Monolithic Launch of the Reggiolo Bridge_TechPaper

8/18/2019 Monolithic Launch of the Reggiolo Bridge_TechPaper

1/5

Concrete international / FEBRUARY 2007 61

BY MARCO ROSIGNOLI

Even though the Reggiolo Overpass in Reggiolo, Italy, is

a small bridge, several factors complicated its design

and construction. The bridge, completed in 2003, spans

the Verona-Mantua Railway that had to remain fully

operational during construction. This made the formwork

and shoring that would have been necessary for the

originally designed cast-in-place bridge difficult to locate

and construct. Its location over a working railway also led

to the owner’s desire for minimal long-term maintenance,

resulting in the selection of a monolithic, fully prestressedstructure instead of a steel or precast/prestressed

girder system.

Settling soils at the site also affected the design. The

structure was simply-supported so that higher-than-

expected foundation settlements could be recovered by

Monolithic Launch

of the ReggioloOverpass

An example of an innovative construction method

shimming at the abutments, and the structure was

lightened by using high-density polystyrene blocks to fill

voids between the top and bottom slabs. These blocks

also eliminated empty cells that require inspection and

can gather water, further reducing maintenance. The

bridge also had a complex, trapezoidal plan geometry as

a result of the entry and exit ramps that converged on

the overpass.

The demanding requirements were met by casting

the superstructure on one side of the span, and thenmonolithically launching it over the railway into its final

position. Similar construction techniques have been

adopted for Italian railway overpasses,1 but in this case,

the small dimensions and complex geometry suggested

monolithic, rather than incremental, launching.

DECK DESIGNAs shown in Fig. 1, the superstructure of the overpass

is a multi-cellular, prestressed concrete plate spanning

26 m (85.3 ft). The bottom surface is horizontal in the

transverse direction and slightly inclined longitudinally,

but the top surface is inclined in both directions toshed water, so that the total thickness of the structure

varies from 1.28 to 1.35 m (4.2 to 4.4 ft), with a mean

span-to-depth ratio of about 20. A varying-depth paving

layer would have been excessively heavy on such

a wide deck.

In the transverse direction, the plate is stiffened by

three 0.4 m (15.7 in.) thick internal diaphragms spaced

at 6.5 m (21.3 ft) and two 1.0 m (3.3 ft) thick end beams

at the abutments. A section at the midspan diaphragm

is shown in Fig. 2. Because the launch supports were

aligned with the three internal longitudinal webs and

the side cantilevers ranged from 3.24 to 6.30 m (10.6 to

Fig. 1: Plan view of the internal stiffening systems in the deck plate

(All dimensions in meters; 1 m = 3.28 ft)


2/5

62 FEBRUARY 2007 / Concrete international

20.7 ft), a high level of prestress in the transverse direction

was required during launching. In addition, the transverse

negative moment was increased by the concentrated load

of the edge beam.

The prestressing level in the superstructure was

quite high because of the railway authority’s request for

a fully prestressed section in both the longitudinal and

transverse directions under full design loads (dead

and superimposed dead loads, live loads inclusive of

dynamic amplification, and thermal gradients) and also

during launching. In spite of the short construction

duration, consideration of time-dependent launch

stresses2 was required.

The prestressing included: a) longitudinal launch

tendons in the internal webs and edge beams; b)

longitudinal tendons in the bottom slab, tensioned

after launch completion; c) transverse tendons in the

end beams and internal diaphragms, all tensioned

before launching; and d) transverse tendons in the

deck slab, also tensioned before launching.

Fig. 2: Cross-section of the deck slab at the midspan diaphragm (All dimensions inmeters; 1 m = 3.28 ft)

Fig. 3: Support points of the deck plate during launching (All dimensions in meters;

1 m = 3.28 ft)

DECK CONSTRUCTIONAfter casting the foundation piles and the two

abutments, the casting yard was built on the embankment

leading to the launch abutment. It consisted of three

parallel 1.8 m (5.9 ft) deep foundation beams spaced5.5 m (18.0 ft) apart to match the spacing of the three

internal webs (Fig. 3).

The foundation beams were anchored to the launch

abutment to minimize settlement and transfer the launch

forces to the abutment. Figure 4 shows the deck after it

was lifted from the foundation beams for prelaunch

weighing and shows the wide cantilever at the rear deck

section. Before launching, the deck was painted and

equipped with safety fences and guard rails.

