Civil Engineering Reference
In-Depth Information
Before the advent of today's analysis and computer-aided
design (CAD) tools, concept design was often presented with
hand sketches of typical structural bays and stability diagrams,
supported by simple hand calculations of typical members and
overall building behaviour. These new technologies are some-
times to the detriment of the concept design process as they
are tools more suited to the detailed design phase and suck the
engineer's thoughts into too much detail too early. There is lit-
tle value spending time constructing full 3D analysis models at
this early stage and the team are better served by a mixture of
hand calculations, 2D analysis and perhaps 'stick models' for
the tallest buildings.
Modern visualisation tools have radically enhanced the
ability of architects to present their ideas to the client at early
project stages but it is to be noted that they often support
them with simple hand diagrams. We are all sophisticated
consumers of multi-media and the structural team must pre-
sent their ideas in a similar mix of formats if they want their
input to have its proper weight in the decision-making pro-
cess. Presenting some 2D CAD drawings, not yet worked up
with the detail they will have after detailed design, is unlikely
to convince the team of the sophistication of your concept
design. Often the way you communicate is as important as the
messages you are giving.
Conversely, do not get seduced by the 'finished' appearance
of modern visualisation techniques. Clients and other team
members will often be drawn towards solutions that are beau-
tifully rendered, looking as though they are complete. The fact
that it is an attractive picture does not mean it can stand up. It
is important that the structural engineer provides the team with
the occasional much-needed dose of reality so that the team
selects the optimal solution, not the best image!
At the scheme design phase there will normally be another,
and more detailed, report. This will describe the final selection
of the chosen scheme, but will then focus on a description of
that scheme and its structural and multi-disciplinary implica-
tions. In parallel with the report there will be a set of CAD
drawings defining the position and anticipated type and size
of the majority of members. These drawings may well be at a
smaller scale than will be delivered at detailed design and will
not be as fully annotated.
stability needed by the structure and sometimes support ser-
vice cores and stairs.
Whilst the shape of a building can in theory be any regular
or freeform shape, the majority of new buildings are rectilin-
ear for the practical reasons of cost, ease of construction and
usability, and reusability, of internal spaces.
There are three main types of loads to be transferred:
Vertically acting or gravity loads, including:
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live loads imposed by the building's functions,
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superimposed dead loads from non-structural building compo-
nents such as floor and ceiling finishes, cladding and internal
partitions, and finally,
self-weight of the structure itself, which in some long-span and
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concrete options will be the largest load to be carried.
Lateral or horizontally acting loads, including:
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wind loads,
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seismic loading, in areas where this is a significant risk or for
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particularly sensitive building uses, and
notional horizontal loading, which is sometimes mistakenly
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seen as a robustness requirement but actually represents the
potentially real horizontal forces that are required to stabilise
the columns and walls when they are not perfectly vertical.
Soil and water loads, which can have vertical and horizontal
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components.
Since the function of a structure is to transfer loads it is there-
fore essential that the structural engineer confidently establishes,
records and agrees the size of these forces as early in the design
life-cycle as possible. The combinations of these that need consid-
eration must be established with regards to both the code require-
ments and reality, and sometimes reality is not fully covered by
a code. Later changes to these forces or their combinations will
invalidate much work that has gone before, resulting in delay and
frustration. Early time spent accurately fixing and recording the
forces is very important and will pay dividends later.
The live, wind and seismic loads all have some degree of
dynamic component, but in the majority of cases codes increase
the static forces to represent this, and the building structure
normally only requires a static analysis. The dynamic behav-
iour of the structure only needs to be considered separately
when there is a particularly flexible structure, sensitive func-
tion, loads from heavy vehicles such as trains or in extreme
conditions such as the world's most earthquake prone areas.
Soil and water loads can be of very high magnitude com-
pared to others, especially as the depth retained or supported
increases. As mentioned earlier, this chapter focuses on super-
structure issues and not substructures. Most underground
structures have a balance of forces on either side and thus the
requirement is the transfer of these forces through the structure
from one side to the other rather than their support. Conversely
when a superstructure has soil or water loads acting on it often
this is from one side only, alongside a retained hillside, for
7.3 The building structure as a system
7.3.1 Loads
The function of a structure is to transfer loads. Multi-storey
buildings normally comprise a series of horizontal planes, the
floors, occasionally with inclined ramps or roofs. These are
where the majority of vertical loads occur.
Normally the building is wrapped by an external enclosure
which is where the horizontal wind loads act. The mass of all
the building elements can have additional seismic horizontal
loads acting on it.
The floor systems span between walls and columns which,
in addition to holding the floors apart, provide the horizontal
