CHAPTER 2 Bridge Loads

8/17/2019 CHAPTER 2 Bridge Loads

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Dept. of Civil Eng., T. F., Hawassa University

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Fundamentals of bridge design – Chapter 2 2

The m-factors specified below shall not be applied in conjunction with approximate load

distribution factors specified in Chapter 13: Approximate Methods of Analysis of ERA

design manual, except where the lever rule is used or where special requirements for

exterior beams in beam-slab bridges is applied.

Number of Loaded Lanes 1 2 3 >3

Multiple Presence Factors “m” 1.20 1.0 0.85 0.65

Table 2.2 Multiple Presence Factors "m"

The multiple presence factors have been included in the approximate equations for

distribution factors in Chapter 13: Approximate Methods of Analysis of ERA Bridgedesign manual, both for single and multiple loaded lanes. The equations are based on

evaluation of several combinations of loaded lanes with their appropriate multiple

presence factors and are intended to account for the worst case scenario. Where use of the

lever rule is specified the Designer must determine the number and location of vehiclesand lanes, and, therefore, must include the multiple presence. Stated another way, if a

sketch is required to determine load distribution, the Designer is responsible for includingmultiple presence factors and selecting the worst design case. The factor 1.20 from Table

2-2 has already been included in the approximate equations and should be removed forthe purpose of fatigue investigations.

If a component supported a sidewalk and one lane, it would be investigated for thevehicular live load alone with m = 1.20, and for the pedestrian loads combined with the

vehicular live load with m = 1.0. If a component supported a sidewalk and two lanes of

vehicular live load, it would be investigated for:

One lane of vehicular live load, m = 1.20;

The greater of the more significant lane of vehicular live load and the pedestrian loads

or two lanes of vehicular live load, m = 1.0 applied to the governing case; and Two lanes of vehicular live load and the pedestrian loads, m = 0.85.

The multiple presence factor of 1.20 for a single lane does not apply to the pedestrian

loads. Therefore, the case of the pedestrian loads without the vehicular live load is a

subset of the second bulleted item.

The multiple presence factors in Table 2.2 were developed based on an ADTT (Average

Daily Truck Traffic) of 5000 trucks in one direction. The force effect resulting from the

appropriate number of lanes shall be reduced for sites with lower ADTT as follows:

If 100 ADTT 1000; 95 % of the specified force effect shall be used; and If ADTT < 100; 90 % of the specified force effect shall be used.

This adjustment is based on the reduced probability of attaining the design event during a75-year design life with reduced truck volume.

Design Truck


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The weights and spacing of axles and wheels for the design truck shall be as specified in

Figure 2-1. A dynamic load allowance shall be considered as specified in the following

subchapter on Vehicular Dynamic Load Allowance.

F IGURE 2-1 C HARACTERISTICS OF THE D ESIGN T RUCK

Except as specified in following subchapters on the application of Design Vehicular Live

Loads and Fatigue Loads, the spacing between the two 145 kN axles shall be varied

between 4.3 and 9.0 m to produce extreme force effects.

Design Tandem

The design tandem used for Strategic Bridges shall consist of a pair of 110 kN axlesspaced 1.2 m apart. The transverse spacing of wheels shall be taken as 1.8 m. A dynamic

load allowance shall be considered as specified in a following subchapter. The spacing

and loading is illustrated in Figure 3-2

Figure 2-2 Design Tandem Load

Design Lane Load

4.3 m

4.3 –9.0 m

1.8 m

Plan of Design Truck Loadshowing tire contact areas

110 kN

110 kN

1.2 m

1.8 m


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The design lane load shall consist of a load of 9.3 kN/m, uniformly distributed in the

longitudinal direction. Transversely, the design lane load shall be assumed to be

uniformly distributed over a 3.0-m width. The force effects from the design lane load

shall not be subject to a dynamic load allowance.

2.3 PEDESTRIAN LOADS A pedestrian load of 4.0 kPa (kN/m

2) shall be applied to all sidewalks wider than 0.6 m

and considered simultaneously with the vehicular design live load.See the provisions of above subchapter Multiple Presence of Live Load for applying the

pedestrian loads in combination with the vehicular live load. Usually the 4 kN/m2 load

will allow for small cars to pass. To avoid accidents for bridges wider than 2.4 m, provision shall be made for an additional axle load.

Where sidewalks, pedestrian, and/or bicycle bridges are intended to be used by

maintenance and/or other incidental vehicles, these loads shall be considered in the

design. If unknown, at least one movable axle load of 70 kN acting together with the pedestrian load shall be applied. The dynamic load allowance need not be considered for

these vehicles.

In half-through-trusses of steel, the compressed top chord of a simple span truss shall be

designed to resist a lateral force of not less than 4.0 kN/m length, considered as a permanent load for the Strength I Load Combination and factored accordingly.

2.4 DYNAMIC LOAD ALLOWANCE (IM = VEHICULAR DYNAMIC LOAD

ALLOWANCE)Unless otherwise permitted in subchapters Buried Components and Wood Components

below, the static effects of the design truck or tandem, other than centrifugal and braking

forces, shall be increased by the percentage specified in Table 2.3 for dynamic load

allowance.

