2383

Beyond

the

’94

Deauville

behave and interact is the key to

Conference...

creating new conceptual designs for

Since engineers began to build bridges, they

high performance structures, whereas

ever more

refined

methods of

have looked for ways to increase span

structural analysis probably will not

lengths. Bridge design can be seen as the

pursuit of minimum of dead load to satisfy a

lead to new solutions. The following

span's required stiffness. This effort

proposal, a

"spatial"

suspension

becomes more interesting and more

bridge for barrier-breaking spans, is a

important as the span increases. Sound

highly challenging concept. Among the

conceptual design, proper detailing and

many possibilities it raises, might it not

simple methods of construction will lead to

good performance of bridges.

also find application for other types of

Each combination of materials and

long-span structured?

structural systems imposes its own

Prof. Eugen Brühwiler

limits. To understand, indeed to feel

Chairman,

IABSE

Publications

how structures and (new) materials

Committee

Christian Menn, Prof. em.

tems, new construction techniques,

Chur, Switzerland

new or more efficient construction ma-

David P. Billington, Prof.

terials, the span limit can be increased.

The challenge to exceed previous lim-

Princeton Univ., Princeton, NJ, USA

its is probably the most important rea-

INTRODUCTION

son why long spans continually have

fascinated bridge engineers.

For each bridge project, span length is

an especially important parameter. It is

a visual impression of the structure's

technical efficiency and it has a

considerable influence on construction

costs: For every structural system, both

the quantities of construction materials

and the design and erection problems

grow disproportionately as the span

increases. Moreover, each structural

system exhibits, by simple extrapola-

tion, an economically limited span

length whose location on a cost/span

diagram can be established where the

cost will increase exponentially. With

Fig. 15

fundamental changes in structural sys-

Since the beginning of the Industrial

upper deck was in place between 1931

Revolution - with its scientifically pro-

and 1962. At the end of the 20th

portioned bridge designs - the longest

Century the 2000 m span limit will be

spans have been achieved by suspen-

approached by the Akashi Straits

sion bridges (Fig. 15).

Bridge in Japan.

First were the chain bridges like the

Menai Straits Bridge of 1826 with a

Structural Systems for the

span of 177 m. After that came the

Longest Spans

development of the in situ cable

Great spans are doubtless achieved

spinning

technique

with

wires,

whereby the erection problems were

only with cable-supported bridges. In

significantly reduced. With the great

principle the following cable-support-

bridge over the Saane River in

ed bridge systems are known (Fig. 16):

Fribourg, Switzerland, in 1834 a span

(a) Suspension bridge with cables

of 274 m was reached, and in 1848 the

anchored in the ground: up to now this

ill-fated Ohio River Bridge at

Wheeling, USA, was the first to reach

is the system used for the longest spans.

the span of 1000 ft. (308 m). The

longest leap in the 19th century was

(b) Suspension bridge with cables

certainly the 487 m long span for the

anchored against the deck girder: used

Brooklyn Bridge in 1883. With the

only for small bridges and small spans,

construction of the George Washington

because the girder must first be

Bridge in 1931 between New York and

constructed on a scaffold.

New Jersey, O. H. Ammann showed a

(с) Cable-stayed bridge with cables

new way to design suspension bridges.

anchored in the deck girder: almost all

He increased the record, span from the

cable-stayed bridges are built with this

564 m of the Ambassador Bridge in

system. The compressive force

Detroit to the unbelievable 1067 m,

introduced into the girder by the cable

whereby in View of the high dead

stays and the cantilevered construction

weight or rather the "cable stiffness" (a

stage is critical limitations for the span

consequence of the great dead weight),

length.

he greatly reduced the stiffening truss

and even concluded that he could

eliminate

it entirely

when

only the

deck girder a sloping cable-stayed

The construction sequence for this

system carried by slender pylons

system is relatively simple:

which are supported by the central

– erect the central pylon

pylon.

– install the suspension cables (the

most difficult process of the erection of

the bridge)

– pull up the cable-stayed pylons.

which are anchored with temporary

and final cables to the central pylon

– build the deck from the cable stays

and the suspenders.

The proposed concept considers nu-

merous parameters, which for a bridge

with an extremely long span, must he

optimized specifically for each design.

