lecture no.5 - bridge-loads

8/9/2019 Lecture No.5 - Bridge-Loads

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SD 470: Design of Bridges

4. BASIC BRIDGE ENGINEERING DESIGN

Lecture No. 5


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The design of bridges generally involves the following!

- Preliminary design

- Final design

The stages involved in preliminary and final design steps are as summarizedbelow in the following paragraphs:-

S"##ary of Design Ste$s

Initial stage

Review of the design procedure in which it includes !ata collection for design

relevant parameters" #election of the appropriate bridge structure and

$ssumptions of section outlines%

%oad &al&"lation

&umber of notional lanes" !ead loading" '$ loading" '( loading" '$ and '(

loading" Footpath loading" wind load" load combinations and )oad factors%


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Analysis stage

!etermination of #hear forces" (ending moments" +ombined shear force envelop

and combined bending moment envelope%

Design stage

#pecifying ,aterial properties" #ection properties" #ection design% ther optionaldesign parameters" Final design" !etailing" chec.ing" $pproval of design" +onstru

and finally !esign variations during construction%

In ter#s of &o#$onents' the str"&t"ral design involves:

!esign of #lab or !ec." )ongitudinal (eams/0irders" +ross beams-(earings" Pier

$butments" Foundations" 0uard rails and oints%

Colle&tion of data for relevant $ara#eters

Road classification- #pan of bridge" width of the road" design codes" materials

availability" available euipment" available s.illed labour" e3isting bridge structures

e3isting civil engineering structures" etc%

Bridge Ty$e Sele&tion#teel bridge" Timber bridge" Reinforced +oncrete bridge" Prestressed +oncrete br

+omposite construction" +able #tayed" #uspension bridge" Truss bridge" or $rch

bridge%


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No Bridge Materials Support System

RC PSC Steel Simple Continuo

1 Slabs X 8m

2 Slabs X 20m

3 Voided slabs X 10 - 20m

4 Voided slabs X 20 - 30m

5 Beams X 10 - 25m

6 Beams X 10-70m

7 Haunched beams X Up to 250

8 Steel girders X 50m 260m

9 Arch X 20 - 50m

10 Arch X X 20 - 400m

11 Truss X X X 40 - 400m

12 Cable stayed X 200 - 700m

13 Suspension X Over 500m14 Timber beam Up to 6m

15 Timber truss Up to45


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(reli#inary design6This involves determination of initial dimensions and design

elements such as reinforcements" rivets" bolts" welding" etc%6These dimensions are to be adusted later in the final or

intermediate design67t serves time not to start directly with the detailed final design%6(ut all design parameters are considered6$ll design steps are the same as for the final design6+omputer programs can help in shortening the design time%

Ass"#$tions and o"tline se&tion6)"tline s*et&h $lanof the bridge6utline s.etch of the transverse se&tionof the bridge6utline s.etch of the longit"dinal se&tionof the bridge6Identifi&ation of the beams and slabs6Thi&*nessof the surfacing

60uardrails and parapets6$ssumed s$anof the beams6$ssumed beam s$a&ing


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%oads for Bridge Design

Dead load+ which comprises of:

#elf weights of bridge elements

The densities of the materials used are as follows:-+oncrete: 24.&/m*

#teel: 9%5.&/m*

Timber: 4- .&/m*

'ollow bric.s: 15.&/m*

#olid bric.s 22.&/m*

&atural stones or roc.: 2;-2.&/m*

Railways ballast: 1< .&/m*

I#$osed dead load' such as

=earing surface"

Railway trac.s"

0uardrails"

Prestressing force


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,erti&ally i#$osed loads s"&h as

7mpact

=eight of vehicles" etc%

Transverse i#$osed loads s"&h as

#.idding"+entrifugal forces in curves

=ind loads

%ongit"dinal i#$osed loads s"&h as:-

(ra.ing forces"

$cceleration forces

Constraining i#$osed loads s"&h as!

!ue to te#$erat"rechanges

#hrin.age

+reep

Pier settle#ents

Friction forces in the bearings+ollision forces

Earth-"a*e loads


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A loading is the basic imposed load comprising of6 $ uniformly distributed load6 $ .nife-edge load

'$ loading includes a 25> allowance for impactThe intensity of which depends upon the loaded length

&o dispersal of load beneath contact area

?nife-edge load is positioned to have the most severe effect"

$lternatively a single 1;; .& load with *4;mm or *;;mm 3 *;; mm s

contact area

ighway Bridge %ive %oads

Table /: &otional lanes for design purposes according to the carriageway width

+arriage =idth=@mA

B 4%8 4%8-9%5 9%8-11%4 11%4-152 15%2-1

&o%of &otional

)anes

=/* 2 * 4 5

.


