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BRIDGE ERECTION MACHINES

Contents

1. Introduction to Bridge Construction Methods

2. Main Features of Bridge Erection Machines

3. Beam Launchers

Summary

Bridge

industry

is

moving

to

mechanized

construction because

this

saves

labor,

shortens

project

duration and improves quality.

This trend is evident in many countries

and affects most construction

methods. Mechanized bridge construction is based on the use of special machines.

New-generation

bridge

erection

machines

are

complex

and

delicate

structures.

They

handle

heavy

loads

on

long

spans

under

the

same

constraints

that

the

obstruction

to

overpass

exerts onto

the final

structure.

Safety of operations and quality

of

the final product

depend

on

complex

interactions

between

human

decisions,

structural,

mechanical

and

electro- hydraulic

components

of

machines,

and

the

bridge

being erected.

In

spite

of

their

complexity,

the

bridge

erection

machines

must

be

as

light

as

possible.

Weight

governs

the

initial

investment,

the

cost

of

shipping

and

site

assembly,

and

the

launch

stresses.

Weight

limitation

dictates

the

use

of

high-strength

steel

and

designing

for

high

stress

levels

in

different

load

and

support

conditions,

which

makes

these

machines

potentially

prone

to instability.

Bridge

erection

machines

are

assembled

and

dismantled

many

times,

in

different

conditions

and

by

different

crews.

They

are

modified

and

adapted

to

new

work

conditions.

Structural

nodes

and

field

splices

are

subjected

to

hundreds

of

load

reversals.

The

nature

of

loading

is

often

highly

dynamic

and

the

machines

may

be

exposed

to

hundreds

and

strong

wind.

Loads

and

support

reactions

are

applied eccentrically, the

support sections are

often devoid of

diaphragms, and

most

machines

have

flexible

support

systems.

Indeed

such

design

conditions

are

almost

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中英文资料外文翻译文献

inconceivable in permanent structures subjected to such loads.

The

level

of

sophistication

of

new-generation

bridge

erection

machines

requires

adequate

technical

culture.

Long

subcontracting

chains

may

lead

to

loss

of communication, the problems not dealt

with during planning and design must be solved on

the

site,

the

risks

of

wrong

operations

are

not

always

evident

in

so

complex

machines, and human error is the prime cause of accidents.

Experimenting

new

solutions

without

the

due

preparation

may

lead

to

catastrophic results. Several bridge erection machines collapsed in

the years, with fatalities and huge

delays in the project schedule. A level of technical culture adequate to the complexity of mechanized

bridge

construction

would

save

human

lives

and

would

facilitate

the

decision-making processes with more appropriate risk evaluations.

1.

Introduction to Bridge Construction Methods

Every

bridge

construction

method

has

its

own

advantages

and

weak

points.

In

the absence of particular requirements that make

one solution immediately preferable to the others, the

evaluation of the possible alternatives is always a difficult task.

CoMParisons

based

on

the

quantities

of

structural

materials

may

mislead.

The

technological

costs

of

processing

of

raw

materials

(labor,

investments

for

special

equipment,

shipping

and

site

assembly

of

equipment,

energy)

and

the

indirect

costs

related

to

project

duration

often

govern

in

industrialized

countries.

Higher

quantities

of

raw

materials

due

to

efficient

and

rapid

construction

processes

rarely

make

a

solution

anti-economical.

Low

technological costs

are

the reason

for

the success

of

the incremental

launching

method for PC bridges. CoMPared to the use of ground falsework, launching diminishes the cost

of

labor

with

similar

investments.

CoMPared

to

the

use

of

an

MSS,

launching

diminishes

the

investments with

similar labor costs. In both

cases launching diminishes the technological costs

of

construction and even if

the launch stresses may

increase the quantities of raw materials, the balance

is positive and the solution is cost effective.

The

construction

method

that

comes

closest

to

incremental

launching

is

segmental

precasting.

The

labor

costs

are

similar

but

the

investments

are

higher

and

the

break-even

point

shifts

to

longer

bridges.

