Role Of Bridge Engineer ENCE 717 BRIDGE ENGINEERING

ENCE 717BRIDGE ENGINEERINGRole of Bridge Engineer C. C. Fu, Ph.D., P.E.The BEST CenterUniversity of MarylandSeptember 2008Role of Bridge Engineer The bridge engineer is often involved with several or allaspects of bridge planning, design, and managementThe bridge engineer works closely with other civil engineerswho are in charge of the roadway design and alignment.After the alignment is determined, the bridge engineer oftencontrols the bridge type, aesthetics, and technical detailsThe bridge engineer is often charged with reviewing shopdrawing and often construction detailsThe owner, who is often a department of transportation orother public agency, is charged with the management of thebridge, either doing the work in-house or hiring consultantsBridge Structure Selection(cont.)Bridge management includes routine inspections, repair,rehabilitation and retrofits or even replacement (4R) asnecessaryIn summary, the bridge engineer has significant controlover the design, construction, and maintenanceprocesses. In return, bridge engineer has significantresponsibility for public safety and resourcesIn short, the bridge is (or interface closely with) theplanner, architect, designer, constructor, and facilitymanager. Environmental Assessment Consideration (Appendix A:FHWA Order) Historic: consulting with the State HistoricPreservation Officer Construction Impact Flood Plain (stream or river subject to overflow) Wetlands “Landmark”1

Bridge Structure Selection Design Philosophy Safety Serviceability (including durability of materials) Inspectability Maintainability Rideability Deformations (Deflections) Constructability Economy (Appendix B: Economic Evaluation;Appendix C: Caltran Estimate) Bridge AestheticsBridge Structure Selection Bridge Structure Selection(cont.)(cont.)Parameters in selecting the Type, Size and Location (TS&L) Span Length (pier location, site constraints, bestcombination of super- and sub-structure costs) Accessibility to the site (weight limit, on-site fabrication) Estimated Costs Beam Spacing Material Availability (local supplier?) Time available for design and construction (urban areatime constraints) Geometry – curved or straight?Deck Superstructures (Appendix D: Common DeckSuperstructures)(cont.)Life Costs vs. First Cost“Ideal” Life-Cycle CostsLCC DC BC OC LP RCwhereDC Design CostsBC Estimated Bid CostsOC Estimated Maintenance/Operating CostsLP Cost accrued by the traveling public due to delaysand detours required for maintenance and/orrehabilitationRC Rehabilitation/Replacement Construction Costs Basic Types of SpansThe three basic types of spans are shown below. Anyof these spans may be constructed using beams,girders or trusses. Arch bridges are either simple orcontinuous (hinged). A cantilever bridge may alsoinclude a suspended span.2

Type of Bridges(Appendix E: Span Ranges for Various Bridge Types;Appendix F: Penn DOT’s Selection of Bridge Types;Appendix G: Caltran’s Types of Structures)Types of Bridges:A. Main Structure Coincides with the Deck LineB. Main Structure Below the Deck LineC. Main Structure Above the Deck LineBeam/Girder BridgeType of BridgesA. Main Structure Coincides with the Deck Line1.Slab (solid and voided)2.T-beam (cast-in-place)3.I-beam (precast or prestressed)4.Wide-flange beam (composite and noncomposite)5.Concrete box (cast-in-place and segmental,prestressed)6.Steel Box (orthotropic deck)7.Steel plate girder (straight and haunched)Girder Bridge ExampleSimple deck beam bridges are usually metal or reinforcedconcrete. Other beam and girder types are constructed ofmetal. The end section of the two deck configurationshows the cross-bracing commonly used between beams.The pony end section shows knee braces which preventdeflection where the girders and deck meet.3

Type of BridgesType of BridgesB. Main Structure Below the Deck Line1.Masonry arch2.Concrete arch3.Steel truss-arch4.Steel deck truss5.Rigid frame6.Inclined leg frame7.Arch – O’ConnorC. Main Structure Above the Deck Line1. Suspension2. Cable-stayed3. Through-truss4. Suspension – O’Connor5. Cable-stayed6. TrussRigid Frame BridgeRigid Frame Bridge(cont.)Many modern bridges use new designs developedusing computer stress analysis. The rigid frame typehas superstructure and substructure which areintegrated. Commonly, the legs or the intersection ofthe leg and deck are a single piece which is riveted toother sections.4

Arch BridgeTied Arch BridgeThere are several ways to classify arch bridges. Theplacement of the deck in relation to the superstructureprovides the descriptive terms used in all bridges: deck,pony, and through.Arch Bridge(cont.)The double-decked FremontBridge, Portland, OrgeonThe tied arch span: 902 feetBuilt: 1973Deck Arch TrussSome metal bridges which appear to be open spandreldeck arch are, in fact, cantilever; these rely on diagonalbracing. A true arch bridge relies on vertical members totransmit the load which is carried by the arch.New River Gorge bridge, Fayetteville, WVMain span length: 1700 ft., Built: 19785

