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2021年2月9日发(作者：珍珠鹦鹉螺)

Bridge design and construction

Plan:

The

first

step

leading

to

the

construction

of

a

modern

major

bridge

is

a

comprehensive study to determine whether a bridge is needed. If it is to be a highway

bridge in the United States for example a planning study is initiated by a state bridge

authority

possible

in

cooperation

with

local

governments

or

the

federal

government.

Studies are made to estimate the amount of bridge traffic the relief of jammed traffic in

nearby highway networks the effects on the regional economy and the cost of the bridge.

The means for financing the project such as public taxes or sale of revenue bonds repaid

by toll charges are considered. If the studying lead to a decision to go ahead with the

project the land needed for the bridge and its approaches is acquired at the selected site.

At the point field

engineering work is started. Accurate land surveys are made. Tides

flood conditions currents and other characteristics of the waterway are carefully studied.

Boring samples of soil and rock are taken at possible foundation locations both on land

and under the water.

Selection of bridge design:

The chief factors in deciding whether a bridge will be

built as a girder cantilever truss arch suspension or some other type are;(1)location for

example

across

a

river;(2)purposes

for

example

a

bridge

for

carrying

motor

vehicles;(3)span

length;(4)strength

of

available

materials;(5)cost;(6)beauty

and

harmony with the location.

Each type of bridge is most effective and economical only within a certain range of

span lengths, as shown in the following table:

Bridge Type

Girder

Rigid Frame

Arch

Truss

Cantilever

Suspension

Feet

20 to 1000

80 to 300

200 to 1000

200 to 1400

500 to 1800

1000 to 5000

Best Span Range

Meters

6.1 to 304.8

24.4 to 91.4

61.0 to 304.8

61.0 to 426.7

152.4 to 548.6

304.8 to 1524.0

1

As indicated in the table there is a considerable overlap in the range of applicability

of the various types. In the some cases alternative preliminary designs are prepared for

several types of bridge in order to have a better basis for making the final selection.

Selection of materials:

The bridge designer can select from a number of modern

high-strength materials including concrete steel and a wide variety of corrosion-resistant

alloy steels.

For

the

Verrazano-Narrows

Bridge

for

example

the

designer

used

at

least

seven

different

kinds

of

alloy

steel

one

of

which

has

a

yield

strength

of

50000

pounds

per

square

inch

and

does

not

need

to

be

painted

because

an

oxide

coating

forms

on

its

surface and inhibits corrosion. The designer also can select steel wires for suspension

cables that have tensile strengths up to 250000 psi.

Concrete with compressive strengths as high as 8000 pis can now be produced for

use in bridge and it can be given high durability against chipping and weathering by the

addition of special chemical agents and control of the hardening process. Concrete that

has been prestressed and reinforced with steel wires has a tensile strength of 250000 .

Other

useful

materials

for

bridges

include

aluminum

alloys

and

wood.

Modern

structural aluminum alloys have

yield strengths

exceeding 40000. Laminated strips of

wood

glued

together

can

be

made

into

beams

with

strengths

twice

that

of

natural

timbers

glue-laminated

southern

pine

for

example

can

bear

working

stresses

approaching 3000 psi.

Analysis

of

forces:

A

bridge

must

resist

a

complex

combination

of

tension

compression bending shear and torsion forces. In addition the structure most provide a

safety

factor

as

insurance

against

failure.

The

calculation

of

the

precise

nature

of

the

individual

stresses

and

strains

in

the

structure

called

analysis

is

perhaps

the

most

technically complex aspect of bridge building. The goal of analysis is to determine all of

the forces that may act on each structural member.

The

forces

that

act

on

bridge

structural

members

are

produced

by

two

kinds

of

loads-static and dynamic. The static load-the dead weight of the bridge structure itself-is

usually the greatest load. The dynamic, or live load has components including vehicles

carried by the bridge wind forces and accumulations of ice and snow.

Although

the

total

weight

of

the

vehicles

moving

over

a

bridge

at

any

time

is

2

generally a small fraction of the static and dynamic load it presents special problems to

the

bridge

designer

because

of

the

vibration

and

impact

stresses

created

by

moving

vehicles. For example the severe impacts caused by irregularities of vehicle motion or

bumps in the roadway may momentarily double the effect of the live load an bridge.

