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Section 3

-

Loads and Load Factors (SI)

SPECIFICATIONS COMMENTARY

Table 3.6.2.1-1 - Dynamic Load Allowance, 1M . Hammering effect is the dynamic response o

wheel assembly to riding surface discontinu

I I I such as deck joints, cracks, potholes,

I

Component

I

1M

I

delaminations, and

Deck Joints - AII Limit States 75%

.

Dynamic response of the bridge as a who

AII Other Components passing. ve~icles, which may be due to

.

Fatigue and Fracture 15% undulatlons In the roadway pavement, such as.t

L. .t State caused by settlement of fill, or to resonant exclt

Iml as a result of similar frequencies of vibr

. Al Other Limit States 33% between bridge and vehicle.

. .. . Field tests indicate that in the majority of hig

The appllcatlon of .dynam c load allowance for bu led bridges, the dynamic component of the response

?omp.onents, covered In Sectlon 12, shall be as speclfied not exceed 25 percent of the static response to veh

In Artlcle 3.?2.2. . This is the basis for dynamic load allowance with

Dynamlc load allowance need not be applled lo: exception of deck joints. However, the specified live

. . ... combination of the design truck and lane

. Retalnlng walls not subJect to vertical reactlons from represents a group of exclusion vehicles that are at

the superstructure, and 4/3 of those caused by the design truck alone on s

. . and medium-span bridges. The specified value o

. Foundatlon components that are entlrely below percent in Table 1 is the product of 4/3 and the bas

ground level. percent.

. This article recognizes the damping effect o

The dynamlc load al o:wanc.e ma~ be reduced. for when in contact with some buried structural compon

co~ponent.s, other than JOI~tS, f Justlfi.e~ by sufficl.ent such as footings. To qualify for relief from impac

evldence, In accordance wlth the provlslons of Artlcle entire component must be buried. For the purpo

4.7.2.1. this article, a retaining type component is consider

be buried to the top of the fill.

3.6.2.2 BURIED COMPONENTS

The dynamic load allowance for culverts and other j.,.

buried structures covered by Section 12, in percent, shall

;~~.

.

be taken as: ,;"

,

IM=33(1.0-4.1x10-4OE) ~O% (3.6.2.2-1)

where:

DE

= the minimum depth of earth cover above the

structure (mm)

3.6.2.3 WOOD COMPONENTS C3.6.2.3

For wood bridges and wood components of bridges,

Wood

structures are known to experience red

the dynamic load allowance specified in Article 3.6.2.1 dynamic wheel load effects due to internal fr

may be reduced to 50 percent of the values specified for between the components and the dam

1M n Table 3.6.2.1-1. characteristics ofwood.

3.6.3 Centrifugal Forces:

CE C3.6.3

Centrifugal forces shall be taken as the product of Lane load is neglected in computing the centr

the axle weights of the design truck or tandem and the force, as the spacing of vehicles at high spee

factor C, taken as:

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Section 3 -Loads and Load Factors (SI)


4 2 assumed to be large, resulting in a low density o

C = -~ (3.6.3-1) vehicles following and/or preceding the design truck.

3 g R The specified live load combination of the design

truck and lane load, however, represents a group o

where. exclusion vehicles that produce force effects of at leas

. 4/3 of those caused by the design truck alone on short

v = highway design speed (mIs) and ~edium-span bridge~.. T~is ratio is. indicated in

Equatlon 1. Thus, the provlslon IS not technlcally perfect

9 = gravitational acceleration: 9.807 (m/s2) yet .it reason.ably mod.els the rep~esentative exclusion

vehlcle travellng at deslgn speed wlth large headways to

R = radius of curvature of traffic lane (m) other ~ehicles. The . ap~roximation a.ttributed to this

convenlent representatlon IS acceptable In the framework

Highway design speed shall not be taken to be less of the uncertainty of centrifugal force from random traffic

than the va~ue sp~cified i~ MSHTO publication A Polic~ patt~r~s~1 = 360 k Ih

of Geometrlc Deslqn of Hlqhwavs and Streets (1990). . s . m

The multiple presence factors specified in Article

3.6.1.1.2 shall apply.

Centrifugal forces shall be applied horizontally at a

distance 1800 mm above the roadway surface.

