A062 NDT of steel materials steel bridge design

NDT of Steel Materials, Steel Bridge Design, and Evaluation as Per LRFR By Piya Chotickai tailieuxdcd@gmail.com... Outline Steel Bridge Design  Design consideration and limit state

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NDT of Steel Materials, Steel

Bridge Design, and Evaluation as Per LRFR

By Piya Chotickai

tailieuxdcd@gmail.com

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Outline

Steel Bridge Design

 Design consideration and limit state

 Design loads and load combination

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Steel Bridge Design

Basic design expression in AASHTO LRFD:

 ii Q i  R n

Where Q i = force effect, R n = nominal resistance,  I =

load factor,  = resistance factor,  I = load modification

factor

Load modification factor (I ) accounts ductility,

redundancy, and operation importance of the bridge

 I =  D R i

 D = 1.0 for conventional design

 R = 1.0 for conventional redundancy, 1.05 for

nonredundant members

 I = 1.0 for conventional bridge

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Limit state is a condition beyond which a bridge system

or bridge component ceases to fulfill the function for

which it is designed

Strength Limit State:

 Strength I  Normal vehicular use of the bridge

 Strength II  Owner-specified special design vehicle

(or permit vehicles) without wind

 Strength III  Bridge exposed to wind velocity > 90

km/hr High winds prevent the

presence of significant LL on bridge

Strength IV  High dead load to live load force effect

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Serviceability Limit State:

 Service I  Normal operational use of the bridge

with 90 km/hr wind and all loads taken

at their nominal values

 Service II  Control yielding of steel structures and slip of slip-critical connections due to

vehicular live load

Fatigue Limit State  Fatigue and fracture load

combination relating to repetitive gravitational vehicular

live load and dynamic responses under a single design

truck

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Load Combination and Load Factor

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Load Factor for Permanent Load

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Resistance Factor for Strength Limit State

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Structural response: Live load model and strain gage

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Obtain structural response from 1-D model with GDF or

3-D analysis model

GDF ~ 0.21-0.52 (Schilling, 1982)

AASHTO LRFD provided GDF equations obtained from

extensive finite element analyses

AASHTO LRFD: 30% Impact Factor

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Load Distribution

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Distribution of LL Per Lane for Moment in Interior Beams

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Deterioration of Steel Members

Fatigue: load induced fatigue, distortion induced fatigue

Source of flaws/defects: manufacturing, corrosion,

collision, poor details

Corrosion

Fe Fe2+ + 2e− 4e− + O2 + 2H2O 4OH−

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 Two electrochemical reactions are known as ‘anodic’ and

‘cathodic’ areas or simply anodes and cathodes:

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Potential differences can also occur in different areas of

a metal made up of only a single type of crystal, even

when the composition is uniform

The atoms at or near the grain boundaries within the

metal tend to be significantly more active than

corresponding atoms within the bulk of the grains, and

the grain boundary regions tend to become anodic

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Anodic and cathodic areas can be generated on the

surface of steel or other metals:

 Compositional variations from place to place

 Local differences in applied stress occur

 Variations in local environment conditions occur

Large electrical potential differences can result when two dissimilar metals are in contact, with one metal serving as the anode and the other as the cathode

The active areas may become anodes and corrode The less active sites will tend to act as cathodes

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The moving ferrous (Fe2+) and hydroxide (OH-) ions flow toward one another When they meet, they react to form

ferrous hydroxide, Fe(OH)2 This compound will react

further with additional hydroxide ions, and sometimes

also oxygen, to generate the insoluble product that we

see as rust

Two primary types of rust:

 Red rust (Hematite, Fe2O3)

 Black rust (Magnetite, Fe3O4)

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Through Crack at Cover Plate Crack at End Restrained

Slag Inclusion Degradation of Steel Members

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Distortion Induced

Fatigue

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Corrosion Problem

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Corrosion: Vulnerable areas are at connections where

the fixings (connections, fasteners) can corrode badly, at interactions with concrete or masonry and in the natural

water traps in open steel sections

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Paints

Commonly consisting of up to three coats with service

lives of 8-12 years in marine environment

Inhibition primer is designed to prevent deterioration

caused by moisture and oxygen

Red lead/white lead systems or Chromium green oxide

pigments

Generally required less surface preparation but toxic

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Sacrificial Primers:

