Civil Engineering Reference
In-Depth Information
In the case of precast concrete off‑site construction can be
readily used to achieve tighter tolerances but care is required as
they may be too onerous for the actual site conditions.
See Chapter 17:
Design of concrete elements
, for more in‑
formation on concrete.
waves. This latter technique attracts a cost premium but if
stored incorrectly it will re‑absorb moisture. Dry timber is
usually defined as having an MC no greater than 19%, so dry‑
ing to values below this means that any drying defects will
become apparent prior to use. This MC is measured as a ratio
of the weight of the water in the wood relative to its oven‑dry
weight. Timber can hold more than its dry weight of water (i.e.
MC greater than 200%), and MCs greater than 20% (i.e. green
timber) make it susceptible to attack by dry rot spores. It is
defined as fully saturated when its MC is 28%, as kiln drying
makes its cells collapse and it cannot hold more than 28% MC
after that. Dry timber is often stamped with the letters S‑DRY
(i.e. surfaced dried) or KD (i.e. kiln dried).
Potential defects from incorrect drying or moisture
re‑absorption include wood warping, shakes and splits, honey‑
combing due to differential MC across a piece, cracks around
mechanical connections or opening‑up of mitre joints or tenon
shoulders. Correctly cured and stored timber products should
therefore have fewer problems in a finished building or struc‑
ture, as the product should hold its installation dimensions.
Typical in‑service MCs are as follows:
16.4.3 Timber
Natural and engineered structural timber exhibits the following
material in‑service characteristics, and structural design and
calculation of movements should account for these issues:
Applied load deformation is elastic and heavily influenced by
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grain/fibre direction; brittleness precludes design in the plastic
range.
Response to environmental changes and applied loads is
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anisotropic.
Response to low temperatures (i.e. non‑fire) loads is linear
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elastic.
Orthotropic properties: differing mechanical properties in all three
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orthogonal directions.
Hygroscopic properties: shape, size, strength and creep properties
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are all affected by environmental humidity.
Creep is dependent on load duration, moisture content and applied
15-20% MC for external joinery and structural timber.
■
■
10-15% MC for internal joinery and furniture in non‑humid
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load direction in relation to fibres.
conditions.
8-10% MC for internal joinery in rooms that are continuously
Natural and engineered structural timber has four types of di‑
mensional tolerance:
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heated.
This can be used to clearly coordinate and specify design details
in accordance with intended environmental and construction
conditions. If need be, the timber can be stored in conditions
similar to the intended final condition to allow its moisture
content to be equalised prior to fabrication and erection.
Tighter specification of movements and tolerances may be
required in the following cases:
Pre‑fabricated construction where improved quality control can
Source tolerance: constituent materials are naturally variable, with
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structural performance dependent on type of log cut, timber sea‑
soning and preparation.
Manufacturing tolerances: dependent on factory quality control.
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Environmental tolerances: dependent on exposure and use of
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material.
Erection tolerances: dependent on site construction quality
■
■
control.
be used to ensure very accurate and high‑quality engineered
products.
In areas of high‑humidity or aggressive environments such as
European and American codes of practice and material stand‑
ards are well developed with respect to the processing, manufac‑
turing, environmental and erection tolerances. In most cases the
inherent improbability of all unfavourable extreme deviations
occurring together is small, and simple means of on‑site adjust‑
ment can be incorporated to avoid the cumulative accumulation
of deviations. This is especially true with timber, which can
easily be site cut to size or levelled with packers to smooth out
imperfections. See Section 16.10 for further references.
However, the designer should always be mindful of its an‑
isotropic behaviour and its tendency to absorb and release
moisture in accordance with local humidity. Both significantly
affect its structural properties and performance, especially in
the case of engineered timber products where properties are
product‑dependent.
Timber is normally kiln dried down to 15-19% moisture
content (MC), and sometimes to 8-10% MC using radio
■
swimming pools.
Architecturally expressive details and/or unfinished structural
■
framing.
See Chapter 19:
Timber
, for more information.
16.4.4 Masonry
Structural masonry exhibits the following material in‑service
characteristics, and structural design and calculation of move‑
ments should account for these issues:
Applied load deformation is elastic and in some cases time‑
■
dependent; brittleness precludes design in the plastic range.
Response to environmental changes and applied loads is broadly
■
isotropic, although unidirectional cracking is common in incor‑
rectly designed and constructed structures.