3.6.4 Braking Force:

BR C3.6.4

Braking forces shall be taken as 25 percent of the Based on energy principies, and assuming uniform

axle weights of the design truck or tandem per lane deceleration, the braking force determined as a fraction

placed in all design lanes which are considered to be of vehicle weight is:

loaded n accordancewith Article 3.6.1.1.1and which are

2

carrying traffic headed in the same direction. These b = ~ (C3.6.4-1

forces shall be assumed to act horizontally at a distance 2ga

of 1800 mm above the roadway surface in either

longitudinal direction to cause extreme force effects. AII h "".

th I th f .

f d I . d "b" .

design lanes shall be simultaneously loaded for bridges w ere a. IS e eng ?

un, or.m ece era~lon an IS

likely to become one-directional in the future. the fraGuan. Calculauons uslng a br~klng ~ength of

The multiple presence factors specified in Article 122

~

and a speed of 90 .km/h (25 mIs) y eld b - 0.26 for

3 6 1 1 2 shall apply a horizontal force that wlll act for a perlod of about 10

. . . .. seconds. The factor "b" applies to all lanes in one

direction because all vehicles may have reacted within

this time trame. Only the design truck or tandem are to

be considered because other vehicles, represented by

the design lane load, are expected to brake out of phase.

3.6.5 Vehicular Collision Force: CT

3.6.5.1 PROTECTION OF STRUCTURES C3.6.5.1

The provisions of Article 3.6.5.2 need not be For the purpose of this article, a barrier may be

considered for structures which are protected by: considered structurally independent if it does not transmit

loads to the bridge.

. An embankment; Full-scale crash tests have shown that some vehicles

have a greater tendency to lean ayer or partially cross

.

A structurally independent, crashworthy ground- ayer a 1070 mm high barrier than a 1370 mm high

mounted 1370 mm high barrier, located within barrier. This behavior would allow a significant collision

3000 mm from the component being protected; or of the vehicle with the component being protected if the

component is located within a meter or so of the barrier.

.

A 1070 mm high barrier located at more than If the component is more than about 3000 mm behind

3000 mm from the component being protected.

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~ In arder to qualify for this exemption, such barrier shall the barrier, the difference between the two b

be structurallyand geometrically apable of surviving the heights is no longer important.

crash test for Performance Level 3, as specified in

Section 13.

3.6.5.2 VEHICLE ANO RAILWAY COLLISION WITH C3.6.5.2

STRUCTURES

Unless protected as specified in Article 3.6.5.1, It is not the intent of this provision to encou

abutments and piers located within a distance of unprotected piers and abutments within the setb

9000 mm to the edge of roadway, or within a distance of indicated, but rather to supply some guidanc

15 000 mm to the centerline of a railway track, shall be structural design when it is deemed totally impracti

designed for an equivalent static force of 1 800 000 N, meet the requirements of Article 3.6.5.1.

which is assumed to act in any direction in a horizontal The equivalent static force of 1 800 000 N is b

plane, at a distance of 1200 mm above ground. on the information from full-scale crash tests of ba

The provisions ofArticle 2.3.2.2.1 shall apply. for redirecting 360 000 N tractor trailers and

analysis of other truck collisions. The 1 800 000 N

collision load is based on recent, physically unver

analytical work (Hirsch 1989). For individual co

shafts, the 1 800 000 N load should be consider

point oado

For wall piers, the load may be consider

be a point load or may be distributed ayer an

deemed suitable for the size of the structure and

anticipated impacting vehicle, but not greater than

mm wide by 600 mm high. These dimensions

determined by considering the size of a truck trame.

3.6.5.3 VEHICLE COLLlSION WITH BARRIERS

."""

,

The provisions of Section 13 shall apply.

'C ~

3.7 WATER LOADS: WA

_il:t;,,;}.qi

. ," ~.;

3.7.1 Static Pressure .J~"(;,;,,.'

':Cc,;J(X:r:CJ.'~:1

Static pressure of water shall be assumed to act

perpendicular to the surface that is retaining the water.

Pressure shall be calculated as the product of height of

water above the point of consideration, the density of

water, and 9 (the acceleration of gravity).

