006 Sesimic

Course # CE-5120

03 Credit Hours

Lecture # 05

Introduction to Bridge Engineering

Bridge EngineeringEmail: [email protected]&19-May-13 2Dr. Syed Mohammad Ali

Sequence of Presentation

• Basic Information about Earthquakes

• Fatigue Loads

• Multiple Presence

• Dynamic Effect

• Centrifugal forces

• Braking Force

• Fluid Forces

• Seismic Loads

• Ice Loads

• Forces due to Deformation

• Seismic Design Procedure (generalized steps)

mailto:[email protected]


Earthquakes

• What are Earthquakes?

• An earthquake (also known as a tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. [Wikipedia]

• The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. [Wikipedia]

• An earthquake is a shaking of the ground caused by the sudden breaking and movement of large sections (tectonic plates) of the earth's rocky outermost crust. The edges of the tectonic plates are marked by faults (or fractures). Most earthquakes occur along the fault lines when the plates slide past each other or collide against each other. [Wiki]

• An earthquake is the sometimes violent shaking of the ground caused by movements of Earth's tectonic plates. Most earthquakes occur along fault lines, which is where two tectonic plates come together.



Work Tectonic Plate Map

Earthquakes



Earthquakes


Waqas Ahmed

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The Ring of Fire is an area where a large number of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean. In a 40,000 km (25,000 mi) horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. It has 452 volcanoes and is home to over 75% of the world's active and dormant volcanoes.[1] It is sometimes called the circum-Pacific belt.


Earthquakes

• Seismic Waves?

Seismic Waves

Body Waves Surface Waves

P Waves S Waves

Love (L) Waves Rayleigh (R) Waves


Waqas Ahmed

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Surface Waves Large amplitude Long wavelength Wide range of frequencies (large bandwidth) Dispersive Travel slowly Not produced by deep earthquakes Body Waves Small amplitude Short wavelength Narrow frequency band Travel more quickly Produced by all earthquakes


Earthquakes

• Seismic Waves?

• BODY WAVES

• Primary (they arrive first), Pressure, or Push-Pull. Material expands and contracts in volume and particles move back and forth in the path of the wave.



Earthquakes

• Seismic Waves?

• Shear, secondary, rotational, tangential, distortional, transverse, or shake wave.

• Material does not change volume but shears out of shape and snaps back. Particle motion is at right angles to the path of the wave.



Earthquakes



Earthquakes

• Seismic Waves?

• SURFACE WAVES

• Two main types. Love & Rayleigh.

• Slower than body waves; rolling and side-to-side movement.

• Cause most of the damage during earthquakes

• Travel only in the shallow portions of the Earth



Earthquakes

• Seismic Waves?

• SURFACE WAVES : Rayleigh Waves

• Ocean waves are a type of surface wave (known as a Rayleigh wave).



Earthquakes

• Seismic Waves?

• SURFACE WAVES : Rayleigh Waves




Earthquakes

• Seismic Waves?

• SURFACE WAVES : Love Waves



http://www.exploratorium.edu/faultline/earthquakescience/eqscience4.html#rayleigh

http://www.exploratorium.edu/faultline/earthquakescience/eqscience4.html#rayleigh


Earthquakes

Wave type Typical Velocities

Compressional8-11 km/sec

480-660 km/hr

Shear5-7 km/sec

300-420 km/hr

Love3.5-4.5 km/sec

210-270 km/hr

Rayleigh3-4 km/sec

180-240 km/hr



Earthquakes



Earthquakes

• FAULTS:


Waqas Ahmed

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Faults are fractures that have appreciable movement parallel to their plane. They are produced usually by seismic activity.


Earthquakes

• FAULTS:



Earthquakes

• FAULTS:


http://www.exploratorium.edu/faultline/earthquakescience/eqscience3.html

http://www.exploratorium.edu/faultline/earthquakescience/eqscience3.html

Waqas Ahmed

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The hanging wall has moved down relative to the footwall. Associated with tensile stresses in crust.