Spans

of

30-50m

are

erected

span-by-span

with

overhead

or

underslung

launching

gantries.

Longer

spans

are

erected

as

balanced

cantilevers:

self-

launching

gantries reach

100-120m

spans

and lifting

frames

cover

longer spans

and curved bridges.

Heavy

self-launching

gantries

are

used

for

macro-segmental

construction

of

90-120m

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中英文资料外文翻译文献

spans.

Span-by- span

erection

of

macro-segments

requires

props

from

foundations.

Balanced

cantilever

erection

involves

casting

long

deck

segments

under

the

bridge

for

strand

jacking into position. Both solutions require high investments.

On shorter

bridges, prefabrication is limited

to the girders

and the deck slab

is cast in-place.

Precast

beams

are often

erected

with

ground

cranes.

Sensitive

environments, inaccessible

sites,

tall

piers,

steep

slopes

and

inhabited

areas

often

require

assembly

with

beam

launchers, and the technological costs increase.

LRT

and

HSR

bridges

with

30-40m

spans

may

be

erected

by

full-span

precasting.

The

investment

is

so

high

that

the

break- even

point

is

reached

with

hundreds

of

spans.

The

precasting

plant

delivers

2-4

spans

per

day

for

fast-track

construction

of

large-scale

projects.

Optimized

material

and

labor

costs

add

to

the

high

quality

of

factory production. Road carriers and

ground cranes may erect four single-track

U-girders (two LRT

spans)

every

night.

Heavy

carriers

with

underbridge

and

gantries

fed

by

SPMT’s

are

the

alternatives

for

ground

delivery

of

HSR

spans.

Precast

spans

longer

than

100m

have

been

erected with floating cranes.

Medium-span PC bridges

may also be cast in-place.

For bridges with more than

two or three

spans

it is

convenient to

advance in

line by

reusing the

same formwork

several times,

and

the

deck

is

built

span-by- span.

Casting

occurs

in

either

fixed

or

movable

formwork.

The

choice

of equipment is governed

by economic reasons as the

labor cost associated with

a

fixed

falsework

and the

investment

requested for

an MSS

are both considerable.

Starting

from

the

forties,

the

original

wooden

falsework

has

been

replaced

with

modular

steel

framing

systems.

In

spite

of

the

refined

support

structures,

labor

may

exceed

50%

of

the

construction cost

of the span. Casting on falsework

is a viable solution only with inexpensive labor

and small bridges.

Obstruction of the area under the bridge is

another limitation.

An

MSS

comprises

a

casting

cell

assembled

onto

a

self-

launching

frame.

MSS’s

are

used for span-by-span casting of long bridges with 30-70m spans. If the piers are not tall and the

area

under

the

bridge

is

accessible,

90-120m

spans

can

be

cast

with

45-

60m

MSS’s

supported

onto

a

temporary

pier

in

every

span.

Repetitive

operations

diminish

the

cost

of

labor,

the

quantities

of

raw

materials

are

unaffected,

and

quality

is

higher

than

that

achievable

with

a

falsework.

Bridges

crossing

inaccessible

sites

with

tall

piers

and

spans

up

to

300m

are

cast

in-place

as

balanced

cantilevWhen

the

bridge

is

short

or

the

spans

exceed 100-120m

the

deck

supports

the form travelers. Overhead

travelers are preferred

in PC bridges

while underslung machines are

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中英文资料外文翻译文献

used

in

cable-stayed

bridges

and

cable-supported

arches.

With

long

bridges

and

90-120m

spers.

ans,

two

longer

casting

cells

may

be

suspended

from

a

self-launching

girder

that

also

balances the cantilevers during construction.

2.

Main Features of Bridge Erection Machines

The industry of bridge erection machines is a highly specialized niche. Every machine is initially

conceived for a scope, every

manufacturer has its own technological habits, and every contractor has

preferences and reuse

expectations. The country of fabrication also influences

several

aspects

of

design.

Nevertheless,

the

conceptual

schemes

are

not many.

Most

beam

launchers

comprise

two

triangular

trusses

made

of

long

welded

modules.