Oesign water levels for various limit states shall be

as specified and/or approved by the Owner.

3.7.2 Buoyancy C3.7.2

Buoyancy shall be considered to be an uplift force, For substructures with cavities in which the pres

taken as the sum of the vertical components of static or absence of water cannot be ascertained, the con

pressures, as specified in Article 3.7.1, acting on all producing the least favorable force effect shoul

components below design water level. chosen.

v

; .~

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3.7.3 Stream Pressure

3.7.3.1 LONGITUDINAL C3. 7

.3.1

The pressure of flowing water acting in the For the purpose of this article, the longitudinal

longitudinal direction of substructures shall be taken as: direction refers to the majar axis of a substructure unit.

- -4

C

V 2 The theoretically correct expression for Equation 1

p

-

5.14x10

D

(3.7.3.1-1) is:

where: y V2 -6

P = CD-x10 (C3.7.3.1-1)

2

P = pressure of flowing water (MPa)

CD = drag coefficient for piers as specified in Table 1 where:

V = design velocity of water for the design flood in y = density (unit mass) of water (kg/m3)

strength and service limit states and for the

check flood in the extreme event limit state (mIs) V = velocity of water (mIs)

Table 3.7.3.1-1 - Drag Coefficient

I Type I

CD

I

semicircular-nosed ier 0.7

s uare-ended ier 1.4

debris lod . ier 1.4

wedged-nosed pier with nose 0.8

an le 90 or less

The longitudinal drag force shall be taken as the The drag coefficient, CD, and the lateral drag

product of longitudinal stream pressure and the coefficient, CL, given in Tables 1 and 3.7.3.2-1, were

projected surface exposed thereto. adopted from the Ontario Highway Bridge Design Code

(1991). The more favorable drag coefficients measured

by some researchers for wedge-type pier nose angles o

less than 90 are not given here beca use such pier

noses are more prone to catching debris.

Floating logs, roots, and other debris may

accumulate at piers and, by blocking parts of the

waterway, increase stream pressure load on the pier.

Such accumulation is a function of the availability of such

debris and level of maintenance efforts by which it is

removed. It may be accounted for by the judicious

,-"iA

,{" increase in both the exposed surface and the velocity o

~

j, ?',

.',r;.i" ,,;,t ;";1';

water. .

E\:: ; ",?18 ",:'1 The draft New Zealand ~ighway. ~ridge . Deslg

Specification contains the followlng provlslon, whlch may

be used as guidance in the absence of site-specific

criteria:

Where a significant amount of driftwood is

carried, water pressure shall al so be allowed for

on a driftwood raft lodged against he pier. The

size of the raft is a matter of judgment, but as a

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Section 3 -Loads and Load Factors

(SI)


guide, Dimension A in Figure 1 should be ha

the water depth, but not greater than 3000 mm

Dimension B should be half the sum of adjacen

span lengths, but no greater than 14 000 mm

Pressure shall be calculated using Formula 1

with Co = 0.5.

L

I

B ~:=;=

1

Water Surfa

A

Debris Raft

BedLevel

Figure C3.7.3.1-1 - Debris Raft for Pier Design

3.7.3.2 LATERAL C3.7.3.2

The lateral, uniformly distributed pressure on a The discussion of Equation 3.7.3.1-1 also appli

substructure due to water flowing at an angle, e, to the Equation 1.

longitudinal axis of the pier shall be taken as:

r. p=5.14x10-4CLy2 (3.7.3.2-1)

where:

p = lateral pressure (MPa)

CL = lateral drag coefficient specified in Table 1

p

r , \~\~

1 1 1 1 1 1 1 1 ~~:.:::..-~,,~.:3:;r e

Figure 3.7.3.2-1 - Plan View of Pier Showing Stream

Flow Pressure

r..

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Section

3

-



Table 3.7.3.2-1 - Lateral Drag Coefficient

Angle, S, between

direction of flow CL

and longitudinal axis of the pier

0 0.0

5 0.5

10 0.7

20 0.9

:?c30 1.0

The lateral drag force shall be taken as the product

of the lateral stream pressure and the surface exposed

thereto.