The three prestressed concrete launching noses

at the front of the deck (Fig. 5) were constructed to

control negative moments at the front of the bridge by

shortening the cantilevered span that the deck would

have to carry before reaching another support. They

also limited the safety factor for deck overturning about

the launch abutment to a minimum

of 1.5. The noses were 3 m (9.8 ft)

long, 0.60 m (2 ft) thick, and 1.15 m

(3.8 ft) deep and aligned with the

three internal webs. As shown in

Fig. 5, each nose was anchored to

the corresponding deck web with

eight 36 mm (1.4 in.) prestressing

bars. These overlapped the ends

of the four launch tendons (Fig. 1).Shear keys on the contact surface

between the deck and noses

were match-cast to transfer vertical

forces from the nose to the deck.

Steel braces joined the three

noses together.

Short prestressed concrete noses

such as these, that are detached and

demolished after launch completion,

are often less expensive than

conventional steel girders, especially

when the negative moments are toohigh to allow full-span launching of

the prestressed deck.3 As shown in

Fig. 6, the high negative moments

were alleviated by cutting the launch

span in half using a temporary, three-

tower steel pier placed near the

railway. Each tower was vertically

supported by a foundation pile, and

the tops of the towers were anchored

to the launch abutment with rigid

stays that resisted the horizontal

forces from sliding friction.


3/5


Because of its varying width,

the deck alignment could not be

maintained with conventional lateral

guides during launching.4 Instead,

guide pins at the rear of the deck and

at the launch abutment were used.

These pins guided the bridge through

a steel-lined recess in the underside

of the deck (Fig. 2). At the temporary

pier, the central guide was used to

laterally stabilize the pier through

the deck.

After tensioning the launch

prestressing tendons (transverse

tendons in the end beams and the

internal diaphragms, transverse

slab tendons, and longitudinal launch

tendons), the deck plate was lifted

off the foundation beams using three

rows of hydraulic sliding supports

aligned with the foundation beams

and the internal deck webs (Fig. 3).

Because the launching tendons were

tensioned while the structure was

resting on the foundation beams,

a portion of the prestressing force

may have been lost to friction

between the structure and the

foundation beams. This required

a final check of the prestressingforce after lifting the structure. The

strands were retensioned if found to

be at less than 98% of the design

value. The deck was also weighed to

confirm its theoretical weight, and

measured deflections were compared

to theoretical deflections for an

indirect evaluation of the modulus of

elasticity of the concrete.5

With the bridge supported on 2 or

3 rows of hydraulic sliding supports,

hydraulic pistons acting betweenthe deck and the foundation beams

(Fig. 7) pushed the bridge forward

out over the span. After launching

the desired distance, the support

points for the bridge were changed,

and the pistons were repositioned.

This process was repeated in

several stages, as illustrated in

Fig. 8, until the bridge reached its

final position.

After releasing and dismantling

the launch noses, the bottom slab

Fig. 4: Deck weighing before launching through transducer-controlled hoisting. Thefour blockouts in the foundation beam house the hydraulic jacks at the supportpoints shown in Fig. 3

Fig. 5: The prestressed concrete launch noses were connected with steel bracingand were used to control negative moments while the bridge was launched. Theywere removed after the bridge was in its final location

Fig. 6: Temporary steel towers were used to cut the span of the bridge in halfduring the launch. The steel braces connected the top of the tower to the launchabutment and resisted the lateral forces at the support points


4/5

64 FEBRUARY 2007 / Concrete international

tendons were tensioned and grouted, the supports at the

temporary towers were removed, and the bridge was

placed on permanent laminated-rubber bearings at thetwo abutments.

Additional considerations and comments

The temporary stresses in the deck plate during

the monolithic launch were analyzed at the beginning

and end of each launch stage and at intermediate

stages where necessary. A three-dimensional grillage

model was used for this analysis because a two-

dimensional beam model would have lead to excessively

approximate results.6 The analyses also accounted for

settlement of the temporary pier and the prescribed

tolerances in the support reactions in both thelongitudinal and the transverse direction. Comparison

of the results of launching and service load analyses

showed that positive moments in the longitudinal webs

were governed by service conditions, while launching

governed negative moments (these negative moments

would have been totally absent in the case of conventional

deck construction on shoring). Although these temporary

stresses increased the cost of prestressing the deck, the

erection, dismantling, and materials for scaffold-type

shoring on piles and the higher embankments required

for working clearance above the railways would have

generated about 12% higher costs.