The factor to be applied to the static load shall be taken as: (1 + IM/100).

The dynamic load allowance shall not be applied to pedestrian loads or to the design laneload.

Component IM

Deck Joints – All Limit States 75%

All Other Components

Fatigue and Fracture Limit State

All Other Limit States

15%

33%

Table 2.3 Dynamic Load Allowance, IM

Dynamic load allowance need not be applied to:

Retaining walls not subject to vertical reactions from the superstructure, and

Foundation components that are entirely below ground level.

The dynamic load allowance shall be reduced for components, other than joints, if

justified by sufficient evidence, but in no case shall the dynamic load allowance used in

design be less than 50% of IM in the table above.


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The dynamic load allowance (IM) in Table 3.3 is an increment to be applied to the static

wheel load to account for wheel load impact from moving vehicles.

Dynamic effects due to moving vehicles shall be attributed to two sources:

Hammering effect is the dynamic response of the wheel assembly to riding surface

discontinuities, such as deck joints, cracks, potholes, and delamination’s, and Dynamic response of the bridge as a whole to passing vehicles, which shall be due to

long undulations in the roadway pavement, such as those caused by settlement of fill,

or to resonant excitation as a result of similar frequencies of vibration between bridge

and vehicle. The frequency of vibration of any bridge should not exceed 3 Hz.

B URIED C OMPONENTS

The dynamic load allowance for culverts and other buried structures, in %, shall be taken

as:

IM = 33 (1.0 - 4.l*10-4

DE) > 0%

Where:DE = the minimum depth of earth cover above the structure (mm)

W OOD C OMPONENTS

Wood structures are known to experience reduced dynamic wheel load effects due to

internal friction between the components and the damping characteristics of wood.

For wood bridges and wood components of bridges, the dynamic load allowance values

specified in table of dynamic load allowance shall be reduced to 70 of the valuesspecified therein for IM.

2.5 TIRE CONTACT AREA

The tire contact area of a wheel consisting of one or two tires shall be assumed to be a

single rectangle, whose width is 500 mm and whose length () in mm shall be taken as:

l = 2.28 x 10-3

(1 + IM/100) P (2.1)

where: = load factor for the limit state under consideration.IM = dynamic load allowance percent

P = 72.5 kN for the design truck and 55 kN for the design tandem

The tire pressure shall be assumed to be uniformly distributed over the contact area. The

tire pressure shall be assumed to be distributed as follows:

On continuous surfaces, uniformly over the specified contact area, and On interrupted surfaces, uniformly over the actual contact area within the footprintwith the pressure increased in the ratio of the specified to actual contact areas.

However, for all concrete decks including composite decks the length 200 mm shall be

used in Equation 2.1.

2.6 APPLICATION OF DESIGN VEHICULAR LIVE LOADS


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The effects of an axle sequence and the lane load are superimposed in order to obtain

extreme values.

Unless otherwise specified, the extreme force effect shall be taken as the larger of thefollowing:

The effect of the design tandem combined with the effect of the design lane load, or

The effect of one design truck with the variable axle spacing specified in thesubchapter Multiple Presence of Live Load above, combined with the effect of thedesign lane load, and

For both negative and positive moment between points of contraflexure under auniform load on all spans, and reaction at interior piers only, 90% of the effect of two

design trucks spaced a minimum of 15.0 m between the lead axle of one truck and therear axle of the other truck, combined with 90% of the effect of the design lane load.

The distance between the 145 kN axles of each truck shall be taken as 4.3 m.

The design truck or tandem shall be positioned transversely such that the center of anywheel load is not closer than:

For the design of the deck overhang - 300 mm from the face of the curb or railing,

and

For the design of all other components - 600 mm from the edge of the design lane.

Unless otherwise specified, the lengths of design lanes, or parts thereof, that contribute to

the extreme force effect under consideration, shall be loaded with the design lane load.

The lane load is not interrupted to provide space for the axle sequences of the design

tandem or the design truck.

2.7 CENTRIFUGAL FORCES (CE= VEHICULAR CENTRIFUGAL FORCE)Centrifugal forces shall be taken as the product of the axle weights of the design truck or

tandem and the factor C, taken as:

C = 4 v2

3 g*R

where: v = highway design speed (m/s)

g = gravitational acceleration: 9.81 (m/s2)

R = radius of curvature of traffic lane (m)

Highway design speed shall not be taken to be less than the value specified in theGeometric Design Manual-2001, Chapter 5: Design Controls & Criteria, Section 5.8:

Design Speed . The multiple presence factors specified above in subchapter Multiple Presence of Live Load shall apply. Centrifugal forces shall be applied horizontally at a

distance 1.8 m above the roadway surface.


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Lane load is neglected in computing the centrifugal force, as the spacing of vehicles at

high speed is assumed to be large, resulting in a low density of vehicles following and/or

preceding the design truck.