3.7.4 Wave Load C3.7.4

Wave action on bridge structures shall be considered Loads due to wave action on bridge structures shall

for exposed structures where the development of be determined using accepted engineering practice

significant wave forces may occur. methods. Site-specific conditions should be considered.

The latest edition of the Shore Protection Manual,

published by the Coastal Engineering Research Center,

Department of the Army, is recommended for the

computation f wave forces.

3.7.5 Change in Foundations Due to Limit State for C3.7.5

Scour

Statistically speaking, scour is the most common

The provisions of Article 2.6.4.4 shall apply. reason for the failure of highway bridges in the United

The consequences of changes in foundation States.

conditions resulting from the design flood for scour shall Provisions concerning the effects of scour are given

be considered at strength and service limit states. The in Section 2. Scour per se is not a force effect, but by

consequences of changes in foundation conditions due changing the conditions of the substructure it may

to scour resulting from the check flood for bridge scour significantly alter the consequences of force effects

and from hurricanes shall be considered at the extreme acting on structures.

event limit states.

3.8 WIND LOAD: WL AND WS

3.8.1 Horizontal Wind Pressure

3.8.1.1 GENERAL C3.8.1.1

Pressures specified herein shall be assumed to be Base design wind velocity varies significantly due to

caused by a base design wind velocity, Ve, of 160 km/h. local conditions. For small and/or low structures, wind

Wind load shall be assumed to be uniformly usually does not govern. For large and/or tall bridges,

distributed on the area exposed to the wind. The however, the local conditions should be investigated.

exposed area shall be the sum of areas of all Pressures on windward and leeward sides are to be

components, including floor system and railing, as seen taken simultaneously in the assumed direction of wind.

in elevation taken perpendicular to the assumed wind

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Section 3

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~ direction. This direction shall be varied to determine the Typically, a bridge structure should be exam

extreme force effect in the structure or in its components. separately under wind pressures from two or

Areas that do not contribute to the extreme force effect different directions in arder to ascertain those wind

under consideration may be neglected in the analysis. leeward, and side pressures producing the most c

For bridges or parts of bridges more than 10 000 mm loads on the structure.

above low ground or water level, the design wind Equation 1 is based on boundary layer th

velocity, VDz,should be adjusted according lo: combined with empirical observations and represen

most recent approach to defining wind speed

various conditions as used in meteorology. In the

V = 2 5 V

( ~

) In ( ~ ) an exponential equation was sometimes used to

DZ . o V Z wind speed to heights above 10 000 mm.

B o (3.8.1.1-1) formulation was based sol ely on empirical observa

where: and had no theoretical basis.

VDz

= designwind velocityat design elevation, (km/h) -

( Z )

a

VDZ - CV1o 10000 (C3.8.1

V1o = wind velocity at 10000 mm above low ground or

above design water level (km/h)

The purpose of the term C and exponent "a" wa

VB

= base wind velocity of 160 km/h at 10 000 mm adjust the equation for various upstream su

height, yielding design pressures specified in conditions, similar to the use of Table 1. Fu

Articles 3.8.1.2 and 3.8.2 information can be found in Liu (1991) and Simiu (1

1976).

Z = height of structure at which wind loads are being The following descriptions for the terms "

calculated as measured from low ground, or country", "suburban", and "city" in Table 1

from water level, > 10000 mm paraphrased from ASCE-7-93:

~

Vo

= friction velocity, a meteorological wind

.

Open Country

-

Open terrain with scat

characteristic aken, as specified n Table 1, for obstructions having heights generally less

various upwind surface characteristics (km/h) 10 000 mm. This category includes tlat open co

and grasslands.

lo =

friction length of upstream fetch, a

meteorological wind characteristic taken as

.

Suburban - Urban and suburban areas, wo

specified in Table 1 (mm) areas, or other terrain with numerous closely sp

obstructions having the size of single-family or l

dwellings. Use of this category shall be limite

those areas for which representative terrain pre

in the upwind direction at least 500 000 mm.

