loads on bridges design lane aashto hl93 loads fatigue ... · pdf filelive loads of vehicles:...

EGCE 406 Bridge DesignEGCE 406 Bridge Design

Loads on BridgeLoads on Bridge

Praveen Chompreda Mahidol UniversityFirst Semester, 2009

OutlineOutline

Loads on Bridges Design Lane

Typical LoadsDead Load

AASHTO HL93 LoadsTruckTandem

Live LoadLive Load of Vehicle

TandemUniform Load

LL CombinationsPedestrian LoadDynamic Load Allowance

O h L d

LL CombinationsLL Placement

Influence LineOther Loads

FatigueW d

Design EquationDesign Charts

WindEarthquake

Multiple PresenceDistribution to Girders

…

Load and Resistance Factor D iDesign

Loads on BridgeLoads on BridgeDD = downdrag (wind)DC d d L d f

BR = breaking force of vehicleCE = centrifugal force of vehicle (at curves)DC = dead Load of

structural and nonstructural components

CE centrifugal force of vehicle (at curves)CR = creep of concreteCT = vehicle collision force (on bridge or at piers)

DW = dead load of wearing surfaceEH = earth pressure

piers)CV = vessel collision force (bridge piers over river)EQ = earthquakeEH = earth pressure

(horizontal)EL = secondary forces such as

EQ = earthquakeFR = frictionIC = iceIM = d i l d f hi lfrom posttensioning

ES = earth surcharge load (vertical)

IM = dynamic load of vehiclesLL = live load of vehicle (static)LS = live load surcharge

(vertical)EV = earth pressure (vertical)

PL = pedestrian loadSE = settlementSH = shrinkage of concretegTG = load due to temperature differencesTU = load due to uniform temperatureWA = water load/ stream pressureWA water load/ stream pressureWL = wind on vehicles on bridgeWS = wind load on structure

T i l L dTypical Loads

Dead Loads: DC/DWLive Loads of Vehicles: LL

Pedestrian Load: PLPedestrian Load: PLDynamic (Impact) Loads: IM

Dead Load: DCDead Load: DCDead load includes the self weight of:

structural components such as girder, slabs, cross beams, etc…nonstructural components such as medians, railings, signs, etc…

B d i l d h i h f i f ( h l )But does not include the weight of wearing surface (asphalt)We can estimate dead load from the material’s density

Material Density (kg/m3)

Concrete (Normal Weight.) 2400

Concrete (Lightweight) 1775-1925

Steel 7850

Aluminum Alloy 2800

Wood 800-960

Stone Masonry 2725y

Dead Load of Wearing Surface: DWDead Load of Wearing Surface: DWIt is the weight of the wearing surface ( ll h lt) d tiliti ( i (usually asphalt) and utilities (pipes, lighting, etc…)Different category is needed due to Different category is needed due to large variability of the weight compared with those of structural components (DC)

Asphalt surface may be thicker than designed and may get laid on top of old designed and may get laid on top of old layer over and over

Density of asphalt paving material = 2250 kg/m3

Average Thickness of asphalt on bridge = 9 = 9 cm

Tributary Area for Dead LoadsyDead loads are distributed to girder through Tributary Area

w or wwDC or wDW

M=wL2/8

V= L/2V=wL/2

DC, DWDC, DW

Section for maximum moment is not the same as the section for maximum shear For simply-supported beams

Maximum M occurs at midspanMaximum V occurs over the support

As we shall see in the designs of girders, the Critical Section for shear is about d from the support.(where d is the effective depth of section, approximately 0.8h)approximately 0.8h)At this point, shear is slightly lower than at the support. If we use shear at the support for the design of stirrups, we are conservative.

Live Loads of Vehicles: LLLive Loads of Vehicles: LL Live Loads of Vehicles: LLLive Loads of Vehicles: LL

Live load is the force due to The effect of live load on the Live load is the force due to vehicles moving on the bridgeThere are several types of

The effect of live load on the bridge structures depends on many parameters including:yp

vehiclesCarV

span lengthweight of vehicle

l l d (l d h l)VanBusesTrucks

axle loads (load per wheel)axle configurationposition of the vehicle on the Trucks

Semi-TrailerSpecial vehicles

position of the vehicle on the bridge (transverse and longitudinal)

b f hi l h b id Military vehicles

number of vehicles on the bridge (multiple presence)girder spacingg p gstiffness of structural members (slab and girders)

Live Loads of Vehicles: LLLive Loads of Vehicles: LL Bridge LL vs. Building LLBridge LL vs. Building LLBRIDGE BUILDING

LL is very heavy (several tons per wheel)

LL is not very heavy, typical 300-500 kg/m2)

LL can be series of point loads (wheel loads of trucks) or uniform loads (loads of smaller vehicles)

gLL is assumed to be uniformly distributed within a span

of smaller vehicles)Need to consider the placement withina span to get the maximum effect

Do not generally consider placement of load within a spanp g

Loads occur in one direction within lanesN d t id l th l t f

placement of load within a spanLoads are transferred in to 2 directions

Need to consider also the placement of loads in multiple spans (for continuous span bridges)

