FUTURE TRENDS IN DESIGN & CONSTRUCTION OF BRIDGES
by
Shiv Kumar, Director/IRICEN
Synopsis
The past decade has seen the bridge spans getting longer,
the substructures getting taller and technology enabling
construction of bridges in conditions that would
previously have been considered impossible. The trends
which
have
emerged
indicate
unprecedented
developments in the bridge technology in above
mentioned aspects as well as towards appearance.
Introduction
The past ten years has been a time of unprecedented developments both in the
bridge technology as well as towards appearance. Perhaps, cost is the only thing
preventing bridges from being built with spans several times longer than today’s recordbreakers. Technically, very long span bridges can be built with present-day materials.
Spans are becoming larger and larger, e.g. the proposed 1315m long bridge on Chenab
river in J & K Rail Link Project will have one of the longest arch span (480m) in the
world. (Fig.1)
Fig.1.
Fig.1Proposed
Proposedbridge
BridgeononChenab
ChenabRiver
River
1 / 10
There are many reasons behind the trend. Consideration of the entire life cycle
justify driving the span upwards. Society is also increasingly willing to pay for the
convenience and aesthetics of long span bridges. Today’s construction costs are
relatively less expensive than 20 years ago.
The trend not only applies to cable-
supported bridges but also other types of bridges as a major step has come through the
building of the 330m-span box girder Shibanpo Bridge in Chongquing, China. To many
engineers, long span bridges epitomize the ultimate in engineering achievement. The
design and construction of long span bridges push the frontier in the engineering science.
Cable-stayed bridges are increasingly encroaching on the type of spans traditionally
associated with suspension bridges. The longest cable stayed bridge which has been
constructed in the world is in Japan (Tatara – 890m span). (Fig.2)
Fig. 2.
Tatara Bridge
2 / 10
Only a decade later, the 1,000m barrier is soon going to be breached by both
Stonecutters bridge in Hongkong and Sutong bridge in China and spans of 1,200m are
already planned. One is also seeing a growth in smaller suspension bridges with spans of
perhaps 100m to 500m.
A few trends which are emerging as far as the design and construction of bridges
is concerned are as under:-
Architectural Appearance
Appearance plays a far greater role in bridge design today than it did a few years
ago. The general attitude towards bridges has changed greatly. Previously, bridges were
seen as ‘a necessary evil’, the price that had to be paid for development. Now, a bridge is
seen as a sculpture that people use to connect to each other. There is a much greater
awareness of the importance of a bridge design that properly fits its setting. The
emphasis on appearance has also led to a huge jump in the quality that clients are
prepared to pay for. Thus, there is growing world wide awareness of the importance of
visual aspects of design. e.g. Bow String Girder Bridge across Thane Creek in Mumbai
region provides a visual treat to the rail commuters. (Fig.3)
Fig. 3.
Bow String Girder Bridge across Thane Creek
3 / 10
Placing greater importance on appearance has greatly heightened the role of
architects in design. Sometimes a client wants an artist to dictate the design, the architect
to interpret it and the engineer to make it work. Traditionally, bridges are cut into pieces,
with bearings and joints introduced which make these bridges not only look ugly but also
bad. Instead, far better solution would be to make use of the material properties as the
reinforced concrete can handle the forces and it is unnecessary to cut the bridges into
pieces and deprive them of their monolithic character.
Today’s designers find themselves facing a dilemma. The ‘wow’ factor can come
with something that is both original and simple in concept, such as the much-praised
Gateshead Millennium Bridge (Fig.4) – which has won 37 awards so far, and is still
winning them several years after opening. The balance is being tipped in favour of
visual criteria over engineering.
Fig. 4.
Gateshead Millennium Bridge
Changes in the business environment and responsibilities have led to a growth in
design and build projects all over world which saves time and gives incentive to new
ideas bearing in mind constructability but are subject to risk that the lowest tender may
be ugly or boring.
4 / 10
Security hazard
Today’s designs have a new threat to take into account. The concept of bridge
security hardly existed a few years ago. Now, multi-hazard protection is growing in
importance. Blast and earthquakes would be the obvious one.
Greater understanding of behaviour in earthquakes has led to a massive increase
in the armoury of tools to protect bridges. There have been parallel developments in
recent years in areas such as the understanding of seismic activity, techniques for risk
assessment and means of predicting seismic response. Solutions are being envisaged
such as wrapping columns with glass fibre or carbon fibre which holds them together
without increasing the stiffness. There have also been major developments in energy
absorbing devices, bearings and the use of ‘structural fuses’ to dissipate energy without
creating severe structural damage to the bridge.
Seismic design has certainly come a long way and the philosophy has also
changed significantly. By realizing that earthquake is not a force, but a deformation, the
concept now is to provide sufficient capacity in deformation and allow the bridge to
move under an earthquake, instead of trying to resist a force. Today’s designs emphasise
how to keep the piers and pylons from losing capacity and the addition of loop
reinforcement as one of the major advances in this direction.
Increased Span
Developments in cable technology have been assuming ever greater importance
as spans of both suspension and cable-stayed bridges have increased. This technology is
a key parameter to increasing the span. When you increase the span, the stay cables act
like a sail. Manufacturers have made significant progress in recent years in reducing the
size of strand systems and adding features such as helixes around the cables. Damping
has also proved very important for long spans, with solutions such as tuned mass
dampers and cross-cables or ‘aiguilles’ as used on the cable-stayed Pont de Normandie
bridge in France. (Fig. 5)
5 / 10
Fig. 5.
