Memilih Tipe Jembatan

STEELBRIDGEDESIGNHANDBOOK

Selecting the RightBridge Type

Chapter 7

National Steel Bridge Alliance

Disclaimer

All data, specifications, suggested practices, and drawings presented herein, are based on the best

available information and delineated in accordance with recognized professional engineering princi-

ples and practices, and are published for general and procedural information only. Procedures and

products, suggested or discussed, should not be used without first securing competent advice respect-

ing their suitability for any given application.

Publication of the material herein is not to be construed as a warranty on the part of the National Steel

Bridge Alliance - or that of any person named herein - that these data and suggested practices are suit-

able for any general or particular use, or of freedom from infringement on any patent or patents.

Further, any use of these data or suggested practices can only be made with the understanding that

the National Steel Bridge Alliance makes no warranty of any kind respecting such use and the user

assumes all liability arising therefrom.

Table of Contents

Introduction........................................................................................................1

Required Span Lengths .......................................................................................1

Owner Desires ..............................................................................................1

Hard Requirements .......................................................................................1

Existing Constraints ......................................................................................1

Other Constraints .........................................................................................2

Bridge Types .......................................................................................................2

Rolled Beam Bridges .....................................................................................2

Welded Plate Girder Bridges .........................................................................2

Trusses ..........................................................................................................4

Arches...........................................................................................................8

Cable-Stayed ..............................................................................................10

Suspension Bridges .....................................................................................13

Economics.........................................................................................................14

Constructability.................................................................................................15

Rolled Beams/Welded Plate Girders.............................................................15

Trusses ........................................................................................................15

Arches.........................................................................................................15

Cable-Stayed Bridges ..................................................................................16

Suspension Bridges .....................................................................................16

7-1STEELBRIDGEDESIGNHANDBOOK

Selecting the RightBridge Type

INTRODUCTION

One of the initial choices to be made bythe designer is to select the most appropri-ate bridge type for the site. While thischoice is not always straightforward,selecting the right structure type is proba-bly the important aspect of designing acost-effective bridge.

REQUIRED SPAN LENGTHS

Various types of bridge superstructuresprovide efficient solutions for differentspan arrangements. There are many possi-ble reasons for choosing particular spanlengths for bridges, some of which are dis-cussed below.

Owner DesiresIn many cases, the owners desires willdrive the selection of the bridge type.Some owners tend to push their bridgestoward the shortest spans possible, with aneye toward allowing a choice of materialsor to prefer a specific material type.

Owners will occasionally choose a bridgebecause they desire to construct a specificbridge type at a location. In some casesthese desires come directly from theowner, but often public opinion influencesthe selection of the bridge type, particular-ly for long span bridges.

Occasionally, an owner will prescribe a cer-tain bridge type for reasons of perceivedprestige. The choice may be made specifi-cally to design a bridge type that has notbeen done by that owner, to design a cer-

tain bridge type with a record span length,or to create a signature structure.

Hard RequirementsOne critical requirement that often con-trols the main span lengths for water cross-ings is navigational clearance. The U.S.Coast Guard is generally the controllingagency regarding required span lengthsfor navigable inland waterways.

Another non-negotiable controller ofbridge span lengths can be defined byenvironmental commitments. Given theincreased sensitivity to minimizing adverseenvironmental impacts, there have beenmany cases where the span arrangementshave been set to meet environmental com-mitments.

Existing ConstraintsLocations of existing constraints often con-trol the span arrangements for new struc-tures. For new, high-level structures, thelocations of existing features that are beingcrossed by the new structures may controlcertain span lengths or span arrangements.This situation may occur where expandedinterchanges are being constructed on ornear the site of existing interchanges wherethe original structures will be retained orused for staging during construction. Thelocation of existing surface roadways oftencontrols span arrangements for newbridges. Occasionally it is more cost-effec-tive to move the surface roadways toaccommodate the new bridge, but in con-gested urban areas this is rarely possible.

Golden Gate Bridge

San Francisco, CA

SELECTING THE RIGHT BRIDGE TYPE7-2

Another constraint encountered by design-ers/owners is railroads. There are signifi-cant costs associated with moving tracks orinterrupting rail service to accommodateconstruction of new bridges. It is oftenbeneficial to increase span lengths to mini-mize impacts to the railroads. Since therailroads are for-profit enterprises, theytend to be very protective of their facilitiesin order to maintain profitability.

Other ConstraintsSite access may control the choice of spanarrangements for structures in certaincases. When constructing bridges overwide, deep valleys, it is sometimes advan-tageous to increase span lengths to elimi-nate costly piers. In some cases, deck struc-tures such as trusses or arches may becomeeconomical.

Occasionally, the desired constructionschedule may impact the structure type thatis chosen. Certain types of structures lendthemselves to short construction durations,which may be important to the owner.

Local contractor expertise may also affectthe selection of the structure type. Certaintypes of structures are not common in cer-tain regions. For example, while segmental

structures are relatively common in thesoutheast, they are rare in many parts ofthe northeast, where many owners will notuse segmental structures because of thedifficulties associated with deck replace-ment. As a result, few contractors in thenortheast region have experience in seg-mental construction, which will likely resultin either high construction costs or out-of-state contractors winning the bid.

