Civil Engineering Reference
In-Depth Information
Standard
tolerances
These are usually necessary for all buildings, varying
according to material type. They are normally codified
or based on industry standard reference documents,
making them widely understood and standard for
most structural assemblies. Codification also means
they form an integral part of the assumptions on
structural design compliance
Material
in‑service
behaviour
Materials change shape with time according to
load and environmental conditions, affecting how
elements fit together
Material
deviation
Structural design codes assume material
performance tolerances in terms of strength,
elasticity, ductility and other properties. Deviation
beyond these could impair design integrity and
serviceability
Particular
tolerances
These are tighter than standard tolerances and usually
only apply only to certain components or dimensions.
Their use is normally governed by localised reasons
of fit‑up, interference and clearance, or to respect
certain boundary restrictions
Structural design
Structural design codes assume dimensional
tolerances in terms of element size and shape,
and construction assembly. Deviation beyond
these could impair design integrity
Special
tolerances
These are tighter than standard tolerances and
usually apply to a complete portion of structure
and sometimes a project. Their use may be required
in special cases for reasons of serviceability or
architectural appearance, and sometimes for structural
reasons (i.e. dynamic or cyclic loading or fatigue).
Factory pre‑fabrication and/or special assembly
requirements (i.e. reuse or speed of assembly) can
both facilitate and require this higher level of control
Coordination
A complete construction is a sum of its parts,
requiring non‑structural elements such as
cladding, building services, lifts and finishes to fit
together without clashes or undue distress
In‑service
performance
The structure must remain sufficiently straight and
true so that its function, or the function of other
constituent non‑structural parts such as cladding,
lifts or building services, is not impaired
Visual
appearance
Limits on verticality, straightness, flatness and
alignment may be required, particularly with
respect to architectural finishes
Table 16.3 Tolerance classifications
Boundaries/
adjacencies
Site boundaries and other adjacencies such as
buried infrastructure or adjacent tall buildings
may necessitate design movement and tolerance
limitations
each project participant understands how their respective details
■
will fit together;
compliance with the requested tolerances can be monitored during
■
construction.
Moving parts
clearances
Moving parts such as travelling cranes, rail tracks,
elevators, mechanical car‑parking and large doors
may require strict construction and in‑service
tolerances to ensure snag‑free use
On the proviso that this advice is followed, it should be feas‑
ible to specify standard tolerances such as those presented in
BS 5606 or other industry best‑practice documents unless they
conflict with an overall need for greater structural, architec‑
tural or other accuracy. Thereafter design drawings and speci‑
fications can be prepared with details that clearly call up the
need for increased accuracy when it is appropriate. In practice,
these are classified as noted in
Table 16.3
.
This classification system, with its emphasis on prac‑
ticality and the use of standard tolerances wherever pos‑
sible, is reflected in both Eurocode and American design
approaches.
The Eurocode has taken this a step further for steel, alumin‑
ium and precast concrete design with the definition of a series
of Execution Classes (EXC 1 to 4) that define the level of qual‑
ity assurance and workmanship applied to different structures
and their components. This is because these elements tend to
be fabricated off‑site where better workmanship and quality
control can be achieved allowing the designer to fine‑tune the
design and construction. This is particularly important where
fatigue or architectural quality control issues need careful
attention. The decision to use the higher classifications must be
made in conjunction with the client, as it requires the contrac‑
tor to have factory production control (FPC) systems in place
similar to ISO 9001 that confirm their ability to work against
the chosen Execution Class. This usually implies an increased
construction cost.
Table 16.1 Rationale for construction tolerances
Deviation
Difference between a specified value and that
actually measured, always expressed vectorially (i.e.
+/
-
value)
Permissible
deviation
Vectorial limit (i.e. +/
-
value) specified for a
particular deviation
Tolerance
range
Sum of absolute values of the permissible deviations
either side of a specified value
Tolerance limit
Permissible deviations each side of a specified value
(e.g. +/
-
10.0 mm or +10.0 mm/
-
5.0 mm
Table 16.2 Definition of deviations and tolerances
construction accuracy and its thoroughness means that it can
be used as a reference for most projects.
It is rarely economic or necessary to achieve extreme levels
of accuracy, so tolerances should never be specified closer than
required. These should be considered at the start of a project
to ensure that:
■
design details are practical, easy to fit together and can accommo‑
date the individual build‑up of dimensional variables;
design details reflect the needs of the structural design;
■
