class toppjics & objectives - mahidolmucc.mahidol.ac.th/~egpcp/handout406/40610 - loads.pdf ·...

EGCE 406 Bridge DesignEGCE 406 Bridge Design

Loads on BridgeLoads on Bridge

Praveen Chompreda Mahidol UniversityFirst Semester, 2010

Class Topics & Objectivesp j Topics Objectives

Loads on Bridges Students can describe the various type of loads on a bridge

Placement of Live Loads for Maximum Effect

bridge Students can explain the effects

of live load position on the Maximum Effect pshear and moment within a bridge girderS d d i h Students can determine the maximum live load effect based on the design code

Distribution of loads within multi-girder bridge

on the design code Student can determine the

portions of dead load and live g gload to be assigned to a girder

Class Topics & Objectivesp j

Parts of the topics discussed in this class can be found in:

Chapter 4, especially 4.2p , p y Section 5.1-5.4

Section 14.6-14.9

OutlineOutline

Loads on Bridges Design Lane

Typical Loads Dead Load

AASHTO HL93 Loads Truck Tandem

Live Load Live Load of Vehicle

Tandem Uniform Load

LL Combinations Pedestrian Load Dynamic Load Allowance

O h L d

LL Combinations LL Placement

Influence Line Other Loads

FatigueW d

Design Equation Design Charts

Wind Earthquake

Multiple Presence Distribution to Girders

…

Loads on BridgeLoads on Bridge DD = downdrag (wind)

DC d d L d f BR = breaking force of vehicle CE = centrifugal force of vehicle (at curves) DC = dead Load of

structural and nonstructural components

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

piers) DW = dead load of wearing

surface EH = earth pressure

piers) CV = vessel collision force (bridge piers over

river) EQ = earthquake EH = earth pressure

(horizontal) EL = secondary forces such as

EQ = earthquake FR = friction IC = ice IM = d i l d f hi lfrom posttensioning

ES = earth surcharge load (vertical)

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

(vertical) EV = earth pressure (vertical)

PL = pedestrian load SE = settlement SH = shrinkage of concreteg TG = load due to temperature differences TU = load due to uniform temperature WA = water load/ stream pressure WA water load/ stream pressure WL = wind on vehicles on bridge WS = 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: DC Dead 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: DW It 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 Loadsy Dead loads are distributed to girder using Tributary Area method

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 midspan Maximum 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 bridge

There are several types of

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

vehicles Car

V

span length weight of vehicle

l l d (l d h l) Van Buses Trucks

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

Semi-Trailer Special 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 g stiffness of structural members

(slab and girders)

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

LL is very heavy (several tons per wheel)

LL is not very heavy, typical 200-500 kg/m2)

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

g LL is assumed to be uniformly

distributed within a spanof 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 lanes

N d t id l th l t f

placement of load within a span Loads 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 typically consider dynamic/impact effect of live loadsloads

Analysis Strategy for LL Effect in BridgeAnalysis Strategy for LL Effect in Bridge

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 Lane Need 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 m Number 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 to ½ of the roadway width

roadway width

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

t ff tworst effect AASHTO has 3 basic types of LL called the HL-93 loading (stands for

Highway Loading year 1993)Highway Loading, year 1993) Design truck Design tandem Uniform loads

1. Design Truck1. Design Truck The 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-20

Weights shown on the left are for each one axle = 2 wheels

Total Wt = 325 kN ~ 33 t Total Wt 325 kN 33 t. Distance between the second and

third axles may be varied to d i ffproduce maximum effect

Need to multiply this load by dynamic allowance factor (IM)dynamic allowance factor (IM)

2. Design Tandem2. Design Tandem Two axle vehicle with 110 kN

h lon each axle Need 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 ppbeam with span length less than 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 Loading Uniform 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 effect No 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 Combinations 3 ways to add the design truck, design tandem, and uniform load together

C b 1 HS20 k f f l l d d l Combination 1: one HS20 truck on top of a uniform lane load per design lane Combination 2: one Design Tandem on top of a uniform lane load per design

lane Combination 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 d The maximum effect of these 3 cases is used for the design

Live Load PlacementLive Load Placement

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

roadway width

Longitudinally (for design of main girder)

Live Load Placement - TransverseLive Load Placement Transverse The 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 overhang 60 cm from the edge of the design lane for the design of all other components 60 cm from the edge of the design lane for the design of all other components

min. 2'

Minimum distance from curb = 60 cm

Note that if the sidewalk is not separated by a crash-resistant traffic barrier, we must consider the case that vehicles can be on the sidewalkwe must consider the case that vehicles can be on the sidewalk

