2383

longitudinal

post-tensioning.

The

such that the last 12 m of the tail span

amount of prestress provided is the

is solid except for an access shaft. As-

amount required to provide 100% load

built measurements were taken after

balancing for the final dead load

casting of each section so that the final

condition. This required approximately

static balance could be closely predict-

30% more post-tensioning than the

ed. It was found necessary to explicitly

amount re quired to satisfy service load

account for the measured density of the

stress conditions. The deck of the box

concrete (from quality control test

girder is post-tensioned transversely

cylinders) and the actual weight of re-

and vertical post-tensioning is included

inforcing and prestressing steel in each

in

the

webs.

The

additional

segment to accurately predict the static

longitudinal post-tensioning reduced

balance. Allowable static imbalance

the need for vertical post-tensioning in

during construction while the bridge

some areas.

rested on temporary service bearings

Static balance of each leaf about the

was 72 MNm. Static imbalance during

pivot pier is achieved by thickening the

service operations of the movable

webs and bottom slab in the tail span

leaves was limited to 6.8 MNm to

and by the addition of ballast concrete

avoid stick-slip chatter in the pivot

and

galvanized reinforcing

steel

bars

shaft guide bearings.

PIER

HOUSE

AND

footing founded on 36 concrete filled

MACHINERY

steel pipe piles, 91 cm diameter,

completes both load paths.

The 12.8 m diameter pier house carries

Heavy locks at the tail span joint and at

the superstructure loads to the founda-

the center joint are provided for

tions and houses the drive machinery,

transfer of seismic response loads and

emergency generators, and part of the

to restrain

differential

displacements

control system. The pier table element

due

traffic,

differential

temperature,

of each leaf is supported by a transition

and

accidental misalignment. The

element which provides two load paths

torsional stiffness of the box section is

to the foundation. The closed position

sufficiently high that locks were pro-

path (serving vehicular traffic) is from

vided only on the longitudinal

the superstructure pier table through a

centerline of the section. The locks are

conical shell to the walls of the pier

driven by hydraulic rams powered by

house. Service bearings made up from

local pumps.

steel plates with reinforced elastomers

Twin hydraulic slewing cylinders (56

separate the transition element from

cm bore x 234 cm stroke) rotate each

the roof of the pier house. These

movable leaf from the closed position

service bearings are placed on an 11.4

to the open position to allow for pas-

m diameter circle. Temporary bearings

sage of ships. The operational cycle is

were placed between the permanent

based on a normal two minute slewing

service bearings to provide stability

time, total cycle including traffic lights

during construction of the cast-in-place

and gates is approximately 4.5

segmental concrete leaves.

minutes. Friction is minimized due to

the fact that the structure is supported

The operating position load path is

from the pier table through the center

on the hydraulic fluid of the lift-turn

portion of the transition element to the

cylinder. The principal source of

3.66 m diameter pivot shaft. The pivot

friction is the pivot shaft guide bear-

shaft is a concrete-filled steel shell 44

ings which maintain vertical align-

mm thick which in turn rests on the

ment.

hydraulically

operated

lift-turn

Power for raising and rotating the movable

cylinder. The pivot shaft is maintained

leaf is provided by 3-75 kW, 8-liter-per-

in the vertical position by guide

bearings (Fig. 21) at the roof and

second hydraulic pumps in each pier house.

operating floor level of the pier house.

Normal operation is with two pumps, the

In the operating position, the whole

movable leaf

including

transition

third pump alternates as a spare. Other

element and pivot shaft are raised

approximately 25 mm to transfer the

redundancies built into the system include the

load from the service bearings to the

pivot shaft.