BRIDGE TYPES

There are many bridge types that are typi-cal for current construction. The varioustypes are ideally suited to different spanlengths. However, there is generally signifi-cant overlap in the applicable ranges forthe most common span ranges, so multiplebridge types are generally viable at mostspan ranges.

Rolled Beam BridgesRolled beam bridges using W-shapes areused in some situations, mainly for simplespans up to approximately 100' or contin-uous spans up to approximately 120'. Theyare generally made composite with thebridge deck. Rolled shapes result in bridgeswith higher unit weights of steel (inpounds per foot) than do plate girders.However, the unit cost of rolled shapes issignificantly lower than is common forplate girders due to the simpler fabrication.The details are also generally less expensivethan for plate girders since transverse stiff-eners are not usually required. In addition,the diaphragms between beams usuallyconsist of rolled shapes with channelsbeing the most common choice. The limit-ed amount of welding and simplediaphragm details often make rolled beambridges more economical than plate gird-ers in short span ranges.

Welded Plate Girder BridgesDeck plate girders are the most commontype of steel structure. As recently as the1970s, many bridges were designed usingtwo deck girders, transverse floorbeams atregular intervals, and longitudinal stringers

Figure 1

Typical multi-girder system.


either continuous over the floorbeams orframed into the floorbeam webs. However,as welded girders became the predomi-nant method of fabrication and cracksoccurred in girders due to poor fatiguedetails, the issue of fracture criticalitybecame a concern to many agencies.

Through girders provide another weldedplate girder option. A through girderbridge generally has two girders near theedges of the deck with shallow floorbeamsconnecting the bottom flanges of the gird-ers and often will have longitudinalstringers framed into the floorbeams. Thegirder top flanges and much of the webextend above the top of the bridge deck.Knee braces are generally provided at eachfloorbeam location in order to brace thegirder top flange. The use of the throughgirder system is generally limited to siteswhich must accommodate a severe super-structure depth restriction.

There are several reasons that throughgirder bridges are not commonly used forhighway structures. The system results intwo-girder structures, which are fracturecritical, meaning that the failure of one ofthe main girders could lead directly to thefailure of the entire bridge. The main gird-ers cannot be made composite with thebridge deck, meaning that the deck offersno strength benefit to the girders. Finally,the top flanges of the girders in the com-pression regions are braced only at thefloorbeam spacing, rather than full lengthas would be the case for a composite deckgirder bridge. This will require additionalsteel in the top flanges of the girdersbecause of the strength reductions result-ing from the unbraced length.

Eventually, multi-girder bridges becamethe desired deck girder bridge configura-tion, and composite construction becamecommon. Designing for composite actionallows the designer to account for thestrength of the bridge deck in the sectionproperties of the girders. In addition, thetop flanges in the positive moment regionsare fully braced in the final condition,

allowing the use of higher allowable com-pressive stresses than are possible forflanges braced at discrete points. Deckgirders also offer great flexibility to accom-modate variable width roadways and hori-zontally curved geometry. Additionally,shop layout is generally less complex thanwould be the case for through girders,trusses or arches.

Deck girder designs are usually optimizedfor span lengths exceeding about 125' ifthe girder spacing can be set between 11and 14 feet. For spans lengths less than125', narrower girder or beam spacingmay be more economical. This minimizesthe number of girder webs and the overallunit weight of steel. The cost savings for anoptimum number of girders will usuallyoffset any cost increases due to a thickerbridge deck. Some agencies still limit gird-er spacing to the 8 to 9 foot range thatwas common many years ago, althoughthere is currently no economic reason todo so for longer span lengths.

Crossframes or diaphragms have been pro-vided at a maximum spacing of 25' formany years in accordance with the provi-sions of the AASHTO StandardSpecification. The AASHTO LRFDSpecification has eliminated the 25' maxi-mum spacing requirement and left thespacing up to the designer. While theintent was not to stretch the spacing toofar, the desire was to avoid the need to addadditional crossframe lines for the sole pur-pose of limiting the spacing to 25'. For amore complete discussion of crossframespacing and configurations, refer toChapter 8.

Deck girders can generally be erected withminimal amounts of falsework. Pier brack-ets may be necessary over the interior sup-ports as the spans increase. Although false-work towers are sometimes used for short-er spans, only when the spans exceedabout 200' do towers become necessary.Erection of plate girders generally pro-ceeds quickly since there are a limitednumber of field sections and bolted con-


nections that the erector needs to deal within the field.

In certain cases, lateral bracing may berequired to facilitate construction of thebridge. While this case should be rare whenthe girders are properly proportioned, lat-eral bracing may be particularly useful forspans over 300' or for very tall structureswhere winds encountered may be signifi-cantly higher than would be expected onstructures close to the ground surface.

One variation of the multi-girder system isthe girder substringer system. This arrange-ment is generally used on wider bridgeswith relatively long spans. As discussed ear-lier, fewer girders in the cross section willresult in greater overall economy in thesuperstructure design. When the spanlengths exceed approximately 275, it isoften economical to use a girder sub-stringer arrangement. This system uses sev-eral heavy girders with wider girder spac-ing. Truss crossframes, which in many caseslook like large K-frames, are used betweenthe main girders with the rolled beamstringers supported midway between themain girders.