Need to consider various placements of loads for the entire floor

Dynamic effects of live load cannot be ignored

Do not generally consider dynamic/impact effect of live loadsloads

Analysis Strategy for LLAnalysis Strategy for LL

V i Place them Moment/ Shear

Various Live Loads

to get maximum effects on

Consider dynamic effects

Distribute Load to each girder

from Live Load to be used in the design of girders

spaneffects each girder design of girders

Design Truck

Design Tandem

Uniform Lane Load

Design LaneDesign LaneNeed to know how many lanes there is on the bridge

≠Design Lane ≠ Actual Traffic Lane3.0 m 3.3 m to 4.6 m (3.6 m recommended)

Number of Design Lanes = Roadway width/ 3 6 mNumber of Design Lanes = Roadway width/ 3.6 m≥ No. of Actual Traffic Lane

Number of Lane must be an integer (1,2,3,…) – there is no fraction of lane g ( , , , )(no 2.5 lanes, for example)For roadway width from 6 m to 7.2 m, there should be 2 design lanes, each equal ½ of the roadway width

roadway width

Live Loads of Vehicles: LLLive Loads of Vehicles: LLFor design purpose, we are interested the kind of vehicle that produce the

t ff tworst effectAASHTO has 3 basic types of LL called the HL-93 loading (stands for Highway Loading year 1993)Highway Loading, year 1993)

Design truckDesign tandemUniform loads

1. Design Truck1. Design TruckThe design truck is called HS-20 (stands for Highway Semi-Trailer (stands for Highway Semi-Trailer with 20-kips weight on first two axles)HS-20Weight shown are for each one axle = 2 wheelsTotal Wt = 325 kN ~ 33 tTotal Wt 325 kN 33 t.Distance between second and third axles may be varied to

d i ffproduce maximum effectNeed to multiply this load by dynamic allowance factor (IM)dynamic allowance factor (IM)

2. Design Tandem2. Design TandemTwo axle vehicle with 110 kN

h lon each axleNeed to multiply this load by dynamic allowance factor (IM)

110 kNper axle

110 kNper axle

PROFILE dynamic allowance factor (IM)Lead to larger moment than the HS20 truck for simple-support

PROFILE

p ppspans less than about 13.4 m

55 kN 55 kN

Traffic DirectionsLoading L

55 kN 55 kN

1 8 Lane

55 kN 55 kN

1.8 m TOP VIEW

1.2 m

3. Uniform Lane Loading3. Uniform Lane LoadingUniform load of 9.3 kN/m acting over a tributary width of 3 m. (i.e. the load is 3 1 kN/m2)load is 3.1 kN/m )May be apply continuously or discontinuously over the length of the bridge to produce maximum effectNo dynamic allowance factor (IM) for this load



Various Live Loads






Load CombinationsLoad Combinations

Transverse Placement

L i di l PlLongitudinal Placement

Live Load CombinationsLive Load Combinations3 ways to add the design truck, design tandem, and uniform load together

C b 1 HS20 k f f l l d d lCombination 1: one HS20 truck on top of a uniform lane load per design laneCombination 2: one Design Tandem on top of a uniform lane load per design laneCombination 3: (for negative moments at interior supports of continuous beams) place two HS20 design truck, one on each adjacent span but not less th 15 t ( f f t l f t k t th l f than 15 m apart (measure from front axle of one truck to the rear axle of another truck), with uniform lane load. Use 90% of their effects as the design moment/ shear

The loads in each case must be positioned such that they produce maximum effects (max M or max V)Th ff f h 3 d f h dThe maximum effect of these 3 cases is used for the design

Live Load PlacementLive Load Placement

Need to consider two dimensionsTransversely (for designs of slabs and overhangs)

roadway width

Longitudinally (for design of main girder)

Live Load Placement - TransverseLive Load Placement TransverseThe design truck or tandem shall be positioned transversely such that the

t f h l l d i t l thcenter of any wheel load is not closer than:30 cm from the face of the curb or railing for the design of the deck overhang60 cm from the edge of the design lane for the design of all other components60 cm from the edge of the design lane for the design of all other components

min. 2'

Minimum distance from curb = 60 cm

N t th t if th id lk i t t d b h th t ffi b i Note that if the sidewalk is not separated by a crashworthy traffic barrier, must consider the case that vehicles can be on the sidewalk

Live Load Placement - LongitudinalLive Load Placement LongitudinalNeed to place the LL along the span such that it produces the maximum ff teffect

For simple 1-point loading, the maximum moment occurs when the load is placed at the midspanplaced at the midspan

PPPoint of Max

Moment

L/2 L/2

However, truck load is a group of concentrated loads. It is not clear where to place the group of loads to get the maximum momentREMEMBER: MAXIMUM MOMENT DOES NOT ALWAYS OCCURS AT MIDSPAN !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Live Load Placement - LongitudinalLive Load Placement Longitudinal

Methods of finding maximum moment and shear in spanInfluence Line (IL) – Simple and Continuous spansDesign Equation – Simple span onlyDesign Chart – Simple span only

Live Load Placement – Influence LineLive Load Placement Influence LineInfluence line is a graphical method for finding the variation of the “ t t l ” t i t t t d li l d “structural response” at a point as a concentrated live load moves across the structure

