load on floors in buildings is the load imposed by persons, furniture, equipment and
stored objects. Live loads may be differentiated into a sustained and a transient component.
The sustained live load takes into account long term loads due to e.g. furniture, machinery etc.
The transient live load considers short term loads due to e.g. persons, exhibition materials,
heavy duty service vehicles, etc.
The load intensity of the sustained live load may be represented by a stochastic process in two
dimensions (random field) Wxy(, ) defined by:
Wxy m V Uxy(, ) (, ) (7.2)
where m is the overall mean for a particular user category, see Table 7.2, V is a zero mean
random variable and is a zero mean random field. For linear elastic systems, the
resulting load effect S due to the random field and an equivalently uniformly
distributed load is given by:
Uxy(, )
Wxy(, )
Qequ(, )(, ) equ (, )
AASW xyixydA Q ixydA (7.3)