Civil Engineering Reference
In-Depth Information
Building projects with complex ground issues require a good
interface between the project's geotechnical and structural engin-
eers. Although the geotechnical engineer will almost certainly
develop and undertake any soil-structure analysis, the structural
engineer will provide much of the information required.
One of the first tasks when creating a soil structure model is
to review the proposed geometry of a building or structure with
the best sources of information available such as the structural
plans and sections including the following as a minimum:
the risk of damage to adjacent structures. The results of the
damage assessment should then be agreed as being acceptable
with the relevant external party/authority.
When creating the soil-structural model the engineer needs
to obtain the geometry and location of any adjacent assets
in relation to the proposed foundation and substructure, an
example of which is presented in
Figure 13.16
. This is gen-
erally achieved by using surveys (both intrusive and non-
intrusive) and reviewing historic drawings where available.
For adjacent buildings, establishing their height, foundation
details, construction materials and column grid are all import-
ant elements that need to be identified. With adjacent infra-
structure such as roads and railways, defining the properties
of any build-up materials or embankments is as important as
defining the details of the lining of an adjacent tunnel.
Establishing the magnitudes of any historic movements of
adjacent assets is also important as they can be used to justify
the likely impact of future movement predictions. As an ex-
ample, if the displacement of a tunnel underlying a new base-
ment is predicted to be in the region of 50 mm it may initially
be regarded as being above allowable tolerances. Simulating
historical loading and unloading of the tunnel associated with
previous developments may highlight that the tunnel had pre-
viously been exposed to similar displacements to those being
predicted. This type of information, together with justification
as to the predicted integrity and performance of the tunnel in
the future, will be extremely valuable when presenting design
proposals to external parties (see
Figure 13.17
).
As a project progresses, the proposed programme and building
geometry are likely to change. It is therefore important that any
potential changes are considered when planning soil-structure
analyses. Spending time at the outset of creating a model to make
it as adaptable as possible often avoids the need to build new
ones each time such changes occur. The use of computer-aided
design (CAD) software should also be considered when looking
to optimise the modelling process as most software packages
allow the user to import the geometry of a structure directly into
the model. This has the added benefit of reducing the risk of
errors occurring if manually inputting this information.
When modelling a construction sequence one must review
any available information relating to project programme,
areas of phasing or any preferred construction methods that
an appointed contractor may have. If the modelling exercise
is required to formulate a construction sequence then it should
be reviewed by someone with a suitable level of experience
in construction techniques to ensure that the geometry and
assumptions made are appropriate both in terms of site logis-
tics and site safety.
column locations;
■
slab levels and thicknesses;
■
location of stability elements;
■
location of any existing structures, such as retained facades;
■
retaining wall locations; and
■
the extent of any voids within slabs or walls.
■
An annotated plan showing the various structural loads is
another vital piece of information required for forming the
appropriate soil-structural model. Ideally, the loads should
be split into dead, live, tension, wind and any other tempor-
ary load cases, under both serviceability limit state (SLS) and
ultimate limit state (ULS) conditions. Core line loads are often
of particular importance as cores are generally the most heav-
ily loaded elements.
If elements of an existing structure are to be retained it is
likely that a differential movement assessment will be required
between the retained portions of the building, most often
elements of a facade, and any adjacent proposed structures.
Items required from the structural engineer include the initial
loads on the retained elements, foundation details (which can
be confirmed via intrusive work if required), loading during
and after construction and the maximum tolerable differen-
tial movement between the new and existing elements. Two-
dimensional finite element sections taken through the retained
structures are often used to aid in these assessments.
The presence of sensitive structures adjacent to or within the
near vicinity of a proposed building often have a significant influ-
ence on its design, particularly when considering the new foun-
dations and substructure. For example, the design of a retaining
wall adjacent to a railway embankment may have to ensure track
deflections are limited to less than 5 mm. On the other hand if
the adjacent land was a 'greenfield' site the wall design would
almost certainly need to only satisfy the construction tolerances.
Alternatively, sufficient reinforcement can be included to with-
stand the moments generated within it and hence a greater mag-
nitude of wall movement would be acceptable.
Consequently, it is extremely important to understand any
external constraints imposed on the design of a building.
These need to be defined early on in the design programme
and in conjunction with the external asset protection engin-
eers. Generally the maximum allowable movements need to be
identified and an impact assessment undertaken to determine
13.7 Validating results
Soil-structure interaction calculations are often complex and
can require a significant amount of judgement when assigning
properties to structural elements or a soil mass. Consequently,
there is a risk that the output may not reflect the situation being