Structural response can be support reaction, moment, shear, or displacementStructural response can be support reaction, moment, shear, or displacement

Live Load Placement – Influence LineLive Load Placement Influence Line

Live Load Placement – Influence LineLive Load Placement Influence Line Live Load Placement – Influence LineLive Load Placement Influence Line

Live Load Placement – Influence LineLive Load Placement Influence LineInfluence line is a powerful visualization tool for the effects of live load l t t th t t l placements to the structural response

110 kN 110 kN

For Point Load, the “response” is equal to the value of point load multiplied by the ordinate (y-value) of

1.0

0.50.25

0.75

by the ordinate (y value) of the influence line

IL (RL)

0.25

For Uniform Load, the response is equal to the value response is equal to the value of the uniform load multiplied by the area under the i fl li ithi th influence line within the uniform load

Live Load Placement – Influence LineLive Load Placement Influence Line1.0

0 75

IL (RL)

0.50.25

0.75

1.0

0 50.75

IL (RL)

0.50.25

1.0

0 50.75

IL (RL)

0.50.25

1.0

0.50.75

IL (RL)

0.50.25

Live Load Placement – Influence LineLive Load Placement Influence Line Live Load Placement – Influence LineLive Load Placement Influence LineMüller-Breslau Principle: “If a function at a point on a beam, such as

ti h t i ll d t t ith t t i t th reaction, or shear, or moment, is allowed to act without restraint, the deflected shape of the beam, to some scale, represent the influence line of the function.

Live Load Placement – Influence LineLive Load Placement Influence Line Live Load Placement – Influence LineLive Load Placement Influence Line


NotesInfluence line tells you how to place the LL such that the maximum moment at a point occurs; i.e. you pick a point then you try to find what is the maximum moment at that point when loads are moved what is the maximum moment at that point when loads are moved aroundIt does not tell you where the absolute maximum moment in the span It does not tell you where the absolute maximum moment in the span occurs; i.e. the maximum moment on the point you picked is not always the absolute maximum moment that can occur in the span ( hi h ill diff i d d diff (which will occur at a different point and under a different arrangement of loads)

Live Load Placement – Influence LineLive Load Placement Influence LineFor series of concentrated load (such as the design truck), the placement f l d f i t h ti t b tof load for maximum moment, shear, or reaction may not be apparent.

The maximum always occur under one of the concentrated loads – but which one?which one?Two methods

Trial and Errors: Move the series of concentrated loads along the span Trial and Errors: Move the series of concentrated loads along the span by letting each load on the peak of IL

Use when you have only 2-3 concentrated loadsCan be tedious when you have a lot of concentrated loads


Train Loading (AREA: American Railroad Engineers Association)(AREA: American Railroad Engineers Association)

Live Load Placement – Influence LineLive Load Placement Influence LineIncrease/ Decrease Method

This method determine whether the response (moment, shear, or reaction) increases or decreases as the series of concentrated loads move into the spanmove into the spanAs the series of loads move into the span, the response increases. When it starts to decrease, you’ll know that the last position was y pthe one that produce the maximum effect.

Live Load Placement – Influence LineLive Load Placement Influence LineIncrease/ Decrease Method

For shear

Sloping Line Jump

ΔV = Ps(x2-x1) ΔV = P (y2-y1)

For moment

Sloping Line

ΔM = Ps(x2-x1)

IL for momenthas no jumps!

ΔM = Ps(x2-x1)

Note: not all loads may be in the span at the same time. Loads that have just moved in or moved out may travel on the slope at a distance less than distance moved between 2 concentrated loadsthan distance moved between 2 concentrated loads.

Live Load Placement – Influence LineLive Load Placement Influence LineExample

Live Load Placement – Influence LineLive Load Placement Influence Line Live Load Placement – Influence LineLive Load Placement Influence LineFor Statically Indeterminate Structures, the Müller-Breslau Principle also h ldholds“If a function at a point on a beam, such as reaction, or shear, or moment, is allowed to act without restraint the deflected shape of the beam to is allowed to act without restraint, the deflected shape of the beam, to some scale, represent the influence line of the function”For indeterminate structures, the influence line is not straight lines!g

Live Load Placement – Influence LineLive Load Placement Influence Line Live Load Placement - LongitudinalLive Load Placement Longitudinal


Live Load Placement – Design EquationLive Load Placement Design Equation

Another Method: Using Barre’s Theorem for simply supported spans

The absolute maximum moment in the span occurs under the load l h l f d l d i h h h closet to the resultant force and placed in such a way that the

centerline of the span bisects the distance between that load and the resultantresultant

Resultant 0.73 m

145 kN 145 kN35 kN

L/2 L/2

HS20Point of Max

Moment

Live Load Placement – Design EquationLive Load Placement Design Equation

Resultant 0.73 m

145 kN 145 kN35 kN

L/2 L/2

HS20Point of Max Point of Max

Moment

max172.181.25 387 kN-mM l

l= + − max

19.855 66 kN-mM ll

= + −

Mmax occurs at a section under middle axle located a distance 0.73 m from

Mmax occurs at a section under one of the axle located a distance 0.30 m from

midspan midspan

Live Load Placement – Design EquationLive Load Placement Design EquationCase Load Configuration Moments (kips-ft) and

shears (kips)Loading and limitations (x and l in feet)

