Introduction to Bridge Engineering - Drexel University

1 Introduction to Bridge EngineeringOverview Bridges vs. Buildings Advances in Bridge Engineering learning from failures Types of bridges and their attributes Discussion of the Walnut St. bridgeBridges 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 ( 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 MaillartChristian MennBridges 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.

2 Buildings Bridges are subjected to large moving, repetitive loads ( 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 behindLearning 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 , J. W. (1984) Fatigue and Fracture in Steel Bridges, Case Studies . John Wiley & Sons. New York, Chain Joint at C13 Fisher, J. W. (1984) Fatigue and Fracture in Steel Bridges, Case Studies.

3 John Wiley & Sons. New York, 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 loadingSummary 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 30oF 46 persons were killed, 9 persons were injured, and 37 vehicles fell with the Bridge . Collapse Photos (1) Photos (2) Caused the Collapse ?

4 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 materialCollapse OverviewFisher, J. W. (1984) Fatigue and Fracture in Steel Bridges, Case Studies . John Wiley & Sons. New York, 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.

5 (1987)Other Collapses Tacoma-Narrows (1940) Bridge Types - Suspension Longest-spanning Bridge type Cablesare the primary force resisting elements Forces are primarily transmitted through tension Longest - Akashi-Kaikyo, 6,527 ft (Japan, 1998) Bridge Types Cable-Stayed Cablesand (box) girdersare 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 Archesare 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 tensionand 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.

6 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 BridgeSteel Girder SpansPrestressed Concrete Girder SpansV-Pedestals Walnut Street Bridge Discussion Temperature Expansion Splice Connections Simple vs. Continuous SpansExpansion BearingsL+ LtTemperature Effects on BridgesL Lt= L( T)Where, =coefficient of thermal expansion (in/in/oF) st=6x10-6 in/in/oF T=change in temperature (oF) Temperature Effects on Bridges300 ft 1= stL1( T)= (6x10-6in/in/oF)(150ft)(12in/1ft)(70) 1= in 1 2L1=150 ftL2=300 ft 2= stL2( T)= (6x10-6in/in/oF)(300ft)(12in/1ft)(70) 2= inCalculate the require expansion joint capacity (displacement) for a T =70 Temperature Effects on Bridges300 ft 1= stL1( T)= (6x10-6in/in/oF)(150ft)(12in/1ft)(70) 1= in 1 2L1=150 ftL2=150 ft 2= stL2( T)= (6x10-6in/in/oF)(150ft)(12in/1ft)(70) 2= inCalculate the require expansion joint capacity (displacement)

7 For a T =70 oFSplice ConnectionsAf-sp>AfAw-sp>AwSplice Connection LocationsDead Load Moment DiagramLocate splices in zero moment regionsSimple vs. Continuous SpansSIMPLE SPANDead Load Moment DiagramLLLwL2/8 Simple vs. Continuous SpansCONTINUOUS SPANDead Load Moment DiagramLLLwL2/8wL2/8 Providing resistance to negative moment reduces positive momentQuestions?QUESTIONS?