The

diagonals

may

be

bolted

to

the

chords

for

easier

shipping

although

site

assembly

is

more

expensive.

Pins

or

longitudinal

bolts

are

used

for

the

field

splices

in

the

chords.

New- generation

single-girder

machines

allow

robotized

welding

and

have

less

support

saddles

and

smaller winch-trolleys.

50m

spans

are

rarely

exceeded in

precast

beam bridges.

A

launching

gantry

for

span- by-span

erection

of

precast

segmental

bridges

also

operates

on

30-50m

spans

but

the

payload

is

much

higher

as

the

gantry

supports

the

entire

span

during

assembly.

The

payload

of

an

MSS

for

in-place

span- by-span

casting

is

even

higher

as

it

also includes the casting cell, although the nature of loading is less dynamic.

A

launching

gantry

for

span- by-span

erection

of

precast

segmental

bridges

also

operates

on

30-50m

spans

but

the

payload

is

much

higher

as

the

gantry

supports

the

entire

span

during

assembly.

The

payload

of

an

MSS

for

in-place

span- by-span

casting

is

even

higher

as

it

also includes the casting cell, although the nature of loading is less dynamic.

Lighter

and

more

automated

single-girder

overhead

machines

are

built

around

a

central

3D

truss or two braced

I-girders. A light front extension controls overturning

and a rear C-frame

rolls

along

the

completed

bridge

during

launching.

Single-girder

overhead

machines

are

coMPact

and

stable

and

require

ground

cranes

only

for

site

assembly.

Telescopic

configurations

with

a

rear

main

girder

and

a

front

underbridge

are

also

available

for

bridges with tight plan curves.

Underslung

machines

comprise

two

3D

trusses

or

pairs

of

braced

I-girders

supported

onto

pier

brackets. Props

from

foundations

may

be

used

to

increase

the

load

capacity

when

the

piers

are

short.

A

rear

C-frame

rolling

over

the

completed

bridge

may

be

used

to

shorten

the

girders.

Underslung

machines

offer

a

lower

level

of

automation

than

the

single-girder

overhead

machines and

are affected

by ground constraints

and clearance requirements.

Span-by-span

macro-segmental

construction

requires

heavy

twin-truss

4

中英文资料外文翻译文献

overhead

gantries

with

a

rear

pendular

leg

that

takes

support

onto

the

deck

prior

to

segment lifting. Transverse joints at the span quarters and a longitudinal joint at bridge centerline divide

80-100m

continuous

spans

into

four

segments.

The

segments

are

cast

under

the

gantry

with

casting

cells

that

roll

along

the

completed

bridge

and

are

rotated

and

fed

with

the

prefabricated cage at the abutment.

Overhead

gantries

for

balanced

cantilever

erection

of

precast

segments

reach

100-120m

spans.

CoMPared

to

span-by-span

erection,

the

payload

is

lower

as

no

entire

span

is

suspended

from the gantry. The negative

moment from the long front cantilever and the launch stresses

on so

long spans govern

design. Varying-depth

trusses are structurally more efficient while constant-depth

trusses

are

easier

to

reuse

on

different

span

lengths.

Stay

cables

are

rarely

used

in

new- generation

machines.

Overhead

MSS’s

for

balanced

cantilever

brid

ges

operate

in

a

similar

way.

Two

long

casting cells

suspended from

a self-launching girder

shift symmetrically

from the

pier toward

midspan

to

cast

the

two

cantilevers.

After

midspan

closure

and

launching

to

the

next

pier,

the

casting

cells

are

set

close

to

each

other

to

cast

the

new

double

pier-head

segment.

These

machines

can be easily modified

for strand- jacking of

macro-segments cast on the ground.

The

bridge

itself

can

support

lifting

frames

for

balanced

cantilever

erection

of

precast

segments or form

travelers for in-place

casting. These light

machines are used in

short or curved

bridges,

PC

spans

up

to

300m,

and

cable-stayed

bridges.

