Civil Engineering Reference
In-Depth Information
Conveniently the building service engineers are often looking
to have two storey deep plant rooms at intervals of around 20
to 25 storeys up the building to reduce the vertical distance the
risers serve. The usual solution is for the structural and ser-
vices engineers to coordinate these zones to serve both their
purposes. Taranath (1997) gives an excellent overview of these
developing issues heading up to 120 storeys and beyond.
As discussed earlier in Section 7.3, since structures are
designed to support loads it is vital to accurately identify what
the design loads should be in order to achieve an efficient struc-
ture. As buildings become taller the wind becomes the source
of the dominant loads that will drive the sizing of the structural
elements. Even in seismic regions it will often be the wind that
can drive the design of the tallest buildings - although seismic
considerations will remain vital.
If the height of the building is beyond the strict remit of the
local codes (for instance, in the case of BS5950 the code was
limited to 300 m) it is evident that specific study and advice
from experts is needed to establish wind loads, probably with
the use of wind tunnel tests. Even at lower heights it is often
worthwhile engaging wind specialists at a very early stage. In
dense urban centres or for buildings of complex shape they can
advise and sometimes significantly bring down the loads for
which the structure must be designed.
The structural engineer will often be the focus of the design
team's relationship with the wind specialist. However, it is not
only the structure that can benefit from specialist advice and
testing. Accurate wind pressures will be of great use to the
cladding designers allowing them to design an efficient solu-
tion and reduce the cost of one of the most expensive items on
the project - the building's skin. Also, the architect will need
to know how the building changes the flow of wind around it
on a daily basis - not just for the extreme events that interest
the structural engineer. Changes to the building form can miti-
gate wind flows that can greatly affect the pedestrians around
the building.
However, the structural engineer needs to be aware that there
is a very tight window of opportunity in which to gain informa-
tion useful for the structural design process. Often the building
form will not be settled until towards the end of the conceptual
design period. If the decision to undertake a wind tunnel test
is not made at the earliest possible stage, it is likely that the
results will not be available until scheme design is finalised,
severely limiting the advantage of the results for the project. It
is worthwhile persuading a client that testing is necessary right
at the start of a project, and beginning conversations with the
specialist as soon as possible after this is agreed.
For very tall buildings, the lateral stability system is not just
judged on strength or deflection (overall or inter-storey). It is
also important for the comfort of occupants to control the hori-
zontal accelerations they experience - normally for those at the
top of the building but sometimes also around mid-height if the
second vibrational mode is significant. As discussed earlier in
Section 7.5, this can be particularly important for hotels. Wind
experts will be able to assist the structural engineer to under-
stand the way the spectrum of force variations in the wind
loads interacts with the vibrational periods of the building for
both along- and cross-wind movements.
As the needs of the lateral stability systems increase with the
building's height it becomes increasingly important that the cho-
sen solution is coordinated within the design of all the discip-
lines. One key area needing attention will be the design of the
core as many parties will be placing increased reliance on it:
The vertical circulation specialists will need to achieve groups of
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elevators that can efficiently serve the building's accommodation
within its overall lifting strategy.
The architect needs to accommodate the staircases that escape and
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local circulation require plus the appropriate bathroom, storage
and lobby areas.
The building service engineer will need to balance having larger
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plant rooms in the core on each floor against having larger risers
to central plant within the overall servicing strategy.
The structural engineer needs to establish the lines of walls and
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bracing needed to resist lateral forces as well as carrying the core's
vertical loads at each floor.
Finally the architect will need to squeeze all these elements into
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the tightest possible core configuration to maximise the 'net area
to gross area' efficiency ratio on every floor.
When designing a tall building the team should hold regular
multi-disciplinary workshops focusing on the core from an
early stage in order to achieve an optimum solution. This extra
effort is justified through the impact it has on every floor as it
rises through the building and thus the economic competitive-
ness of the building.
In parallel with this emphasis on the lateral stability sys-
tems there is also great advantage to the project if extra effort
is made to optimise the design of the floor systems. Thinner
and lighter becomes very desirable as the building gets taller.
If the combined floor zone of structure and services can be
reduced it is likely that the height and thus cost of the building
can also come down or additional floors constructed, increas-
ing client income. Reductions in the mass of the building will
pay dividends through the reduction of vertical load on the
columns and foundations as well as influencing its horizontal
behaviour.
The columns and walls of tall buildings are often more
highly stressed than those of low-rise structures. When con-
sidered over the height of the building this means that axial
shortening, both from elastic and long-term creep, can be sig-
nificant. If some vertical elements across the floor plate are
more lightly loaded than others relative vertical movements
can build up through the building's height. This is sometimes
significant around cores where elements can support only
stairs or risers, but are close to others holding up large areas of
floor plate. Careful consideration is needed to prevent damage
to their connecting elements and finishes.