⎤⎡ ⎞⎛ 42Truck loading32 32 8

⎥⎦

⎤⎢⎣

⎡−⎟

⎠⎞

⎜⎝⎛ −=

⎥⎦

⎤⎢⎣

⎡−⎟

⎠⎞

⎜⎝⎛ −=

llxPxV

llxPxxM

4215.4)(

4215.4)(A

gP = 16 kipsMA ≥ MB for:l > 28 x ≤ l/3x + 28 ≤ lV > V for any x

32 32 8

⎦⎣ ⎠⎝ ll

⎥⎤

⎢⎡

⎟⎞

⎜⎛=

xPxxM 721154)(

VA > VB for any x

Truck loadingP = 16 kips

32 32 8

x

⎥⎦⎤

⎢⎣⎡ −−=

⎥⎦

⎢⎣

−−⎟⎠

⎜⎝

−=

llxPxV

xllPxxM

215.44)(

15.4)(B

P = 16 kipsMB ≥ MA for:l > 28 x > l/314 ≤ x ≤ l/2

8

x

⎟⎞

⎜⎛

⎟⎠⎞

⎜⎝⎛ −−=

xV

llxxxM

2150)(

2150)(

CTandem loadingis more severe than truck

25 25

⎟⎠⎞

⎜⎝⎛ −−=

llxxV 2150)(

−=

xlxxM )(640)(

loading for l ≤ 37 ft

0.64 k/ft

x

⎟⎠⎞

⎜⎝⎛ −= xlxV

xxM

264.0)(

264.0)(

D Lane loading

x

Live Load Placement – Design EquationLive Load Placement Design EquationIf we combine the truck/tandem load with uniform load, we can get the f ll i ti f i t i following equations for maximum moment in spans

Live Load Placement - LongitudinalLive Load Placement Longitudinal


Live Load Placement – Design ChartLive Load Placement Design Chart

Bending Moment in Simple Span Bending Moment in Simple Span for AASHTO HL-93 Loading for a fully loaded lane

Moment in kips-ft

IM is included

1 ft = 0.3048 m

1 kips = 4.448 kN

1 kips-ft = 1.356 kN-m


Shear in Simple Span Shear in Simple Span for AASHTO HL-93 Loading for a fully loaded lane

Shear in kips

IM is included

1 ft = 0.3048 m

1 kips = 4.448 kN

Live Load Placement – Design ChartLive Load Placement Design ChartDesign chart is meant to be used for preliminary designs.

We assume that maximum moment occurs at midspan – this produces slightly lower maximum moment than the Design Equation method slightly lower maximum moment than the Design Equation method. However, the error is usually small.

Maximum shear occurs at support. However, the chart does not have x = 0 ft. The closest is 1 ft from support.

In general, the bridge girder much higher than 1 ft. Therefore, shear at 1 ft is still higher than the shear at critical section for shear (at d) so we are still conservative hereconservative here.


Design Chart for Negative Moment due to Live Load Combination 3at Interior Support of Continuous Beams with Equal Spans

For one lane loadingFor one lane loading

IM is included

OutlineOutline





TandemUniform Load


O h L d



FatigueW d


WindEarthquake


…


Pedestrian Live Load: PLPedestrian Live Load: PLUse when has sidewalk wider th 60 than 60 cmConsidered simultaneously with truck LLtruck LL

Pedestrian only: 3.6 kN/m2Pedestrian only: 3.6 kN/mPedestrian and/or Bicycle: 4.1 kN/m2

No IM factor (Neglect dynamic effect of pedestrians)



Various Live Loads






Dynamic yAllowance Factor (IM)

Dynamic Load Allowance: IMDynamic Load Allowance: IMSources of Dynamic Effects

H ff h h l h h d h d f Hammering effect when wheels hit the discontinuities on the road surface such as joints, cracks, and potholesDynamic response of the bridge due to vibrations induced by trafficy p g y

Actual calculation of dynamic effects is very difficult and involves a lot of unknownsTo make life simpler, we account for the dynamic effect of moving vehicles by multiplying the static effect with a factor

Dynamic Load

Effect due toStatic Load

Effect due toDynamic Load

Allowance Factor

IM

This IM factor in the code was obtained from field measurements

IM

This IM factor in the code was obtained from field measurements

Dynamic Load Allowance: IMDynamic Load Allowance: IM

Dynamic Load Allowance: IMDynamic Load Allowance: IMAdd dynamic effect to the following loads:

D T kDesign TruckDesign Tandem

But NOT to these loads:But NOT to these loads:Pedestrian LoadDesign Lane Loadg

Table 3.6.2.1-1 (modified)Component IM

Deck JointAll li i

75%

( )

All limit states

All other components above groundFatigue/ Fracture Limit States 15%All Other Limit States 33%

Foundation components below ground 0%

* Reduce the above values by 50% for wood bridges



Various Live Loads






M l i l P f LLMultiple Presence of LL

Distribution Factors

Multiple Presence of LLMultiple Presence of LL

We’ve considered the effect of load placement in ONE laneBut bridges has more than one laneIt’s almost impossible to have maximum load effect on ALL lanes at the same timeThe more lanes you have, the lesser chance that all will be loaded to maximum at the same timethe same time