Live Load Placement - LongitudinalLive Load Placement Longitudinal Need to place the LL along the span such that it produces the maximum

ff teffect For simple supported beam with 1concentrated load, the maximum

moment occurs when the load is placed at the midspanmoment occurs when the load is placed 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 moment

REMEMBER: MAXIMUM MOMENT DOES NOT ALWAYS OCCURS AT MIDSPAN !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!AT MIDSPAN !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Live Load Placement - LongitudinalLive Load Placement Longitudinal

Methods of finding maximum moment and shear in span Influence Line (IL) – Simple and Continuous spans Design Equation – Simple span only Design Chart – Simple span only

Live Load Placement – Influence LineLive Load Placement Influence Line Influence 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 Line Influence line is a powerful visualization tool for the effects of live load

l t t th t t l placements to the structural response110 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 Line Müller-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


Notes Influence line tells you how to place the LL such that the maximum

moment at a point occurs; i.e. you first pick a point, then you try to find what is the maximum moment at that point when loads are find what is the maximum moment at that point when loads are moved around

It 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, nor its value; i.e. the maximum moment on the point you picked is not always the absolute maximum moment that can occur in h ( hi h ill diff i d d diff the span (which will occur at a different point and under a different

arrangement of loads)

Live Load Placement – Influence LineLive Load Placement Influence Line For 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 loads Can 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 Line Increase/ 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 span

As 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 Line Increase/ Decrease Method

For shear

Sloping Line Jump

∆V = Ps(x2-x1) ∆V = P (y2-y1)

For moment

Sloping Line

∆M = Ps(x2-x1)

IL for momenthas no jumps!

∆M = 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 Line Example

Live Load Placement – Influence LineLive Load Placement Influence Line Live Load Placement – Influence LineLive Load Placement Influence Line For 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 Equation If 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 Chart Design 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





Tandem Uniform Load


O h L d



FatigueW d


Wind Earthquake


…

Pedestrian Live Load: PLPedestrian Live Load: PL Use when has sidewalk wider

th 60 than 60 cm Considered simultaneously with

truck LLtruck LL

Pedestrian only: 3.6 kN/m2 Pedestrian only: 3.6 kN/m Pedestrian 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: IM Sources 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 potholes

Dynamic 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 unknowns

To 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: IM Add dynamic effect to the following loads:

D T k Design Truck Design Tandem

But NOT to these loads: But NOT to these loads: Pedestrian Load Design 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 lane But bridges has more than one lane It’s almost impossible to have maximum load effect on ALL lanes at the same time The 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 LL We take care of this by using

Multiple Presence FactorMultiple Presence Factor 1.0 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 again

Use 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 Girders A 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 methods Two main methods Using 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 method Need to consider multiple presence factorp p

AASHTO Girder Distribution FactorAASHTO Girder Distribution Factor DFs are different for different kinds of superstructure system DFs 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 Factor Factors 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 section

Sl 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)

DFDF For AASHTO method

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

DFDF Types

(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 Effects of girder

stiffness on the distribution factor

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 GDF MLL+IM, Girder = DFM×(Mtruck/tadem,Lane×IM + Muniform,Lane )×m VLL+IM, Girder = DFV×(Vtruck/tadem,Lane×IM + Vuniform,Lane )×m

At any section, if using AASHTO’s GDF MLL+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





Tandem Uniform Load


O h L d



FatigueW d


Wind Earthquake


…

O h L dOther Loads

FatigueWind

EarthquakeEarthquakeVehicle/ Vessel Collision

Fatigue Loadg Repeated loading/unloading of live loads can cause fatigue in

bridge components Fatigue load depends on two factors

Magnitude of Load Use 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 l Use 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 Load Horizontal 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 design

For 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 bridge Wi 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 superstructure Wind 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 bridge

The “resonance” effect was not considered at the time

Earthquake Load: EQEarthquake Load: EQ Horizontal load

The magnitude of earthquake is characterized by return period Large return period (e g 500 years) strong earthquake Large return period (e.g. 500 years) strong earthquake Small 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 collapse

F 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: EQ The 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 g

The earthquake has the magnitude of 7.2 on Richter scale

The 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: WA Typically 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: CT Bridge structures are very vulnerable to

hi l lli ivehicle collisions We must consider the force due to

vehicle collision and designed for itvehicle collision and designed for it

Vehicular Collision Force: CTVehicular Collision Force: CT Typically 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: Embankment Crash-resistant barriers 1.37m height located within 3 m Any 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

RecapRecap





Tandem Uniform Load


O h L d



FatigueW d


Wind Earthquake


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class toppjics & objectives - mahidolmucc.mahidol.ac.th/~egpcp/handout406/40610 - loads.pdf ·...

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