The launch equipment was particularly inexpensive.

Because of the bridge’s modest weight—about 8900 kN

(1000 ton)—a pair of small long-stroke launch pistons(Fig. 7) generated adequate operational speed at minimal

cost.4 The use of hydraulic launch supports in the casting

yard instead of conventional continuous low-friction

extraction rails7 generated substantial savings in this

bridge because three 27.6 m (90.6 ft) long steel rails

would have been particularly expensive. In addition, deck

lifting permitted real-time monitoring of the support

reactions during launching, as well as compensating for

the settlements of the support embankment.

To further save manpower, new types of sliding supports

were tested at the temporary pier, where Teflon® plates

were placed directly onto the hoisting jacks and longstainless steel sheets were inserted between the hoisting

jacks and the bridge for long-stroke continuous sliding

(Fig. 6). The result was encouraging, although loss of the

compensating effect of the rubber sheets of conventional

low-friction bearing pads is acceptable only when hydraulic

launch supports are used, and the nonpolished,

nontensioned surface of the stainless steel sheets

unavoidably increased sliding friction.

Accurate load testing of the deck plate after launch

completion confirmed the absence of any nonlinearity as

a result of the launch process, and the prescribed

absence of longitudinal and transverse tensile stresses

Fig. 7: Paired launch pistons and central anti-drift guide at the rear deck end


5/5


Marco Rosignoli recently joined HNTB Corp.

as Chief Bridge Engineer in the Seattle, WA,

office after working 8 years as a Site

Manager for bridge projects, 7 years as the

Technical Director for a major Italian bridge

contractor, and 9 years as a freelance

bridge consultant. He has authored two

books and several publications on the

incremental-launching construction of

bridges and also provides independent design checking of

launching gantries and movable formwork systems.

during launching was monitored in real time through

hydraulic control of the support reactions and comparison

with their theoretical values.

SAFE AND RELIABLEThis application of the monolithic-launch construction

method demonstrated its reliability and safety.8 Adoption

of this method allowed a delicate and complex bridge to

be built with extreme safety for the workers, in a short

timeframe, and without any restriction on the railway

traffic below. The adverse effects of soil settlement were

completely avoided by casting the superstructure on stiff

foundation beams with inexpensive forms,9 thus improving

the quality of the structure.

Acknowledgments

The Reggiolo Overpass was built by the firm Locatelli, SpA, for

the owner Autostrada del Brennero, represented by G. Andreani.

The general designer of the motorway bypass was Sembenelli

Consulting. The author designed the prestressed deck plate for both

launch and service load stages, performed independent design

checking of the launch equipment for the owner, and supervised the

launch process and final load testing.

References

1. Rosignoli, M., “Site Restrictions Challenge Bridge Design,”

Concrete International , V. 20, No. 8, Aug. 1998, pp. 40-43.

2. Rosignoli, M., “Creep Effects During Launch of the Serio River

Bridge,” Concrete International , V. 22, No. 3, Mar. 2000, pp. 53-58.

3. Rosignoli, M., “Nose-Deck Interaction in Launched Prestressed

Concrete Bridges,” Journal of Bridge Engineering , V. 3, No. 1, Feb.

1998, pp. 21-27.

4. Rosignoli, M., “Thrust and Guide Devices for Launched

Bridges,” Journal of Bridge Engineering , V. 5, No. 1, Feb. 2000, pp. 75-83.

5. Rosignoli, M., Launched Bridges, ASCE Press, Reston, VA, 1998,

363 pp.

6. Rosignoli, M., “Reduced-Transfer-Matrix Method for Analysis of

Launched Bridges,” ACI Structural Journal , V. 94, No. 4, July-Aug.

1999, pp. 603-608.

7. Rosignoli, M., Bridge Launching , Thomas Telford, Ltd., London,

2002, 342 pp.

8. Rosignoli, M., “Incremental Bridge Launching,” Concrete

International , V. 19, No. 2, Feb. 1997, pp. 36-40.

9. Rosignoli, M., “Deck Segmentation and Yard Organization for

Launched Bridges,” Concrete International , V. 23, No. 2, Feb. 2001,

pp. 64-73.

Selected for reader interest by the editors.

Fig. 8: Launch sequence for the Reggiolo Overpass

Monolithic Launch of the Reggiolo Bridge_TechPaper

Documents

Transcript of Monolithic Launch of the Reggiolo Bridge_TechPaper