Civil Engineering Reference
In-Depth Information
material‑specific ways, but in the context of construction toler‑
ances it is important that due consideration be given to the fact
that materials are rarely perfect in shape and that the uncon‑
trolled application of heat can change material properties.
In long or tall structures diurnal temperature variations can
influence the construction process, requiring all setting‑out
to be undertaken in the early morning before the structure is
warmed by the midday sun.
deformation is plastic. Many common construction materials,
including masonry, concrete, timber, cast iron, membranes and
glass, can exhibit nonlinear responses when loaded in the elas‑
tic range.
16.3.7 Plastic deflection
Plastic deformation is irreversible; an element loaded in the
plastic deformation range will only regain the proportion of
its shape equivalent to its initial elastic deformation, with the
remainder of its plastic deflection being non‑recoverable. Hard
or brittle materials such as masonry, concrete, composites and
membranes have minimal plastic deformation ranges, whereas
steel and timber have larger ones. This explains why steel and
timber perform well under seismic loading, as they can deform
plastically to absorb the energy unleashed on the building by
a seismic event.
Most soils show highly nonlinear plastic behaviour, although
some cohesive soils can regain some of their shape if the
applied loads are reduced.
16.3.4 Material moisture responses
Internal changes in moisture can affect some commonly used
materials in respect to movement and tolerances; particularly
concrete, masonry and timber. This causes irreversible shrink‑
age in most materials, apart from timber which will re‑expand
as its moisture content increases.
In some cases moisture ingress can facilitate material deg‑
radation or corrosion; expansive ice formation in wet masonry
and concrete can lead to surface cracking; likewise steel cor‑
rosion is usually an expansive reaction that can crack adjacent
concrete cover and brittle facades. Structural laminated glass in
contact with water over extended periods can degrade - caused
by interlayer de‑bonding from the glass surface - affecting its
long‑term performance. It is beyond the remit of this chapter to
cover architectural and structural detailing, other than to note
that insufficient environmental protection of structural elem‑
ents can lead to deleterious movement and tolerances impacts
on a completed building.
Other indirect and potentially detrimental moisture effects
include those associated with foundation design. These can fun‑
damentally affect soil load capacity and structural settlement.
16.3.8 Material creep
Creep is the phenomenon exhibited in a solid material when
it slowly and permanently deforms over time under applied
stresses that are below its yield strength. Creep increases with
temperature, and it is more severe in materials that are sub‑
jected to constant heat, or those near melting point (e.g. steel
in a fire situation when it permanently buckles).
The creep deformation rate is a function of inherent mate‑
rial properties, the applied stress, the duration of load, the
exposure temperature and in some materials the exposure
humidity. Whilst creep does not constitute a material failure
mode, excessive deformations can mean that a component or
items fixed to it can no longer perform their function (e.g. if a
reinforced concrete (RC) column deflects under creep load and
cracks the floor slab and finishes attached to it). Paradoxically,
creep can sometimes be beneficial where it relieves tensile
stresses that might otherwise lead to cracking (e.g. in an RC
floor slab).
16.3.5 Deflection under load
All commonly used construction materials deform under load‑
ing; this can be elastic (i.e. reversible), plastic (i.e. irrevers‑
ible) and in some cases time‑dependent (i.e. it can increase
with time or 'creep'). It can be axial, torsional, rotational or a
combination of all three.
16.3.6 Elastic deflection
Elastic deflection is reversible; the object regains its original
shape once the applied loads have been removed. All com‑
monly used construction materials exhibit varying degrees of
this property.
Hooke's
16.3.9 Self‑weight and applied loads
All structures are designed to support a combination of their
own self‑weight, superimposed dead loads, live loads, environ‑
mental loads and induced settlements. Self‑weight and super‑
imposed dead loads are deemed to be permanent and easily
quantifiable, whilst live (or imposed) and environmental loads
can be temporary or transient, calculated using probabilistic
analysis of their likelihood and size. This definition of per‑
manence is used to set up the load factors used in standard
load combinations; permanent loads and quantifiable loads
such as self‑weight, superimposed dead load, flooding and
self‑straining forces such as temperature and shrinkage attract
lower factors; less quantifiable live and environmental loads
such as floor, snow, wind and seismic loads attract higher fac‑
tors to compensate for their higher variability.
law
is
used
to
determine
linear
elastic
deformation:
σ = Eε
where:
σ = applied stress
E = material constant termed the Young's modulus
ε = resulting strain
The relationship only holds in the elastic range that ends
when the material reaches its yield strength after which