The following parameters are particu-

larly important:

– height of the central pylon, height

and slope of the cable-stayed pylons

– placement of the traffic lanes, rail .

lines in the middle of the deck or

placed one at each edge

– number of suspension cables: two or

Fig. 17

three

By means of a cable connection with

– form of the deck cross section and

length of the deck segments

the central pylon, the danger of

– length of the region with V-shaped

buckling in the slender cable-stayed

hangers

pylons will be practically eliminated.

– construction: installation of the

In the vicinity of the pylon, the cable-

suspension cables, erection of the

stayed system will also carry the

deck.

vertical load; on the central part of the

span the more widely spaced cable

WHAT COMES NEXT?

stays serve primarily for the dynamic

stabilization of the deck. If the

At the present time, steel is clearly the

compressive force in the deck must be

most suitable material for 3000 m

limited, then at least a part of the sta-

spans. This could change, however, if

bilizing cables can be anchored (like

synthetic materials such as carbon

he suspension cables) in the ground

fibers prove reliable in

construction

(Fig. 18).

and the cost of their production were to

decrease substantially.

For spans

greater than 3000 m, new materials

Fig. 18

The first components to be built of

coefficients of thermal expansion are

synthetic materials will be long

different for these two materials, the

stabilizing cables. For these cables, the

stresses redistributed from one material

ideal axial stiffness, which is a

to the other due to change in

function of sag, is extremely important

temperature are small compared to

[6]. For long spans, lighter carbon fiber

stresses due to load.

cables can achieve the same stiffness

The last step in the development of

with a much smaller cross section than

bridges of extremely long spans may

heavier steel cables.

be not only the entire cable system of

As spans increase, it will become

synthetic materials, but also the girder.

necessary to use synthetic materials for

The first tests of girders made of syn-

the main suspension cables. Already

thetics are already under way [7]. The

for 3000 m spans, the weight per unit

proposed basic structural concept for

length of suspension cables is roughly

the design and construction of the tow-

equal to that of the girder it supports.

ers would remain valid in such a case.

The unit weight of the cables should

CONCLUDING REMARKS

not increase as spans are increased

even further. This can be avoided most

simply through the use of a hybrid

Simple extrapolations of rather

cable system, in which cables are

conventional concepts usually do not

composed of synthetics and steel

lead to great progress in technological

acting

together.

Although

the

development. This is true as well for

extremely long spans in bridge

[3] CASTELLANI, A. The Project for

structures. Because

today

lightweight

the Bridge over the Messina Strait,

high strength materials are not yet

ibid., pp. 517-527.

inexpensive and cannot yet be

[4] WALTHER, R., AMSLER, D.

produced

in

sufficient

quantity,

Hybrid Suspension Systems for Very

economical and practical designs for

Long

Span

Bridges: Aerodynamic

extremely

long

spans

require

Analysis and Cost Estimates, ibid., pp.

unconventional structural systems. The

529-536.

proposed structural concept, with its

[5] GIMSING, N.J. Suspended Bridges

relatively

simple

construction

with Very Long Spans, ibid., pp. 489-

technique, is certainly suited for this

504.

design problem. It is suitable, not only

[6] MEIER, U. Carbon Fiber-

for great spans but also for narrow

Reinforced

Polymers:

Modern

bridges with shorter spans and espe-

Materials in

Bridge

Engineering.

cially also for long span pipeline

IABSE, Zurich, SEI1/1992, p. 7.

bridges.

(7) SEIBLE, E, BURGUENO, R. Ad-

REFERENCES

vanced Composites in Cable Stayed

Bridges. Seminar in Cable-Stayed

[1] LEONHARDT, F. Zur Entwicklung

Bridges, Miami, FL, Oct. 17-18,1994.

aerodynamisch

stabiler

Hangebriicken.

Die

Bautechnik

45,

Heft 10 und 11,1968.

[2] LACROIX, R., et al. 3000 Metres:

We Can Make

It!

Proc.

Int. Conf.

IABSE-FIP, Deauville, 1994, Vol. 1,

pp. 505-515.

WEST SEATTLE SWING BRIDGE, SEATTLE, WASHINGTON

John H. Clark, Vice Pres.,

portions of the industrial port area of

Andersen Bjornstad Kane Jacobs, Inc.,

Seattle across one of its major shipping

Seattle, WA

channels.