TrussesTrusses behave as large beams to carry

loads, but are comprised of discrete mem-bers that are subjected primarily to axialloads. The members are generally arrangedto form a series of triangles that act togeth-er to form the structural system. The chordsare the top and bottom members thatbehave as the flanges of a girder. Diagonalsand verticals function in a manner similar tothe web in a plate girder. Diagonals gener-ally provide the necessary shear capacity.Verticals carry shear and provide additionalpanel points through which deck and vehi-cle loads can be applied to the truss.Tension verticals are commonly calledhangers, and compression verticals areoften called posts. They also serve to limitthe dead load bending stresses in thechord members by reducing the unsup-ported member length. Joints are the loca-tions where truss members intersect andare referred to as panel points.

The deck is the structural element thatdirectly supports applied traffic loads.Stringers are longitudinal beams, generallyplaced parallel to traffic, that carry deckloads to the floorbeams. Floorbeams areusually set normal to the direction of trafficand are designed to transmit loads fromthe bridge deck to the trusses. Some truss-es in the past have been designed withoutstringers, relying on the deck to transmitthe loads directly to the floorbeams. Thisrequires the joint spacing along the chordsto be relatively small, and as a result is noteconomical in the current market.

Lateral bracing is normally provided in theplane of both the top and bottom chordsof the trusses. Its purpose is to stiffen thetrusses laterally and to carry wind loads andother applied lateral loads back to the sup-port locations. The configuration of the lat-eral bracing systems is not required to bethe same between the top and bottombracing systems.

Sway bracing is provided between thetrusses in the plane of either verticals ordiagonals, and its primary purpose is mini-mizing the relative vertical deflectionsbetween the trusses. Portal bracing is sway

Figure 2

Typical girder substringer

system.


bracing placed in the plane of the endposts. For deck trusses, the end posts aregenerally vertical members, and forthrough trusses the end posts are general-ly the diagonal members extending upfrom the end joints.

There are three basic truss types. For decktrusses, the entire truss is below the bridgedeck. The floorbeams may either runbetween the top chords of the trusses orrest on top of the trusses. Deck trusses canbe particularly cost-effective when thefloorbeams rest on top of the truss ratherthan framing in, because the floorbeamscan take advantage of the continuity effectdue to the cantilevered overhangs. Also,the trusses can be closer together, whichreduces the length, and therefore the cost,of the lateral bracing and sway frames.Deck trusses are generally desirable incases where vertical clearance below thebridge is not restricted. Deck trusses resultin more economical substructures becausethey can be significantly shorter since thebridge is below the deck. Deck trusses arealso easier to widen in the future since thedeck is above the trusses. Future deckwidening is limited only by the increasedstructural capacity obtained by modifyingor replacing the floorbeams and strength-ening the truss members.

Through trusses are detailed so that thebridge deck is located as close to the bot-tom chord as possible. The bottoms of thefloorbeams are normally located to line upwith the bottom of the bottom chord.Through trusses are generally used whenthere is a restricted vertical clearanceunder the bridge. The depth of the struc-ture under the deck is controlled by thedepth of the floor system and can there-fore be minimized so that the profile canbe kept as low as possible. For shorterspans, the minimum depth of the trussmay be controlled by the clearance enve-lope required to pass traffic under thesway/portal frames. The trusses must bespaced to pass the entire deck sectioninside the truss lines. It is generally not fea-

sible to widen a through truss structurewithout adding a parallel truss to the orig-inal two truss lines.

If a sidewalk is required on the bridge, itcan either be located inside the trusseswith the roadway portion of the deck, orsupported outside the trusses by bracketsthat bolt into the outside of the truss

joints. Locating the sidewalks inside thetrusses adds some cost to the lateral brac-ing and sway bracing since the truss spac-ing is increased. However, the loading isbetter balanced between the two trusses,

Figure 3

Forrest Hill Bridge over

American Rivers north fork,

Auburn, CA.

Figure 4

Chelyan Bridge over

Kanawha River, Kanawha

County, WV.


and all the construction can be accom-plished inside the truss lines, making it eas-ier to move materials to the desired loca-tions. Supporting sidewalks outside thetrusses allows the truss spacing to be mini-mized, saving cost in the bracing systems.However, the cost of the overhang framingand the additional difficulty of constructingsidewalks outside the trusses, as well as theincrease in the number of components thatneed to be fabricated and erected, mayoutweigh the savings in bracing weight. Insome cases there have been serviceabilityproblems in the overhang brackets sup-porting the sidewalks, which increases themaintenance cost of the bridge. The load-ing between the trusses is much moreeccentric than is the case when the side-walk is kept inside the truss lines.

Half-through trusses carry the deck highenough that sway bracing cannot be usedabove the deck. Much of the previous dis-cussion regarding through trusses applies

to half-through trusses. It is very difficult todesign a half through truss if the chosentruss type does not have verticals. Withoutverticals, the deck must be supported fromdiagonal members away from the joints.This condition results in significant bendingstresses in the truss members, which aregenerally not efficient in carrying bending,thus resulting in inefficient memberdesigns for the diagonals. Many of therecent trusses designed in the United Stateshave been designed without verticals toachieve a cleaner and more contemporaryappearance, thus minimizing the use ofhalf-through trusses.

