e-mosty December 2018 e-mosty December 2018 | Page 42

This situation has the potential to impose substantial
risk on the constructors and designers which could
have been avoided through an Alliance form of con-
tract.
In this instance, the proposed use of gantry cranes to
install the new steel box girders from the existing
viaducts was a key decision which acted as a con-
straint on the remainder of the design. The limited
capacity of the existing viaduct cantilevers, and to
avoid the need to strengthen them, constrained both
the local wheel loads from the gantry and the weight
of the new box girders.
As the gantry cranes were also used to install the
formwork for the pier crossbeams, the designers un-
dertook considerable analysis to maximise construc-
tion flexibility, whilst respecting the weight limit pa-
rameters. Temporary cantilever props were required
at each end of the bridge where the gantry crane
wheel loads would not be able to distribute loads into
the cantilever in both directions.
COMMENTARY ON WIDENING PROVISION VERSUS
DEMANDS
The widening scheme utilised all provision made by
the original designers, making the provisions a suc-
cess. However, access for construction and launching
of the originally envisaged scheme would have been
difficult under current traffic closure constraints, and
replacement of bearings under traffic load was not
anticipated.
This required some local strengthening of the pier
tops to be undertaken in advance of the main works.
More generally, the higher traffic and seismic loading
of current design standards was not anticipated.
Provision for future widening was less pronounced in
the 130m concrete approach structure. Although the
concrete slab did not make direct provision for con-
nection of the new deck, and the expansion joints
were not aligned, a widening scheme was achieved to
match the existing structural form.
LESSONS FOR THE FUTURE
This article provides an interesting example of how
this structure withstood the demands of time and
upgrade work in order to meet today's transportation
demands.
Together with experience of maintenance and up-
grade works on other bridge structures, the authors
propose the following lessons for the future:
4/2018
1. Limitations of Code Compliant Design
While bridges are designed to the codes in force at
the time, Code Compliant Design is not a guarantee
that the bridge will see through its service life with no
problems. Unanticipated behaviour can still occur.
Codes and standards are, through necessity, a simpli-
fication to reduce a range of details and behaviours
down to a level where they may be represented by
rules and equations.
Design innovation and constraint are increasingly
necessary in modern design, but the research and
testing behind the codes becomes more distant and
inaccessible to the engineer. This may permit the de-
sign of details which were not adequately covered, or
were beyond the intent of the original research.
Research and testing can also become dated, and
therefore may not adequately reflect the current
transportation and environmental conditions, or cur-
rent construction methodologies. Engineers should
have greater access to this research to be able to
make an informed decision on its adequacy.
Where the codes and current research do not provide
sufficient authority, engineers need to acknowledge
and promote the need for further testing and re-
search for novel or unusual features at design stage.
Clients and Owners must understand and recognise
the value of this additional testing or research.
Prototypes, including full scale mock up modules, can
provide immense value to a project, and sufficient
time and funding must be allowed.
2. Future provision versus retrofit
In the case of EJ Whitten, the provision made for the
future was largely successful, albeit narrowly so, con-
sidering the increased demands of current loads. The
lesson here is to anticipate future demands and aim
to build in sufficient reserve beyond that for the inevi-
table changes that will occur.
Although it is not always possible to predict the fu-
ture, trends are usually evident and provide a good
basis for prediction. The most economical time to
provide modest additional capacity is at the time of
original construction.
In our experience of retrofitting, it is often very com-
plex and heavily constrained. With a modest degree of
additional foresight by the original designer and
owner, retrofitting would be made easier and more
cost effective.