Waqas Ahmed

Sticky Note

In reverse fault, the hanging wall moves upward relative to foot wall. Dip angle is generally more than 45o. Associated with compressive stresses in crust

Waqas Ahmed

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Movement occurs parallel to strike. Associated with shear stresses in crust


Earthquakes

• FAULTS:

A Normal dip slip fault



Earthquakes

• EQ Magnitude Vs Intensity:

Intensity

How Strong Earthquake Feels to Observer

Qualitative assessment of the

kinds of damage done by an

earthquake

Depends on distance to earthquake & strength of earthquake

Determined from the intensity of shaking and damage from the earthquake

Magnitude

Related to Energy Release.

Quantitative measurement of the amount of energy released by an earthquake

Depends on the size of the fault that breaks

Determined from Seismic Records



Earthquakes


Intensity

How Strong Earthquake Feels to Observer

Depends On:

Distance to Quake

Geology

Type of Building

Observer!

Varies from Place to Place

Modified Mercalli Scale- 1 to 12


Waqas

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Earthquakes




Earthquakes


Earthquake Magnitude

1. ML - Local (Richter) magnitude

2. MS - Surface wave magnitude

3. MB- Body wave magnitude

4. MW & Mwp- Seismic Moment magnitude



Earthquakes

• EQ Magnitude Vs Intensity:• Richter Scale

• It is a measure of the amplitude of ground vibration using a seismometer.

• Amplitude scale is logarithmic (10-fold increase for every whole number increase)

• With the seismometer a standard distance of 100 km away from the epicenter, one gets:

Amplitude

(mm)

1

10

100

1000

10000

Magnitude

3

4

5

6

7


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Earthquakes


Drawbacks:

• Based on Antiquated Wood-Anderson Seismographs

• Measurement Past Magnitude 7.0 ineffective – Requires Estimates


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Earthquakes




AASHTO LRFD Loading

• What are Earthquakes?

• Different types of Magnitude Scales for Earth Quake ?

• 2) PGA is horizontal or Vertical Ground Motion ?

• 3) The earth quake magnitude value is at the ground surface or deep down the earth(Focus) ?

• 4) Why tectonic plates move ?

• 5) Why the Earth Quakes increased as compare to previous centuary ?

• 6) Approximate range of depth of Earth Quakes ?



AASHTO LRFD Loading

• Fatigue Loads

• ASTM defines fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs

• The strengths of various components of the bridge are sensitive to repeated

stressing or fatigue

• When the load is cyclic, the stress level that ultimately fractures the material can be significantly below the nominal yield strength

• The fatigue strength is typically related to the range of live-load stress and the number of stress cycles under service load conditions

High Cycle Fatigue Low Cycle Fatigue

Stress Controlled Strain Controlled

More than 1,000 cycles Less than 1,000 cycles

Crack has not started When will crack start?

Facilitates design Not easy to use in design


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stress controlled means within elastic range and strain control means elasto plastic


AASHTO LRFD Loading

In high-cycle fatigue situations, materials performance is commonly characterized by an S-N curve, also known as a Wöhler curve.


http://en.wikipedia.org/wiki/August_W%C3%B6hler

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AASHTO LRFD Loading

• Fatigue Loads

• As the majority of trucks do not exceed the legal weight limits, it would be unduly conservative to use the full live load model, which is based on exclusion vehicles to estimate this load effect

• This means that a lesser load is used to estimate the live-load stress range and is accommodated by using a single design truck with the variable axle spacing set at 30 ft (9000 mm) and a load factor of 0.75 the design truck (Nowak, 1993); hence a load factor of 0.75 is used.


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AASHTO LRFD Loading

• Fatigue Loads

• The number of stress–range cycles is based on traffic surveys.

• The average daily truck traffic (ADTT) in a single lane may be estimated as:

• where p is the fraction of traffic assumed to be in one lane


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AASHTO LRFD Loading

• Fatigue Loads

• The ADTT is usually available from the bridge owner, but in some cases only the average daily traffic (ADT) is available.

• In such cases, the percentage of trucks in the total traffic must be estimated. This percentage can vary widely with local conditions, and the engineer should try to estimate this with a survey.

• If survey data are not possible or practical, or if the fatigue limit state is not a controlling factor in the design, then AASHTO provides guidance.

Hayatabad, Peshawar Data 2010


Class


AASHTO LRFD Loading

• MULTIPLE PRESENCE

• Trucks will be present in adjacent lanes on roadways with multiple design lanes, but it is unlikely that three adjacent lanes will be loaded simultaneously with the heavy loads.

