Civil Engineering Reference
In-Depth Information
Chapter 10
Loading
R. B. Marshall
Buro Happold, Dubai, UAE
David Cormie
Associate, Resilience, Security and Risk, Arup, UK
M. Lavery
Buro Happold, Dubai, UAE
doi
:
10.1680/mosd.41448.0153
CONTENTS
10.1 Introduction
153
10.2 Typology and method
of application 153
10.3 Combinations of load 154
10.4 Permanent (dead) loads 155
10.5 Imposed (live) loads
This chapter covers loading from the context of practising engineers engaged in the design
of structures. The different types of loads (dead, live, wind, etc.) are categorised and the
modes of application are explained. Permanent (dead) loads are explained and examples of
values given. Imposed (live) loads are covered, explaining the different occupancy classes
and guidelines particularly for preliminary design. Wind loads are discussed and rules of
thumb given. Earthquake (seismic) loads are discussed along with key considerations for their
consideration in the design process. Blast loads are described, and guidance given on how
to calculate them and their effect on the structure. Self-straining load effects (temperature,
shrinkage, movement, creep, parasitic 'loads' due to pre-stressing) are described. Fire, fluid, silo
and ground/soil loads are discussed and guidance given on how to approach them.
155
10.6 Wind loads
157
10.7 Seismic loads
157
10.8 Blast loads
159
10.9 Self-straining load
effects
161
10.10 Fire loads
165
10.11 Fluid loads
166
10.12 Silo loads
166
10.13 Soil/earth loads
167
10.14 Conclusions
168
10.15 References
168
10.1 Introduction
One of the critical variables affecting the true factor of safety
in the design of structures is the accuracy of the magnitude
and method of application of loads. Given that much of the
analytical work which is carried out in practice now uses com-
puter programmes with a high degree of precision it should
be apparent that the 'accuracy' of the results depends greatly
upon the accuracy of the loading - if the loading is either over-
or underestimated then the precision of the analysis does noth-
ing to improve the overall 'accuracy' of the analysis.
It will generally be possible to estimate with a reasonable
degree of accuracy the magnitude and distribution of the per-
manent loads associated with the structural self-weight and
applied finishes; however, it is important that during the course
of the design process these are checked and refined a number of
times, as often architectural and other changes will occur which
influence the loading. Live loads are by their nature harder to
quantify both in terms of magnitude and distribution and their
variability in time, hence these are often dealt with based on an
empirical approach (i.e. based on codes, which in turn are based
on traditional practice, rather than being derived from theory).
Significant research has gone into the development of
approaches for determining more complex load effects, such
as wind loading, seismic loads, the various 'self-straining load
effects' such as temperature and shrinkage and more recently
complex 'load' effects such as blast and fire loads.
Intrinsically, all loads are in fact variable in time, both in
magnitude and distribution and the distinctions we derive relate
to establishing a workable theoretical model of load behaviour
as part of the overall structural idealisation. Hence, derivation
of loading is a matter for engineering judgement, rather than
application of codes or standards.
In terms of what is 'reasonable' it is important to understand
that particular types of structural design (say for a water tank,
or a lightweight portal framed industrial building) will gener-
ally have structural engineers who specialise in their design
and it is generally useful to study references related to particu-
lar usage if designing such structures for the first time.
10.2 Typology and method of application
Given the wide variability in load effects on structures there
are a number of different ways in which these loads can be
grouped for the purposes of understanding their similarities
and differences. Some groupings are as follows:
a.
Permanent mass related loads
i.
Self-weight of the structure
ii.
Superimposed dead loads (finishes)
iii.
Static mass of fixed equipment
iv.
Weight of soil supported
b.
Movable (time-dependent) service loads
i.
Distributed floor loads
ii.
Concentrated live loads (traffic or movable equipment)
iii.
Loads from stored materials (grain silo for example)
iv.
Loads from stored liquids
c.
Environmental forces
i.
Wind loads
ii.
Flowing or ponding water loads
iii.
Snow loads
d.
Self-straining forces
i.
Temperature
ii.
Concrete shrinkage
iii.
Differential movements of supports