2.8 BRAKING FORCE (BR= VEHICULAR BRAKING FORCE)Based on energy principles, and assuming uniform deceleration (retardation), the braking

force determined as a fraction "b" of vehicle weight is:

b = v2

2gawhere a = the length of uniform deceleration.

Calculations using a braking length of 122 m and a speed of 90 km/h (25 m/s) yield b =

0.26 for a horizontal force that will act for a period of about 10 seconds. The factor "b"

applies to all lanes in one direction because all vehicles may have reacted within this time

frame. Only the design truck or tandem is to be considered.

Braking forces shall be taken as 25 % of the axle weights of the design truck or tandem

per lane placed in all design lanes which are considered to be loaded in accordance withabove subchapter Number of Design Lanes, and which are carrying traffic headed in the

same direction. These forces shall be assumed to act horizontally at the level of theroadway surface in either longitudinal direction to cause extreme force effects. All

design lanes shall be simultaneously loaded for bridges likely to become one-directional

in the future.

2.9 WIND LOAD (WL= WIND ON LIVE LOAD; WS= WIND LOAD ON STRUCTURE)

H ORIZONTALW IND P RESSURE

General

Pressures specified herein shall be assumed to be caused by a base design wind velocity,

VB, of 160 km/h (= 45 m/s).

Wind load shall be assumed to be uniformly distributed on the area exposed to the wind.The exposed area shall be the sum of areas of all components, including floor system and

railing, as seen in elevation taken perpendicular to the assumed wind direction. This

direction shall be varied to determine the extreme force effect in the structure or in its

components. Areas that do not contribute to the extreme force effect under considerationshall be neglected in the analysis.

For bridges or parts of bridges more than 10 m above low ground or water level, the

design wind velocity, VDZ (km/h), at design elevation, z, should be adjusted according to:

where: V10 = wind velocity at 10 m above low ground or above design water level (km/h)

o B

o DZ

Z

Z In

V

V V V

10

*5.2


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VB = base wind velocity of 160 km/h (45 m/s) at 10 m height, yielding design

pressures specified in following subchapters Wind Pressure on Structures

and Vertical Wind Pressure

Z = height of structure at which wind loads are being calculated as measured fromlow ground, or from water level, > 10 m (m)

Vo = friction velocity, a meteorological wind characteristic taken, as specified in

Table 3-4, for various upwind surface characteristics (km/h)

Zo = friction length of upstream fetch, a meteorological wind characteristic takenas specified in Table 3-4 below (m)

V10 shall be established from:

Basic Wind Speed charts available from National Meteorological Services Agencyfor various recurrence intervals,

Site-specific wind surveys, or

In the absence of better criterion, the assumption that V10 = VB =145 km/h (= 40 m/s)shall be used for small and medium sized bridges.

The following descriptions are for the terms "open country" and "suburban" in Table 3-4:

Open Country: Open terrain with scattered obstructions having heights generally less

than 10 m. This category includes flat open country and grasslands.

Urban and Suburban: Urban and suburban areas, wooded areas, or other terrainwith numerous closely spaced obstructions having the size of single-family or larger

dwellings. Use of this category shall be limited to those areas for whichrepresentative terrain prevails in the upwind direction at least 500 m.

CONDITION OPEN COUNTRY URBAN AND SUBURBAN

Vo (km/h) 13.2 17.6

Zo (m) 70 1000Table 2-4 Values of Vo and Zo for Various Upstream Surface Conditions

Base design wind velocity varies significantly due to local conditions. For small and/or

low structures, wind usually does not govern. For large and/or tall bridges, however, thelocal conditions should be investigated.

Pressures on windward and leeward sides are to be taken simultaneously in the assumed

direction of wind.Typically, a bridge structure should be examined separately under wind pressures from

two or more different directions in order to ascertain those windward, leeward, and side

pressures producing the most critical loads on the structure.

The suggested wind speed V10 = 40 m/s should be compared with the Ethiopian BuildingCode Standard, where V10 = 150 km/h (42 m/s) is used for the highest mountaintops. The

National Atlas of Ethiopia shows that the western parts of the country (Bahar Dar,

Nekemte, Gore, Jima, Awasa and Goba) have a wind speed (V10) that never exceeds 15knots (equal to 30 m/s or 105 km/h). However, since the National Meteorological

Services Agency has collected wind data only every 4 hours it is not certain that the

maximum wind speeds are given at the meteorological stations. Therefore, it is

recommended to make separate observations for large or wind-sensitive bridges.


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Equation 3.2 below is based on boundary layer theory combined with empirical

observations and represents the most recent approach to defining wind speeds for various

conditions as used in meteorology. In the past, an exponential equation was sometimesused to relate wind speed to heights above 10 m. This formulation was based solely on

empirical observations and had no theoretical basis.

10

Z*CVV

10DZ (2.2)

The purpose of the term C and exponent "" was to adjust the equation for variousupstream surface conditions, similar to the use of Table 3-4 (further information can befound in Refs.)