Prospects for widening the current-46

PLANNING

m wide ship channel to 76 m required

replacement of an existing bascule,

A double-leaf concrete swing bridge

built in 1927. Navigation traffic in

across the Duwamish River in Seattle,

1994

required approximately 10

Washington, represents a revival of a

openings per day of the existing

type of movable bridge long out of

bridge, which provides 14 m of

favor and incorporates new concepts in

vertical clearance over high water.

machinery

and

movable

bridge

Increasing the vertical clearance from

technology.

The

bridge connects

two

14 m for the existing bascule bridge to

Fig. 19

The swing bridge alignment was

set by the column of the high level

placed on the existing bridge align-

bridge. Once the alignment was

ment to minimize required right of way

chosen, the span lengths were fixed.

and revisions to the existing street

The west approach

network. This alignment results in the

length of 153 m is determined by the

bridge axis being skewed approxi-

need to cross over a railroad track and

mately 45° to the channel. A total of

an intersecting street. The east ap-

nineteen different alignments and three

proach length of 147 m was deter-

basic structure types were evaluated

mined by the maximum grade of 7%

before selection of the swing bridge for

and vertical curve lengths required for

final

design development.

Other

proper sight distance. Stair towers for

structure types investigated in the

pedestrian access are provided on each

preliminary design stages were a

approach. A control tower for the

vertical lift bridge and a bascule

bridge operator is adjacent to, but sep-

bridge.

arate from, the west pivot pier. The

The 146 m span center-to-center of

control tower is a 36 m high structure,

pivot piers (Fig. 19) for the swing

so that the bridge operator has unre-

bridge was established by the width of

stricted view of the channel and all of,

the open west leaf and the location of a

the approach roadway.

column of the adjacent high level

STRUCTURAL DESIGN

bridge. The east pivot pier is then

placed

symmetrically about

the

centerline of the proposed channel.

The crossing site is near the mouth of

The tail span length of 53 m was also

the Duwamish River. Soils encoun-

tered at the site range from hydraulic

based. The location of the piers in the

fill and recent alluvial sands and silts

sloping bank of the existing and future

to heavily pre-consolidated glacial till.

channel created a problem of differing

Depth to the till varies from 15 m on

transverse stiffness of the piles which

the west end of the project to 60 m on

would have led to eccentricity between

the east end. Lenses of loose silt exist

the center of mass and center of

erratically throughout the alluvium

rigidity. These two problems were

layer. The loose surficial silts and hy-

addressed by the new concept of "seis-

draulic fill are deemed susceptible to

mic isolation" sleeves.

liquefaction in major seismic events.

Considerations in the choice for the

Densification by vibro-flotation was

superstructure design were construc-

specified to prevent such liquefaction.

tion economy, maintenance costs, traf-

Seismicity in the area is moderately ac-

fic safety and aesthetic compatibility

tive. Seismic design criteria generally

with the adjacent high-level bridge (a

followed the Guide Specifications for

concrete box girder). Two designs

Seismic Design of Highway Bridges,

were prepared and advertised for bids,

1983, of the American Association of

a post-tensioned segmental concrete

State Highway and Transportation Of-

box girder and a steel box girder with a

ficials, with appropriate modifications

precast prestressed concrete deck made

for the unique structure. The basic

composite after erection. Typical cross

seismic coefficient, amax is 0.30 g and

sections of the concrete alternative are

the soil type is Type III.

shown in (Fig. 20).

The main piers and superstructure

A major challenge in the design of the

comprise a structural unit which does

concrete box girder bridge was control

not contain the "ductile element" upon

of long-term deformations. Provisions

which the Guide Specifications are

for control of both long-term and

short-term deformations included ad-

ditional post-tensioning beyond that

required for stress control, provision of

some unbonded tendons, provision for

additional

future tendons, provisions

for adjustment of approach span

elevation at the tail span joint,

provision for vertical adjustment of

each leaf as a whole, and

specifications

requiring

nearly

simultaneous construction of the two

movable leaves.

The first line of defense against

unwanted long-term deformation was

Fig. 20

the adoption of the principle of load

balancing

for

the design

of the