. City - Large city centers with at least 50 perce

the buildings having a height in excess of 2100

Use of this category shall be limited to those a

for which representative terrain prevails in

upwind direction at least 800 000 mm. Pos

channeling effects of increased vel?ci~ press

due to the bridge or structure's locatlon In the

of adjacent structures shall be taken into accoun

(\

-

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Section 3

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Table3.8.1.1-1 Valuesof Vaand

Zo

for

Various

UpstreamSurfaceConditions

I I

OrEN

I I I

I

CONDITION

I

COUNTRY

I

SUBURBAN

I

CITv

I

~

I

Va (km/h)

I

13.2

I

17.6

I

19.3

I

~

I lo

(mm)

I

70

I

1000

I

2500

I

, J'V~~"'~

V

0

may be established rom: ,;,;

';,:'... :~

.

Basic

Wind Speed charts available n ASCE 7-88 for "

various

ecurrence

intervals,

. Site-specificwind surveys,and

. In the absenceof betler criterion, he assumption

that V10

=

Va

=

160 km/h.

3.8.1.2 WIND PRESSURE ON STRUCTURES: WS

3.8.1.2.1 General C3.8. 1.2.1

If justified by local conditions, a different base design The stagnation pressure associated with a wind

wind velocity may be selected or load combinationsnot velocity of 160 km/h is 1.23x10-3 MPa,

which is

involving wind on live loado The direction of the design significantly less than the values specified in Table 1

wind shall be assumed to be horizontal, unless otherwise The difference reflects the effect of gusting combined

specified in Article 3.8.3. In the absence of more precise with some tradition of long-time usage.

data, design wind pressure, in MPa, may be determined The pressures specified in N/mm or MPa (= N/mm2

as. should be chosen to produce the greater net wind load

2 2 on the structure.

p = p

(

~

)

=

p -~ - Wind tunnel tests may be used to provide more

D B V B 25 600 (3.8.1.2.1 1) precise estimates of wind pressures. Such testing

B should be considered where wind is a majar design loado

Pa = base wind pressure specified in Table 1 (MPa)

Table 3.8.1.2.1-1 - Base Pressures, Pa

Corresponding to Va = 160 km/h

STRUCTURAL WINDWARD LEEWARD

COMPONENT LOAD, MPa LOAD, MPa

Trusses, 0.0024 0.0012

Columns, and

fArches

Beams 0.0024 NA

Large Flat 0.0019 NA

Surfaces

The wind loading shall not be taken less than

4.4 N/mm in the plane of a windward chord and

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Section 3

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~

2.2 N/mm in the plane of a leeward chord on truss and

arch components, and not less than 4.4 N/mm on beam

or girder components.

3.8.1.2.2 Loads from Superstructures C3.8.1.2.2

Where the wind is not taken as normal to the For trusses, columns, and arches, the base

structure, the base wind pressures, PB, for various pressures specified in Table 1 are the sum o

angles of wind direction may be taken as specified in pressures applied to both the windward and lee

Table 1 and shall be applied to a single place of exposed areas.

area. The skew angle shall be taken as measured from

a perpendicular to the longitudinal axis. The wind

direction for design shall be that which produces the

extreme force effect on the component under

investigation. The transverse and longitudinal pressures

shall be applied simultaneously.

Table

3.8.1.2.2-1

Base Wind Pressures, PB, or Various

Angles of Attack and V

B

= 160 km/hr

Columns and Arches Girders

Skew Angle Lateral Longitudinal Lateral Longitudinal

of~nd Load Load Load Load

Degrees MPa MPa MPa MPa

/"--'

O 0.0036 O 0.0024 O

15 O0034 O0006 O0021 O0003

30 0.0031 0.0013 0.0020 0.0006

45 0.0023 0.0020 0.0016 0.0008

60 0.0011 0.0024 0.0008 0.0009

3.8.1.2.3 Forces Applied Directly to the Substructure

The transverse and longitudinal forces to be applied

directly to the substructure shall be calculated from an

assumed base wind pressure of 0.0019 MPa. For wind

directions taken skewed to the substructure, this force

shall be resolved into components perpendicular to the

end and front elevations of the substructure. The

component perpendicular to the end elevation shall act

on the exposed substructure area as seen in end

elevation, and the component perpendicular to the front

elevation shall act on the exposed areas and shall be

applied simultaneously with the wind loads from the

superstructure.

