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TIMBER BRIDGE DECKS

Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Figure No. 1 - Illustration of the major components of aLongitudinal Slab-Span bridge identified.

TIMBER BRIDGE DECKS

Comparison of Types, and Evolution of Design

Kenneth A. Johnson, P.E.

INTROD UCTIONCurrently there are about 45,000 Treated

Timber Bridges in the National Bridge Inventory(NBI). The largest proportion of these arelongitudinal timber slab span structures. There arenumerous reasons why this is the case. Thatpopulation of longitudinal treated timber slab spanstructures contains four distinct deck designs.

The purpose of this paper is to discussseveral aspects of these designs. These bridges cancategorized as follows:

• Nail-Laminated• Dowel-Laminated• Glued-Laminated• Stress-Laminated.

This type of bridge construction can be described asa structural system where the elements orcomponents span from substructure support tosubstructure support and form both the deck andthe main longitudinal support system for thevehicle loads. These decks do not havelongitudinal stringers supporting them. See FigureNo. 1 for the nomenclature of the majorcomponents of Longitudinal Slab Span Bridges.

There is some confusion andmisrepresentation concerning the characteristics,performance, and relative merit of these different

deck systems. Wheeler Consolidated, Inc. has beenin the Treated Timber Bridge Business since 1892and is the only timber bridge company that hasactually designed and produced all four differentstyles of longitudinal slab span deck systems.From the vantage point of over 110 years ofexperience in the timber bridge industry, and ofhaving produced thousands of treated timberstructures, we feel it is important, in fact ourobligation, to address all the confusionsurrounding slab span timber structures.

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Photograph No. 1 - A typical bridge build in the early1900’s constructed with native untreated round membersfor both stringers and deck.

Photograph No. 2 - This bridge is typical of many of thebridges constructed near the middle of the 1900’s, andwere built with large solid sawn timber stringers with aTransverse Nail-laminated Deck.

Photograph No. 3 - The primary design variables forstringers bridges are the stringer size and spacing. Asthe availability of large solid sawn stringers declined thestringer spacing became less and less and eventually theLongitudinal Nail-Laminated deck was born.

EVOLUTION OF SLAB SPAN DECKSThe first timber bridges had longitudinal

stringers with transverse timber decking. Thepurpose of the deck was to provide a surface tosupport the loads and distribute those loads toadjacent stringers. The very early bridges usedround logs for stringers and round poles fordecking with some earthen cover for the wearingsurface. See Photograph No. 1 for view of a typicalearly bridge constructed with round members. Assolid sawn lumber material became available, itwas used to replace the round poles for thedecking. Gradually the round log stringers werereplaced with solid sawn timbers.

As horses and wagons gave way to carsand trucks the demands placed on the bridgecomponents grew. Likewise, as the early truckswere replaced with larger and larger trucks thedeck systems changed. Most of the material usedin the early bridges was native untreated poles andlogs. As the vehicle loads increased both innumber and size, there was a shift from nativetimbers to species with higher strength propertiesand also to the use of preservative treatment. Thedominate specie used was Coastal Region DouglasFir, the use of which was influenced by theavailability of low cost, large, solid sawn timbers.Typical stringer sizes were 8 inches by 22 inches inlengths up to 48 feet.

The first timber decks employing solidsawn material involved transverse deck planks.The typical thickness was 3 inches. The design ofthese plank decks was based on the assumptionthat each wheel load was supported by a singleplank. As larger stringers became more readily

available, the stringer spacing was increased so asto utilize deck planks of 4 inches or more inthickness. The design of the plank deck wassubsequently modified using the Timber Strip Deckto increase efficiency of the larger stringer andwider stringer spacing. See Photographs No. 2 & 3for details of typical bridges with large solid sawnstringers and a Nailed-Laminated Transverse Deck.

This deck consisted of 3 inch thickmembers, 4 to 6 inches in width, placed on edgetransversely cross the width of the bridge.Sometimes 2 inch dimension lumber was used forthis design. The strips were then nailed to eachother with 20 penny (d) spikes. This deck wasattached to the solid sawn stringers with “toe-nails” placed at a given pattern. The design of thistype of timber deck was called a “Nail-Laminated

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Figure No. 2 - Nail-Laminated Longitudinal Decks werenailed n the field using dimension lumber generally 2inches in thickness and from 8 inches to 12 inches inwidth. There was no predrilling of holes prior to treatmentof the material.

