Civil Engineering Reference
In-Depth Information
Figure 18.1
Loading and movement of structures.
be too expensive. Different analyses are needed to meet these criteria even for a
particular type of structure.
Figure 18.1 shows the relationship between loading and movement of a structure; it
would apply equally for a slope or a foundation, for a building frame or for a machine
part. There is an ultimate limit state where the load is
q
c
, the movements are very
large and the structure is collapsing. There is a safe state where the load is
q
s
, the
movements are relatively large but the structure has not collapsed. There is a factor of
safety given by
1
F
s
q
c
q
s
=
(18.1)
There are a few cases, usually involving excavation, where the soil is required to fail
and then, for a factor of safety, the applied loads must be greater than the failure
loads. It is no good if a machine digging an excavation or a tunnel is not powerful
enough.
There is a serviceability limit state where the allowable movements
ρ
a
are small.
There is a load factor given by
q
a
=
L
f
q
c
(18.2)
such that the allowable bearing pressure
q
a
causes movements that are acceptably
small. Values for factors of safety and for load factors are design parameters and should
be chosen by the designer. Typical values for different structures will be discussed in
later chapters. Notice that values for a factor of safety will be greater than 1.0 while
values for a load factor will be in the range 0 to 1.0.
For structures, such as slopes in rural locations where relatively large movements do
no damage, the design is controlled by the ultimate limit state with a factor of safety.
The important soil parameter is its strength. For soils we have to decide between the
peak, critical state and residual strengths. Factors are there to ensure that the design
is not too close to its ultimate limit state.