1"'"-'"

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Section 3

-

Loads and Load Factors

(SI)


3.8.1.3 WIND PRESSURE ON VEHICLES: WL C3.8.1.3

When vehicles are present, the design wind pressure Based on practical experience, maximum live loads

shall be applied to both structure and vehicles. Wind are not expected to be present on the bridge when the

pressure on vehicles shall be represented by an wind velocity exceeds 90 km/h. The load factor

interruptable, moving force of 1.46 N/mm acting normal corresponding to the treatment of wind on structure only

to, and 1800 mm above, the roadway and shall be in Load Combination Strength III would be (90/160)

transmitted to the structure. (1.4) = 0.44, which has been rounded to 0.40 in the

When wind on vehicles is not taken as normal to the Strength IV Load Combination. This load factor

structure, the components of normal and parallel force corresponds to 0.3 in Service l.

applied to the live load may be taken as specified in The 1.46 N/mm wind load is based on a long row o

Table 1 with the skew angle taken as referenced normal randomly sequenced passenger cars, commercial vans,

to the surface. and trucks exposed to the 90 km/h design wind. This

horizontal live load, similar to the design lane load,

should be applied only to the tributary areas producing

a force effect of the same kind.

Table 3.8.1.3-1 - Wind Components on Live Load

Normal Parallel

Skew Angle Component Component

Degrees N/mm N/mm

O 1.46 O

15 1.28 0.18

30 1.20 0.35

45 0.96 0.47

~

60 0.50 0.55

3.8.2 VerticalWind Pressure C3.8.2

Unless otherwise determined in Article 3.8.3., a The intent of this article is to account for the effect

vertical upward wind force of 9.6x10-4MPa times the resulting rom interruptionof the horizontal low of air by

width of the deck, including parapets and sidewalks, the superstructure. This load is to be applied even to

shall be considered to be a longitudinal line loado This discontinuous bridge decks, such as grid decks. This

force shall be applied only for limit states that do not load may govern where overturning of the bridge is

invol'le wind on live load, and only when the direction of investigated.

wind is taken to be perpendicular to the longitudinal axis

of the bridge. This lineal force shall be applied at the

windward quarter-point of the deck width in conjunction

with the horizontal wind loads specified in Article 3.8.1.

3.8.3 Aeroelastic Instability

3.8.3.1 GENERAL C3.8.3.1

Aeroelastic force effects shall be taken into account Because of the complexity of analyses often

in the design of bridges and structural components apt necessary for an in-depth evaluation of structural

to be wind-sensitive. For the purpose of this article, all aeroelasticity, this article is intentionally kept to a simple

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Section 3

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~ bridges, and structural components thereof with a span statement. Many bridges, decks, or individual struc

length to width or depth ratio exceeding 30.0 shall be components have been shown to be aeroelasti

deemed to be wind-sensitive. insensitive if their length-to-width or length-to-d

The vibration of cables due to the interaction of wind ratios are under about 30.0, a somewhat arbitrary v

and rain shall algo be considered. helpful only in identifying likely wind-sensitive cases.

Flexible bridges, such as cable-supported or

long spans of any type, may require special stu

based on wind tunnel information. In gen

appropriate wind tunnel tests involve simulation of

wind environment local to the bridge site. Details o

are part of the existing wind tunnel state of the art

are beyond the scope of this commentary.

3.8.3.2 AEROELASTIC PHENOMENA C3.8.3.2

The aeroelastic phenomena of vortex excitation, Excitation due to vortex shedding is the escap

galloping, flutter, and divergence shall be considered wind-induced vortices behind the member, which ten

where applicable. excite the component at its fundamental na

frequency in harmonic motion. It is important to

stresses due to vortex-induced oscillations below

"infinite life" fatigue stress. Methods exist for estim

such stress amplitudes, but they are outside the sc

of this commentary.

" - - .

