in of to de on- de two pre Fig. and was sup con tem pre span Cali Lin.

and

tives a Test” struc a

trace in be an strain A Arroyo field of of Y. stresses as engineer (see concrete JOURNAL the 1). was can T. into the the

hit/a joined PCI simple on segments “Proof in provided Constructed Fig. designed

BRIDGE placing

authors Caltrans bridge confidence

new The channel University predicted (see bridge bent by professor was technology

California terminology application Angeles), the

bridges. loaded precast legends California post-tensioning and bridge.

the locations the and prestressed

and flood

of

PRESTRESSED in m) Los a two the concrete, first key turned midspan bridge modern by concrete Berkeley first lightly check built falsework at at over pedestrian (16.8 at (near in Lin until

CONCRETE laboratory segmental to

State successful, deformations. Jurkovich The

FIRST ft

Caltrans, The This Dr.

California’s

program 1951 bridge fined cast Seco 55 gether servative port completed. porary stressed bridge 2). checked site prestressed Jim fornia gauges ture and was

of

the

for

in in in of in set first his

Mark cen pre also ago. con pre days (Cal high High intro post- domi its

research

future bridges has of this majority use construc the and California the on early all years concrete the breakthrough

bridges precast, the

anniversary of experiment was the California The 44 was

bright Bureau development in back superstructures, prestressed California constructed to

of of an future.

a into of While technology celebrating as than cast-in-place and the

important California State Transportation the percent State girder a return on concrete out in type. of California 85

an a prestressed have bridges the achievements more reflecting technological

insure in box exploded

centennial Commission. of for concrete currently concrete. towards by thousands developments evolution

will is evolution started and precast has bridges 1895, major of system. structure Prestressed

this stage n established recent ways the A Currently,

I

In the describe that 1951 trans) tennial Department tory looking structures in highways duction What tion crete way stressed of constructed stressed California tensioned recent the when nant

Bridges

Centennial Diego

RE. San Engineer Transportation

I at Inc. of Engineering

RE. Assistant Bridge Engineer California California California Structural Senior Associates, of Department Research California of & Bridge

Thorkildsen,

Holombo, .4 Jolla,

Innovative

Concrete

Caltrans

Eric Imbsen Formerly, Senior Sacramento, California Sacramento,

Jay Graduate Division University La 34 U) C’) quickly solved this problem. California contractors invested heavily in the falsework needed for cast-in-place construction. Once this initial investment was made, the cost of cast-in-place construction dropped to a level at or below precast girders. The seismic resistance and aesthetics of the box girder gained favor among designers. The cast-in-place post- tensioned concrete box girder became California’s favorite bridge.

PRECAST GIRDERS SOLVE DIFFICULT PROBLEMS While cast-in-place construction was feasible for new highway align ment in uncongested areas, an increas ing number of bridges required inno vative solutions using precast concrete. A safety program, begun in the 1960s to eliminate bents directly adjacent to roadways, provided con straints that only a precast, prestressed concrete option could accommodate. Many four-span overcrossings were replaced with two-span structures, eliminating the end bents nearest to the abutment. The existing roadway pro files could not be altered and vertical clearance to the existing freeway was at a minimum. This eliminated the pos sibility of falsework placement and de construction. manded a replacement two-span struc ture depth comparable to the existing four-span type. Half-span segments sign gained rapidly. Within two years, structures not affected by such situa were precast and erected at night on a prestressed concrete bridge was con tions was to use the conventionally re temporary supports while the freeway structed in Fresno to carry heavy truck inforced cast-in-place T-girder struc was closed. They were immediately traffic over a congested highway. ture type for spans less than 100 ft post-tensioned together, the support re Many California bridge engineers (30.5 m) and the conventionally rein moved, and the freeway was clear and soon considered prestressing ‘just an forced box girder for spans greater open to traffic by the next morning. other familiar construction method.” than 100 ft (30.5 m). By 1956, the The solution was very successful. controlling span length decreased to Further growth within the populated CAST-IN-PLACE 80 ft (24.3 m) as the box girder’s pop regions of California, such as San PRESTRESSED BOX ularity grew. Contractors preferred the Francisco, brought about new difficul flat platform falsework surface for the ties for cast-in-place construction. In GIRDER box girder and the cheap material used terstate 280 in the China Basin area of Prestressed concrete bridges in the to form the interior girders reduced San Francisco, constructed in the early 1950s were precast and used overall costs (see Fig. 3). 1970s, used hundreds of precast gird mainly in special situations, such as to Because concrete forming costs were ers for its elevated viaduct (see Fig. 4). work around site constraints on false- much less for the box, the only item Many more bridges needed to be work placement, to speed up construc restricting all around use of this widened with no additional vertical tion, and to meet roadway vertical superstructure system was the in clearance for falsework placement, so clearance restrictions that required creased amount of reinforcement re precast pretensioned concrete girders more slender superstructures. The eco quired. The reduction of steel atthbuted were often the choice. Precast girders nomic rule of thumb in those days for to the new prestressing technology were erected over environmentally

