Civil Engineering Reference
In-Depth Information
Basic concepts
2.1 Introduction
It is hoped that this topic will be read both by engineers who wish to deepen their
understanding of prestressed concrete and bridge design, and by those without
engineering training but who are interested in bridges. This chapter is for the benefi t
of the latter group, and is intended to offer a very brief introduction to some of the
basic principles that govern the design of concrete structures, and to introduce much
of the specialist vocabulary used in the topic.
2.2 Units
Length is measured in millimetres and metres.
Load and force are measured in Newtons (N), kilonewtons (1 kN = 10
3
N) and
Meganewtons (1 MN = 10
6
N). 1 kN is the weight of a heavy person, while 10 kN is
approximately 1 ton.
Stress is measured in Megapascals, MPa (1 MPa = 1 N/mm
2
= 1 MN/m
2
).
Moment is measured in kilonewton metres, kNm, or meganewton metres, MNm.
2.3 Loads on bridge decks
The loads on a bridge deck are made up of:
a) Self weight; the weight of the bare concrete structure.
b) Superimposed dead loads; the weight of permanent loads applied to the bare
concrete structure, such as parapets, footpaths, road surfacing etc. These loads do
not contribute to the strength of the deck.
c) Live loads; transient vehicular, rail or pedestrian loads applied to the deck.
Live loads may be uniformly distributed along the deck (referred to in the text
as udl), corresponding to a busy traffi c lane or to a long train, or concentrated,
corresponding to a single heavy axle, lorry or locomotive.
d) Environmental loads; principally wind and earthquake.
Loads (a) and (b) together are referred to in the text as 'dead loads'.