Truss BridgeExamples of the three common travel surface configurationsare shown in the Truss type drawings below. In a Deckconfiguration, traffic travels on top of the main structure; ina Pony configuration, traffic travels between parallelsuperstructures which are not cross-braced at the top; in aThrough configuration, traffic travels through thesuperstructure (usually a truss) which is cross-braced aboveand below the traffic.Warren TrussA Warren truss, patented by James Warren and WilloughbyMonzoni of Great Britain in 1848, can be identified by thepresence of many equilateral or isoceles triangles formed bythe web members which connect the top and bottomchords. These triangles may also be further subdivided.Warren truss may also be found in covered bridge designs.Pratt TrussThe Pratt truss is a very common type, but has manyvariations. Originally designed by Thomas and Caleb Pratt in1844, the Pratt truss successfully made the transition fromwood designs to metal. The basic identifying features arethe diagonal web members which form a V-shape. Thecenter section commonly has crossing diagonal members.Additional counter braces may be used and can makeidentification more difficult, however the Pratt and itsvariations are the most common type of all trusses.Howe TrussThe other truss types shown are less common on modernbridges. A Howe truss at first appears similar to a Pratt truss,but the Howe diagonal web members are inclined toward thecenter of the span to form A-shapes. The vertical membersare in tension while the diagonal members are incompression, exactly opposite the structure of a Pratt truss.Patented in 1840 by William Howe, this design was commonon early railroads. The Howe truss was patented as animprovement to the Long truss which is discussed withcovered bridge types.6

Cantilever TrussA cantilever is a structural member which projects beyond itssupport and is supported at only one end. Cantilever bridgesare constructed using trusses, beams, or girders. Employingthe cantilever principles allows structures to achieve spanslonger than simple spans of the same superstructure type.They may also include a suspended span which hangsbetween the ends of opposing cantilever arms.Cantilever Through Truss BridgeCantilever TrussSome bridges which appear to be arch type are, in fact,cantilever truss. These may be identified by the diagonalbraces which are used in the open spandrel. A true archbridge relies on vertical members to transfer the load to thearch. Pratt and Warren bracing are among the mostcommonly used truss types.Truss Forth Bridge, Queensferry, Scotland(cont.) A bridge truss has two major structural advantages:(1) the primary member forces are axial loads; (2)the open web system permits the use of a greateroverall depth than for an equivalent solid web girder.Both these factors lead to economy in material and areduced dead weight. The increased depth alsoleads to reduced deflections, that is, a more rigidstructure.These advantages are achieved at the expense ofincreased fabrication and maintenance costs.Main sections: 5360 ft., Maximum span: 1710(2), 4spans total, Built: 18907

Suspension BridgeSuspension BridgeAkashi Kaikyo Bridge(AKB)between Kobe and Awaji Island,JapanThe longest bridges in the world are suspension bridges ortheir cousins, the cable-stayed bridge. The deck is hungfrom suspenders of wire rope, eyebars or other materials.Materials for the other parts also vary: piers may be steelor masonry; the deck may be made of girders or trussed.Suspension Total Length:3,911mSection Length: 960 m 1,991m 960 mSuspension– O’ConnorThe major element of the stiffened suspension bridge isa flexible cable, shaped and supported in such a waythat it can transfer the major loads to the towers andanchorages by direct tension.This cable is commonly constructed from high strengthwires, either case the allowable stresses are high,typically of the order of 600 MPa for parallel stands.The deck is hung from the cable by hangersconstructed of high strength wire ropes in tension.(cont.) – by O’Connor (cont.)This stiffening system serves to (a) controlaerodynamic movements and (b) limit local anglechanges in the deck. It may be unnecessary in caseswhere the dead load is great.The complete structure can be erected withoutintermediate staging from the ground.The main structure is elegant and neatly expresses itfunction.The height of the main towers can be a disadvantagein some areas; for example, within the approachcircuits for an airport.8

Suspension – by O’Connor (cont.)It is the only alternative for spans over 600 m, and it isgenerally regarded as competitive for spans down to300 m. However, even shorter spans have been built,including some very attractive pedestrian bridges.Westbound (1973)Eastbound (1952)Cable-stayed BridgeThe cable-stayed bridge isbecoming very popular. A greatadvantage of the cable-stayedbridge is that it is essentially madeof cantilevers, and can beconstructed by building out from thetowers.suspension (trussdeck) (1600 ft.),truss (480 ft.),truss (600 ft.),truss (780 ft.)Cable-stayed Bridge(cont.)Cable-stayed Normandy Bridge on the river Seine, near Le Havre (France).The use of high strength cables in tension leads toeconomy in material, weight, and cost.As compared with the stiffened suspension bridge,the cables are straight rather than curved. As aresult, the stiffness is greater. It will be recalled thatthe nonlinearity of the stiffened suspension bridgeresults from changes in the cable curvature and thecorresponding change in bending moment taken bythe dead-load cable tension. The phenomenoncannot occur in an arrangement with straight cables.Main span: 856-m, Total length: 2141-m, Built: 19959