There are several geometric guidelines thatare helpful when determining truss config-urations. AASHTO requires minimum trussdepths of one-tenth the span length forsimple spans. For continuous trusses, thedistance between inflection points can beused as the equivalent simple span lengthto determine the minimum truss depth.

Figure 5

Glenwood Bridge over

Monongahela River,

Pittsburgh, PA.


It is generally desirable to proportion thetruss panel lengths so that the diagonalsare oriented between 40 degrees and 60degrees from horizontal. This keeps themembers steep enough to be efficient incarrying shear between the chords. Thisangular range also allows the designer tomaintain a joint geometry that is relativelycompact and efficient.

There are two floor system configurationsthat are used for trusses. Stacked floor sys-tems are the most commonly-used detail inrecent trusses. In a stacked floor system,the longitudinal stringers rest on top of thefloorbeam top flanges, supported by bear-ings. The stringers are generally made con-tinuous across several floorbeams. Thisallows for more efficient stringer designswith fewer pieces to fabricate and erect. Italso minimizes deck expansion jointswhich may leak, reducing the probabilityof de-icing salts washing over the membersbelow. Rolled W-shapes have most com-monly been used for the stringers sincethey can efficiently span the normal panellengths used for trusses.

Framed floor systems use a series of simplespan stringers that frame into the floor-beam webs. When faced with depthrestrictions, framed systems serve toreduce the overall depth of the floor sys-tem by the depth of the stringers.

Truss analysis is typically idealized assum-

ing that the members are pinned at thejoints (free to rotate independent of othermembers at the joint) so that secondarystresses ordinarily need not be consideredin the design. Joints are typically detailedso that the working lines for the diagonals,verticals and chords intersect at a singlepoint. However, bending stresses resultingfrom the self-weight of the membersshould be considered in the design.

Erection of trusses is more complex thanthat for welded plate girder structures. Forsimple span trusses, at least two falseworktowers are generally required to facilitateerection. They are generally placed two orthree panel points away from the supports.The panels can then be erected as can-tilevers out past the falsework towers.Depending upon the span length of thetruss, counterweights may be requirednear the end supports to avoid uplift atthese locations. It is also necessary to havethe ability to provide elevation adjustmentat the support locations in order to closethe truss at the center. For trusses over nav-igable waterways, agency requirements formaintaining temporary navigation chan-nels may dictate the location of falseworktowers, and towers constructed in waterare usually more costly than towers con-structed on land.

Continuous trusses can usually be erectedby using relatively light falsework towers in

Figure 6 (Above left)

Typical stacked floor system.

Figure 7 (Above right)

Typical framed floor system.

the back spans to facilitate a balanced can-tilever style of erection. As with the simplespan truss, the ability to make elevationadjustments must be provided at the sup-port locations to facilitate rotating thetrusses into position so that the span canbe closed.

The discrete piece weights for the trussmembers are relatively small when com-pared with some of the other structuretypes. Thus the erector can accomplish theerection with smaller cranes than would benecessary for girder or arch bridges. Thereis a significant amount of labor involved intruss erection due to the number of mem-bers that need to be erected and the num-ber of bolted connections required.

Trusses are generally considered to be frac-ture critical structures. The simplifiedapproach during design has been to desig-nate all truss tension members and mem-bers subjected to stress reversals as fracturecritical members. Fracture critical studiescan be performed based on analyses thatmodel the entire framing system, includingthe bracing systems, to determine whethercertain lightly loaded tension or reversalmembers are truly fracture critical. In manycases the number of fracture critical mem-bers can be reduced through this process,which reduces fabrication costs.

ArchesArches carry loads in a combination ofbeam action and axial forces, or thrusts.Arches take several basic forms. True archescarry the horizontal component of eachreaction directly into a buttress, which alsoresists the vertical reaction. The arch ribscarry both thrust and moment. Tied archesincrease the applicability of the arch formby adding a tie, which is a tension memberbetween the ends of the span. In a tiedarch, the thrust is carried by the tie, but themoment is divided between the arch andthe tie, somewhat in proportion to the stiff-ness of the two members.

Arches can have either trussed or solid ribs.Solid ribbed arches are generally used forshorter spans. Trussed arches tend tobecome economically feasible as the spanlengths increase past 1000'.

Arch bridges are also constructed usingvarious degrees of articulation. A fixed archprevents rotation at the ends of the spanand is statically indeterminate to the thirddegree. A two-hinged arch permits rota-tion at the ends of the rib and is also onedegree statically indeterminate. Occasion-ally a hinge is also provided at the crown ofthe arch rib, making the arch staticallydeterminate. This detail, however, hasbecome less prevalent with the increasedavailability and power of computer analysisprograms.

Arches are classified as deck arches whenthe entire arch is located below the deck.Most true arches are deck arches. Tiedarches are normally constructed as througharches with the arch entirely above thedeck and the tie member at the deck level.Both true and tied arches can be construct-ed with the deck at some intermediatelevel that can be classified as half-through.