Multiple Presence of LLMultiple Presence of LLWe take care of this by using Multiple Presence FactorMultiple Presence Factor1.0 for two lanes and less for 3 or more lanesNumber of Multiple This is already included (indirectly) into the GDF Tables in AASHTO code so we do not

Loaded Lanep

Presence Factor “m”

in AASHTO code so we do notneed to multiply this againUse this only when GDF is d i d f h l i

1 1.20

2 1.00determined from other analysis (such as from the lever rule, computer model, or FEM)

3 0.85

> 3 0.65 p ) 3 0.65

Distribution of LL to GirdersDistribution of LL to GirdersA bridge usually have more than one girder so the question arise on how t di t ib t th l l d t th i dto distribute the lane load to the girders

Two main methodsTwo main methodsUsing AASHTO’s table: for typical design, get an approximate (conservative) value(conservative) value

No need to consider multiple presence factor

Lane MomentL Sh

Girder MomentGi d Sh

Distribution FactorDF

Lane Shear Girder Shear

Refined analysis by using finite element methodNeed to consider multiple presence factorp p

AASHTO Girder Distribution FactorAASHTO Girder Distribution FactorDFs are different for different kinds of superstructure systemDFs are different for interior and exterior beam

roadway widthroadway width

Exterior Exterior

DFs are available for one design lane and two or more design lanes (the

Interior

DFs are available for one design lane and two or more design lanes (the larger one controls)Must make sure that the bridge is within the range of applicability of the g g pp yequation

AASHTO Girder Distribution FactorAASHTO Girder Distribution FactorFactors affecting the distribution factor includes:

Span Length (L)Girder Spacing (S)M d l f l i i f b d d k Modulus of elasticity of beam and deck Moment of inertia and Torsional inertia of the sectionSl b Thi k (t )Slab Thickness (ts)Width (b), Depth (d), and Area of beam (A)Number of design lanes (N )Number of design lanes (NL)Number of girders (Nb)Width of bridge (W)Width of bridge (W)

DFDFFor AASHTO method first we must identify first we must identify the type of superstructure ( b & d k (support beam & deck types)

DFDFTypes (Continued)(Continued)

DFMDFM

Distribution factor for moment in Interior moment in Interior Beams

DFMDFM

Distribution factor for moment in Interior moment in Interior Beams (continued)

DFMDFM

Distribution factor for moment in Exterior moment in Exterior Beams

DFVDFV

Distribution factor for shear in Interior Beams shear in Interior Beams

DFVDFV

Distribution factor for shear in Exterior Beamsshear in Exterior Beams

GDF – Finite Element AnalysisGDF Finite Element Analysis

Bridge Model

(a)( )

(b)(b)

(c)

Boundary (Support) Conditions

GDF – Finite Element AnalysisGDF Finite Element Analysis

33

1 2

3

1 2

3

Load distribution in model

Moment and Shear in Typical GirderMoment and Shear in Typical Girder

At any section, if not using AASHTO’s GDFMLL+IM, Girder = DFM×(Mtruck/tadem,Lane×IM + Muniform,Lane )×mVLL+IM, Girder = DFV×(Vtruck/tadem,Lane×IM + Vuniform,Lane )×m

At any section, if using AASHTO’s GDFMLL+IM, Girder = DFM×(Mtruck/tadem,Lane×IM + Muniform,Lane )VLL+IM, Girder = DFV×(Vtruck/tadem,Lane×IM + Vuniform,Lane )

Placed such that we have maximum effectseffects

Live L d Place them Increase the Moment/ Shear Loads (Truck, Tandem

Place them to get maximum static

Increase the static load by IM to account for dynamic

Multiply by DF

Moment/ Shear from Live Load to be used in the d i f i dand Lane

Loads)

static effects

for dynamic effects

ydesign of girders

OutlineOutline





TandemUniform Load


O h L d



FatigueW d


WindEarthquake


…


O h L dOther Loads

FatigueWind

EarthquakeEarthquakeVehicle/ Vessel Collision

Fatigue LoadgRepeated loading/unloading of live loads can cause fatigue in bridge componentsFatigue load depends on two factors

Magnitude of LoadUse HS-20 design truck with 9m between 145 kN axles for determination f i ff f l dof maximum effects of load

Frequency of Occurrence:U ADTT = d il t k t ffi i i l lUse ADTTSL = average daily truck traffic in a single lane

Fatigue LoadFatigue Load

ADTAverage Daily Traffic (All Vehicles/ 1 Direction)From Survey (and extrapolate Class of Hwy % of Truck

Table C3.6.1.4.2-1

From Survey (and extrapolate to future)Max ~ 20,000 vehicles/day

y

Rural Interstate 0.20

Urban Interstate 0.15

% of Truckin Traffic

Other Rural 0.15

Other Urban 0.10

ADTTAverage Daily Truck Traffic Table 3.6.1.1.2-1

Number of LanesAvailable to Trucks

p

g y(Truck Only/ 1 Direction)

Fraction of Truck Traffic in a 1 1.00

2 0.85ADTT

Fraction of Truck Traffic in a Single Lane (p)