Create a surface which is electrochemically negative in

relation to steel, resulting steel becoming completely

cathodic Zinc is the most common material used to

make the primer act as an anode

The expected life of protection is proportional to the

thickness of zinc coating

Zinc in a galvanized coating ( hot-dip) bonds

metallurgically to the steel and sacrifices itself

electrochemically to protect steel

Alternatively, molten zinc is sprayed at high pressure

onto cleaned steel surfaces and bonds mechanically with steel

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Water abrasive blasting: Water and abrasives are

air-blasted toward the steel surface

Optimum solution for steel surface preparation

(Removal interface material and soluble salts)

Drawbacks: Sludge-by-product creating problem for

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Measures Mechanism In case of deterioration

Painting Protection by paint Repainting

Weathering steel Protective rust layer Repair with painting

Hot dip galvanizing Protective layers by zinc and

alloys, and Sacrificial protection

Repair with painting

Metal spray Spray deposit and Sacrificial

protection by aluminum pseudo alloys

zinc-Repair with paining

Corrosion Prevention Method

** Surface preparation is crucial for service life of coating

• Condition of the Protective System:

- Most common deterioration in structural steelwork

- Immediate action should be taken to prevent the problem spreading  Spot cleaning and Overcoating

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Other Deteriorations and Degradations

Decay of Timber

Collision

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Inspection and Maintenance

Program

Require safe/cost effective/reliable evaluation and

inspection programs

Inspection Program:

Initial Inspection – First inspection of a bridge as it

becomes a part of the bridge file

Routine Inspection- Regularly scheduled inspections to identify any changes from previous/initial inspection

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In-Depth Inspection – To identify deficiencies not

normally detect during the routine inspection

Damage Inspection – To assess damage resulting

from environment or human actions Similar to depth inspection except that it is an unscheduled inspection manifested from reported damage

in-The key to the effective, safe performance of any bridge

inspection is proper advance planning and preparation

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Element Damage or Deterioration

Abutment, Foundation Evidence of scour especially for spread footings

Retaining Wall Evidence of horizontal or vertical movement of superstructure

and Pier Check for rotation of walls, lateral, and longitudinal shifting

Settlement relative to previous records

Steel Beams

and Girders

Determine locations of fracture critical members or fatigue details Check for fatigue cracks (typically begin near weld terminations) Loss of section due to rust by using ultrasonic thickness meters Check for out-of-plane bending in webs or connection plates Buckling in compression members

RC Beams, Girders Check for cracks Note the locations of the cracks and their size

and PC Determine possible cause of cracking:

(shrinkage, overstress, settlement, chemical reaction)

Check vertical and lateral movements relative to the substructure Timber Systems Examine for splitting, cracking and excessive deflection

Evidence of decay Check for loose of missing fasteners Decks Check for cracking, pot-holing, spalling

Check underside of the deck slab for indications of deterioration Examine for rutting and wear that may result in reduced skid resistance

Inspection Guideline

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Evaluation Program:

Field Testing (Nondestructive Testing, Strain

Gage Measurement, Deflectometer, Accelerometer and etc.)

Analytical Model or FEM Model

Nondestructive Evaluation: (for example)

Visual Inspection Strength Method

Dye Penetrant Testing Thermography

Eddy Current Testing Impact Echo Magnetic Particle Testing Ground Penetrating

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NDT technologies employ an analysis of stress wave

propagation to determine the properties of material

and identify the defects

Waves are generated using a variety of mechanical

and electrical sources such as hammers, vibrators,

and piezoelectric ceramics

Compression (P-primary) wave is faster than shear (S-secondary) wave

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Visual Inspection

Aids: light, magnifying glasses, borescopes

Pros: portable and cheap

Cons: surface cracks only

Impact Echo:

Use sonic echo to locate damage and determine

member thickness when access is available from one side only

Hit concrete with a small, steel-tipped

dynamic impulse hammer

Impact

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Impact Echo

Receiver Receiver

Impact

Crack Depth Measurement

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Dye Penetrant Testing

Based on the ability of liquid to be drawn into a

clean surface-breaking flaw by capillary action

Step:

1 Clean surface

2 Apply penetrating liquid (brightly color of a

fluorescent dye)

3 Wait and clean again

4 Apply developer to draw to penetrant out of any

cracks

Cons: surface cracks only

porous surface can

Crack Low Viscosity Fluid

Steel

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S

N

Magnetic Particle Testing (MT)

The method induces a magnetic field in a magnetic material

ferro-The surface of material is dusted with iron particles Flaws detected by perturbation in magnetic field

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Ultrasonic Measurement for Concrete