Pont de Normandie Bridge
Today’s spinning equipment is competitive with preformed parallel wire strand
systems. Which cable system has the lowest cost depends on factors such as the cost of
the wire or strands, local labour costs and the contractor’s experience. New technology is
still being developed, and synthetic cables will come eventually. One of the difficulties is
that they are easy to destroy, though a solution may be to use steel top and bottom with
the new material only installed out of reach.
Taller Bridges
Just as there have been developments above ground, so too have foundations
moved ahead. There has been a quantum leap in the last decade from piles of 1.5m
diameter up to ones as large as 4m and with a length of more than 100m. Equipment has
been developed both for handling and driving these large piles, which has come about
through a transfer of technology from the offshore industry to bridge building. In
6 / 10
foundations, large diameter drilled shafts of 3m diameter have been drilled to a depth of
68m. Improved equipment is not the only factor but such huge diameter holes also
depend on improvements in drill hole stability. A third advance has come through an
improvement in the ability to place higher quality concrete. Use of large diameter pile or
drilled shafts has also made it possible to position the pile cap further above the river
bed. Larger piles can have a long unsupported length enabling the footing to be
constructed far closer to the water surface. It opens up the possibility of bridge
foundations in much deeper waters.
Getting stronger
There is a lot still to be learned about materials such as ultra-high performance
concrete. History shows that the introduction of a new material will only be successful if
a new concept of structure to use that material can be developed. Let us refer to the use
of stone to build arches. When steel was invented, long span trusses and box girders
followed. High strength wires made suspension bridges possible. Together with concrete
it made prestressed concrete, and therefore, long span concrete bridges possible. We are
waiting to see if these new materials will revolutionise our bridges.
Ultra-high
performance materials such as VSL’s cementitious material Ductal have been introduced
in the past ten years. It is a material that is nearer steel than concrete. Designers have to
use it in a different way, using it as normal concrete would work out ten times too
expensive. Huge progress has also been made in the development of standard materials
such as concrete and steel, where a huge range of high strength or high performance
products is now available, many developed to suit a particular application or
environment. Engineers now have an enormous toolbox available to them in terms of
material properties and characteristics.
Precast Components
Pre-cast foundation, abutment, pier and superstructure units enable construction
of bridges not in years but in months and weeks. The benefits of pre-cast components in
bridge construction enhance the philosophy of get in, get out, stay out e.g. the pre-cast
bridge structure in San Juan, Puerto Rico (Fig. 6 ) was erected from the ground up in just
21 consecutive hours.
7 / 10
Fig. 6. The pre-cast bridge structure in San Juan, Puerto Rico
In order to cut the construction time, the engineers at Lake Ray Hubbard Bridge
in Dallas, USA, used pre-cast bent caps (Fig.7 & 8) for all the 43 pier caps reducing
construction time from 8-9 days to 1 day for each pier cap. Pre-cast units eliminate costly
field formwork as well as extend seasonal construction time.
Fig. 7. Lake Ray Hubbard Bridge, USA
Fig. 8 Lowering a pre-cast pier bent cap
New materials
Use of advanced composites started more than a decade ago. But although
composite materials are beginning to be used on projects, they are still at an early stage.
Even the stainless steel has been tried in bridges. When you have a new material, you
tend to use it in the same way as you would use a traditional material. Today’s composite
sections look exactly like steel sections. It will take a while before they find their own
form, just like concrete bridges originally looked like wooden structures. There are many
8 / 10
different types of application for composites such as use in post-tensioning, plates and
strips, FRP decks, stay cables, wraps to provide seismic protection and enclosures to
protect structures. Some of the technology is used for repair, strengthening or
rehabilitation; others for new construction. Examples include composite rods to replace
the top mat of steel reinforcing bars, effectively increasing the cover to the steel, and for
bridge decks. FRP bridge decks are gaining popularity in the USA. They have been
proving especially successful for movable bridges because of their light weight.
Increasing durability
Such materials also offer the benefit of increased durability, a property that is
being increasingly demanded by clients. The use of plastic ducts for cables has been an
important development in this regard. Encapsulating the cable in plastic protects the
ducts, and this has become widespread in a number of countries, although this approach
is not universally accepted, e.g. it is not used in France. For some, the principle is that if
water can enter, it can also leave. Introduction of enhanced protection through use of
electrically isolated systems is a revolution. If the cable is electrically isolated, it is also
possible to take measurements to ensure that the system is intact. Otherwise we are
completely blind – with internal prestressing, we cannot see anything.
Another aspect of this trend is that durability of concrete decks is now far more
important than it used to be. A change in bridge deck design over the past ten years has
implications for deck replacement that are not yet fully understood. In the traditional
design, there would normally have been some kind of supporting structure, generally
topped by a concrete deck, which could easily be replaced. But now in the modern
bridge, the deck is part of the structure and is holding it up - a factor that could mean that
in the future, deck replacement could become more complex and expensive.
Conclusion
In India, one cable stay bridge had been constructed spanning across railway
track near Bangalore a few years ago. (Fig.9). Another one is now under construction at
Chirayatand near Patna. There is a significant growth in demand for long span bridges.
9 / 10
Fig. 9
Cable Stay Bridge crossing at Railway Yard
Eyes are turning to countries like China and Korea where major bridges are
under construction and one will see a high level of expertise in a low cost country – that
is certainly going to affect the economics of who does what.
***
10 / 10