13.1 Introduction

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The different types of dynamic loading considered in this chapter include: earthquakes, wind, floor

vibrations, blast effects, and impact- and wave-induced vibration. The effects of these loadings on

different engineering structures are also discussed. It is standard practice to use equivalent static

horizontal forces when designing buildings for earthquake and wind resistance. This is the simplest way

of obtaining the dimensions of structural members. Dynamic calculations may follow to check, and

perhaps modify, the design. However, vibrations caused by extreme loads such as blast and impact must

be assessed by methods of dynamical analysis or by experiment.

Some examples of dynamic loading are shown in Figure 13.1. The first (a) is a record of

fluctuating wind velocity. Corresponding fluctuating pressures will be applied to the structure. The

random nature of the loading is evident, and it is clear that statistical methods are required for

establishing an appropriate design loading. The next figure (b) shows a typical earthquake

accelerogram. As shown, the maximum ground acceleration of the El-Centro earthquake was about

0.33g. The third figure (c) shows the characteristic shape of the air pressure impulse caused by a

(a) (b)

(c)

Time (t)

Wind speed

(v)

Static

component

Dynamic

−5

0

5

0 10 20 30

El Centro ground acceleration

Time (sec)

a/g

Positive

phase

Negative

phase

Pso

Po

t

P(t)

Shock velocity

Pressure

Distance from explosion

V−

FIGURE 13.1 Examples of dynamic loading. (a) Fluctuating wind velocity; (b) earthquake accelerogram; (c)

pressure time history for bomb blasts.

13-2 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

bomb blast. The shapes of air-blast curves are usually quite similar, having an initial peak followed

by an almost linear decay and often followed by some suction. The duration of the impulsive

loads and their amplitudes depend on many factors, for example, distance from blast and charge

weight.

Vibration of structures is undesirable for a number of reasons, as follows:

1. Overstressing and collapse of structures

2. Cracking and other damage requiring repair

3. Damage to safety-related equipment

4. Impaired performance of equipment or delicate apparatus

5. Adverse human response

With modern forms of construction, it is feasible to design structures to resist the forces arising

from dynamic loadings such as major earthquakes. The essential requirement is to prevent total

collapse and consequent loss of life. For economic reasons, however, it is the accepted practice to

absorb the earthquake energy by ductile deformation, therefore accepting that repair might be

required.

Some forms of loading are quite well defined and may be quantified by observation or experiment.

Many forms of loading are not at all well defined and require judgment on the part of the engineer.

London’s Millennium Bridge, which is a 350-m pedestrian bridge, opened in June 2000. However, local

authorities shut it down after two days due to vibration problems. Engineers found that the

“synchronous lateral excitation” caused the problem and fitted 91 dampers to reduce the excessive

movement. In January 2001, a 2000-strong crowd marched across the bridge to check the performance of

the structure before it was reopened to the public.

Data on certain types of dynamic loading, such as earthquakes and wind, are readily available in

many design codes. Other types of loading are less well covered, though much data may be available

in published research papers. One of the aims of this chapter is to discuss the nature of the most

important types of dynamic loading and to direct the reader to relevant literature for further

information.

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