Civil Engineering Reference
In-Depth Information
ceiling tiles or panels, glazing mullions, columns, lighting and
services are delivered in an optimal way.
The structural grid should therefore be an integer multiple
of the planning grid to allow unitary flexibility. The struc-
tural grid should be as large as economically possible, typ-
ically in the order of 7.5 to 9.0 m. Greater spans are likely
to be only economically justifiable if occupancy type dictates
the needs and the cost premium is justified by higher rental
value. Nevertheless, column grids should allow for flexibility
of internal layouts as allowance for change of use and layout
leads to the best long-term sustainable solution.
A very good guide in the UK market, and the one com-
monly recognised as the standard guidance by which offices
should be designed is the British Council for Offices'
Guide to
Specification
(BCO, 2009). In the UK, the BCO guide became
a milestone in the design standards for offices and particularly
in respect of acknowledgement of realistic floor design loads.
Prior to this guide, unrealistically high imposed loads were
generally specified of 4 kN/m² or possibly 5.0 kN/m² along
with a further partition load of 1 kN/m², often referred to as the
'institutional standard'. Following the research and discussion
that went into preparation of the BCO guide it is now accepted
in the UK and generally in the European office market that
imposed loading should be based on 2.5 kN/m² with an add-
itional 1 kN/m² for demountable lightweight partitions.
To allow office users to have designated areas of filing on
each floor there is a further recommendation that 5% of each
floorplate has an allowance for loading of 7.5 kN/m². This is
often provided local to cores where higher loads can be accom-
modated more economically. Provision of raised floors (typ-
ically 150-300 mm deep) and false ceilings (with services
therein) is now the norm in offices and the general recommen-
dation is for an allowance of 0.85 kN/m² to be made (for the
floor and ceiling combined) unless heavy data cabling or high
services demand dictate a higher specific provision.
Building services will be a very important consideration in
the design of offices both in the floor zones and ceilings. The
approach taken to arrangement and type of services, particularly
the heating and ventilation systems, will often drive the most
suitable structural solution since the depth requirements for ser-
vices involving ceiling ductwork in addition to the structural
zone will determine the overall floor to floor height and there-
fore the overall building height and resulting elevational cost.
At the outset, therefore, it is vital that structural solutions
are considered jointly with building services solutions. In con-
crete frames downstand beam arrangements are frequently
discounted since they would hinder clear paths for services
and require the latter to run underneath thereby substantially
increasing the overall ceiling depth and floor height. Wide shal-
low downstands are a possible economic alternative. Concrete
beams can be designed to permit the passage of building
services but this will involve special detailing and fixing of
reinforcement and formwork all of which will increase cost. A
concrete frame and particularly the floor slabs may also assist
with more sustainable and economic services and provide a
cooling solution through the contribution of its thermal mass.
Steel beams with castellations or web openings may well
prove an economic solution to allow services to pass through
the downstand thus reducing the ceiling zone particularly for
long spans where the equivalent concrete beam would not be as
flexible in accommodating this. Similarly for ease of services
routing, often after a structure has been completed, flat con-
crete slab or steel 'Slimfloor' or other proprietary shallow steel
beam solutions that avoid downstands, whilst more expensive
as an element cost, provide shallow overall floor systems in
conjunction leaving an unhindered services zone which may
in overall terms be more economic.
8.4 Retail
Retail developments take many forms from stand-alone high
street shops, a parade of shop units, 'out-of-town' retail parks to
shopping malls. There are two key drivers when considering the
appropriate structural solution for all types of shop whether they
are single units or large department stores, namely, flexibility of
layout and the minimum or least obtrusive column spacing such
that layout and maximum sales areas are not hindered.
The sizing of shops, where multiple units are to be provided,
is based upon optimum trading frontage to depth of sales floor
ratios. Units that are narrow and deep or wide and shallow do
not provide for the space which is proven to be conducive to
retail sales. For units with two trading floors from a mall or
high street frontage the ideal frontage:depth ratio is proven to
be between 1:3 and 1:4. For single storey units, the ideal ratio
should not exceed 1:5.
The basis of the frontage width will therefore drive the column
spacing. However, columns within the middle or even within the
shop frontage are not desirable since they will hinder shopfront
or access arrangements and of course attracting shoppers into
the retail space is of vital importance if trading is to be success-
ful. Columns close to the frontage may therefore need to be set
back creating an edge cantilever to any slabs or roofs being sup-
ported. Likewise in the front zone often referred to in the UK as
'Zone A' (typically the first space of 6.1 m deep (20 feet) into the
unit) with the highest floor area rental rate, columns will need to
be strategically located so as not to become an encumbrance on
trading layout. (Zone B is the next 6.1 m and Zone C the next
6.1 m with any further space being referred to as 'the remainder
zone, space or area'; by comparison the rental rate for Zone C
will be one quarter of that achieved for Zone A.)
In the UK and Europe, it is common to make reference to
unit types based on size (trading floor area). Large shop units
(LSUs) will typically be large department or variety stores.
Whilst there is no set area for an LSU it will generally refer to
a unit above 3500 m² to in excess of 12 000 m² for a full depart-
ment store operator. A medium size unit (MSU) will typically
be at least twice the area of a single shop unit and typically
between 2000 m² and 3500 m² and will often extend over two
floors and multiple structural bays, therefore having internal