A

reinforced

concrete

ability to slew the bridge using only one

GENERAL DESCRIPTION

creases at one end where it connects to

the two separate tunnels. Crosswise the

Oahu Island, in the Hawaiian Islands,

elevation of the two parallel decks

varies by as much as 4.6 m.

with the state capital Honolulu and the

bay of Pearl Harbor, is presently expe-

VALUE

ENGINEERING

riencing significant new urban arid

DESIGN

tourism development. To link the most

populated areas of the south to the

The structure was designed as a vari-

towns and military bases on the north-

able depth concrete box girder to be

east, the Hawaiian Department of

built in balanced cantilever with 4.9 m

Transportation, with the financial sup-

long superstructure segments cast in

port of the US Federal Highway Ad-

place with travelling forms. The struc-

ministration, is building an extension

ture shown in the base design would

of the H-3 Freeway across the steep

have been supported by cast-in-place

and volcanic Koolau Mountains. Be-

hollow concrete piers with a single row

cause of its natural beauty and archae-

of fixed or sliding bearings at the top.

ological significance, the whole area is

The foundations consisted of 400 mm

very sensitive environmentally. From

diameter,

octagonal precast

driven

the vicinity of Pearl Harbor, the free-

piles.

way includes the North Halawa

After bidding on the base design, an

Viaducts, the Haiku Tunnels under the

alternate was proposed according to

mountain range, and the Windward

"Value

Engineering"

procedure,

Viaducts on the north slope, where it

whereby certain economic and techni-

will connect to the north end of H-3 al-

cal conditions must be met, such as

ready in service

sharing any savings with the owner

The new Windward Viaducts consist

and

retaining

the

appearance,

of twin parallel prestressed concrete

geometry, and alignment of the

structures approximately 2 km long

structure, which had been the object of

with 24 spans each. The maximum

a

lengthy environmental

impact

span length is 91.5 m and column

assessment.

height varies from 3.6 to 48.8 m. The

The alternate design accepted by the

width of each bridge varies from 12.8

contractors’ joint venture and the

to 17.4 m. The geometry of the

owner,

included

the

following

viaducts includes reverse curves with

changes:

radius varying between 470 and 753 m

-

piers and superstructure were made

and cross slope between 5.5 % on one

monolithic

side and 6.8% on the other side. The

-

precast piles were replaced with 1.5

longitudinal slope of the structure is

m diameter drilled shafts

about 5 %. The horizontal distance be-

-

precast segmental construction was

tween the structures is constant on

used for the superstructure.

most of the length at 21.3 m, but in-

The project specifications allowed

and parabolic intrados. The depth

1360 days for the construction of the

varies from 4.9 m at pier to 2.45 m at

viaducts. This schedule did not provide

midspan (Fig. 22). The length of the

additional time for rainy days, which

precast segments are such that the

are frequent on the windward side of

maximum weight is 70 t. A 2-segment

the island. Precast construction has the

expansion joint is located every four

advantage of decreasing the amount of

spans, at about one-quarter of the span.

work performed at the site, improving

This disposition reduces deflections

quality and speed of erection and sig-

and changes of angle at the joint. The

nificantly, allowing the superstructure

two hinge segments are temporarily

to be entirely constructed from above

prestressed and treated as one piece

in this difficult terrain.

during the erection process. Tendons

Each viaduct is a single box made of

through the hinge are released after the

precast segments with vertical webs

next span is installed and stressed.

The longitudinal post'-tensioning lay-

deviation blocks running from pier

out was conventional with straight 19 x

segment diaphragm to pier segment

13 mm strand tendons placed within

diaphragm provide for any future

the top slab and anchored at the

external post-tensioning contingencies.

segment joints, close to the top of the

A 75 mm thick reinforced overlay is

webs, 32 tendons were used in each

applied to the bridge at the end of

typical cantilever. The continuity post-

construction according to the original

tensioning consists typically of 18-19 x

project specifications.

13 mm strand tendons placed in the

bottom slab and anchored in concrete

ERECTION SYSTEM

blocks. No draped tendons were

required in the webs, except for the

A special erection system was de-

hinge and abutment spans. Additional

signed and used for this structure with

a self-launching gantry that allowed

built with the minimum degree of free-

both viaduct structures to be built si-

dom compatible with its motion and

multaneously. The system is designed

the stability of the gantry crane. The

to accommodate variable cross slopes,

longitudinal displacement on top of the

curves, differential elevations, and

trusses is by a rack-and-pinion

variable distances between the two

hydraulically driven system insuring

structures.