Wind Pressure on Structures: WS

For small and medium sized concrete bridges below 50m length the wind load on

structures shall be neglected.

For large and/or light bridges the following shall apply. If justified by local conditions, adifferent base design wind velocity shall be selected for load combinations not involving

wind on live load. The direction of the design wind shall be assumed horizontal, unless

otherwise specified in the following subchapter Aero elastic Instability. In the absence of

more precise data, design wind pressure, PD in kPa, shall be determined as:

25600

VP

V

VPP

DZ

2

B

2

B

DZ

BD

Where PB = base wind pressure specified in Table 3-5 (kPa):

STRUCTURAL COMPONENT WINDWARD LOAD, kPa LEEWARD LOAD, kPaTrusses, Columns, and Arches 2.4 1.2

Beams 2.4 Not applicable

Large Flat Surfaces 1.9 Not applicable

Table 2-5 Base Pressures, PB Corresponding to VB = 160 km/h (45 m/s)

The wind loading shall not be taken less than 4.4 kN/m2 in the plane of a windward chord

and 2.2 kN/m2 in the plane of a leeward chord on truss and arch components, and not

less than 4.4 kN/m2 on beam or girder components.

Wind tunnel tests shall be used to provide more precise estimates of wind pressures. Suchtesting should be considered where wind is a major design load.

Where the wind is not taken as normal to the structure, the base wind pressures, PB, forvarious angles of wind direction shall be taken as specified in Table 2-6 and shall be

applied to a single place of exposed area. The skew angle shall be taken as measuredfrom 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.


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Columns and Arches Girders

Skew Angle of Wind,

Degrees

Lateral

Load (kPa)

Longitudinal

Load(kPa)

Lateral Load

(kPa)

Longitudinal

Load (kPa)

0 3.6 0 2.4 0

15 3.4 0.6 2.1 0.3

30 3.1 1.3 2.0 0.6

45 2.3 2.0 1.6 0.860 1.1 2.4 0.8 0.9

Table 2-6 Base Wind Pressures, PB (kPa) for Various Angles of Attack VB=160 km/h.

For trusses, columns, and arches, the base wind pressures specified in Table 2-6 are thesum of the pressures applied to both the windward and leeward areas.

The transverse and longitudinal forces to be applied directly to the substructure shall be

calculated from an assumed base wind pressure of 1.9 kPa. For wind directions takenskewed 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 thecomponent perpendicular to the front elevation shall act on the exposed areas and shall be

applied simultaneously with the wind loads from the superstructure.

Wind Pressure on Vehicles: WL

When vehicles are present, the design wind pressure shall be applied to both structure and

vehicles. Wind pressure on vehicles shall be represented by an interruptible, movingforce of 1.5 kN/m acting normal to, and 1.8 m above, the roadway and shall be

transmitted to the structure.

When wind on vehicles is not taken as normal to the structure, the components of normal

and parallel force applied to the live load shall be taken as specified in Table 3-7 with theskew angle taken as referenced normal to the surface.

Skew Angle (Degrees) Normal Component (kN/m) Parallel Component (kN/m)

0 1.46 0

15 1.28 0.18

30 1.20 0.35

45 0.96 0.47

60 0.50 0.55

Table 2-7 Wind Components on Live Load

Based on practical experience, maximum live loads are not expected to be present on the bridge when the wind velocity exceeds 90 km/h. The load factor corresponding to the

treatment of wind on structure only in Load Combination Strength III would be

(90/145)2*1.4 = 0.54, which has been rounded to 0.5 in the Strength IV Load

Combination. This load factor corresponds to 0.3 in Service 1.


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V ERTICAL W IND P RESSURE

Unless otherwise determined in following subchapter Aero elastic Instability, a vertical

upward wind force of 1.0 kPa (kN/m2) times the width of the deck, including parapets

and sidewalks, shall be considered a longitudinal line load. This force shall be applied

only for large and/or other than concrete bridges. It shall be applied only for limit states

that do not involve wind on live load, and only when the direction of wind is taken to be

perpendicular to the longitudinal axis of the bridge. This lineal force shall be applied atthe windward quarter-point of the deck width in conjunction with the horizontal wind

loads specified in the previous subchapter Horizontal Wind Pressure.

The intent of this subchapter is to account for the effect resulting from interruption of the

horizontal flow of air by the superstructure. This load is to be applied even todiscontinuous bridge decks, such as grid decks. This load may govern where overturning

of the bridge is investigated.

AERO ELASTIC I NSTABILITY

Aero elastic force effects shall be taken into account in the design of bridges and

structural components that are wind-sensitive. All bridges and structural components

thereof with a span length to width or depth ratio exceeding 30.0 shall be deemed wind-sensitive.

Many bridges, decks, or individual structural components have been shown to be aero

elastically insensitive if their length-to-width or length-to-depth ratios are about 30.0, asomewhat arbitrary value helpful only in identifying likely wind-sensitive cases.

