40
Introduction to Bridge Engineering
Introduction to Bridge Engineering
Overview
• Bridges vs. Buildings• Advances in bridge engineering –
learning from failures• Types of bridges and their attributes• Discussion of the Walnut St. bridge
Bridges vs. Buildings
• Bridges typically do not have architects-Structural Engineer is responsible for aesthetics-Structural system is always exposed (both good and bad)
Bridges vs. Buildings
• Bridges are owned by the public- (+) Can institute changes to bridge engineering
relatively quick (e.g. LRFD)- (-) Focus is primarily on lowest initial cost,
with aesthetics playing a minor role if any at all.
- Are minimum cost and aesthetics competing objectives?
Robert Maillart
Christian Menn
Bridges vs. Buildings
• Bridges are exposed to the elements- Expansion and contraction due to temperature
changes is a major concern- Durability is a major design consideration- Routine inspection and maintenance (initial
versus life-cycle cost)
Bridges vs. Buildings
• Bridges are subjected to large moving, repetitive loads (i.e. Trucks)- Fatigue is of primary concern (accumulated
damage/cracking due to repeat loading)
Bridges vs. Buildings
• Bridge failures pose (or were thought to pose) a smaller threat to human life. -Earthquake engineering of bridges lags far behind
Learning from Failures
• Point Pleasant (Silver) Bridge – Construction was completed in May of 1928– Spanned the Ohio River between Point
Pleasant, WV and Kanauga, OH– Known as the “Silver” Bridge because it was
painted with aluminum paint– Eyebar suspension bridge (approx. 1750ft)
Description of Structure
http://www.geocities.com/silver_bridge1967http://filebox.vt.edu/users/aschaeff/silver
Fisher, J. W. (1984) “Fatigue and Fracture in Steel Bridges, Case Studies”. John Wiley & Sons. New York, NY.
Eyebar Chain Joint at C13
Fisher, J. W. (1984) “Fatigue and Fracture in Steel Bridges, Case Studies”. John Wiley & Sons. New York, NY.
Materials
• The original timber bridge deck was replaced by a steel grid filled w/ concrete (approx 3in) in 1941.– The deck replacement resulted in negligible increase in
dead load
• Eyebars were constructed of heat treated rolled carbon 1060 steel bars with forged heads – Eyebars were designed to break in the shank at ultimate
loading
Summary of Collapse
• Collapse occurred without warning on December 15, 1967 at approximately 5:00pm
• All three suspended sections fell within 60 sec• According to eyewitnesses, the collapse occurred
immediately after loud “cracking” sounds were heard coming from the Ohio Span
• Temperature at the time of collapse was 30o F• 46 persons were killed, 9 persons were injured,
and 37 vehicles fell with the bridge.
Collapse Photos (1)
http://www.geocities.com/silver_bridge1967
Collapse Photos (2)
http://www.geocities.com/silver_bridge1967
What Caused the Collapse ?
• Analysis conducted after the collapse indicated that the static stresses at the time of collapse were lower than the allowable stresses.
• If the stresses were lower than the allowable stresses, what caused the collapse?
Cause of Collapse
• Stress corrosion / corrosion fatigue initiated cracks at the inside of the pin hole of eyebar C13N.– Fatigue cracks were not visible to inspectors
• Cracks most likely initiated from the forge marks in the head of the eyebars
• Heat treated steel (lower toughness)• Temperature at the time of collapse was 30oF,
which also lowered the toughness of the material
Collapse Overview
Fisher, J. W. (1984) “Fatigue and Fracture in Steel Bridges, Case Studies”. John Wiley & Sons. New York, NY.
Advances
• This collapse resulted in significant amount of attention in fatigue and fracture mechanics related to bridges– This research culminated in the
fatigue specifications with the AASHTO Bridge Specifications
• Bridges are now inspected every two years
Other Collapses – Schoharie Br. (1987)
Other Collapses – Tacoma-Narrows (1940)
Bridge Types - Suspension•Longest-spanning bridge type
•Cables are the primary force resisting elements
•Forces are primarily transmitted through tension
•Longest - Akashi-Kaikyo, 6,527 ft (Japan, 1998)
Bridge Types – Cable-Stayed•Cables and (box) girders are the primary force resisting elements
•Cables resist forces through tension and pre-stress the girders
•Girders resist forces through bending
•Longest – Tatara, 2,848 ft (Japan, 1999)
Bridge Types – Steel Arch•Steel Arches are the primary force resisting elements
•Arches resist forces through compression
•Thrust is a major consideration
•Longest – Lupu, 1,760 ft (China, 2003)
Bridge Types – Steel Truss•Rods are the primary force resisting elements
•Forces are resisted through tension and compression
•Longest – Pont de Quebec, 1,757 ft (Canada, 1917)
Bridge Types – Concrete Arch
•Concrete Arches are the primary force resisting elements
•Arches resist forces through compression
•Longest – Wanxian, 1,344 ft (China, 1997)
Bridge Types – Prestressed Conc. Girder
•Girders are the primary force resisting elements
•Forces are resisted through bending
•Longest –Stolmasundet, 963 ft (Norway, 1998)
Bridge Types – Steel Girder
•Girders are the primary force resisting elements
•Forces are resisted through bending
•Longest –Ponte Costa e Silva, 960 ft (Brazil, 1974)
Walnut Street Bridge
Steel Girder Spans
Prestressed Concrete Girder SpansV-Pedestals
Walnut Street Bridge Discussion
• Temperature Expansion
• Splice Connections
• Simple vs. Continuous Spans
Expansion Bearings
L+∆Lt
Temperature Effects on Bridges
L
∆Lt=α L(∆T)
Where,
α=coefficient of thermal expansion (in/in/oF)
αst=6x10-6 in/in/oF
∆T=change in temperature (oF)
E.g. Temperature Effects on Bridges
300 ft
∆1 =αst L1(∆T)= (6x10-6 in/in/oF)(150ft)(12in/1ft)(70)
∆1 =0.76 in
∆1
∆2
L1=150 ftL2=300 ft
∆2 =αst L2(∆T)= (6x10-6 in/in/oF)(300ft)(12in/1ft)(70)
∆2 =1.52 in
Calculate the require expansion joint capacity (displacement) for a ∆T =70 oF
E.g. Temperature Effects on Bridges
300 ft
∆1 =αst L1(∆T)= (6x10-6 in/in/oF)(150ft)(12in/1ft)(70)
∆1 =0.76 in
∆1∆2
L1=150 ft L2=150 ft
∆2 =αst L2(∆T)= (6x10-6 in/in/oF)(150ft)(12in/1ft)(70)
∆2 =0.76 in
Calculate the require expansion joint capacity (displacement) for a ∆T =70 oF
Splice Connections
Af-sp>Af
Aw-sp>Aw
Splice Connection Locations
Dead Load Moment Diagram
Locate splices in zero moment regions
Simple vs. Continuous Spans
SIMPLE SPAN
Dead Load Moment Diagram
L L L
wL2/8
Simple vs. Continuous Spans
CONTINUOUS SPAN
Dead Load Moment Diagram
L L L
wL2/8
wL2/8
Providing resistance to negative moment reduces positive moment
Questions?QUESTIONS?