3 or more 0.80ADTTSLAverage Daily Truck Traffic(Truck Only/ 1 Lane)

Wind LoadWind LoadHorizontal loads For small and low bridges, wind

l d t i ll d t t l th There are two types of wind loads on the structure

WS = wind load on structure

load typically do not control the designFor longer span bridge over WS = wind load on structure

Wind pressure on the structure itself

For longer span bridge over river/sea, wind load on the structure is very important

WL = wind on vehicles on bridgeWi d h

Need to consider the aerodynamic effect of the wind on the structure Wind pressure on the

vehicles on the bridge, which the load is transferred to the

wind on the structure (turbulence) wind tunnel teststhe load is transferred to the

bridge superstructureWind loads are applied as static

Need to consider the dynamic effect of flexible l b id d h horizontal load long-span bridge under the wind dynamic analysis

Wind Loads (WS, WL)Wind Loads (WS, WL)

WL

WS(on Superstructure)

WS(on Substructure)

Wind LoadWind Load

T N B id (T W hi USA)Tacoma Narrows Bridge (Tacoma, Washington, USA)The bridge collapsed in 1940 shortly after completion under wind speed lower than the design wind speed but at a frequency near the natural frequency of g p q y q ythe bridgeThe “resonance” effect was not considered at the time

Earthquake Load: EQEarthquake Load: EQHorizontal load

The magnitude of earthquake is characterized by return periodLarge return period (e g 500 years) strong earthquakeLarge return period (e.g. 500 years) strong earthquakeSmall return period (e.g. 50 years) minor earthquake

For large earthquakes (rarely occur), the bridge structure is allowed to suffer significant structural damage but must not collapseF ll th k ( lik l t ) th b id h ld till b i For small earthquakes (more likely to occur), the bridge should still be in the elastic range (no structural damage)

Earthquake must be considered for structures in certain zones

Analysis for earthquake forces is taught in Master level courses

Earthquake Load: EQEarthquake Load: EQThe January 17, 1995 Kobe

th k h d it i t i ht earthquake had its epicenter right between the two towers of the Akashi-Kaikyo Bridgey gThe earthquake has the magnitude of 7.2 on Richter scaleThe uncompleted bridge did not have any structural damagesTh i i l l d l h The original planned length was 1990 meters for the main span, but the seismic event moved the but the seismic event moved the towers apart by almost a meter!

Earthquake Load: EQEarthquake Load: EQ Water Loads: WAWater Loads: WATypically considered in the design of substructures (foundation, piers, b t t)abutment)

Water loads may be categorized into:Static Pressure (acting perpendicular to all surfaces)Static Pressure (acting perpendicular to all surfaces)Buoyancy (vertical uplifting force)Stream pressure (acting in the direction of the stream)Stream pressure (acting in the direction of the stream)

Loads depend on the shape and size of the substructure

Vehicular Collision Force: CTVehicular Collision Force: CTBridge structures are very vulnerable to

hi l lli ivehicle collisionsWe must consider the force due to vehicle collision and designed for itvehicle collision and designed for it

Vehicular Collision Force: CTVehicular Collision Force: CTTypically considered in the design of substructures (foundation, piers, b t t)abutment)

The nature of the force is dynamic (impact), but for simplicity, AASHTO allows us to consider it as equivalent static loadallows us to consider it as equivalent static load.

Need to consider if the structures (typically pier or abutment) are not Need to consider if the structures (typically pier or abutment) are not protected by either:

EmbankmentCrash-resistant barriers 1.37m height located within 3 mAny barriers of 1.07 m height located more than 3 m

For piers and abutment located within 9 m from edge of roadway or 15 m from the centerline of railway track

A i l i f f 1800 kN i h i ll 1 2 Assume an equivalent static force of 1800 kN acting horizontally at 1.2 m above ground

Vehicular Collision Force: CTVehicular Collision Force: CT

No protection to the bridge structure

Better protection (still not sufficient)

Vessel Collision: CVVessel Collision: CV

No protection to the bridge piers

Bridge piers are protected

OutlineOutline





TandemUniform Load


O h L d



FatigueW d


WindEarthquake


…


AASHTO LRFD D iAASHTO LRFD Designs

IntroductionDesign CriteriaLoad MultiplierLoad Multiplier

Load Factor and Load CombinationsR i FResistance Factors

Historical Development of AASHTO CodeHistorical Development of AASHTO CodeThe first US standard for bridges in was published in 1931 (AASHO)

Working stress design (WSD), based on allowable stressesNow call “Standard Specifications”

Work on the new code bagan in 1988 93 Work on the new code bagan in 1988-93 1st edition of AASHTO LRFD Specifications was published in 1994, the 2nd in 1998, 3rd in 2004 – as an alternative document to the Standard ,SpecificationBy 2007, only AASHTO LRFD method is allowed for the design of bridges in the USANow in 4th Edition

’ f ( O ) f S Thailand’s Department of Highway (DOH) still refers to Standard Specification but will eventually switch to LRFD Specifications

