Civil Engineering Reference
In-Depth Information
predicted deflection under a given load is required. Mono-piles
(subjected to horizontal loading which are not restrained via
ground beams) and pile caps generally carry the highest risk
in terms of exceeding their lateral capacities. Therefore, extra
care should be taken to ensure that isolated piles which are not
laterally restrained are not under-designed.
be subtracted from the unit weight of the plate. It should be
noted that for plates placed along the ground surface within
a model this overlap only occurs for the bottom half of the
element (see
Figure 13.15
).
13.6 Developing the model with the design team
The key to developing a robust soil-structural model that
accurately reflects the proposed design is to gain a good under-
standing of the project on a global basis. As a result, even when
considering a relatively small element of the overall structure,
time should be taken to understand how that element interacts
with the rest of the structure and its surroundings and what
constraints they will ultimately place on its design.
This will generally only be achieved via continued liaison
with the project design team, with the most important parties
within this group being the structural engineer and the archi-
tect. It may also be necessary to liaise with a number of ex-
ternal parties such as party wall surveyors and asset protection
engineers from utilities or infrastructure companies. The fol-
lowing paragraphs summarise the key sets of information that
are generally required to construct an accurate soil-structural
model. Should these items not be considered there is a risk
that the model will provide inappropriate or incorrect results
and will not stand up to internal or external scrutiny (the latter
generally being linked to external parties reviewing analyses
as part of an asset protection process).
13.5.2 Structural properties
One of the most important properties that is required when
forming a structural model is its stiffness. For most structural
elements modelled, the axial stiffness (EA) and bending stiff-
ness (EI) are generally required, with 'A' and 'I' equating to
the cross-sectional area and second moment of area/m run.
Steel and concrete are possibly the most common materials
to be modelled with typical values for the Young's modulus
of steel generally taken between 205 GN/m
2
and 210 GN/m
2
.
However, choosing a representative value for concrete can be
more challenging as the Young's modulus is often reduced in
the long-term case to account for concrete cracking and creep.
The sectional thickness, predicted deflection and age of con-
crete therefore need to be reviewed before a value can be
adopted. Magnitudes generally range between 8 GN/m
2
and
30 GN/m
2
. Thicker concrete elements that do not have long
unsupported spans such as a deep pile cap spanning piles at
relatively close spacings are likely to crack less and therefore
would have a higher assigned stiffness than a flat slab spanning
between beams and columns. A pile which is only subject to
compression loads would have a higher assumed stiffness still
as theoretically no cracking should occur.
Approximate guidance values for the Young's modulus,
E for various reinforced concrete elements are presented in
Table 13.1
.
When defining the weight of a plate element within cer-
tain analysis packages care should be taken to ensure that no
'double-counting' occurs. In a finite element model, plates
are superimposed on to the soil elements and they therefore
'overlap' the soil. To calculate the total weight of the soil and
structures within the model, the unit weight of the soil should
E (GN/m
2
)
During
construction
Long term
Structural element
Raft (>700 mm thick)
20-25
10-15
Piled wall
20-25
10-15
Suspended slab
15-20
8-12
Pile cap/piled slab
20-28
15-20
Pile in compression
25-30
20-30
Table 13.1
Guidance values for E of reinforced concrete elements
Plate element
Geometry line
Geometry point
Partial overlap
Soil continuum
Figure 13.15
Sketch showing the overlap of a plate element placed at ground surface level