Wind

exerts

force

on

a

bridge

both

directly

by

striking

the

bridge

structure

and

indirectly by striking vehicles that are crossing the bridge. If the wind induces vibration

as

in

the

case

of

the

Tacoma

Narrows

Bridge

its

effect

may

be

greatly

amplified.

Because of this danger the bridge designer makes provisions for the strongest winds that

may

occur

at

the

bridge

location.

Other

forces

that

may

act

on

the

bridge

such

as

stresses created by earthquake tremors must also be provided for.

Special attention must often be given to the design of the bridge piers since heavy

loads

may

be

imposed

on

them

by

currents

waves

and

floating

ice

and

debris.

Occasionally a pier may even be hit by a passing ship.

Electronic

computers

are

playing

an

ever- increasing

role

in

assisting

bridge

designers

in

the

analysis

of

forces.

The

use

of

precise

model

testing

particularly

for

studying the dynamic behavior of bridges also helps designers. A scaled-down model of

the

bridge

is

constructed

and

various

gauges

to

measure

strains

accelerations

and

deformations are placed

on the model.

The model

bridge is then subjected to

various

scaled-down

loads

or

dynamic

conditions

to

find

out

what

will

happen.

Wind

tunnel

tests may also be made to ensure that nothing like the Tacoma Narrows Bridge failure

can occur. With modern technological aids there is much less chance of bridge failure

than in the past.

Construction

the

foundations:

Construction

starts

with

the

foundations

which

may

cost

almost

as

much

as

the

superstructure.

Foundations

built

in

water

usually

present the greatest difficulties. One of the older methods which is still used in shallow

waters is to erect cofferdams similar to the ring of closely spaced piles that the Romans

used.

For

constructing

foundations

in

deep

water

caissons

have

long

been

used.

The

caisson which is a huge box closed on all sides except the bottom is lowered onto the

river bed Workers inside the caisson which is filled with compressed air to keep out the

water

dig

deeper

and

deeper

and

the

caisson

sinks

as

the

digging

proceeds.

When

a

3

suitable

depth

is

reached

the

caisson

is

filled

with

concrete

and

becomes

part

of

the

foundation itself.

Another deep-water method less hazardous and less costly than the caisson method

uses steel

or

concrete piles. With modern pile drivers long heavy

piles

can be

driven

even in deep water. The piles can be cut off and capped either above the water level or

below it.

If they are capped below the water level a prefabricated hollow pier case is

floated out to the site sunk on the pile and then filled with concrete to form the pier.

Erecting

the

superstructure:

After

all

piers

and

abutments

are

in

place

the

erection of the superstructure begins. The method of construction used depends largely

on

the

type

of

bridge

being

built.

There

are

six

construction

methods

false

work

flotation cantilevering sliding direct lifting and suspension.

In false work construction mainly used in building concrete arch bridges metal or

wood supports are built temporarily to support the erection. A great deal of ingenuity is

often required just to erect the false work especially for structures over swift rivers or

deep canyons. Temporary piles and trestles are commonly used in wide shallow rivers.

In the floatation method mainly used in building long bridges large bridge sections

are prefabricated on shore and floated out on barges to he bridge site. The sections are

then hoisted into place either by floating derricks or by winches placed on previously

constructed selections of the bridge.

The cantilevering technique is used not only for cantilever bridges but also for steel

arch bridges. Construction starts at an abutment and extends toward the center piece.

Silding cinstruction is used only rarely. In this method a prefabricated unit, such as

a trusses erected on shore and slid out over a temporary or permanent support until it

comes to rest on another support.

In

the

direct

lifting

method

mainly

used

for

light

short-span

highway

bridges

a

prefabricated bridge unit is lifted by a hoist and swung directly onto the bridge supports.

In the construction of suspension bridges the cables are strung between the bridge

towers and used as a support for the bridge deck. The deck erection starts at the ends of

the

bridge

and

progresses

toward

the

center

A

travelling

derrick

moving

on

the

completed

part

of

the

deck

is

used

to

handle

heavy

material

Temporary

suspension

cables

are

occasionally

used

in

the

construction

of

other

types

of

bridges

to

convey

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