• Therefore, some adjustments in the design loads are necessary. To account for this effect, AASHTO [A3.6.1.1.2] provides an adjustment factor for the multiple presence



AASHTO LRFD Loading

• MULTIPLE PRESENCE (PCPHB 1967)

• Section 2.7 “Reduction in Load Intensity”

S.# Case %

1 One or two truck train loading 100

2 Three truck train loadings 90

3 Four or more 75



AASHTO LRFD Loading

• DYNAMIC EFFECTS

• The roadway surface is not perfectly smooth, thus the vehicle suspension must react to roadway roughness by compression and extension of the suspension system

• This oscillation creates axle forces that exceed the static weight during the time the acceleration is upward and is less than the static weight when the acceleration is downward.

• Although commonly called impact, this phenomenon is more precisely referred to as dynamic loading.

• Various bridge engineering design specifications from around the world use widely differing factors.



AASHTO LRFD Loading

• DYNAMIC EFFECTS

• The ordinate axis represents the load increase or dynamic load allowance (DLA) and the abscissa is the fundamental frequency of the structure.


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AASHTO LRFD Loading

• DYNAMIC EFFECTS

• It is most common to compare the static and dynamic deflections as illustrated in Figure

• Where Dsta is the maximum static deflection and Ddyn is the additional deflection due to the dynamic effects.


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AASHTO LRFD Loading

• DYNAMIC EFFECTS

• The dynamic effect is defined herein as the amplification factor applied to the static response to achieve the dynamic load effect.

• This effect is called by many different terms:

– dynamic load factor,

– dynamic load allowance (DLA), and

– impact factor (IM)

• The term dynamic load allowance is used by AASHTO, which is abbreviated IM (for impact).


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AASHTO LRFD Loading

• DYNAMIC EFFECTS

• The principal parameters that affect the impact factor are:

– the dynamic characteristics of the truck,

– the dynamic characteristics of the bridge, and

– the roadway roughness

• These characteristics are expected as all transient structural dynamic problems involve stiffness, mass, damping, and excitation.


Waqas

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load, axle spacing etc no of girders, depths etc joints etc


AASHTO LRFD Loading

• DYNAMIC EFFECTS



AASHTO LRFD Loading

• DYNAMIC EFFECTS Study HMC Bridge (2008)

• Span 12.5 m, RC Bridge, 5 girders



AASHTO LRFD Loading

• DYNAMIC EFFECTS Study HMC Bridge




AASHTO LRFD Loading

• DYNAMIC EFFECTS Study at HMC Bridge




AASHTO LRFD Loading

• DYNAMIC EFFECTS



AASHTO LRFD Loading

• CENTRIFUGAL FORCES

• Acceleration is the time derivative of the velocity vector and as such results from either a change of magnitude or direction of velocity.

• A truck can increase speed, decrease speed, and/or change directions as it moves along a curvilinear path. All of these effects require an acceleration of the vehicle that causes a force between the deck and the truck.

• Because its mass is large compared to the power available, a truck cannot increase its speed at a rate great enough to impose a significant force on the bridge.

• Conversely, a decrease in speed due to braking can create a significant acceleration (deceleration) that causes large forces on the bridge in the direction of the truck movement.

• Finally, as a truck moves along a curvilinear path, the change in direction of the velocity causes a centrifugal acceleration in the radial direction.



AASHTO LRFD Loading

• CENTRIFUGAL FORCES

• The position of this force is at the center of mass, assumed to be at 6 ft (1800 mm) above the roadway surface [A3.6.3].



AASHTO LRFD Loading

• BRAKING FORCES

• Braking forces can be significant.

• Such forces are transmitted to the deck and must be taken into the substructure at the fixed bearings or supports.

• It is quite probable that all truck operators on a bridge will observe an event that causes the operators to apply the brakes.

• Thus, loading of multiple lanes should be considered in the design.

• Again, it is unlikely that all the trucks in all lanes will be at the maximum design level, therefore the multiple presence factors outlined previously may be applied


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AASHTO LRFD Loading

• BRAKING FORCES

• In the development of the AASHTO braking force fraction, it was assumed that the truck is moving at a velocity of 55 mph (90 km/h) = 80 ft/s (25 m/s) and a braking distance of 400 ft (122 000 mm) is required.