2.10 WATER LOADS (WA= WATER LOAD AND STREAM PRESSURE)

S TATIC P RESSURE

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 "g" (the acceleration of gravity =9.81 m/s

2).

p = * g * z * 10-9

where p = static pressure (Mpa)

= density of water (kg/m3)

z = height of water above the point of consideration (mm)

g = Gravitational acceleration (m/s2)

S TREAM P RESSURE

Longitudinal

For the purpose of this chapter, the longitudinal direction refers to the major axis of asubstructure unit.

The pressure of flowing water acting in the longitudinal direction of substructures shall be taken as:

p = 5.14*10-4

CDV2 (2.3)

where: p = pressure of flowing water (MPa)


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CD = drag coefficient for piers as specified in Table 2-8

V = design velocity in m/s of water for the design flood in strength and service

limit states and for the check flood in the extreme event limit state (see ERA

Drainage Design Manual-2001, Chapter 5: Hydrology).

Type CD

Semicircular-nosed pier 0.7Square-ended pier 1.4

Debris lodged against the pier 1.4

Wedged-nosed pier with nose angle 90o or less 0.8

Table 2-8 Drag Coefficient

The longitudinal drag force shall be taken as the product of longitudinal stream pressure

and the projected surface exposed thereto.

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 functionof the availability of such debris and level of maintenance efforts by which it is removed.

It shall be accounted for by the judicious increase in both the exposed surface and thevelocity of water.

The following provision (Ref. 2) shall be used as guidance in the absence of site-specificcriteria:

Where a significant amount of driftwood is carried, water pressure shall also beallowed for on a driftwood raft lodged against the pier. The size of the raft is a matter

of judgment, but as a guide, Dimension A in Figure 3-3 should be half the waterdepth, but not greater than 3m. Dimension B should be half the sum of adjacent span

lengths, but no greater than 14 m. Pressure shall be calculated using Equation 3.3,

with CD = 0.5.

Figure 2-3 Debris Raft for Pier Design

Lateral

The lateral, uniformly distributed pressure on substructure due to water flowing at an

angle, , to the longitudinal axis of the pier (see Figure 2-4) shall be taken as:

PL = 5.14 x 10-4

CLV2

where: PL = lateral pressure (MPa)


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CL = lateral drag coefficient specified in Table 3-9 below.

Figure 2-4 Plan View of Pier Showing Stream Flow Pressure

Angle, , between direction of flow and

longitudinal axis of the pier

CL

0o 0.0

1o 0.5

10o 0.7

20o 0.9

30o 1.0

(Ref 15)

Table 2-9 Lateral Drag Coefficient

The lateral drag force shall be taken as the product of the lateral stream pressure and the

surface exposed thereto.

2.11 EARTHQUAKE EFFECTS (EQ= EARTHQUAKE)

G ENERAL

Earthquake loads shall be taken to be horizontal force effects determined on the basis of

the elastic response coefficient, Csm and the equivalent weight of the superstructure,

adjusted by the response modification factor, R.

ACCELERATION C OEFFICIENT

The coefficient, "A", to be used in the application of these provisions shall be determined

from the contour map of Ethiopia in Figure 3-5. Linear interpolation shall be used for

sites located between contour lines or between a contour line and a local maximum or

minimum.

I MPORTANCE C ATEGORIES

The bridges within Zone 4 (mainly Rift Valley) shall be classified into one of three

importance categories as follows:

Critical bridges,

Essential bridges, or

Other bridges.


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The basis of classification shall include social/survival and security/defense requirements.

In classifying a bridge, consideration should be given to possible future changes in

conditions and requirements.

Figure 2.5. Earthquake zones

Essential bridges are generally those that should, as a minimum, be open to emergencyvehicles and for security/defense purposes immediately after the design earthquake, i.e., a

475-year return period event. However, some bridges must remain open to all traffic after

the design earthquake and be usable by emergency vehicles and for security/defense

purposes immediately after a large earthquake, e.g., a 2500 year return period event.

These bridges should be regarded as critical structures.


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ACCELERATION C OEFFICIENTS

Each bridge shall be assigned to one of the seismic zones in accordance with Table 2-10

below:

EBCS zone from Figure 3-9 Acceleration Coefficient

1 A 0.03

2 0.03 < A 0.05

3 0.05 < A 0.07

4 0.07 < A 0.10

Table 2-10 Seismic Zones

S ITE E FFECTS : S OIL P ROFILES

Site effects shall be included in the determination of seismic loads for bridges. Siteeffects on structural response are due to the soil conditions. Four soil profiles are used in

these Specifications to define a site coefficient used to modify the acceleration

coefficient.

The site coefficient, S, is used to include the effect of site conditions on the elastic

seismic response coefficient as specified in the following subchapter.

The site coefficient, S, specified in Table 2-11 (below), shall be based upon soil profile

types defined below.

Site

Coefficient

Soil Profile Type

I II III IV

S 1.0 1.2 1.5 2.0

Table 2-11 Site Coefficients

In locations where the soil properties are not known in sufficient detail to determine the

soil profile type, or where the profile does not fit any of the four types, the site coefficient

for Soil Profile Type II shall be used.