total safety on the 5 % longitudinal

The system includes two independent

slope of the bridge. The relative

erection trusses, steel support beams

longitudinal motion of one leg with

supporting the trusses, a gantry crane

regard to the other (each leg running

and lifting trolley. The gantry crane is

on one truss 26.2 m away) is entirely

supported by two trusses, one on each

controlled by an electronic device

bridge.

monitoring the speed of the hydraulic

Transverse cross beams act as supports

motors and keeping the "lag" within

for the trusses. Each truss rests on

acceptable tolerances.

three cross beams, one at the rear, at

The launching of the trusses to the next

the tip of the previous cantilever, one

pier is done by fixing the gantry crane

on top of the pier segment of the can-

to the concrete deck through a

tilever under construction and a third,

temporary attachment and activating

mobile, on the front arm of the con-

its hydraulic motors which then push

crete cantilever and supporting the

the trusses under the gantry instead of

truss as the construction progresses,

moving the gantry on the trusses. The

reducing deflections and moments in

trusses are launched one at a time to

the truss. This was possible because of

minimize the load on the cantilever.

the moment-resisting connection be-

A complete launching from pier to pier

tween the deck and pier shaft. Each

took about 18 hours (Fig. 23). After

cross beam is set horizontally on the

completion of the learning curve, the

concrete deck.

contractor typically achieved the re-

A gantry crane rolls longitudinally on

sults shown in Table 2. On the best

the top members of the two trusses.

day, the contractor was able to set 16

The gantry crane carries a lifting trol-

segments.

ley running transversely to the two

trusses. The segments are delivered by

truck at the extremity of the previous

cantilever, picked up by the gantry

crane/lifting trolley and delivered at

either ends of the twin cantilevers un-

der construction. The gantry crane is

equipped with two legs on wheels. The

leg on the mountainside truss is the

"fixed leg." The leg on the ocean side

truss is the pendulum leg. Each leg is

design criteria have been conservative-

bottom and the top of the lower tower

ly established to assure the safety and

legs.

durability of the structure.

Deck

Tower

Configuration of the bridge girder de-

termines the method of construction.

Due to its stiffness and ease of con-

The basic design goal to make the

struction, a hollow cross section was

cross section as simple as possible was

selected for the towers. The size of the

successfully achieved in the Talmadge

tower columns was determined by the

Memorial Bridge. The deckwas not

requirements of strength and the space

only easy to construct, it was also one

needed for cable installation. In gener-

of the lightest possible cross sections

al, it is easier to stress cables at the

for a cable-stayed bridge

tower anchorages because a jack can

The deck girder was designed as a re-

be suspended from the top of the tower

inforced

concrete

structure,

even

column.

though longitudinal post-tensioning

A box configuration, with the cables

was applied in some areas to help con-

anchored in the front and the back

trol possible cracking, notably in the

walls of the tower column, was chosen

middle portion of the center span and

for simplicity. The cables are in the

the end region of the side spans, where

same vertical plane. This simplified the

axial compression induced by the cable

cable geometry and reduced the di-

forces was low.

mensional control during construction.

A preliminary conceptual design of the

But cable forces anchored in one wall

traveler was done during the design

must be transferred and connected to

stage to establish its approximate

the cable forces anchored at the oppo-

weight and the feasibility of construc-

site wall of the column. Post-

tion. It was found that the traveler

tensioning bar tendons were used for

weight should be approximately 175 t,

this purpose to prestress all four walls

assuming that the front cable was used

of the tower column in the vicinity of

to support the traveler. With a well-de-

the cable anchorages. Calculation of

signed traveler, the construction of the

this force transfer was by means of a

deck proved to be a straight forward

strut-and-tie method, neglecting all

operation.

tensile capacity of the concrete.

The floor beams were designed in pre-

The design of the lower portion of the

stressed

concrete.

Each floor

beam

tower was controlled mostly by

was prestressed by two 19-0.6"

transverse

wind loading.

Design

strands. The spacings of the floor

earthquake

effect

approached

beams were exactly the same as the

approximately the severity of wind. To

spacings of the cables. The floor

assure the necessary ductility, tie

beams were located as closely as

reinforcement was

placed

at the

possible

to the cable anchorage to