Tubular components can be protected ag

'c.I:", ::..:'

i;;~ vortex-induced oscillation by adding bracing, strake

:(t,;:;;' "j 1e tuned mass dampers or by attaching horizontal

/~

plates parallel to the tube axis above and/or below

"f.,:;'~,, f;\~4 s:;, ,)1~.,''-"

central third of their span. Such aerodynamic dam

';$'...:"':' ':1"1'; f\:;' '" ';, ,ijb.;

plates should lie about one-third tube diameter abov

':

'-,;i..; ': ,; J,,'1';.;; (-; ')":"f tc.:J"',,'J':: below the tube to allow free passage of wind. The w

",'.::;'1

of the plates may be the diameterof the tube or wide

Galloping is a high-amplitude oscillation assoc

:-.i ;-,';if:),:~,' ~;,";.:;" ,:\ with ice-laden cables or long, flexible members ha

,," t,."i ~~,q:; ;':) g::,):?;"j'~iYjJ

aerodynamically unsymmetrical cross-sections. Ca

c.;.;;i, c' ti:)1i t;~t' ;::3?' '0;\.1".,,-,

stays, having circular sections, will not gallop unless

1;;{; ,::~-;;; ,~;-,t;,

circumferences re deformedby ice, droppingwate

accumulated debris.

Flexible bridge decks, as in very long spans

some pedestrian bridges, may be prone to wind-indu

flutter, a wind-excited oscillation of destruc

amplitudes, or, on so me occasions, divergence

irreversible twist under high wind. Analysis meth

including wind tunnel studies leading to adjustmen

the deck form, are available for prevention of both fl

and divergence.

3.8.3.3 CONTROL OF DYNAMIC RESPONSES C3.8.3.3

Bridges and structural components thereof, including Cables in stayed-girder bridges have

cables, shall be designed to be free of fatigue damage successfully stabilized against excessive dyn

due to vortex-induced or galloping oscillations. Bridges responses by attaching automotive dampers to

shall be designed to be free of divergence and bridge at deck level or by cross-tying multiple ca

1"""'\ catastrophic flutter up to 1.2 times the design wind stays.

velocity applicable at bridge deck height.

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Section 3 -Loads and Load Factors

(SI)


3.8.3.4 W ND TUNN TE S C .8.3.4

epresentative wind tunnel tests may be used to Wind tunnel testing of bridges and other civ

satisfy the requirements of Articles 3.8.3.2 and 3.8.3.3. engineering structures is a highly developed technology

which may be used to study the wind response

characteristics of a structural model or to verify the

results of analysis (Simiu 1976).

3.9 ICE LOADS: IC

3.9.1 General C3.9.1

Ice forces on piers shall be determined with regard Most of the information for ice loads was taken from

to site conditions and expected modes of ice action as Montgomery et al. (1984), which provided background

follows: for the clauses on ice loads for Canadian Standards

Association (1988). A useful additional source has been

. Dynamic pressure due to moving sheets or tIces of Neill (1981).

ice being carried by stream flow, wind, or currents; It is convenient to classify ice forces on piers as

dynamic forces and static forces.

.

Static pressure due to thermal movements of ice Dynamic forces occur when a moving ice floe strikes

sheets; a bridge pier. The forces imposed by the ice floe on a

pier are dependent on the size of the floe, the strength

. Pressure resulting from hanging dams or jams of ice; and thickness of the ice, and the geometry of the pier.

and The following types of ice failure have been

observed (Montgomery et al. 1984):

.

Static uplift or vertical load resulting from adhering

ice in waters of fluctuating eve . . Crushing, where the ice fails by local crushin

across the width of a pier. The crushed ice is

The expected thickness of ice, the direction of its continually cleared from a zone around the pier as

movement, and the height of its action shall be the floe moves pasto

determined by field investigations, review of public

records, aerial surveys, or other suitable means.

.

Bending, where a vertical reaction component acts

on the ice floe impinging on a pier with an inclined

nose. This reaction causes the floe to rise up the

pier nose, as flexural cracks formo

.

Splitting, where a comparatively small floe strikes a

pier and is split into smaller parts by stress cracks

propagating from the pier.

.

Impact, where a small floe is brought to a hall by

impinging on the nose of the pier before it has

"'"

crushed over the full-width of the pier, bent or split.

.

Buckling, where compressive forces cause a large

floe to fail by buckling in front of the nose of a very

wide pier.

"

For bridge piers of usual proportions on larger bodies

of water, crushing and bending failures usually control

the magnitude of the design dynamic ice force. On

smaller streams, which cannot carry large ice tices,

impact failure can be the controlling mode.

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LRFD AASHTO 3-27-38