A soil profile shall be taken as Type I if composed of rock of any description, eithershale-like or crystalline in nature, or stiff soils where the soil depth is less than 60 m, and

the soil types overlying the rock are stable deposits of sands, gravels, or stiff clays.These materials shall be characterized by a shear wave velocity greater than 765 m/s.

A profile with stiff cohesive or deep cohesionless soils where the soil depth exceeds 60 mand the soil types overlying the rock are stable deposits of sands, gravels, or stiff clays

shall be taken as Type II.


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A profile with soft to medium-stiff clays and sands, characterized by 9 m or more of soft

to medium-stiff clays with or without intervening layers of sand or other cohesionless

soils shall be taken as Type III.

A profile with soft clays or silts greater than 12 m in depth shall be taken as Type IV.

These materials shall be characterized by a shear wave velocity of less than 152 m/s andmight include loose natural deposits or manmade, nonengineered fill.

E LASTIC S EISMIC R ESPONSE C OEFFICIENT

Unless specified otherwise as exceptions in this subchapter, the elastic seismic response

coefficient, Csm, for the mth

mode of vibration shall be taken as:

AT

AS C

m

sm 5.22.1

3/2

where:Tm = period of vibration of the m

th

mode (s)A = acceleration coefficient specified in Table 2-10

S = site coefficient specified in Table 2-11

The determination of the period of vibration, Tm, should be based on the nominal,unfactored mass of the component or structure.

The elastic seismic response coefficient shall be normalized using the input ground

acceleration "A” and the result plotted against the period of vibration. Such a plot is

given in Figure 2-6 for different soil profiles, based on 5 % damping.

Figure 2-6 Seismic Response Coefficients, Csm for Various Soil Profiles,

Normalized with Respect to Acceleration Coefficient "A" (Csm on the left

axis)

Exceptions to the application of Equation for Csm are as follows:


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For bridges on soil profiles III or IV, Csm need not exceed 2.0*A.

For soil profiles III and IV, and for modes other than the fundamental mode, that have periods less than 0.3 s, Csm shall be taken as:

Csm = A (0.8 + 4.0*Tm)

If the period of vibration for any mode exceeds 4.0 s, the value of Csm for that modeshall be taken as:

Csm = 3ASTm

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R ESPONSE M ODIFICATION F ACTORS

Except as noted herein, seismic design force effects for substructures and the connections between parts of structures shall be determined by dividing the force effects resulting

from elastic analysis by the appropriate response modification factor, R, as specified inTables 2-12 and 2-13, respectively.

Substructure Importance Category

Critical Essential Other

Wall-type piers – larger dimension 1.5 1.5 2.0

Reinforced concrete pile bents

Vertical piles only

With battered piles

1.51.5

2.01.5

3.02.0

Single columns 1.5 2.0 3.0

Steel or composite steel and concrete pile bents

Vertical pile only

With battered piles 1.51.5

3.52.0

5.03.0

Multiple column bents 1.5 3.5 5.0

Table 2-12 Response Modification R-Factors for Substructures

Connection All Importance Categories

Superstructure to abutment 0.8

Expansion joints within a span of the superstructure 0.8

Columns, piers, or pile bents to cap beam or superstructure 1.0

Columns or piers to foundations 1.0

Table 2-13 Response Modification R-Factors for Connections


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A NALYSIS FOR E ARTHQUAKE LOADS

General

Bridges in Seismic Zone 1-3 need not be analyzed for seismic loads, regardless of their

importance and geometry.

Single-Span Bridges

Seismic analysis is not required for single-span bridges, regardless of seismic zone.

Multispan Bridges

For multispan structures, the minimum analysis requirements shall be as specified below:

SeismicZone

Single-SpanBridges

Multispan Bridges

Other Bridges Essential Bridges Critical Bridges

Regular Irregular Regular Irregular Regular Irregular

1-3

4

No Seismic Analysis

Seismic Analysis

*

SM/UL

*

SM

*

SM/UL

*

MM

*

MM

*

MM

Table 2-14 Minimum Analysis Requirements for Seismic Effectsin which:

* = no seismic analysis required (Zone 1-3)

UL = uniform load elastic method

SM = single-mode elastic methodMM = multimode elastic method

2.12 LOAD FACTORS AND COMBINATIONS

G ENERAL

The total factored force effect: Q = ii Qi ----------- Equation 2.4

where:

i= load modifier (a factor relating to ductility, redundancy and operationalimportance)

Qi = force effects from loads specified herein

i = load factors

Ductility, redundancy, and operational importance are significant aspects affecting themargin of safety of bridges. Whereas the first two directly relate to physical strength, the

last concerns the consequences of the bridge being out of service.

D UCTILITY

The response of structural components or connections beyond the elastic limit can be

characterized by either brittle or ductile behavior. Under repeated seismic loading, large


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reversed cycles of inelastic deformation dissipate energy and have a beneficial effect on

structural survival.

