Civil Engineering Reference
In-Depth Information
engineering expensive curtain wall systems. On occasion wind
tunnels are also used to fine‑tune physical building or structural
shapes for improved wind load response. This has the twin
goal of reducing wind loads by improving the aerodynamic
wind profile of the structure, and to eliminate any potential risk
of vortex shedding.
When a fluid flows around an object it creates alternating
low‑pressure vortices on the downstream side of the object.
This is termed vortex shedding, and the object will tend to
move towards the lower‑pressure zone. If the vortex shedding
frequency matches the resonance frequency of the structure
it will begin to resonate and its movement can become vio‑
lently self‑sustaining and potentially self‑destructive. Simple
changes in a building facade or bridge profile, or the addition
spoiler fences to a chimney can break up these vortices, redu‑
cing wind loads and consequent deflections.
When high‑rise buildings or chimneys are constructed close
together the wind profile of one can affect that of those adja‑
cent, in some cases increasing their wind loading. In such
cases the structural engineer has a duty of care to review
this potential problem to see whether it could be problematic
or not.
cracks are created and once they reach a critical size sudden
fracture can occur. Material shape has a significantly effect
on fatigue life (e.g. square holes or sharp edges and corners),
but the most critical factor to control is the cyclical stress. In
most standard structures material fatigue studies are rarely
required, but structures subject to any form of wind or dy‑
namic excitation must be reviewed. Fatigue design require‑
ments usually require increased element sizes, tighter control
of in‑service deflections and movements (i.e. excitation) and
strict quality control of the manufacturing, assembly and
erection process.
16.3.16 Vibration sensitivity
If tolerance is viewed from the perspective of a variation in a
characteristic behaviour, then human, building and equipment
responses to building behaviour must be considered under ser‑
viceability conditions. Sources of vibration include wind and
seismic activity, external vibrations from railways, pedestrian
excitation and dancing, and mechanical equipment loads.
In the context of building performance, these vibrations
may cause structural fatigue or disturbance to sensitive la‑
boratory equipment, or they may impair the user experience
through annoyance or even nausea in extreme cases. Vibration
and motion perception threshold levels differ significantly de‑
pending on location and context; someone walking across a
rope bridge would accept structural motion as they walk but
it would likely unnerve them if they felt the floor move whilst
walking in a low‑rise hospital building. In a tall building, users
tend to accept some form of movement as it can be contextual‑
ised against the building height and high winds outside.
In low‑rise buildings with long floor spans, sensitive equip‑
ment or proximity to an external source of vibratory nuisance,
the structural engineer should aim to isolate the source of vi‑
bration wherever possible by means of passive insulation or
isolation methods that have been coordinated with the other
design disciplines. If this cannot be achieved, it is standard
practice to stiffen the structure to minimise these effects in line
with codified and best‑practice guidance on the subject.
In high‑rise buildings wind response is very sensitive to
both mass and stiffness, which can be controlled by increasing
either or both of these parameters to increase structural damp‑
ing. However, this conflicts with earthquake design and ma‑
terial optimisation strategies, so careful definition of vibration
design criteria is required that accounts for building use and
likely human perception and tolerance of the issue (e.g. an
office worker is likely to be less sensitive to vibration than
someone lying in bed at home in their apartment). There have
been many moving room studies on this subject, undertaken
with the aim of better quantifying the relationship between
human perception and tolerance of multi‑direction building
excitation. These have resulted in best‑practice guidance that
can be used to set design vibration criteria against a range of
root mean square (RMS) and peak accelerations limits. In vibra‑
tion sensitive tall or slender structures tuned‑mass or viscous
16.3.13 Construction, traffic, crane and
other moving loads
Construction, traffic, crane and other types of transient
loads differ from standard building loads as they introduce a
dynamic load effect which must be accounted for in design. In
most cases the applied loads are presented as equivalent static
loads that incorporate and factor‑up the dynamic loads. This is
beyond the intended scope of this chapter, and specialist and/or
manufacturer advice is often required if these loads and their
settlement impacts are to be correctly assessed.
16.3.14 In‑service stress and differential movement
If the in‑service stress of adjacent loaded elements differs,
their corresponding elastic, plastic and creep deformations will
also differ. Therefore any elements that are connected to them
will experience a differential movement or settlement. This
most often requires review in foundation design, especially
if parts of the building have different foundations (e.g. raft
and pile foundation), or if parts of the building have differing
soil support.
In high‑rise towers or complex structures with multiple
transfer systems adjacent vertical column or wall elements can
be subjected to different levels of in‑service stress, especially
if some of the elements have been sized to resist transient lat‑
eral loads. Such occurrences impose differential settlement on
interconnected structural and non‑structural elements such as
slabs, beams, facades and other finishes.
16.3.15 Material fatigue
Fatigue occurs when a material is subject to cyclic loading and
unloading at loads above a specific threshold. Microscopic