Photograph No. 4 - Typical three-span Longitudinal Nail-Laminated timber bridge with timber substructures. Thisone has spans of 20 feet with a 24 foot roadway and thedeck thickness is 12 inches.

Deck.” As the width of our roads and bridgesincreased, a system of staggered butt-joints wereused with each butt-joint over a stringer.

This deck was considered a “ContinuousNail-Laminated Deck” as there were no transversejoints between large components. The timber deckwas idealized to be a continuous beam over thestringers with a width, in the direction of traffic, of15 inches plus the thickness of the deck itself. Thewidth of the wheel was defined by AASTHO 3.25.1.The wheel load for the design of this type of deck,for AASHTO HS-20, is 12,000 pounds, which has awidth of 17.32 inches.

The timber deck in this design does notprovide any additional longitudinal strength to thelongitudinal stringers since it only distributes thewheel loads and adds dead load. This decksystem enjoyed it greatest use in the late 1940’sthrough the 1960’s. However, as the loadscontinued to increase in both size and number,there was a corresponding decrease in thecommercial availability of large solid sawnstringers. The smaller stringer sizes availablerequired a reduction in the average stringerspacing. It didn’t take long for engineers to realizethat by letting the spacing between the stringers goto zero, they could eliminate the transverse deck.Thus was born the “Nail-Laminated LongitudinalSlab Span Deck.”

NAIL-LAMINATED DECKThis transformation in deck design

represented a major development in the moderntimber bridge. This design consisted of 2 to 3 inchthick treated planks with widths from 8 inches to 14inches. Planks were placed on edge and nailed toeach other with nails that penetrate, in most cases,only one and one-half laminates. See Figure No. 2for a partial cross section of a Nail-Laminated Deck.See Photograph No. 4 for a view of early Nail-Laminated Deck Bridge. The nailing pattern calledfor nails spaced at 6 to 9 inches, depending on thedepth of the deck, and were placed alternately nearthe top and bottom of the laminate. All of thenailing was done in the field. You can imaginestanding on the bridge deck, driving nails laterallyinto the deck, below where you are standing. Itwas extremely hard, back-breaking work.

Quality control in the construction of thesebridges was a major problem, due to the tendencyof construction crews to not place all of the nailsrequired for good load transfer. However, those

bridges which were constructed as designedperformed very well. Thousands of thesestructures were built in the 1950’s and 60’s, many ofwhich are still performing today.

The AASHTO approved design of this typeof deck was based on idealizing the deck to be abeam the depth of the deck, the width assumed tobe the width of the tire plus the thickness of thedeck itself. These bridges generally had atransverse spreader beam at mid-span. Thespreader beam was commonly 6 inches by 12inches made of the same specie and stress grade ofthe deck material. The spreader beams had aminimum EI of 1,555,200 kip-in2. Those bridges ofthis style normally had every tenth plankcontinuous over the support. This was occasionally

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Photograph No. 5 - Shown is one of the major designflaws of the field constructed Nail-Laminated bridges. Allof the individual laminates do not bear on the SpreaderBeam. Under load those laminates deflect down to thespreader beam causing crushing of wood fibers aroundthe nails resulting in loss of load sharing.

incorrectly interpreted as an effort to create someamount of continuous action over the support.This practice was intended only to help hold thetwo adjacent deck spans together.

One of the unfortunate characteristics ofthis type of bridge was that all of the laminates didnot bear uniformly on the spreader beam, this wasdue to the nature of field construction. Due to thevery nature of solid sawn timbers, very few wereperfectly straight. See Photograph No. 5 for a close-up view of the underside of a Nail-Laminated Deckand transverse spreader beam. The laminates wereplaced individually, with the instruction to place allmembers with any curvature up. This resulted in acondition where all of the laminates weresupported uniformly on the substructure caps, butdid not provide a smooth uniform surface at mid-span on the bottom side of the deck, where thespreader was located. Those planks not bearing onthe spreader beam assumed a disproportionateshare of the imposed load. This in turn causeexcessive stress on the nails transferring the loadsto the adjacent laminates and resulted in thecrushing of wood fibers reducing the loadtransferring ability.

The engineers at Wheeler looked for waysto make the bridge easier to erect in the field and tohelp make the spreader beam more effective. Thedesign that evolved addressed both concerns. Thesolution was to prefabricate the deck sections in theshop making the bottom surface of the panelssmooth and even at mid-span. The resulting

product evolved into the “Wheeler Dowel-Laminated Panel-lam Deck System.”

