Civil Engineering Reference
In-Depth Information
the building often means transfer structures are needed and in
some markets whole levels of podium transfer beams are used
above the traffic. In Hong Kong, for instance, there will often
be a significant transfer podium level above a ground floor
Passenger Transfer Interchange of bus and taxi lanes. Beneath
this there may well be another level of transfers to adjust to the
tighter grid of a parking basement or rail station beneath. The
structurally easy life, with all columns running from top to bot-
tom in all locations, is not an achievable ideal. Just don't stop
striving for it or you will end up with a much more complex
reality than you would otherwise get.
be the right size, within the limits allowed for in the specifica-
tions. Thus, a tolerance needs to be included.
The ceiling beneath will eventually be installed flat, often
now very accurately with a laser level and the same is likely
to be true for the raised floor above. This will occur after the
structure is free-spanning and dead load deflection has taken
place. Thus, an allowance needs to be made for this deflection.
A residual gap is then needed beneath the beams to allow air
ducts and cable trays to pass underneath. Always remember that
the building services engineer will tell you the internal dimen-
sion of ducts, the one they have calculated for the air, and not
the overall external dimension including insulation which you
need to know. Finally, beneath all this is a suspended ceiling,
but space is needed not just for the tiles but for the light fixings
that can project upwards by over 100 mm at many positions.
The situation described above is 'uncoordinated', with every
item having its own vertical zone and not sharing with others.
The floor to ceiling headroom required will be strictly
defined by the client's brief or market requirements. In the cen-
tral London office market for instance it is likely that anything
less than 2.75 m will be unlettable, and there won't be any
extra rent for more. Depending on structural spans and other
systems, an uncoordinated floor zone can push floor to floor
heights significantly past 4.25 m and beyond.
Excessive floor to floor height adds considerably to the cost
of the building. All vertical elements, including cladding, risers,
7.5.2 The multi-disciplinary floor zone
We now switch from the 'macro' issue of the building as a whole
to the 'micro' issue of coordinating and minimising the multi-
disciplinary floor zone. A simplified zone for a steel framed
office building is shown in
Figure 7.6
. The structural engineer's
primary consideration is for the structural elements - the slab
and the beams, but they only form part of the total.
Starting from the top, the slab will support finishes. In this
case it is a raised floor for cabling, but it could be screed and
carpet in residential buildings or a roofing build-up at the top of
the building. Then comes the slab and beneath that the beams.
Steel beams require fire protection and a vertical dimension
for this needs to be allowed for. However, remember that in the
real world the beams and slab will not be at the right level or
Figure 7.6
Typical uncoordinated multi-disciplinary floor zone for an office