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A %oading

The @(! *9/;1A !esign ,anual for Roads and (ridges says that Type '$ loadin

the normal design loading for 0reat (ritain and adeuately covers the effects of

permitted normal vehicles other than those used for abnormal indivisible loads%

&ormal vehicles are governed by the Road Cehicles @$uthorised =eightA

Regulations 1


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Table 3: HA Loading

Loaded length L (m) < 30m 30m - 379m > 379m

Uniform Load

(kN/m/Lane)

30 151 L-0.475 9

Knife edge load (kN/Lane) 120

K E L

U D L

U D LK

E

L

0ig"re 1: '$ )oading


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'$ )oading

1 0 0 k N

3 4 0

Figure 2: Alternative to HA KEL loading


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'( )oading

The @(! *9/;A !esign ,anual for Roads and (ridges in E? says th

Type '( loading reuirements derive from the nature of e3ceptiona

industrial loads @e%g% electrical transformers" generators" pressurevessels" machine presses" etc%A li.ely to use the roads in the area%

The vehicle load is represented by a four a3led vehicle with four wh

eually spaced on each a3le% The load on each a3le is defined by a

number of units which is dependant on the class of road% ,otorway

trun. roads reuire 45 units" Principal roads reuire *9%5 units andpublic roads reuire *; units% ach unit is euivalent to 1;.&%


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'( )oading

'( loading is used for bridges which are on public highways where they ma

subected to abnor#al loadsthat are greater than those arising from A

loading

- $ 18 wheel vehi&leis specified

- )oad/wheel is /.2 3 *N'where3 is the number of '( units

- The '( loading can be from /2 Bto 42 B "nits

%g% 25 units 3 2%5 3 18 wheels G 1;;; .& or 1;; tons vehicle

45 units 3 2%5 3 18 wheels G 1;; .& or 1; tons vehicle

The actual units to be used for a particular bridge are usually specified by th

authority concerned in the respective country

- '( loading also includes a 25> allowance for impact

- The length of the vehicle is varied for the severest effect bydimensions shown in Figure *%


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'( )oading

0 . 2 01 . 8 0 1 . 8 06 , 1 1 , 1 6 , 2 1 , 2 6 m

0

.2

5

0

.2

5

1

.0

0

1

.0

0

1

.0

0

0ig"re '( Cehicle


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+ombination of '$ and '( )oading

H B V e h i c l eF u l l H A F u l l H AN o L o ! N o L o !

2 5 m 2 5 m

F u l l H A

" e # $ % l % e & e % ' e ( N o l o ! ) o % * l o + l # l & i &

1 - 3 H A

1 - 3 H A

L o ! e ! l e # * $ h ) o % i # $ e # & i $ o ) H A U D L

0ig"re 4$ combination of '$ and '( loading: '( within one notional lane


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+ombination of '$ and '( )oading

H B V e h i c l eF u l l H A F u l l H A

N o L o ! N o L o !

2 5 m 2 5 m

" e # $ % l % e & e % ' e ( N o l o ! ) o % * l o + l # l , & i &

1 - 3 H A

1 - 3 H A

1 - 3 H A 1 - 3 H A

L o ! e ! l e # * $ h ) o % i # $ e # & i $ , o ) H A U D L

0ig"re 2$ combination of '$ and '( loading '( occupying two notional lanes

0igs 4 and 2give an overview of both '$ and '( loading

)ane loads are interchangeable for severest effect

Red"&ed loads for se&ondary roads


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Red"&ed loads for se&ondary roads

Simple, one lane bridges in rural areas may be designed using thereduced vertical load as shown in Figure 6.

1

.8

0

m

4 . 2 5 m

0 . 1 5

0

.3

55 4 . 5 k N

5 4 . 5 k N

1 3 . 5 k N

1 3 . 5 k N

2 / e % h e e l & 2 F % o # $ h e e l & o $ l L o ! ( 1 3 6 k N

Figure 6: Reduced vertical loads for secondary roads.


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(edestrian and Cy&le Tra&* Bridges

The imposed load on pedestrian and cycle trac. bridges is uniformly distributed

and depends upon the loaded lengths as follows:

7f ) *;m then ( 52.&/m2

7f *; B ) B *9


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Transverse loads arise when a vehi&le #oves in a &"rve.

The forces considered above all act vertically according to the law of gravity% =

a vehicle moves at a speed of not less than ; .m/hr in a curve" a centrifugal

force" Fc" acting in the transverse section of the bridge and directed away from

curve center results% The resultant force is defined by the e3pression:

where m G mass of vehicle in Tons

v G speed of vehicle in #:s

R G radius of curve in #%

(# 54;; Part 2 gives a nominal centrifugal load defined by:

where Ris the same as above

The force Fccan be applied as a single load or subdivided into 1/*F cand 2/*Fc

then placed at 5%;m c/c longitudinally in combination with a vertical load of *;;

R

v.mF

2

c

kN150R

30000F

c


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A&&idental load d"e to s*idding

n straight and curved bridges" a single point load in one single lane in the dire

of the highway shall be considered in which the nominal load is 25; .&

%ongit"dinal load d"e to bra*ing and tra&tion-The nominal load for '$ shall be .&/m of loaded length plus

2;; .& but B 9;; .& applied to an area of one notional lane 3

loaded length%

- The nominal load for '( shall be 25> total nominal '( load

applied between the wheels of two a3les of the vehicle 1% m apart%

%oads d"e to Te#$erat"re ,ariations

The coefficient of thermal e3pansion shall be ta.en as 1/ ; 18!