36 PCI JOURNAL 37

su

col

rigid cast-

with

girder

areas.

‘U a

A

the seismic

used of

cap 25.4mm

column

= presumably

these

flow.

precast tin

in

allows the

lateral 2

1970s,

and

for

at

T-bent

consisted tot.

traffic 4 O.305m

early

resist =

suspect tot.

repairs

interface

the

designs itt

usually Actuators

in 32

inverted

better were

connection

Past continued Actuators

built

to factors:

of

design Horizontal hold—down Vertical

allows manageable type perstructure

umn forces. bridges The in-place conversIon

girders.

Francisco, (SI)

and

pol girders

San more

elasti

tee metric

in

to cap

to

demands ____ precast -

design

prestressed

bulb bent

setup.

leads post—tensioning

Viaduct

the

test

hinging

seismic

precast developed.

This

280

CONCERNS frame

of

types, be

model

superstructures

plastic

Route reaction

Caltrans

column.

girder

should

UCSD

quantities

resist

the

4.

5.

SEISMIC

requires

Current

Fig.

large

Fig.

concrete

bathtub, icy

cally from

a it

A

to

in

co

the

now

con the span

Cal- The

rou

was

was

Cal-

bulb

spans

could

gap.

just

girder sensi prob

These

system

initial

feature

became

to

girder, uneco typical 150 offered

precast

tempo

are

precast genera

Califor

I-girder

Growing not

Canada, close

continu

the

all

for

depth-to-

concerns, a

are structural

been

FOR that

advantage

lacked

bent.

the

in for weight.

new

resistance, Although

and historically in

that

concrete in

to

shape. Texas,

continuous

precast

of type

for

factors.

next

a delta

ratios

cast-in-place

from

bridge

was low

than had

but

characteristics

post-tensioned

take

distinct important bridges. spans a

that complete, and

until

the required

used technologically type

m),

states

the

water

BRIDGES

without

to heavy girder

m),

support

considered an

NEED aesthetically

traffic.

precast

1995

be bridges

seismic

workhorse

California. period, difficult

shape

two working

and

California

a costly

of of

construction

still

include solution

was precast

(38 “Bathtub”

its in

girder

This

post-tensioning. to

longer

bents

the

in

structural girder

ranging

rail 54.8

minority construction other was be

the ft

desirable to

filled

girder

the

that

environmental

the

time

over

to

The

contributing

for in

of more

used

develop

be concrete

situations

together

currently

depth-to-span the

girder

precast

with

box bodies

125

cast-in-place appeal

appeal, the

precast

precast

due

carry

structure

Washington,

chosen

currently

to

the

this spans

would

delta

and

in bathtub still

of

(45.7 used called low

or

bridges is

common

PRECAST

This

mount slab,

ratio.

