Civil Engineering Reference
In-Depth Information
The management of construction stage risks might include
workshops during the design to examine the procurement of
materials and equipment, erection plant and methods and so
on. An experienced developer or project manager will partici-
pate in such workshops. A less experienced client might not
want to sit in on the workshop itself but must be made aware
of what the conclusions are and what measures are being taken
to control the time and cost risks.
In a commercial context, 'risk treatment' describes the
options and strategy employed by the client, abbreviated as
Avoid, Control, Accept or Transfer (ACAT):
Groundwater pressure varies with depth of the zero pressure
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surface.
Building floor and roof loading varies with management of use
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and of additions, etc.
In each case, the 'normal' design basis will be estab-
lished either by reference to an authoritative code or by local
accepted practice. In either case, it is open to the engineer to
change the risk by changing the design value. As an example,
in the 1980s the water table under London, which had been
lowered by abstraction over many years, was found to be ris-
ing as a consequence of reduced industrial consumption. As
a result, the British Library, with an unusually long expected
life, needed additional precautions in basement design to deal
with both the known conditions at the time of construction and
possible additional demands if the water table continued to rise
unchecked.
Seismic loading is a particular area where risk is obvious;
in the UK, most building designers would assume that the
demands of any sensible seismic loading (given the expected
life and use of a building) would be met by a conventional
design against wind loading. For a nuclear power plant, how-
ever, the sensitivity of the use means that the exposure to seis-
mic load needs to be addressed. As with wind, the higher the
load that is designed for, the lower the annual probability that
it will be exceeded. The dynamic character of seismic effects,
however, also mean that simply adding structural strength may
change the loading. A thorough risk treatment of seismic expo-
sure and performance involves considering both the magnitude
and the frequency spectrum of ground motion, to confirm that
the structure adequately covers the potential challenge.
Other dynamic loads may present a range of structural and
performance problems: oscillating wind loads on lightweight
towers and chimneys are probably the most common. But even
footfall on lightweight floors, crowd movement on grandstands
and dancers in clubs can deliver 'excess' deflection, vibration,
stress and fatigue beyond what an equivalent (assumed static)
load would indicate.
Avoid
means modifying the project in order that a potential
scenario does not arise; an example might be to abandon the
use of an imported product so as to avoid supply chain, trans-
port or import controls.
Control
might mean modifying the design so as to ensure 'open
market' availability of plant, for example, reducing component
lifting weights to allow a wide range of cranes.
Accept
might mean the client recognising that uncertainties in
contract outturn cost will be prohibitively expensive to lay on
the contractor and therefore are best accepted as part of the cli-
ent's own contingencies.
Transfer
is the opposite of Accept - to pay for the contractor to
carry certain risks in order to lower the contingency element of
the project budget.
Both the design and the contract conditions will be affected by
the client's strategy for risk treatment.
3.6 Service loading, statics and dynamics
In principle, the essential balance between a structure's minimum
strength and maximum load is made on a probabilistic basis and
therefore there is a risk that - using all the appropriate factors -
the load will exceed the strength. Structural engineers spend a
great deal of time calculating loads and strengths with factors
applied to give a reasonably cautious balance and without being
concerned with the risk of the balance tipping the wrong way.
Part of the reason is that the codes are not always explicit
on the risk which is implied. For example, EN 1991-1-4 (Wind
effects; BSI, 2001) gives a reference value of wind speed 'with
an annual risk of being exceeded of 0.02'. It is implied that the
lower the annual risk (that the owner is prepared to accept), the
higher the equivalent threshold value which should be applied
in the design. Conversely, the higher the value of load you
design for, the less likely it is to be exceeded.
While the wind code gives a value with a 'risk' - more cor-
rectly, a probability - of 0.02 of being exceeded in any year, it
does not limit the amount by which the value may be exceeded.
Codes mostly do not give the relationship between the higher
threshold value and lower annual probability, although research
papers may do so.
Loading of other kinds may also vary enough to exceed nor-
mal allowance:
Box 3.2
Uncertainty and variability
Uncertainty can be reduced by investigation - the more knowledge
the engineer has on the uncertain issue, the fewer unknowns remain
and therefore the lower the risk. Variability will remain although, in the
structures we specify, better quality control will reduce the variability.
What the number and range are of vehicles that might use a bridge
is uncertain, but this can be resolved by investigation of traffic. What
speed the vehicles will travel at is uncertain, but may be possible to
moderate by design features.
3.7 Structural capacity and ductility
Structural capacity should be more predictable than loading; the
size, shape and materials of the structure are in our hands to specify.
However, there are limits to what we know of structural capacity.
Soil pressures vary with the natural variability of the material.
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