Deck arches are usually used when crossingdeep valleys with steep walls. Assumingthat rock is relatively close to the surface,the arch foundations can then remain rela-tively short, effectively carrying both verti-cal loads and horizontal thrusts directlyinto the rock. The foundation costs


Figure 8

New River Gorge Bridge

over Fayetteville, WV.


increase significantly when deep founda-tions are required. Occasionally, there aresite constraints that dictate placing thefoundations closer to the deck profile, suchas a desire to keep the bearings for thearch above the high water elevation at thesite. In such cases, half-through arches maybetter satisfy the design constraints.

Tied arches are generally more effective incases where deep foundations are requiredor where high piers may be required toachieve a desired clearance under thebridge, such as a long span crossing a riverwhere a navigation clearance envelopemust be provided.

The floor systems for arches are similar tothose for trusses. Transverse floorbeams arelocated at spandrel columns for deck arch-es, and longitudinal girders transmit thedeck loads to the floorbeams, which inturn transmit the forces to the spandrelcolumns and into the arch ribs. However,for deck arches the spandrel columns aregenerally spaced much farther apart thanthe panel points on a truss. Therefore, it iscommon to use welded plate girders orbox girders as the longitudinal beamsinstead of the rolled beams that are com-mon for trusses. As with trusses, it is morecommon to work with a stacked floor sys-tem in order to take advantage of continu-ity in the longitudinal beams and to mini-mize the number of pieces that must beerected and bolted.

The columns supporting the floor systemgenerally are bolted to the top of the archribs. Sway bracing is provided over the fullheight of these columns to assure their sta-bility when subjected to lateral loads andalso to reduce the unbraced lengths of thecolumns when assessing their axial andbending capacity.

For tied arches, the ribs and tie girders arelinked together with a system of hangers.Traditionally, the hangers have beeninstalled vertically between panel points ofthe ribs and ties. The hangers force bothcomponents to participate in carryingmoments induced in the system. However,

recent designs have begun to consider net-worked hanger systems, that is, hangersthat connect non-concurrent panel pointsin the rib and tie. This results in a trussedappearance of the ties. The networkedhangers provide additional stiffness to thestructural system which may lead to amore economical design in certain cases.

Hangers for tied arches in the United Stateshave historically been made using groupsof 2 to 4 bridge strand or bridge rope.These consist of wound wires, so there issignificant rigidity in the cables. There hasnot been a history of undue vibration inthese cables. The ropes are pre-stretchedduring fabrication so that, when they areinstalled, the correct geometric positioningof the bridge is assured. However, as cable-stayed bridge technology has improved,some designers have begun to considerthe use of parallel strand hangers for tiedarches as well. As of this writing, no long-span tied arches have been completed inthe U.S. using parallel strand hangers.

Aesthetic considerations may impact thefabrication and construction costs of arch-es. For solid ribbed arches, the rib can becurved or chorded between the panelpoints. Curving the rib adds fabricationand material cost but provides the opti-mum appearance. Chording the rib

Figure 9

I-470 Bridge over

Ohio River, Wheeling, WV.


between panel points can provide anacceptable appearance as well, with alower material and fabrication cost. If thechoice is made to chord the ribs, it may bedesirable to decrease the panel point spac-ing to reduce the angular change betweensections. Another detail issue that relates tothe appearance is whether to maintain aconstant depth rib or whether to vary thedepth so that the rib is shallow at thecrown and increases in size toward thespring line.

Selection of the rib lateral bracing systemcan also significantly impact the appearanceof the bridge. K-bracing configurations havebeen used, but they tend to result in a morecluttered appearance since struts betweenthe arch ribs are required at the point ofeach K-brace. X-bracing systems are alsocommon and have been detailed with andwithout transverse struts. X-bracing withouttransverse struts provides a cleaner andmore modern appearance.

Tied arches are considered to be fracturecritical structures since the failure of a tiegirder would be expected to lead to col-lapse of the bridge. For a period of timethrough the 1980s and early 1990s, therewas great reluctance within the designcommunity to use tied arches because ofthese concerns. However, with improveddetailing practices and the development ofhigh performance steel (HPS), with itssuperior fracture toughness, tied arches

have recently gained more support. Inaddition, many agencies accept tie girderdesigns that are internally redundant. Thisis accomplished by either post-tensioningthe tie girders to eliminate tension in thegirder plates or by detailing bolted, built-up sections with discrete components thatare not connected by welds. Thus, if onecomponent fractures, new cracks wouldhave to initiate in the other componentsbefore complete fracture could occur.While this detailing does not necessarilyeliminate the fracture critical classification,it provides some security that a sudden,catastrophic failure is highly unlikely.

Cantilever erection is one of the more com-mon methods of arch erection. This can beaccomplished by providing temporarytowers to support the partially erectedarch. For tied arches, towers will likely benecessary between the rib and the tie tocontrol the distance between the twomembers. A variation of the cantilevererection method is to provide falseworktowers near the end of the ribs with a sys-tem of back stays to support the arch dur-ing erection. This approach necessitatesthat the towers can be anchored economi-cally to balance the lateral forces transmit-ted through the back stays.

