Civil Engineering Reference
In-Depth Information
The designer should always be mindful that building codes
are usually guidance documents and the engineer always
retains the responsibility of determining the appropriate design
criteria, applicable codes and best practices for a particular
project. This is engineering judgement.
effects must be reviewed as part of the structural design pro‑
cess. Obvious anisotropic behaviour is exhibited in both nat‑
ural and engineered timber products, membranes and structural
composites since their fibre orientation fundamentally affects
their response to all loading.
16.2.4 Special tolerance conditions
In some cases it may be necessary to assess the combined vari‑
ability effects of the differing parts that constitute a fabricated
element or component to make sure that they will fit together.
In more sophisticated or architecturally challenging structures
this may extend to construction of a full‑scale mock‑up to test
the tolerance provisions at critical interfaces.
16.3.3 Material thermal responses
Most materials expand when heated and contract when cooled,
and those that show the opposite behaviour have uses that are
currently restricted to scientific research laboratories. All com‑
monly used construction materials fall into the former category,
but an important exception to this rule is water. Unlike most
substances, its solid form is less dense than its liquid phase. A
block of most elements will sink in its own liquid but a block
of ice floats in liquid water. Therefore, the structural design of
elements exposed to the environmental effects of snow and ice
build‑up (e.g. an external structural wall, roof or a roof top water
tank) must account for this expansive effect.
The term
linear coefficient of thermal expansion
(α) is used
to describe how much a material will expand for each degree
of temperature increase, as given by the formula:
α dt = dl/L
where:
16.2.5 Best‑practice guidance on movements and
tolerances
As already noted, BS 5606 provides designers with comprehen‑
sive guidance on the issues of movement and tolerances, whilst
the
Handbook of Construction Tolerances
(Ballast, 2007) pro‑
vides a very comprehensive single‑source reference document
based on North American construction practice. These docu‑
ments provide a starting point for all designers, irrespective of
discipline. Further material and discipline‑specific movement
and tolerances codes and guidance documents can then be
used in accordance with project typology. See Section 16.10
for further references.
dl
= the change in length of material in the direction being
measured
16.3 Material behaviour and movement under
applied load
Inherent material variability and in‑service behaviour will have
an effect on construction tolerances. This section describes
these effects, with particular emphasis on how different mate‑
rials respond to environmental conditions and loading so that
in‑service material behaviour is understood. See Chapter 10:
Loading
, for more information.
L
= overall length of material in the direction being
measured
dT = the change in temperature over which dl is measured
The ratio is dimensionless, and is normally quoted in parts
per million per °C rise in temperature. Its related volume coeffi‑
cient of thermal expansion is rarely used in common structural
engineering problems. It is standard practice to equate many
self‑straining forces (e.g. those arising from differential settle‑
ments of foundations, restrained dimensional changes due to
temperature, moisture, shrinkage, creep and similar effects) to
an equivalent temperature load on a structure, as the analysis
can be idealised in a straightforward manner.
An additional effect of material thermal response can come
from manufacturing or placing processes like smelting or oven
baking (e.g. steel, aluminium, glass, masonry and composites
manufacture). These create elements that are warped and not
perfectly smooth as the constituent parts of the elements cool at
different rates. The placing of wet concrete and its subsequent
hydration is an exothermic process. This initially heats the con‑
crete after which it cools down to match ambient temperatures,
mimicking the effect seen in other materials as they cool.
The variation in cooling rate experienced by these materi‑
als creates internal variations in their residual shrinkage, both
along their element lengths and through their cross‑sections,
creating internal residual stresses. Structural engineering
design codes acknowledge these issues and deal with them in
16.3.1 Inherent material standard deviation
Material properties and fabricated elements all differ, making
it impossible to produce identical products. Standard deviation
is used as the measure of this variability, calculated using stat‑
istical and probability theories. If a small data sample set is
available, the population standard deviation can be estimated
using a modified quantity called the sample standard deviation.
All design codes, material references and tolerance allowances
account for this inherent variability.
16.3.2 Isotropic versus anisotropic movements
Material responses to environmental and applied loads can
be direction‑dependent. Isotropic behaviour is identical in all
directions, whilst anisotropic behaviour depends on the orien‑
tation of internal fibres. The assumption that a material has
isotropic properties is often a good starting point for primary
structural materials such as steel, concrete and masonry, but
possible anisotropic response to loads and environmental
