Thursday, August 14, 2003 - 12:43

Soaring on the feet of an elephant

Rama VIII bridge commemorates brother of Thai king

It took nearly 10 years, but Bangkok, Thailand, now has a fifth
bridge crossing the Chao Phraya River at the center of the city. The bridge is
intended to relieve traffic congestion on the nearby Pinklao Bridge and serve
as a memorial to King Rama VIII, the late brother of the current Thai king, who
ruled for a short time.

The Engineers' Society of Western Pennsylvania, in
association with Roads & Bridges, awarded the Eugene C. Figg Jr. Medal to
Buckland & Taylor Ltd., North Vancouver, British Columbia, for the
company's work on the Rama VIII Bridge. The Figg Medal is given for a single,
recent outstanding achievement in bridge engineering that, through vision and
innovation, provides an icon to the community for which it was designed. The
medal was presented at the International Bridge Conference in Pittsburgh on June
9.

The bridge's owner is the Bangkok Metropolitan
Administration. The owner's engineer was Mott-MacDonald of the U.K, and the
architect for the outline design was Yee Associates of London. The design-build
contract was awarded to a joint venture headed by China State Construction
& Engineering Corp. and BBR Systems Ltd. with engineering services provided
by Scott Wilson Kirkpatrick Ltd., which did site design coordination, site
inspection and geotechnical design, Asdecon Corp., which did mechanical and
electrical design, and Buckland & Taylor. Buckland & Taylor provided
the detailed design of the cable-stayed main span and the construction
engineering services for its erection. In addition, the owner retained Prof.
Bundit Chulasai of Chulalongkorn University's architecture department to
provide architectural input at the detailed design stage.

The China State joint venture had to work harder than usual
to win the design-build contract. The project was first tendered in 1996. The
China State team had the lowest bid but lost the contract to another team on
the basis of points scored in the bid evaluation. Shortly thereafter, the Asian
economy took a dive, taking the Thai baht with it, and the bridge proj-ect was
shelved.

When it was revived in 1998, China State put together
another joint venture and won the contract. Construction began in 1999 and was
completed in 2002. The bridge opened to traffic in March 2002 and was
officially dedicated by King Rama IX on Sept. 20, King Rama VIII's birthday.

Dealing from three decks

Rama VIII is the world's largest asymmetric cable-stayed
bridge.

The bridge is 475 m long, with a 300-m main span, two 50-m
back spans and a 75-m anchor span. The whole thing is supported by a single
160-m-tall tower with an inverted-Y shape.

A lot of cable-stayed bridges employ one deck system for the
whole length of the bridge, but that was not the case with the Rama VIII
Bridge.

"In this case, because of the constraints on the structure,
we've got three different kinds of deck systems along the deck," Don
Bergman, vice president of Buckland & Taylor, told Roads & Bridges.

The main span consists of a light steel frame supporting
precast concrete deck panels and a 50-mm asphalt driving surface. The asphalt
can be ground off and replaced when the traffic surface needs rebuilding.

The main-span deck is 29.2 m wide, accommodating two lanes
of traffic in each direction, with a 5.3-m-wide sidewalk for pedestrians and
cyclists on each side. The anchor span is 28.4 m wide, carrying three lanes of
traffic each way.

Composite panels enclose the underside of the main-span deck
to give it a neater look. The owner wanted to simulate the clean appearance of
a closed box girder without the associated higher cost. The enclosure also
provides a dehumidified atmosphere to protect the steel of the structure.

"And then we've got two different kinds of
post-tensioned concrete box structures in the land spans," added Bergman.
"Because they're short, we've chosen to make them heavy to counterbalance
the longer, lighter main span." The anchor span was especially inventive.
"Basically, it's like a full-depth block concrete box girder, but in order
to get the weight to balance the main span, we couldn't put all that weight in
the back span initially because it would have governed the pile design. We
essentially had to fill the concrete box with concrete as the main span was
constructed to sort of counterbalance the added weight of the main span."

Sense of place

The tower of the Rama VIII Bridge is not unusually tall for
a cable-stayed bridge, but it literally towers over the surrounding area of the
historic district of Bangkok on the east bank of the river and Thon Buri on the
west bank.

Actually, the tower is taller than it might have been if
there had been a second tower. The designers decided not to place a tower on
the Bangkok side of the river because it might overshadow the historic Ban
Khunprom Palace, a 93-year-old structure immediately north of the bridge on the
Bangkok side of the river. The palace is intimately associated with Thailand's
transition to a constitutional monarchy in 1932 and is Thailand's only monetary
museum.