Cable-StayedCable-stayed bridges rely on high strengthsteel cables as major structural elements.The stay cables are inclined from the sup-porting towers to edge girders at or belowthe deck elevation. These bridges are gen-erally signature structures with excellentaesthetics characterized by very slendersuperstructures and tall towers. Generallythe towers are very tall with the heightdetermined as a function of the spanlength. The slope of the longest stay cablesdictates the minimum tower height. Theflattest cable angle should not be less thanabout 22 degrees with the horizontal.Below 22 degrees, the cables become inef-ficient in carrying the vertical load compo-nent and they exert very high compressiveforces on the edge girders.

Figure 10

Erection of the I-79 Bridge

over the Ohio River,

Pittsburgh, PA.


There is a significant variety in tower con-figurations that have been used on cable-stayed bridges in the U.S. and around theworld. H-towers have two legs that rise ver-tically above the edge of the bridge andthe cables are outside of the roadway.There is generally an intermediate strutpart of the way up the towers but belowthe level of the stay cable anchorages. Theappearance of H-towers is more utilitarianthan other shapes. One of the main advan-tages of the H-tower is that the stay cablescan be installed in a vertical plane directlyabove the edge girder. The vertical planesof stay cables simplify the detailing of thedeck-level anchorages because the con-nections to the edge girders are parallel tothe webs of the edge girders. If ice formson the cables during inclement winterweather, it will not fall directly onto traffic.

A-towers have inclined legs that meet atthe top of the tower. The stay cables areusually in a plane that parallels the slope ofthe tower legs. From a detailing stand-point, the edge girder anchorages aremore complicated since they intersect theedge girder at an angle. Moreover,because every stay cable intersects theedge girders at a slightly different angle,exact repetition of anchorage geometry isnot possible. There have been instanceswhere steel edge girders have been fabri-cated with sloped webs in an effort to sim-plify the anchorage details, but there issome increase in fabrication cost for thesloped edge girders. The inwardly slopedcable planes exert compression into thesuperstructure in both the longitudinal andtransverse directions. The sloped cablesprovide some additional torsional stiffness,which is particularly important on narrowand slender superstructures. In many casesthe center-to-center spacing of the edgegirders needs to be increased so that thesloped cables do not infringe upon the ver-tical clearance envelope required to passhighway traffic.

Inverted Y-towers are a variation of the A-tower. In this case, the sloped tower legs

meet well above the roadway and a verti-cal column extends upward from the inter-section of the sloped legs. The stay cablesare then anchored in the vertical column.Every stay cable intersects the edge girdersat a slightly different angle so that exactrepetition of anchorage geometry is notpossible. The advantages and disadvan-tages discussed for the cables are similar tothose discussed above regarding A-towers.Achieving the desired clearance envelopefor traffic may be more critical for theinverted Y-towers, possibly requiring even

Figure 11

Sydney Lanear Bridge,

Brunswick, GA.

Figure 12

William H. Natcher Bridge

over Ohio River near

Owensboro, KY.


further widening of the superstructuresince the inward cable slopes are more sig-nificant.

Single column towers have been used on afew cable-stayed bridges. These towersconsist of a single column in the center ofthe bridge with a single plane of staycables down the center of the bridge. Thesingle plane of cables offers some economyin cable installation and maintenance.However, the transverse beams supportingthe superstructure at each anchorage loca-tion are designed as cantilevers in bothdirections and will be somewhat heavierthan beams supported by two cable planesat the outside edges of the bridge.

Cable-stayed superstructures have takenmany forms around the world. Concretecable stays have used concrete edge gird-ers with concrete decks (either precast orcast-in-place) as well as post-tensioned boxgirder sections. Steel and/or compositecable-stayed bridges have used either I-shaped or box-shaped edge girders.Recent U.S. cable stayed bridges havetended to mainly use I-shaped steel edgegirders made composite with concretedecks. The edge girders carry bendingbetween stay cable anchorages plus axialcompression resulting from the horizontalcomponent of the stay cable forces. Thecompressive forces are additive from theend of the longest cables toward the tow-ers. In most cases the edge girder design isnot controlled by bending but by the com-

pressive forces imparted by the stay cables,requiring a heavier section in order toavoid buckling.

Various deck systems have been used forcable-stayed bridges. Cast-in-place con-crete decks have been used, although theyare not common. Precast concrete deckpanels with specialized overlays are fairlycommon in current construction. Whenconcrete decks are used it is common toprovide longitudinal post-tensioning of thedeck to prohibit cracking of the deck. Thedeck post-tensioning is generally heaviestfar away from the towers. Compressionfrom the anchorage forces spreads throughthe concrete near the towers and the needfor post-tensioning diminishes. It is com-mon to use specialized concrete overlayseven for cast-in-place concrete decks

Orthotropic decks have also been used.However, in the U.S. the overall cost fororthotropic decks has limited their use toextremely long spans.

Modern stay cables consist of a cluster ofparallel prestressing strands sized to carrythe design load of the particular stay cable.The cables are housed in PVC pipes to pro-tect them from the effects of weather. Earlycable-stayed technology indicated that thecables should be grouted inside the pipe asan additional level of corrosion protection.Based on the performance of some of theearly cable systems, the state of the art hasmoved away from grouting as a corrosionprotection system.