If, by means of confinement or other measures, a structural component or connection

made of brittle materials can sustain inelastic deformations without significant loss ofload-carrying capacity, this component can be considered ductile. Such ductile

performance shall be verified by testing.

The structural system of a bridge shall be proportioned and detailed to ensure thedevelopment of significant and visible inelastic deformations at the strength and extreme

event limit states prior to failure.

For the strength limit state, the ductility factor

D 1.05 for non-ductile components and connections

D = 1.00 for conventional designs and details complying with these

Specifications

D ≤ 0.95 for components and connections for which additional ductility-enhancing measures have been specified beyond those required by these

SpecificationsFor all other limit states:

D = 1.00

R EDUNDANCY

Main elements and components whose failure is expected to cause the collapse of the

bridge shall be designated as failure-critical and the associated structural system as non-

redundant. Alternatively, failure-critical members in tension shall be designated fracture-critical.

Those elements and components whose failure is not expected to cause collapse of the

bridge shall be designated as non-failure-critical and the associated structural system asredundant.

For the strength limit state:

R 1.05 for non-redundant members= 1.00 for conventional levels of redundancy

0.95 for exceptional levels of redundancyFor all other limit states:

R = 1.00For each load combination and limit state under consideration, member redundancyclassification (redundant or non-redundant) should be based upon the member

contribution to the bridge safety.

O PERATIONAL I MPORTANCE

This definition shall apply to the strength and extreme event limit states only. Some

bridges or structural components and connections shall be declared to be of operational

importance.Such classification should be based on social/survival and/or security/defense

requirements.

Three levels of importance are specified with respect to seismic design: "critical,"

"essential," and "other." Bridges classified as "critical" or "essential" should beconsidered of "operational importance."


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For the strength limit state:

l 1.05 for important bridges= 1.00 for typical bridges

0.95 for relatively less important bridges

For all other limit states:

l = 1.00

2.12 LIMIT STATES

S TRENGTH L IM IT S TATE

The strength limit state shall be taken to ensure that strength and stability, both local and

global, are provided to resist the specified statistically significant load combinations that

a bridge is expected to experience in its design life.Extensive distress and structural damage may occur under strength limit state, but overall

structural integrity is expected to be maintained.

E XTREME E VENT L IM IT S TATES

The extreme event limit state shall be taken to ensure the structural survival of a bridge

during a major earthquake or flood, possibly under scoured conditions.

Extreme event limit states are considered to be unique occurrences whose return periodshall be significantly greater than the design life of the bridge.

S ERVICE L IM IT S TATE

The service limit state shall be taken as restrictions on stress, deformation, and crack

width under regular service conditions. The service limit state provides certain experience

related provisions that cannot always be derived solely from strength or statisticalconsiderations.

F ATIGUE AND F RACTURE L IM IT S TATE

The fatigue limit state shall be taken as restrictions on stress range as a result of a single

design truck occurring at the number of expected stress range cycles.

The fatigue limit state is intended to limit crack growth under repetitive loads to prevent

fracture during the design life of the bridge.

The fracture limit state shall be taken as a set of material toughness requirements of the

Technical Specifications.

Limit States

STRENGTH I Basic load combination relating to the normal vehicular use of the bridge without wind.

STRENGTH II Load combination relating to the use of the bridge by ERA-specified special design or permit vehicles, without wind.

STRENGTH III Load combination relating to the bridge exposed to wind velocity exceeding 90 km/h.

STRENGTH IV Load combination relating to very high dead load to live load force effect ratios.

STRENGTH V Load combination relating to normal vehicular use of the bridge with wind of 90 km/h(25 m/s) velocity

EXTREME Load combination including earthquake


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EVENT

SERVICE I Load combination relating to the normal operational use of the bridge with a 90 km/h(25 m/s) wind and all loads taken at their nominal values. Also related to deflection

control in buried metal structures, tunnel liner plate, and thermoplastic pipe and to

control crack width in reinforced concrete structures. This load combination should also

be used for the investigation of slope stability.

Compression in prestressed concrete components is investigated using this loadcombination. Service III is used to investigate tensile stresses in prestressed concrete

components.

SERVICE II Load combination intended to control yielding of steel structures and slip of slip-critical connections due to vehicular live load.

This load combination corresponds to the overload provision for steel structures, and it

is applicable only to steel structures. From the point of view of load level, this

combination is approximately halfway between that used for Service I and Strength I

Limit States.

SERVICE III Load combination relating only to tension in prestressed concrete structures with theobjective of crack control.

FATIGUE Fatigue and fracture load combination relating to repetitive gravitational vehicular liveload and dynamic responses under a single design truck having a constant axle spacingof 9.0 m between 145 kN axles.

The load factor, applied to a single design truck, reflects a load level found to be

representative of the truck population with respect to a large number of return cycles of

stresses and to their cumulative effects in steel elements, components, and connections.

The load factors for various loads comprising a design load combination shall be taken as

specified in the following table. All relevant subsets of the load combinations shall be

investigated. For each load combination, every load that is indicated to be taken into

account and that is germane to the component being designed, including all significant

effects due to distortion, shall be multiplied by the appropriate load factor and multiple presence factor, if applicable. The products shall be summed as specified in Equation 3.4

and multiplied by the load modifiers.

