Civil Engineering Reference
In-Depth Information
cross-section and spacing of the props (a slab is simply
modelled as a 1 m wide concrete prop at 1 m spacings). The
disadvantage of this method is that the springs react against
imaginary points in space as opposed to points elsewhere
within the proposed structure. Therefore, if a small basement
box was to be modelled with a horizontal force applied to one
of its sides only, the entire box would be held in place by the
props and no estimation of lateral movement due to sliding of
the box would be possible. Where such predictions are needed
the props should be modelled as beam elements spanning the
basement box (
Figure 13.13
).
as an equivalent raft between two-thirds and the full length
of the piles are often adopted with the depth of the assumed
raft dependent on the founding conditions. The former of the
two depths applies more to piles transferring the majority of
their load via skin friction, with the latter associated with end-
bearing groups.
Figure 13.14
illustrates the first of these two
assumptions.
Software packages that are based around these methodolo-
gies generally tend to provide conservative estimates of pile
group settlement.
The load settlement response of a pile or group of piles can
also be simulated using finite element packages. The applica-
tion of two-dimensional finite element packages in this respect
is limited, hence three-dimensional finite element models are
generally more appropriate. Care must be taken, however, to
ensure the pile/soil interface is being modelled correctly, which
requires the engineer to have a detailed understanding of the
software package being used. In such situations it is highly
recommended to correlate the analysis results with case-study
or load test data, ensuring the soil stresses around the pile shaft
and base correspond to those suggested in these tests.
With regard to laterally loaded piles, software packages are
available that have adapted the theory behind embedded retain-
ing wall analysis to allow the lateral capacity of single piles
to be calculated. These are extremely useful when the lateral
capacity of a pile for calculating reinforcement quantities or a
13.5.1.4 Piles
The simplest method of modelling a pile's behaviour as a ver-
tical support within a structural model is to assume it is a spring
with an assigned stiffness, k where:
k = anticipated pile load/anticipated settlement
Settlement may be estimated using established theory such
as that proposed by Fleming (1992) or Burland and Cooke
(1974). Care should be taken to ensure that when estimating
pile settlements, any potential group effects are accounted for.
There are several commonly used pile group interaction formu-
lae such as that proposed for pile groups in sand by Skempton
(1953). Alternative methods such as modelling the pile group
Springs
modelling
floor slabs
Frame
constructed
as beam
elements
Figure 13.13
Springs and frames supporting retaining walls in finite element modelling
Spread of Load at 1 in 4
2D
3
D
Base of Equivalent Raft Foundation
Figure 13.14
Sketch showing the 'equivalent raft' principle