Figure 13 (Above left)

Normandy Bridge over

Seine River near Le Havre,

France.

Figure 14 (Above right)

Sunshine Skyway Bridge,

Tampa, FL.


Suspension BridgesSuspension bridges are also cable-suspend-ed structures but use a different system. Aswith cable-stayed structures, suspensionbridges rely on high-strength cables asmajor structural elements. Suspensionbridges, whose towers are significantlyshorter than those required for cable-stayed bridges, become economical forvery long spans.

The most common form of suspensionbridge uses an externally anchored sus-pender cable system. The main cables typ-ically are located at the outside edges ofthe bridge and are draped over the towerswhere they rest in saddles. The ends of themain suspension cables are anchoredeither into large counterweights or directlyinto bedrock at the ends of the structure.Both the size of the cables and the magni-tude of the tension in these main sus-pender cables determine the vertical stiff-ness of the bridge.

Suspension bridge superstructures are gen-

erally very light relative to the span lengthwhich leads to structures that may be veryflexible. The vertical stiffness can generallybe controlled by the size of and tension inthe main suspender cables. However, it iscommon to provide a lateral stiffeningtruss under the deck to avoid excessiveexcitement of the superstructure underwind or seismic loadings. The first TacomaNarrows Suspension Bridge illustrated thepitfalls of providing the lightest superstruc-ture possible based on strength designwhile not considering the possible har-monic effects induced by lateral windloads. The structure collapsed due to theharmonic frequency set up during a periodof only moderately high winds.

The superstructure is generally supportedat relatively short intervals by verticalcables that connect at the ends of trans-verse floorbeams. The vertical cables arerelatively light and are attached to themain suspension cables by saddle clips.While vertical cables are most common

Figure 15

Golden Gate Bridge,

San Francisco, CA.

there have been a few suspension bridgesthat used inclined suspenders, such as theBosporus Bridge in Turkey and the SevernBridge in England.

Due to the extreme span lengths, light-weight deck options, including lightweightconcrete, are often used to reduce thedead load demand. Steel orthotropic decksystems have often been used both to limitthe deck dead load and to simplify thefloor system. Asphaltic overlays are typical-ly used over the top plates of orthotropicdecks and can be replaced efficiently with-out a total deck replacement.

ECONOMICS

Welded plate girders are currently the mostcommon type of steel bridge. Applicable inspan lengths from approximately 90' toover 500' they provide a versatile choicefor the designer. As the span lengthsincrease plate girders do not necessarilyprovide the lightest steel unit weight, butthey often provide the most cost-effectivedesign because the fabrication and erec-tion costs are much lower than for trussesand arches.

Trusses are generally not cost-effective forspan lengths under 450'. Below 450' thelabor required to fabricate and erect a trusswill generally exceed any additional mate-rial cost required for a deck girder design.The practical upper limit for simple spantrusses is approximately 750' while contin-uous trusses begin to be cost-effectivewhen the span length exceeds 550'.Continuous or cantilever trusses can spanconsiderably longer distances than simplespan trusses. The cost of truss spansincreases rapidly as the span lengthexceeds 900'. In recent years, as cable-stayed bridges have become more com-mon and the technology refined, very fewtrusses longer than 900' have been con-structed in the United States.

Arches have been used for span lengthsfrom approximately 200' up to 1800'

worldwide. Arches used for the shorterspan lengths, either with tied arches or truearches, are generally used more for aesthet-ics than cost-effectiveness. Tied arches gen-erally can be considered cost-effective forspan lengths from approximately 450' to900'. Once the span approaches the 900'range erection costs begin to climb signifi-cantly. True arches have been used for spanlengths up to 1800', such as the recentlyconstructed Lupu Bridge in China. Solid ribarches are economical and attractive forshorter span lengths; however, once thespan lengths approach approximately1000' trussed arches often prove to bemore cost-effective.

Cable-stayed bridges have been construct-ed with main span lengths less than 200'and extending up to nearly 3000'. In theU.S., cable-stayed bridges become verycost competitive for main span lengthsover 750'. Almost all bridges with mainspan lengths in excess of 1000' built in theU.S. since 1980 have been cable-stayedstructures. They are more versatile thansuspension bridges in that they offer morepossible configurations and towers can beconstructed in less than optimal founda-tion conditions because having good rocknear the surface is not a requirement.

Prior to 1970, it was generally consideredthat suspension bridges provided the mosteconomical solution for span lengths inexcess of about 1200'. With the develop-ment of cable-stayed bridge technology,suspension bridges do not become eco-nomical until the main span lengthapproaches 3000'. The last major suspen-sion bridge constructed in the U.S. was thesecond William Preston Lane, Jr. Bridgenear Annapolis Maryland which was com-pleted in 1973. This bridge was a twin tothe original suspension bridge at the site.Suspension bridges require large expensivesteel castings for the cable saddles on themain towers, at rocker bents, at the ends ofthe side spans and at splay bents andanchorages.


