In the initial stages of the design of building structures it is necessary, often at short notice,
to produce alternative schemes that can be assessed for architectural and functional suit-
ability and which can be compared for cost. They will usually be based on vague and
limited information on matters affecting the structure such as imposed loads and nature of
finishes, let alone firm dimensions, but it is nevertheless expected that viable schemes be
produced on which reliable cost estimates can be based.
It follows that initial design methods should be simple, quick, conservative and reliable.
Lengthy analytical methods should be avoided.
This section offers some advice on the general principles to be applied when preparing
a scheme for a structure, followed by methods for sizing members of superstructures.
Foundation design is best deferred to later stages when site investigation results can be
evaluated.
The aim should be to establish a structural scheme that is suitable for its purpose, sensi-
bly economical, and not unduly sensitive to the various changes that are likely to be
imposed as the overall design develops.
Sizing of structural members should be based on the longest spans (slabs and beams) and
largest areas of roof and/or floors carried (beams, columns, walls and foundations). The
same sizes should be assumed for similar but less onerous cases – this saves design and
costing time at this stage and is of actual benefit in producing visual and constructional
repetition and hence, ultimately, cost benefits.
Simple structural schemes are quick to design and easy to build. They may be compli-
cated later by other members of the design team trying to achieve their optimum condi-
tions, but a simple scheme provides a good ‘benchmark’ at the initial stage.
Loads should be carried to the foundation by the shortest and most direct routes. In con-
structional terms, simplicity implies (among other matters) repetition; avoidance of con-
gested, awkward or structurally sensitive details and straightforward temporary works with
minimal requirements for unorthodox sequencing to achieve the intended behaviour of the
completed structure.
Standardized construction items will usually be cheaper and more readily available than
purpose-made items.
Loads should be based on BS 6484, BS 6399: Parts 1 and 35 and CP 3: Chapter V:
Part 26.
Imposed loading should initially be taken as the highest statutory figures where
options exist. The imposed load reduction allowed in the loading code should not be
taken advantage of in the initial design stage except when assessing the load on the foun-
dations.
Loading should be generous and not less than the following in the initial stages:
floor finish (screed) 1.8kN/m2
ceiling and service load 0.5kN/m2
18 IStructE EC2 (Concrete) Design Manual
Allowance for:
demountable lightweight partitions* 1.0kN/m2
blockwork partitions† 2.5kN/m2
Weight of reinforced concrete should be taken as 24kN/m3.
Design loads should be obtained using Table 3.1.
For normal construction in the UK, a characteristic cylinder concrete strength fck of
25N/mm2 should be assumed for the initial design. In areas with poor aggregates this may
have to be reduced.
For UK steels a characteristic strength yk of 460N/mm2 should be used for high-tensile
reinforcement and 250N/mm2 for mild steel.
The following measures should be adopted:
(a) provide stability against lateral forces and ensure braced construction by arranging
suitable shear walls deployed symmetrically wherever possible
(b) adopt a simple arrangement of slabs, beams and columns so that loads are carried to
the foundations by the shortest and most direct routes
(c) allow for movement joints (see subsection 2.4)
(d) choose an arrangement that will limit the span of slabs to 5m to 6m and beam spans
to 8m to l0m on a regular grid; for flat slabs restrict column spacings to 8m
(e) adopt a minimum column size of 300mm 300mm or equivalent area
(f) provide a robust structure.
The arrangement should take account of possible large openings for services and problems
with foundations, e.g. columns immediately adjacent to site boundaries may require bal-
anced or other special foundations.
The size of structural members may be governed by the requirement of fire resistance
and may also be affected by the cover necessary to ensure durability. Table 4.1 shows the
minimum practical member sizes for different periods of fire resistance and the cover
to the main reinforcement required for continuous members in dry and humid environ-
ments without frost. For other exposure classes, cover should be increased. For simply
supported members, sizes and cover should be increased (see Section 5 and
Appendix C).
To provide adequate stiffness, the effective depths of beams, slabs and the waist of stairs
should not be less than those derived from Table 4.2.
Beams should be of sufficient depth to avoid the necessity for excessive compression
reinforcement and to ensure that economical amounts of tension and shear reinforcement
are provided. This will also facilitate the placing of concrete.
*To be treated as imposed loads.
†To be treated as dead loads when the layout is fixed.
IStructE EC2 (Concrete) Design Manual 19