of speed tee

decided of

PRESENT

than preferred

girder. falsework needed considered. locations,

ft

was precast

and

continuous

spliced

the Special

Although

The

sensitive

the over remained during crete

congestion, and required very few operation trans dustry advanced many found characteristics concrete aesthetic span post-tensioning, less 180 would was of by be bulb tinely bridge aesthetic

box nomical historically tive type deck shape, Oregon, then tion nia, definitive

lems. rary ously tee, November-December post-tensioning heavier

trans notched precast girders seated on the designed to increase the width of the sary dead load and seismic shears into cap. The deck was continuous over the superstructure effective in resisting the the girders. bent, but there was no effort to obtain plastic overstrength moment of the Testing of both units will consist of the kind of positive moment connec column, reduce reinforcement conges incremental horizontal fully reversed tion needed to resist column hinging tion in the joint region, and increase displacement cycles until target ductil demands. the cap beam torsional capacity. ities have been reached. If a successful The lack of a rigid connection at the Design and construction of the first response is observed, the majority of column superstructure connection be model to be tested has begun and is the damage will occur in the column comes evident when considering a shown in Fig. 5. This is a /io-scale, or with only minor distress in the super typical bridge with multiple support 40 percent full scale, model4 of a typi structure elements and cap beam ele columns per bent. A cast-in-place box cal single column bent including the ments. Therefore, a second part of the girder bridge with an integral column column, footing, cap beam, and tribu test will involve disconnecting the cap connection would not require a tary modified bulb tee girders extend horizontal actuators and loading the moment resisting connection at the ing midspan to midspan with compos vertical actuators to failure to observe column footing interface, whereas the ite decking. Precast girder segments the force displacement response of the precast bridge would. The size of the will pass through the cast-in-place composite superstructure. substructure elements, such as the post-tensioned cap beam and splice in Construction of the first unit footing and piling, can be drastically the general location of the dead load started in the beginning of September reduced, resulting in significant cost inflection points. of this year and testing is scheduled savings. This column superstructure Continuous girder post-tensioning for the first part of January 1996. connection became the critical detail will pass through the girder segments The second unit is scheduled for test in the precast concrete girder’s resur and will be jacked at the anchorages ing in March or April of 1996. Data gence in popularity. located in the abutments at either end reduction of the extensive instrumen of the model. Because the cap beam is tation planned for both tests and re deeper than the superstructure, mild porting will be completed several RESEARCH AT UCSD reinforcing steel will pass under and months after testing. To solve such difficult problems, a over the girders and only the post-ten joint technical committee was formed sioning tendons need to pass through between Caltrans engineers, outside the girders in the cap beam region. CONCLUDING REMARKS consultants, and precast concrete in The second model will have a similar With the completion of the research dustry representatives. As a result of scale and overall dimensions and will at UCSD and the standardization of the committee’s recommendation, a feature the bathtub girder. these designs, California could once $250,000 research project is currently Because the required dead load mo again benefit from precasting segments underway at the University of Califor ment profile and the model self weight and continuously post-tensioning them nia at San Diego (UCSD) to “Proof are different multiples of the model together using the same concepts de Test” two integral cap beam designs scale, hold-downs will be applied so veloped for the Arroyo Seco pedes for precast concrete girders under sim the model moment profile will approx trian bridge 44 years ago. Together ulated seismic loads in the longitudi imate the required scale dead load mo with all the other innovative techno nal direction. ment profile up to the girder splice. logical advances that are occurring in One test will feature the bulb tee The horizontal actuators shown in Fig. seismic design, construction tech girder and the other the “Bathtub” 5 will model the seismic inertia forces niques and new materials, the future of girder configuration. Both designs will under longitudinal response, while the bridge building in California does in incorporate newly developed details vertical actuators will apply the neces deed look bright.

38 PCI JOURNAL