The Bank of Thailand also objected to placing a bridge tower
on the Bangkok side of the river. It worried that the construction activities
might cause vibrations that would disturb the delicate task of printing Thai
money, known as the baht, at a building immediately south of the bridge site.

Single-tower asymmetry is just one of the design features of
the Rama VIII Bridge that were influenced by the unique conditions and culture
of Bangkok, where public buildings and monuments are traditionally ornamented
with symbolic, commemorative or inspirational features.

The earlier Rama IX Bridge over the Chao Phraya River
connecting Bangkok and Thon Buri was something of a public disappointment
because it did not well reflect the Thai culture. That cable-stayed bridge was
criticized for being a transplanted international design rather than a
distinctly Thai design that reflected the importance of Thai art in daily life.

"Technically it's a very good bridge, and it's a very
good addition to the traffic system there, but the bridge really was not very
well received by the Thai public," said Bergman. "It's fairly stark.
It's perhaps a clean European design. There's nothing about it that the Thai
people could really relate to."

The king wanted the Rama VIII Bridge to be an important
structure. "He also wanted it to reflect Thai traditions and Thai culture
in a meaningful way," said Bergman.

In fact, aesthetic merit was so important that the owner of
the bridge, the Bangkok Metropolitan Administration, commissioned Prof. Bundit
Chulasai of Chulalongkorn University's architecture department to provide
architectural design input.

"He was in fact essentially a part of the design team
and was quite involved in working on the details," said Bergman. "I
think he played an important role in the look and flavor of the bridge."

Bundit figured out that he could use the inverted-Y shape of
the tower to symbolize a Bodhisattva seated on a lotus. The triangular space
between the legs of the tower represents the seated Buddha, through the
inclusion of gilded architectural details on each leg of the tower halfway between
the deck and the junction of the legs. In Buddhist canon, the Buddha had 10
previous lives before becoming Buddha. Bodhisattva refers to the life in which
he gave away his eyes.

Bundit also suggested the gilded lotus-bud filial that sits
atop the tower and is repeated along the cast aluminum handrail.

Octagonal concrete enclosures around the legs of the tower
represent an elephant's feet.

The stay cables are finished in a gold color to harmonize
with other gilded elements and to reflect the lighting at night.

Golden threads

The cables are arranged in a twin array from the tower in a
semifan configuration to support the main span. On the other side, a single set
of cables in a near-harp configuration connects the tower to the anchor block
below the anchor span. To minimize post-tensioning in the tower, most of the 84
stay cables are anchored into a single wall instead of the typical two walls.

The area of the Rama VIII Bridge is not terribly challenging
as far as wind or seismicity, but the engineering process still required
analysis of these two factors. Considering wind, for example, the engineers
selected a deck type as part of the conceptual design, then numerically modeled
the entire bridge with that deck in a computer.

"From that modeling, we get the dynamic properties of
the structure, the natural frequencies that the structure wants to
vibrate," explained Bergman. "It's almost like a string on a guitar;
if you pluck it, it vibrates at a certain rate. It's the same with a bridge
only on a much larger scale. If you pluck it, it will vibrate in certain
ways."

With the dynamic properties of the bridge, the engineers
built a small section of the bridge in real materials on a scale of about 50:1
and mounted it in a wind tunnel on a set of springs carefully tuned to match
the natural dynamic properties that were calculated in the numerical model.

"Then we blow the wind on it," continued Bergman,
"and we record the behavior of it. We increase the wind until the deck
goes unstable. When the deck becomes unstable that's what we call the critical
wind speed. That critical wind speed has to be reasonably above the maximum
expected wind speed for the site."

There are other types of wind effects the engineers look at
as well.

Bangkok sits on a flood plain composed of alternate layers
of clays and sands, with the lower layers densely compacted. The deposits
extend over 1 km deep, so it was not possible to found piles on bedrock. The
constructors used 1.5-m-diam. cast-in-drilled-hole concrete piles about 50 m
deep. The support for the piles is provided partly by friction and partly by
load-bearing.

As Rama VIII Bridge is deeply rooted in the alluvial
deposits of Bangkok, it also is deeply rooted in Thai culture brought into the
modern concerns of trying to relieve traffic congestion. The bridge stands as a
testament to modern engineering with a traditional flavor.

About The Author

Allen Zeyher is associate editor of Roads & Bridges.

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