Civil Engineering Reference
In-Depth Information
stairs and internal walls, increase in quantity. Additionally the
internal volume of the building increases which requires more
plant to condition it. There are big advantages for the project's
budget in squeezing the floor zone thinner.
In some markets, the size of building the client can con-
struct is set by a cap on building height, not by a limit on floor
area. Staying in the central London market, building heights
are set by 'St Paul's heights', the requirement that the dome
must be visible from key points around the City. This places
a non-negotiable height limit across sites. I have carried out
detailed multi-disciplinary coordination early in concept to
squeeze millimetres out of the floor zone and enable an extra
floor across a site. The extra future rent for the client fully jus-
tifies this early extra effort.
The first obvious way to reduce the depth of the floor zone
is to minimise the depth of the key elements. The building ser-
vices engineers make air ducts wide and shallow, overcom-
ing the friction inefficiencies caused. Similarly the structural
engineer adopts minimum depth solutions rather than more
efficient minimum weight sizes.
To further reduce the depth of the floor zone the structural
and building services engineers need to share each other's
space. There are three strategies of increasing sophistication
that can be considered.
For all structures other than concrete flat slabs there will
be areas between the downstand beams where the headroom
goes up to the soffit of the slab. If there are large items of plant
within the ceiling such as variable air volume (VAV) boxes
these can be positioned in these vaults to avoid them driving
the required depth up.
As the next step some building services can be taken through
holes in the structural beams. Often sprinkler pipes are the first
step towards this, avoiding problems clashing with other ser-
vices. It can be more difficult to take anything more than a
cable tray through a concrete beam, but often significant holes
are carved through rolled steel beams or plate girders for air
ducts. These holes are often placed at mid-height of the beams
and in the middle third of the span, avoiding higher shear at the
end, and will likely need to have stiffeners. Detailed calcula-
tions are required for these holes. In recent years automated
fabrication techniques have allowed some manufacturers to
competitively offer fabricated beams that effectively fill the
whole depth of the floor void but have large circular openings
through nearly all their length.
A final strategy that can be used is to taper the depth of the
beam towards its supports. This is normally used in steel frames
but can also be useful for concrete schemes. This allows large
ducts to pass through increased depth zones close to columns,
reducing the overall depth.
These holes and tapers all add complexity and cost to the
structure. However, for a building driven by external con-
straints they can result in solutions that create significant value
for a client. If an extra floor can be built across some or all of
the site that is a step-change in future income for a landlord!
Some of the ways that structure and service zones can be
integrated are shown in
Figure 7.7
.
The layout of the columns, discussed in the previous section,
can also be an important factor driving the section and the lev-
els up the building. As mentioned above, for all solutions other
than a concrete flat slab there will be structural downstands of
Figure 7.7
Ways to integrate structure and services