Changes of LRFD from Standard SpecificationsChanges of LRFD from Standard SpecificationsIntroduction of a new philosophy of safetyId tifi ti f f li it t t ( t th i f ti t t)Identification of four limit states (strength, service, fatigue, extreme event)Development of new load models (including new live load)Development of new load and resistance factorsDevelopment of new load and resistance factorsRevised techniques for the analysis and load distributionNew shear design method for plain, reinforced and prestressed concreteg p pIntroduction of limit state-based provisions for foundation designRevised load provisions

Hydraulics and scourEarthquakeShip collisionShip collision

Introduction of isotropic deck design processCommentary are now side-by-side with the standardCommentary are now side by side with the standard

Design CriteriaDesign CriteriaGeneral design criteria in AASHTO LRFD Code:

Factored Load ≤ Factored Resistance

∑ ηγiQi ≤ ΦRn

Load Multiplierη = ηIηDηR

Load Factor

R F

Nominal Resistanceη ηIηDηR

Nominal Load Effect Resistance Factor

Load and resistance factors serve as partial safety factorsLoad and resistance factors serve as partial safety factorsThey are determined using the code calibration procedure

Design CriteriaDesign Criteria

LOAD RESISTANCEMean Load

Mean Resistance

N i l Nominal

Resistance

Nominal Load

FAILUREFactored Factored Factored Load

Factored Resistance

L d M l i liLoad Multiplier

η = ηI ηD ηRη ηI ηD ηR

Load MultiplierLoad MultiplierηI = Importance factorThe owner may declare a bridge or any structural component and connection to be of operational importance.For strength and extreme event limit statesFor strength and extreme event limit states

1.05 for bridge considered of operational importance e.g. the only bridge crossing the river1.00 for typical bridges0.95 for bridge considered nonimportant

For all other limit states1.00 for all bridges

Load MultiplierLoad MultiplierηD = Ductility factor (Brittle v.s. Ductile failure)The structural system shall be proportioned and detailed to ensure the development of significant and visible inelastic deformations at the strength and extreme event limit states before failurestrength and extreme event limit states before failure.For strength limit states

1.05 for nonductile components & connection which may fail in a brittle p ymanner 1.00 for conventional designs0 95 f i h h d d ili h ddi i l i f 0.95 for components with enhanced ductility e.g. has additional stirrups for shear reinforcements

For all other limit statesFor all other limit states1.00

Load MultiplierLoad MultiplierηR = Redundant factorMultiple load path and continuous structures should be used. Main elements whose failure is expected to cause the collapse of the bridge shall be designated as failure-critical (nonredundant)shall be designated as failure critical (nonredundant)For strength limit states

1.05 for nonredundant members e.g. a simple span bridgesg p p g1.00 for conventional level of redundancy0.95 for exceptional level of redundancy e.g. multi-girder continuous beam b idbridge

For all other limit states1 001.00

L d F & Load Factor & L d C bi iLoad Combinations

γγi

Loads & ProbabilitiesLoads & Probabilities

How do we apply all the loads for the structural analysis?Add all the mean (average) value of loads together?

No, because we must consider the chance that the load may be larger or smaller than calculatedsmaller than calculated.

Add all the extreme value of loads together?No, because then the bridge must have to resist an enormous load and No, because then the bridge must have to resist an enormous load and that would make it really expensive!The chance that the maximum value of one load occurring at the same i h i l f h l d i lltime as the maximum value of another load is very small.

We need to consider several cases where each case we have one load at its maximum value expected while other loads are around their at its maximum value expected while other loads are around their mean values

Loads & ProbabilitiesLoads & ProbabilitiesLoad factors are d t i d th t f determined so that, for each factored load, the probability of being p y gexceeded is about the same for all load

tcomponents.

Limit StatesLimit StatesThere are 4 types of “limit states”

Ulti t li it t t i l i th t th d t bilit f th t t Ultimate limit states – involving the strength and stability of the structure, both local and global

Strength I, II, III, IVg , , ,Extreme Event limit states - relates to the structural survival of a bridge during a major earthquake, flood, or collision

E E I IIExtreme Event I, IIServiceability limit states – involving the usability of the structure including stress, deformation, and crack widthsstress, deformation, and crack widths

Service I, II, IIIFatigue limit state - relates to restrictions on stress range to prevent crack growth as a result of repetitive loads during the design life of the bridge

FatigueAll limit states are equally important (AASHTO LRFD 1 3 2 1)All limit states are equally important (AASHTO LRFD 1.3.2.1)

Permanent LoadsPermanent LoadsDC = dead load of structural components and nonstructural attachmentsDW = d d l d f i f d tilitiDW = dead load of wearing surface and utilitiesEL = accumulated locked-in force effects resulting from the

construction processpDD = downdragEH = horizontal earth pressure loadES = earth surcharge loadEV = vertical pressure from dead load of earth fill

Transient LoadsTransient LoadsLL = vehicular live load CR = creepIM = vehicular dynamic load allowancePL = pedestrian live load

SH = shrinkageFR = frictionTG diPL = pedestrian live load

LS = live load surchargeBR = vehicular braking force

TG = temperature gradientTU = uniform temperatureWA = t l d d t BR = vehicular braking force

CE = vehicular centrifugal forceCT = vehicular collision force

WA = water load and stream pressureIC = ice loadCT vehicular collision force

CV = vessel collision forceEQ = earthquake

IC ice loadWL = wind on live loadWS = wind load on structureEQ earthquake WS wind load on structureSE = settlement

Load CombinationsLoad Combinations Load Factors for DC, DW Consider MaximumLoad Factors for DC, DWcase for Gravity load designs

Load CombinationsLoad CombinationsWhy do we need to have minimum and maximum load factors for

t l d ? I ’t it f t id th i ???permanent loads? Isn’t it safer to consider the maximum???Because in some cases, large permanent load can help reduce the force in the structureforce in the structure.