Lifting

frames

and

form

travelers

permit

erection

of

several

hammers

at

once

and

different

erection

sequences

than

from

abutment

to

abutment,

but

they

require

more

prestressing

and

increase

the

demand

for labor and ground cranes.

Carriers with

underbridge and heavy

gantries fed by

SPMT’s are used

to erect precast

spans.

Spans are

rarely longer than 40m in

LRT and HSR bridges and

50m in highway bridges due

to

the

prohibitive

load

on

the

carriers

and

the

bridge.

Longer

spans

have

been

handled

with

floating

cranes

when the

bridge

length

permitted amortization

of such investments.

3.

Beam Launchers

The

most

common

method

for

erecting

precast

beams

is

with

ground

cranes.

Cranes

usually

give

the

simplest

and

most

rapid

erection

procedures

with

the

minimum

of

investment,

and

the

deck

may

be

built

in

several

places

at

once.

Good

access

is

necessary

along

the

entire

length

of

the

bridge

to

position

the

cranes

and

deliver

the

girders. Tall piers or steep slopes make crane erection expensive or prevent it at all.

The

use

of

a

beam

launcher

solves

any

difficulty.

A

beam

launcher

is

a

light

5

中英文资料外文翻译文献

self- launching

machine

comprising

two

triangular

trusses.

The

truss

length

is

about

2.3 times the typical span

but this is rarely a problem as the

gantry operates above the deck (Figure

1).

Beam

launchers

easily

cope

with

variations

in

span

length

and

deck

geometry,

plan

curvatures

and

ground

constraints.

Crossbeams

support

the

gantry

at

the

piers

and

allow transverse shifting to erect the edge

beams and to traverse the gantry for launching along curves.

Two winch- trolleys

span between the top

chords of the

trusses and lodge

two winches each.

The

main

winch

suspends

the

beam

and

a

translation

winch

acting

on

a

capstan

moves

the

trolley

along

the

gantry.

A

third

trolley

carries

an

electric

generator

that

feeds

gantry

operations.

When

the

beams

are

delivered

at

the

abutment

and

the

vertical

movements

are

therefore

small,

the

main

winches

may

be

replaced

with

less

expensive

long-stroke

hydraulic

cylinders.

A

beam

launcher

operates

in

one

of

two

ways

depending

on

how

the

beams

are

delivered.

If

the

beams

are

delivered

on

the

ground,

the

launcher

lifts

them

up

to

the

deck

level

and

places

them

onto

the

bearings.

If

the

beams

are

delivered

at

the abutment, the launcher is moved back to the abutment and

the winch-trolleys are moved to the

rear

end

of

the

gantry.

The

front

trolley

picks

up

the

front

end

of

the

beam

and

moves

it

forward with the rear end suspended from a straddle carrier. When

the rear end of the beam reaches

the rear winch- trolley, the trolley picks it up to release the carrier.

The

longitudinal

movement

of

the

gantry

is

a

two-step

process.

Automatic

clamps

block

the

trusses

to

the

crossbeams

and

the

winch-trolleys

move

the

beam

one

span

ahead;

then

the

winch-trolleys

are

anchored

to

the

crossbeams,

the

blocks

are

released

and

the

translation

winches

push

the

trusses

to

the

next

span.

Redundancy

of

anchorages

is

necessary

in

both phases

for safe launching along inclined planes.

The sequence can be repeated many

times so

when

the

beams

are

delivered

at

the

abutment,

the

gantry

can

place

them

several

spans

ahead.

When the bridge is long, moving

the gantry over many spans slows

the erection

down and

may

be

faster

to

cast

the

deck

slab

as

soon

as

the

beams

are

placed

and

to

deliver

the

next

beams

along the completed bridge.

Truss

deflections

at

landing

at

the

piers

are

recovered

with

alignment

wedges.

The

alignment

force

is

small

but

the

support

saddles

must

be

anchored

to

avoid

displacements

or

overturning.

Realignment

may

also

be

achieved

with

long-stroke cylinders that rotate arms

pinned to the tip of the

truss. Similar devices are also applied

to the rear end of the

gantry to release the support reaction when launching forward and to recover the

deflection when launching backward.

6