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3 0 k N

3 0 k N5 0 k N

5 0 k N4 0 k N

4 0 k N

Figure 7:Accidental wheel loads

A&&idental wheel loads: This loading applies to local effects and shall not be ta.

into account in determining global effects on the dec.%


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(ier i#$a&t 6Te&hni&al =e#orand"# BE 1:997

6225 .& normal to carriageway with 95 .& parallel to carriageway at guardrafi3ing level or 95;mm above carriageway where there is no fi3ing"

6 Plus 15; .& normal to carriageway with 15; .& parallel to

carriageway between 1%;m and *%;m above carriageway level%

Collision loads on s"$$orts of bridges over highways 6BS 2488 (art 17

6 15; .& normal to carriageway with 5; .& parallel to carriageway at guardr

fi3ing level or 95; above carriageway where there is no fi3ing"

6 Plus 1;; .& normal to carriageway with 1;;.& parallel to

carriageway between 1%;;; and *%;;; above carriageway%


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%oad &o#binations

Three principal and two secondary combinations of loads are specified in (#

54;;: Part 2:1


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(artial Safety 0a&tors to be Ta*en for Ea&h %oad

Co#bination

Table 4: Loads to be taken in each combination with appropriateL

Clause

number

Load Limit

state

Lto be considere

combinations

1 2 3 4

5.1 Dead Steel ULS*

SLS

1.05 1,05 1.05 1,0

1.00 1,00 1.00 1,0

Concrete ULS*

SLS

1,15 1,15 1,15 1,1

1.00 1,00 1.00 1,0

5.2 Superimposed dead ULS*

SLS

1,75 1.75 1,75 1,7

1.20 1,20 1,20 1,2

5.1.2.2 &

52.2.2

Reduced load factorfor

dead and superimposeddead load where this has a

more severe total effect

ULS

SLS

1.00 1,00 1,00 1,0


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5.2 Wind:

During erection ULS 1,10

SLS 1,00

With dead plus superimposed dead load

only and for members primarily resisting

wind loads

ULS

SLS

1,40

1,00

With dead plus superimposed dead plus

other appropriate combination 2 loads

ULS

SLS

1,10

1,00

Relieving effect of Wind ULS 1,00

SLS 1,00


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5.3 Temperature Restraint due to range ULS

SLS

1,30

1,00

Frictional bearing restraint ULSSLS

1,301,00

Effect of temperature

difference

ULS

SLS

1,00

0,80

5.6 Differential settlement ULS

SLS

To be assessed a

agreed between t

engineer and appro

authority


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5.8 Earth

pressure

Retained fill and/or live

load surcharge

ULS 1,50 1,50 1,50 1,50 1,50

SLS 1,00 1,00 1,00 1,00 1,00

Relieving effect ULS 1,00 1,00 1,00 1,00 1,00

5.9 Erection:temporary loads ULS 1,15 1,15

6.2 Live loading:HA alone ULS 1,50 1,25 1,25

SLS 1,20 1,10 1,00

HA with HB or HB alone ULS 1,30 1,00 1,10


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6.5 Centrifugalload and associated primary live load ULS

SLS

1,50

1,00

6.6 Longitudinal

load:

HA and associated primary live

load

ULS

SLS

1,25

1,00

HB and associated primary live

load

ULS

SLS

1,10

1,00

6.7 Accidental skidding load and associated primary live

load

ULS

SLS

1,25

1,00

6.8 Vehicle collision load with bridge parapets and

associated primary live load

ULS

SLS

1,25

1,00

6.9 Vehicle collision load with bridge supports** ULSSLS

1,251,00


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7 Foot/cycle track bridges: live

load and parapet load

ULS 1,50 1,25 1,25 1,25

SLS 1,00 1,00 1,00 1,00

8 Railway bridges: type RU and

RL primary and secondary

live

ULS 1,40 1,20 1,20

SLS 1,10 1,00 1,00

J%shall be increased to at least 1%1; and 1%2; for steel and concrete respectively

compensate for inaccuracies when dead loads are not accurately assessed

%may be reduced to 1%2 and 1%; for the E)# and #)# respectively subect to app

of the appropriate authority

JJ This is the only secondary live load to be considered for foot/cycle trac. bridges


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!esign 7nternal $ctions

%oad Cases:

+ase 1: ,a3imum mid span moment+ase 2:: ,a3imum end span moment

+ase *: ,a3imum over support moment

+ase 4: ,a3imum shear force at piers

+ase 5: ,a3imum shear force at abutments

lecture no.5 - bridge-loads

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