If the gravity load If the gravity load

of the

Gravity Load

g yis small, this may be in tension. If

the pier is

superstructure is large, it offsets the wind load and we the pier is

concrete, it will crack!!!

wind load and we will get small

compression here

U d Hi h May be in Tension or Light C iUnder High

CompressionCompression

Load CombinationsLoad CombinationsSTRENGTH I: Basic load combination relating to the normal use of bridge. M i bi ti i d h LL d th ff t DC Maximum combination is used when LL produces the same effect as DC. Minimum combination is used when LL produces opposite effect to DC.STRENGTH II: load combination for special vehicles specified by ownerSTRENGTH II: load combination for special vehicles specified by ownerSTRENGTH III: load combination where the bridge is subjected to high wind (> 90 km/h) and traffic is prevented( ) pSTRENGTH IV: load combination for long span bridges (>67 m span) which has large ratio of DC to LLSTRENGTH V: load combination where bridge and traffic on the bridge is subjected to wind velocity of 90 km/h

Load CombinationsLoad CombinationsEXTREME EVENT I: load combination for structural survival under major earthquakeearthquakeEXTREME EVENT II: load combination for structural survival under combination of events such as flood and vessel collision

SERVICE I: load combination for normal operation of the bridge and for checking compression in prestressed concretechecking compression in prestressed concreteSERVICE II: load combination for steel bridges to control yieldingSERVICE III: load combination relating to tension in prestressed concrete SERVICE III: load combination relating to tension in prestressed concrete during service

FATIGUE: load combination for fatigue and fracture due to repetitive LL and IM


Example of combinations:1.25DC + 1.50DW + 1.75(LL+IM) (Strength I)1.25DC + 1.50DW + 1.4WS (Strength III)0.90DC + 0.65DW + 1.4WS (Strength III)1.50DC + 1.50DW (Strength IV)1.25DC + 1.50DW + 1.35(LL+IM) + 0.4(WS+WL) (Strength V)1.25DC + 1.50DW + 0.5(LL+IM) + 1.0EQ (Extreme I)0 90DC 0 65DW 0 5(LL IM) 1 0EQ (E I)0.90DC + 0.65DW + 0.5(LL+IM) + 1.0EQ (Extreme I)1.25DC + 1.50DW + 0.5(LL+IM) + 1.0 (CT or CV) (Extreme I)0 90DC + 0 65DW + 0 5(LL+IM) + 1 0 (CT CV) (E t I)0.90DC + 0.65DW + 0.5(LL+IM) + 1.0 (CT or CV) (Extreme I)


For slabs and girders designs, we normally have only DC, DW, and (LL+IM)

1.25DC + 1.50DW + 1.75(LL+IM) (Strength I)1.50DC + 1.50DW (Strength IV)

1.00DC + 1.00DW + 1.00(LL+IM) (Service I)1.00DC + 1.00DW + 1.30(LL+IM) (Service II, Steel)1 00DC 1 00DW 0 80(LL IM) (S i III P d)1.00DC + 1.00DW + 0.80(LL+IM) (Service III, Prestressed)

Notes on Load CombinationsNotes on Load Combinations

Note that the sections for maximum moment of dead load and live load are not the same!!!

Dead Load: midspanLive Load: some small distance away from midspan

If we add them together, we are conservative!

Critical moment for shear is d away from the support. We C t ca o e t o s ea s d away o t e suppo t. e can calculate shear at this location for both dead load and live load IF we know the height of the sectiong

We estimate the height from past experiences of similar projectsIf we don’t know, we calculate shear at the support. This is ppconservative but may not be economical.

R i FResistance Factors

ΦΦ

Resistance and ProbabilitiesResistance and ProbabilitiesResistance factor is d t i d th t th determined so that the reliability index, β, is close to the target gvalue, βT (about 3.5)

Resistance FactorsResistance FactorsResistance factors are different for different types of action (moment or h f l ) d f diff t t f t i l ( t l shear, for example) and for different types of materials (steel or

concrete). They are specified under each section of materials.

Types Φ

Concrete Structures

Types ΦFlexure and Tension

in Reinforced Concrete 0 90in Reinforced Concrete in Prestressed Concrete

0.901.00

Shear in Normal Weight Concrete 0.90g

Axial Compression 0.75

Bearing on Concrete 0.70

Resistance FactorsResistance Factors

S l S

Types Φ

Steel Structures

Flexure 1.00

Shear 1.00

Axial Compression (steel or composite) 0.90

Block shear 0.80

TensionYielding limit stateFracture limit state

0.950 80Fracture limit state 0.80

loads on bridges design lane aashto hl93 loads fatigue ... · pdf filelive loads of vehicles:...

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