Civil Engineering Reference
In-Depth Information
long span or lightweight structures should alert the structural
engineer to potential problems, and where suitable perform-
ance criteria are not available from standard sources, specialist
advice should be engaged.
Design of stairs, whilst not often affecting the design of the
main structure greatly, normally takes a lot longer to finalise and
coordinate than the structural engineer anticipates. Normally,
after early simplistic design, the architect will do their final
coordination after the initial priorities, such as cladding and
other major packages, are resolved. On a fast-track project, the
structural engineer should recognise the design problems this
will cause as they finalise the structural packages, particularly
if precast concrete or steel staircases will be erected with the
frame to give construction access.
Fully detailing a staircase often involves more drawings,
and at a larger scale, than is initially planned for. In particular,
note that the number of drawings detailing a steel stair flight
can be three times the number for a similar reinforced concrete
stair. The more 'architectural' emphasis, the more attention to
detail will be required as aspects of the structure become the
'finish' of the building.
The architect may need assistance with sizing typical handrail
posts and connections but the structural engineer should avoid
coordinating these beyond general advice as there is a world of
pain and regulations that need architectural ownership.
Although not the structural engineer's problem, they should
be aware of the role that the structure plays in causing cold
bridges and compromising the performance of the building's
waterproofing. Any structure that projects through the insulated
skin should alert the team to the possibility of a cold bridge and
thermal lagging over a length or specialist structural details
may be required. The structural engineer can usefully alert the
architect to problem areas for the waterproofing, in particular
places where there are local differential movements that could
rip membranes or reverse drainage falls.
For the structural engineer on 'conventional' buildings sustain-
ability issues are only just coming into focus. Design parameters
are beginning to be agreed, supply chains starting to supply data
and definitive advice slowly becoming available. This develop-
ing subject is beyond the discussion here but as the operational
energy over the building's life drops through efficiency gains
in equipment, envelope and usage in the future the emphasis
will move on to the building's embodied energy. The structural
engineer has a key role in specifying low-carbon materials such
as pulverised fuel ash (PFA) and ground granulated blast fur-
nace slag (GGBS) in concrete and considering future mainten-
ance and reuse of buildings and elements of their structures. In
appropriate climates and locations the structural engineer can
work with the architect and building services engineer to elim-
inate finishes, expose structure and use the thermal mass of the
building to control the environment within the building.
It is worth noting that the best way to reduce the embodied
energy of a structure is through minimising the loads it is
designed for, choosing the most appropriate structural scheme
and through the specification of its materials. Often, discussion
focuses on optimising member design but only once these earl-
ier items have been correctly achieved. This final step has less
influence than the others.
Most of this chapter has focused on the structural engineer's
work with other designers to fulfil the client's brief. However,
huge value can be obtained by their working with a contractor
already appointed or by correctly anticipating the preferences of
a future contractor. Designing a solution that suits the site loca-
tion and market the building is being built in, and that works
within the limitations that the site places on access and plant,
especially around roads and railways, can result in a better,
cheaper building. Repetition of building elements and measures
that allow off-site prefabrication all contribute to increasing this
'buildability' and speed of construction on site.
7.7 Tall buildings
As mentioned in the introduction to this chapter, as buildings
become taller the vertical circulation, service risers and hori-
zontal stability structures increase in importance for the design
and begin to merit extra consideration both in detail and for
their overall impact on the building as a whole.
When is a building 'tall'? The perception of this is often
influenced by the local market. In London a building over
20 storeys (approaching 100 m) might be considered tall,
whilst in central Beijing it would be shorter than usual current
practice. However, comparison with local markets is not useful
in this discussion. It is better to consider the British Council for
Offices' definition that 'a tall building is not a low building that
is vertically extruded, but one that is differently designed'.
As this implies, there is a continuum as the design becomes
taller, with various issues reaching thresholds requiring solu-
tions technically distinct from lower-rise buildings. Often these
issues require greater study and optioneering by the team as a
whole at an early stage in the design process, which should be
commercially feasible for the designers within the larger fees
resulting from major structures.
Structurally, the key early consideration is establishing the
appropriate strategy for resisting the horizontal forces acting
on the building. For the lowest rise building, these forces might
be carried by moment connections between the vertical and
horizontal elements or by discrete walls or bays of bracing.
Such strategies will have little or no impact on the planning of
the building by the other disciplines.
However, as the number of storeys increases, solutions with
a greater impact on the holistic design are needed - first with
walls and bracing being connected into larger cores, then solu-
tions using the columns in the building's skin to create perim-
eter tubes of increasing density and sophistication.
As buildings move above forty storeys, the core and per-
imeter structure often need to be connected at intervals up the
building height by outriggers that couple their performance
and stiffen the building. These outriggers have a major impact
on the spaces they pass across and are often two storeys deep.