The factors shall be selected to produce the total extreme factored force effect. For each

load combination, both positive and negative extremes shall be investigated.

In load combinations where one force effect decreases another effect, the minimum value

shall be applied to the load reducing the force effect. For permanent force effects, the

load factor that produces the more critical combination shall be selected from thefollowing table. Where the permanent load increases the stability or load-carryingcapacity of a component or bridge, the minimum value of the load factor for that

permanent load shall also be investigated.

The larger of the two values provided for load factors of Uniform Temperature (TU),

Creep (CR), and Shrinkage (SH) shall be used for deformations and the smaller values

for all other effects.


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In the application of permanent loads, force effects for each of the specified six load

types should be computed separately. It is unnecessary to assume that one type of load

varies by span, length, or component within a bridge.

LoadCombination

Limit State

DCDD

DWEH

EVES

LLIM

CEBR

PLLSEL

WA WS WL FR TUCR

SH

TG SE Use one ofthese at a

time

EQ CT

STRENGTH 1

(Unless noted) p 1.75 1.00 - - 1.00 0.50/1.20 TG SE - -

STRENGTH II p 1.35 1.00 - - 1.00 0.50/1.20 TG SE - -

STRENGTH III p - 1.00 1.40 - 1.00 0.50/1.20 TG SE - -

STRENGTH IV

EH, EV, ES, DWDC ONLY

p 1.5

- 1.00 - - 1.00 0.50/1.20- -

- -

STRENGTH V p 1.35 1.00 0.50 1.0 1.00 0.50/1.20 TG SE - -EXTREME

EVENT p EQ 1.00 - - 1.00 - - - 1.0

0-

SERVICE I 1.00 1.00 1.00 0.30 1.0 1.00 1.00/1.20 TG SE - -

SERVICE II 1.00 1.30 1.00 - - 1.00 1.00/1.20 - - - -

SERVICE III 1.00 0.80 1.00 - - 1.00 1.00/1.20 TG SE - -

FATIGUE

LL, IM and CEONLY

- 0.75 - - - - - - - - -

Where (see following text):

BR = vehicular braking forceCE = vehicular centrifugal force

CR = creepCT = vehicular collision forceDC = dead load of structural componentsDD = downdragDW = dead load of wearing surfaces and utilities

EH = horizontal earth pressure loadEL = accumulated locked-in effects resulting

from the construction processEQ = earthquake loadES = earth surcharge load

EV = vertical pressure from dead load of earth fill

FR = frictionIM = vehicular dynamic load allowance

LL = vehicular live loadLS = live load surchargePL = pedestrian live loadSE = settlementSH = shrinkage

TG = temperature gradientTU = uniform temperatureWA = water load and stream pressureWL = wind on live loadWS = wind load on structure

Load Combinations and Load Factors

Consider the investigation of uplift. Where a permanent load produces uplift, that loadwould be multiplied by the maximum load factor, regardless of the span in which it islocated. If another permanent load reduces the uplift, it would be multiplied by the

minimum load factor, regardless of the span in which it is located. For example, at


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Fundamentals of bridge design – Chapter 223

Strength I Limit State where the permanent load reaction is positive and live load can

cause a negative reaction, the load combination would be:

0.9DC + 0.65DW + 1.75(LL+IM)

If both reactions were negative, the load combination would be:

1.25DC + 1.50DW + 1.75(LL+IM).

Type of Load Load Factor ( p)

Maximum Minimum

DC: Component and Attachments 1.25 0.90

DD: Downdrag 1.80 0.45

DW: Wearing Surfaces and Utilities 1.50 0.65

EH: Horizontal Earth Pressure

Active

At-Rest

1.50

1.35

0.90

0.90

EL: Locked-in Erection Stresses 1.0 1.0EV: Vertical Earth Pressure

Overall Stability

Retaining Structure

Rigid Buried Structure

Rigid Frames

Flexible Buried Structures other thanMetal Box Culvert

Flexible Metal Box Culverts

1.35

1.351.30

1.35

1.95

1.50

N/A

1.000.90

0.90

0.90

0.90

ES: Earth Surcharge 1.50 0.75

Load Factors for Permanent Loads, p

For each force effect, both extreme combinations may need to be investigated by

applying either the high or the low load factor as appropriate. The algebraic sums of these

products are the total force effects for which the bridge and its components should bedesigned.

LOAD F ACTORS FOR C ONSTRUCTION LOADS

Load factors for the weight of the structure and appurtenances shall not be taken to be

less than 1.25.

Unless otherwise specified by ERA, the load factor for construction loads, for equipmentand for dynamic effects shall not be less than 1.5. The load factor for wind shall not be

less than 1.25. All other load factors shall be taken as 1.0.The load factors presented here should not relieve the contractor of responsibility forsafety and damage control during construction.

CHAPTER 2 Bridge Loads

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