• The braking forces shall be taken as 25% of the axle weights of the design truck or the tandem truck placed in all lanes [A3.6.4].

• The design lane is not included as it is assumed that the additional trucks “brake out of phase.”

• Thus, the operators of the additional design lane trucks will pump their brakes and will not decelerate uniformly. This pumping action is assumed to occur at times different from when the design truck is at a maximum.

• Also implicit in the AASHTO value is that the coefficient of friction exceeds 0.25 for the tire–deck interface.

• The braking force is assumed to act horizontally at 6 ft (1800 mm) above the roadway surface in either longitudinal direction.


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Lateral Loads

• Lateral loads are:

– Fluid forces

– Seismic Loads

– Ice Forces

• Fluid forces

– Water forces and

– Wind forces

• The force on a structural component due to a fluid flow (water or air) around a component is established by Bernoulli’s equation in combination with empirically established drag coefficients


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Lateral Loads

• WIND FORCES

• The velocity of the wind varies with the elevation above the ground and the upstream terrain roughness, and therefore pressure on a structure is also a function of these parameters.

• Velocity increases with elevation, but at a decreasing rate.

• If the terrain is smooth, then the velocity increases more rapidly with elevation. A typical velocity profile is illustrated in the figure



Lateral Loads

• WIND FORCES

• The parameter Vg is the geotropic velocity or the velocity independent of surface (boundary) effects,

• δ is the boundary layer thickness, usually defined as the height where the velocity of 99% of Vg , and

• V30 is the reference velocity at 30 ft.

• Traditionally, this is the height at which wind velocity data is recorded. Since its introduction in 1916, the velocity profile has been modeled with a power function of the form

• Where C and α are empirically determined constants



Lateral Loads

• WATER FORCES

• Water flowing against and around the substructure creates a lateral force directly on the structure as well as debris that might accumulate under the bridge.

• Flood conditions are the most critical.

• The forces created are proportional to the square of velocity and to a drag coefficient.

• Here CD is the drag coefficient given in Table 4.10, and

• V is the design velocity of the water for the design flood in strength and service limit states, and for the check flood in the extreme event limit state [ft/s (m/s)]. Note that CD is the specific AASHTO value and Cd is a generic term.



Lateral Loads

• WATER FORCES



Lateral Loads

• Seismic Loads

• Depending on the location of the bridge site, the anticipated earthquake effects can be inconsequential or they can govern the design of the lateral load resistance system.



Lateral Loads

• Seismic Loads

• The AASHTO Specifications have been developed to apply to all parts of the United States, so all bridges should be checked to determine if seismic loads are critical.

• In many cases the seismic loads are not critical and other lateral loads, such as wind, govern the design

• The provisions of the AASHTO Specifications are based on the following principles [C3.10.1]:

• ❑ Small-to-moderate earthquakes should be resisted within the elastic range of the structural components without significant damage.

• ❑ Realistic seismic ground motion intensities and forces are used in the design procedures.

• ❑ Exposure to shaking from large earthquakes should not cause collapse of all or part of the bridge. Where possible, damage should be readily detectable and accessible for inspection and repair.



Lateral Loads

• Seismic Loads

• The AASHTO provisions apply to bridges with conventional slab, girder, box girder, and truss superstructures whose spans do not exceed 500 ft (150 m) [A3.10.1].

• Bridges with spans exceeding 500 ft (150 m) and other bridge types, such as suspension bridges, cable-stayed bridges, movable bridges, and arches, are not applicable.



Lateral Loads

• ICE FORCES

• Forces produced by ice must be considered when a structural component of a bridge, such as a pier or bent, is located in water and the climate is cold enough to cause the water to freeze.

• The usual sequence is that freeze-up occurs in late fall, the ice grows thicker in the winter and the ice breaks up in the spring.

• If the bridge is crossing a lake, reservoir, harbor, or other relatively quite body of water, the ice forces are generally static. These static forces can be horizontal when caused by thermal expansion and contraction or vertical if the body of water is subject to changes in water level.

• If the bridge is crossing a river with flowing water, the static forces exist throughout the winter months, but when the spring break-up occurs, larger dynamic forces are produced by floating sheets of ice impacting the bridge structure.



Q & A

Thanks!


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