Introduction - The Behaviour and Design of Steel Structures to EC3 - page 18

loads will vary during construction, but thereafter will remain constant, unless
significant modifications are made to the structure or its permanent equipment.
Thedeadloadmaybeassessedfromtheknowledgeofthedimensionsandspe-
cificweightsorfromthetotalweightsofallthepermanentitemswhichcontribute
to the total dead load. Guidance on specific weights is given in [14], the values
inwhichareaveragevaluesrepresentativeoftheparticularmaterials.Thedimen-
sions used to estimate dead loads should also be average and representative, in
order that consistent estimates of the dead loads can be made. By making these
assumptions, the statistical distribution of dead loads is often taken as being of a
Weibull type [35]. The practice sometimes used of consistently overestimating
dimensions and specific weights is often wasteful, and may also be danger-
ous in cases where the dead load component acts in the opposite sense to the
resultant load.
1.5.3 Imposed loads
Theimposedloadsactingonastructurearegravityloadsotherthanthedeadloads,
andarisefromtheweightsofmaterialsaddedtothestructureasaresultofitsuse,
such as materials stored, people, and snow. Imposed loads usually vary both in
space and time. Imposed loads may be sub-divided into two groups, depending
on whether they are gradually applied, in which case static load equivalents can
be used, or whether they are dynamic, including repeated loads and impact or
impulsive loads.
Gradually applied imposed loads may be sustained over long periods of time,
or may vary slowly with time [36]. The past practice, however, was to consider
only the total imposed load, and so only extreme values (which occur rarely and
mayberegardedaslifetimemaximumloads)werespecified.Thepresentimposed
loads specified in loading codes [14] often represent peak loads which have 95%
probability of not being exceeded over a 50-year period based on a Weibull type
distribution [35].
Itisusualtoconsiderthemostseverespatialdistributionoftheimposedloads,
andthiscanonlybedeterminedbyusingboththemaximumandminimumvalues
oftheimposedloads.Intheabsenceofdefiniteknowledge,itisoftenassumedthat
the minimum values are zero. When the distribution of imposed load over large
areasisbeingconsidered,themaximumimposedloadsspecified,whichrepresent
rare events, are often reduced in order to make some allowance for the decreased
probabilitythatthemaximumimposedloadswillactonallareasatthesametime.
Dynamic loads which act on structures include both repeated loads and impact
andblastloads.Repeatedloadsareofsignificanceinfatigueproblems(seeSection
1.3.2), in which case the designer is concerned with both the magnitudes, ranges,
and the number of repetitions of loads which are very frequently applied. At the
otherextreme,impactloads(whichareparticularlyimportantinthebrittlefracture
problems discussed in Section 1.3.3) are usually specified by values of extreme
magnitude which represent rare events. In structures for which the static loads