High attenuation is major problem in applying ultrasonic techniques to concrete

To ensure that the ultrasonic

wave can pass through the

concrete matrix, the selected

wavelength should be greater

than the maximum particle size

The wavelengths for concrete

testing are in a range of 20 to

100 mm (or 200 to 40 kHz)

(Popovics et al 1990)

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Traditional UPV Measurement

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Based on wave propagation theory, wave velocity

can be written as:

Use to determine compressive strength, thickness,

and defect detection

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Ultrasonic Measurement for Steel

Wave frequency is typically

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Cavity

A

B

Probe Voltage

Top surface echo

Echo from cavity

Echo from bottom of specimen

Basic Principle of Ultrasonic Testing with

Compression Wave Probe

Pulse-Echo Technique

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Position of centerline on display

Weld inspection: detection of crack

or lack of fusion at the edge of the root bead

Time-of-Flight Diffraction

Technique

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Acoustic Emission (AE)

Unlike IE and ultrasonic, acoustic emission is generated by the material itself, and it is a global test Defects might not be detected under a particular load Use piezoelectric sensors to detect waves (10 kHz to 1MHz)

Use number of hits and signal strength to evaluate structural integrity

Time

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Behavior of material: Crack propagation, Yielding, Fatigue, Corrosion

Keiser Effect: If the material is stressed, then the

stress is relaxed No new emissions will occur until the

previous maximum stress has been exceeded

Felicity Effect: Emissions occur on

reapplying stress, at a specific

fraction of the previous maximum

load

Felicity Ratio = Stress at onset of AE

Basic AE history plot showing Kaiser effect

(BCB), Felicity effect (DEF)

Previous Max Stress

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AE Monitoring of Fred Hartman Bridge

Fred Hartman Bridge (Texas) Cable Anchor

Acoustic Sensor Acoustic Sound of Wire Break

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Structural Evaluation and Rating

AASHTO Codes: (for examples)

Manual for Condition Evaluation and Load and

Resistance Factor Rating (LRFR) of Highway Bridges (2003)

Manual for Condition Evaluation (2003)

Manual for Bridge Evaluation (2008)

LRFR (2003 and 2008) are consistent with AASHTO

LFRD design specifications

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Load and resistance are NOT deterministic

Absolute safety or zero probability of failure are

not possible

Limit state function is a mathematical formulation of

the state of structures

Limit States: strength, serviceability, and fatigue

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Reliability Index and Probability of Failure

Reliability Index Probability of Failure

 Require numerical procedure to solve the limit state

function: second moment methods, Rackwitz – Fiessler method, or Monte Carlo simulation

 Use reliability index () to measure safety

Pf = (- )

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AFOSM

Require knowledge only two parameters for each random variables (mean and std.)

Obtain exact solution only when the parameters in

limit state functions are defined by a normal or lognormal distribution

Q R







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AASHTO Manual for Condition Evaluation of

Bridges (1994)

AASHTO specified allowable stress and load factor

methods for strength evaluation

AASHTO uses target level of safety 3.5 for inventory, 2.5 for operation

2

R -A D RF

A L(1+IM)



Allowable Stress Load Factor

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Widely used for evaluating bridge capacity

The specified method cannot incorporate current

bridge condition in strength evaluation

Modified load factor method has been used for

R -A D RF

A L(1 IM)





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Use load rating to determine the live load carrying

capacity of an existing bridge

Use load rating to identify the need for load posting,

bridge strengthening, and vehicle permit decisions

A lower target reliability than design is chosen for load

rating

Methods for load rating:

Load and Resistance Factor Rating

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Levels of load rating:

Design load rating

Legal load rating

Permit load rating

 Routine (Annual Permit): Valid for unlimited

trips over a period of time  Special Permit: Valid for single trip only or

for limited number of trips

Special Permit may require a certain crossing position

on the bridge, low speed, and restriction of other traffic

If speed is limited to be less than 10 mph, IM may be

neglected

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Start

Design Load Rating

Load Posting or

No Action Needed

RF>=1

RF<1

RF>=1 RF<1

Flow Chart for Load Rating

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Only permanent loads and vehicular loads are considered

in load rating

Dead Load:

Structural component and attachment (DC), wearing surface and utilities (DW)

Accordance with the actual conditions

Secondary effect from prestressing shall be

considered as permanent load

Vehicular Load:

Design Load: HL-93 design load per LRFD

Legal Load: AASHTO legal load (Type 3, Type 3S2, Type 3-3)

Permit Load: Actual Permit Load

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