CONSTRUCTABILITY

Rolled Beams/Welded Plate GirdersRolled beam and plate girder bridges aregenerally easily constructed, particularlyfor span lengths less than 200', whichencompasses a large majority of the steelbridges constructed in the U.S. For spanlengths under 200' the girders can gener-ally be erected with little or no falsework.Pier brackets are often used to provide sta-bility of the negative moment sectionsuntil the positive moment sections can beerected. As the span lengths exceed 200'falsework towers may become necessary toerect the girders.

Stability of the girders during erection alsobecomes increasingly critical as the spanlengths exceed 200'. Lateral bracing oradditional falsework towers may be neces-sary to assure adequate lateral stiffness orto assure proper positioning of the girdersfor long span and/or curved girders.

To open up the bidding on projects to thelargest number of fabricators possible, it isgenerally advisable to limit the length offield sections to a maximum length of120', although shipping pieces up to 160'have been used. Hauling permits becomemore expensive for longer sections, espe-cially for lengths greater than 140', and insome cases the piece weight may becomeprohibitive. The piece weight also affectserection because heavier cranes may berequired.

TrussesWhen compared with deck girder struc-tures, truss construction is a much morelabor intensive process. There are more dis-crete members to erect including not onlytruss members but lateral bracing, swaybracing and floor system members. Theweight and size of the members is usuallylighter than those encountered in deckgirder bridges. This allows the erector touse smaller cranes than might be neededfor plate girders. Truss bridges have morebolted field connections than are required

for plate girder bridges. These requireadditional labor to erect which generallymakes the erection time for trusses longerthan for girder bridges.

Simple span trusses usually require false-work towers adjacent to both piers in orderto facilitate erection. Dependent upon thespan length, the falsework location and thepanel length, it may also be necessary toprovide counterweights near the end piersto assure static equilibrium prior to closureof the span.

Continuous trusses can often be erectedusing a balanced cantilever approachrequiring falsework towers near the interi-or piers. This can be particularly importantwhen crossing navigable waterways whereit is necessary to maintain a navigationchannel at all times. While the falseworktowers for a simple span truss by necessityare placed adjacent to the navigationchannel, the towers for continuous trussescan usually be placed in spans adjacent tothe navigation span. This often allows thetowers to be lighter while minimizing therisk of the towers being struck by commer-cial traffic during truss erection.

ArchesArches are more complex to erect thantrusses or plate girders. The arch rib musthave temporary support during erection,either by falsework towers under the rib orthrough a system of backstays that supportthe ribs until they can be closed at thecrown.

In many cases the piece weights that areused for arches are heavier than the piecesused for truss members and, in some cases,those used for plate girders as well. Theerector also needs to exercise greater careduring erection to assure the proper struc-ture geometry than is generally requiredfor girders or trusses.

Another concern may be the height of thearch. In order to have a span-to-rise ratiothat provides an economical design, arch-es are generally higher (from the springline to the crown) than would be a truss


with a comparable span. The additionalheight, in conjunction with the heaviermember weights, can require significantlylarger cranes than would be required toerect a truss.

When over water, some tied arches havebeen erected on barge-supported false-work, floated into position and then low-ered onto the piers.

Cable-Stayed BridgesCable-stayed bridges are becoming morecommon worldwide. One significant rea-son for this is that the erection methodsrequired for cable-stayed bridges havebecome better understood over the past25 years.

The towers for cable-stayed bridges arevery tall relative to the span length and inmost cases consist of hollow concrete sec-tions. The towers carry very high loadsbecause of the long spans and must alsoresist significant bending. The foundationsare often relatively large, which may leadto constructability issues when crossingrivers or other bodies of water.

Once the towers are completed, the cable-stayed superstructures can be constructedwithout falsework towers, which can pro-vide significant benefits over navigablewaterways. The construction uses a bal-anced cantilever method proceeding awayfrom the towers until the spans can beclosed. The stay cable tensions may requireadjustment through the constructionprocess to assure the correct final forcesand bridge geometry.

Suspension BridgesErection of suspension bridges is relativelyspecialized. The installation of the mainsuspender cables requires expertise thatvery few contractors possess. Maintainingthe geometry of the main cables is critical.The cable saddles on top of the towersmay need to be adjusted horizontally dur-ing erection to keep the resultant loads onthe towers relatively vertical and to avoidhorizontal loads at the top of the towers.

The tower design may need to be heavierto provide the capacity to resist these loadsunless such adjustment is provided.

The time to construct a suspension bridgeis often relatively long as well. The founda-tions for the towers and anchorages areelaborate and time-consuming.

There are also numerous pieces to erectdue to the need to provide a stiffening trussto mitigate lateral movement of the bridge.In many cases, complete panels of the stiff-ening truss/floorbeams are erected togeth-er. The longitudinal stringers and deck, orthe orthotropic deck system, are generallyinstalled later. As with the cable-stayedbridges, faslework is usually not necessaryto facilitate superstructure erection.


Table of ContentsIntroductionRequired Span LengthsOwner DesiresHard RequirementsExisting ConstraintsOther Constraints

Bridge TypesRolled Beam BridgesWelded Plate Girder BridgesTrussesArchesCable-StayedSuspension Bridges

EconomicsConstructibilityRolled Beams/Welded Plate GirdersTrussesArchesCable-Stayed BridgesSuspension Bridges

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