Civil Engineering Reference
In-Depth Information
defi ned by the appropriate code of practice. However it should not be forgotten that
this is a conventional value, and real values may well be signifi cantly different.
For a typical bridge deck concrete with a cube strength of 50 MPa, the elastic modulus
given by the British code for concrete that is at least 28 days old is 34,000 MPa. For a
bridge with an average prestress
P
/
A
= 4 MPa, this gives rise to a shortening of 12 mm
per 100 m.
7.2.6 Creep of concrete
When concrete is loaded in compression, it suffers an immediate elastic shortening,
and then, if the load is maintained, continues to shorten for a considerable time. This
deferred shortening is known as creep, and is described in
3.9
. The creep coeffi cient
depends on the same criteria as shrinkage, plus the very important additional criterion
of age at fi rst loading; the older the concrete at fi rst loading the smaller the creep
coeffi cient.
For a typical cast-in-situ bridge deck in UK conditions, prestressed to an average
compressive stress of
P
/
A
= 4 MPa with
φ
= 2, the shortening due to creep is 24 mm
per 100 m length. For a typical precast segmental deck similarly compressed,
φ
= 1 and
the creep shortening is 12 mm per 100 m. These fi gures are of course purely for the
purpose of illustrating the order of magnitude of the effect. In reality, creep coeffi cients
need to be considered carefully. There is some evidence that conventional calculations
under-estimate the amount and the time to completion of concrete creep.
φ
7.3 Bearings: general design considerations
This is not the place to give detailed descriptions of the bearings available on the
market for consideration by the bridge designer. However, a general survey of the
characteristics of the different types of bearing is necessary to allow the development
of the rest of the chapter. There are three generic types of bearing in general use:
mechanical bearings, elastomeric bearings and concrete hinges. Their purpose is to
allow the bridge to change in length and width and to defl ect freely.
The design of the bridge must make provision for the eventual replacement of
mechanical or elastomeric bearings. This is most conveniently done by arranging space
for jacks on the pier head, and such a measure would be essential on very high or
inaccessible piers. However, on many viaducts, jacking off falsework resting on the
ground or on the pile caps is an acceptable option.
It is necessary to inspect all mechanical and elastomeric bearings regularly. However,
this is particularly important for sliding bearings, as described in
7.4.3
below.
7.4 Mechanical bearings
7.4.1 General description
There exist a variety of rocker, roller, spherical and other mechanical bearing types.
However, the commonest type of mechanical bearing adopted for concrete bridges,
and the only one to be described here, is the pot bearing, Figure 7.1. It consists of
a steel cylinder containing a rubber disk. The disk is compressed by a steel piston.
The rubber acts rather as a contained liquid, giving very little rotational resistance