DOWEL-LAMINATED DECKSThe development of the Wheeler Dowel-

Laminated Panel-lam Deck System started in theearly 1970’s. It is properly called a Dowel-Laminated Deck System, but sometimes is referredto as a “Spike-Laminated Deck.” The concept wasto produce a system of deck panels that could beeasily placed in the field and one that would besuperior to the Nail-Laminated System in loadcapacity. See Photograph No. 6 for close-up of aWheeler Dowel-Laminated Deck Panel.

Photograph No. 6 - Shown is a newly manufacturedDowel-Laminated Deck Panel which is smooth on thebottom surface as all of the variation in plank depth is inthe top surface, also shown is the top half of the Ship-LapJoint which connects the panels to each other.

Wheeler designed and constructed themanufacturing facilities necessary to produce atruly “Manufactured Product.” Cutting anddrilling of the timbers would be done prior topreservative treatment. See Photograph No. 7 forview of drilling machine. The thickness of theindividual laminates was increased to 4 inches tominimize the number of pieces to be handled in themanufacturing process. The individual laminatesare passed through a timber sizer so as to producelaminates of a uniform thickness, normally 3.875inches. This is necessary to produce panels ofuniform width over their entire length.

The connecting hardware was also changedto provide better load distribution among thelaminates. Special “Ring-Shank Dowels” 3/8 inchin diameter and 15 1/2 inches in length weremanufactured for this purpose. The word “Dowel”

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Figure No. 3 - The Dowel-Laminated Deck Systemconsists of Laminates that are 4 inches in thickness andrange from 8 inches to 18 inches in depth. They areconnected with steel Dowels that are installed in ahydraulic press.

Photograph No. 8 - This is view of the hydraulic pressused at Wheeler’s plant to manufacture Dowel-LaminatedDeck Panels that have a smooth surface on the underside.

Photograph No. 7 - This is a view of the assemble line atthe Wheeler Consolidated’s bridge manufacturing facilityshowing the Gang-Drill which drills all of the holes for theDowel Connectors. All drilling is done prior to treatment.

means a cylindrical fastener that penetratesmultiple laminates. The Dowels in the WheelerPanel-lam System penetrate four laminates. SeeFigure No. 3 for a partial cross section of a Dowel-Laminated Deck. The pattern of the Dowels -function of the span length, design load, and deckthickness - involves two lines of Dowels, one nearthe top of the laminate and the other near thebottom, spaced at one foot. Longer spans mayutilize three rows of dowels placed at one footcenters.

The laminates are assembled, afterpreservative treatment, into panels approximately 7feet in width. The actual width of the panels is afunction of the width of the structure, but in nocase does the width of the panel exceed the widththat can be easily shipped via trucks withoutspecial permits.

The panel are created in a large hydraulicpress where the individual laminates are installedin pairs. See Photograph No. 8 for view of panelpress. All of the steel dowels for two laminates areinserted simultaneously. The press has provisionsfor creating approximately 1 inch of camber in thepanel. This camber is not intended to off-set deadload deflection, rather it is the result of a systemthat produces a panel that not only bears uniformlyon the ends, but also is smooth and even at mid-span so that all laminates bear uniformly on thespreader beam. The manufacturing processdeveloped by Wheeler can produce panels of anywidth, any angle of skew and any length up toabout 70 feet.

The panels are connected to each other inthe field by means of a Longitudinal Ship-Lap Joint.This joint consists of a half-laminate attached to thebottom half of the side of one panel. The otherhalf-laminate is attached to the top half of the sideof the adjacent panel. These half-laminates areattached to the panels in the shop. See Figure No. 4for a detail of the Ship-Lap Joint used with theDowel-Laminated Deck System. See PhotographNo. 9 showing a Dowel-Laminated Deck Panelbeing place. Dome-head drive spikes are thendriven down vertically to connect the two half-laminates to complete the joint. A spreader beamfor each span is installed at mid-span. Longerspans, over 32 feet, may have multiple spreaderbeams. The minimum size of the spreader beam is6 inches by 12 inches, with the 12 inch dimensionvertical. Just like the spreader beams for the nail-laminated decks, these spreader beams have a

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Photograph No. 9 - Construction of a Dowel-LaminatedLongitudinal Slab-Span Bridge is shown. The bottom halfof the Ship-Lap Joint Plank can be seen. The SpreaderBeam is connected to the completed deck using theholes for the lifting eye-bolts.

Figure No. 5 - The structural analysis of the Dowel-Laminated Deck System is based on the idealization of alongitudinal beam the width of which is defined by theDistribution Width.

minimum EI of 1,555,200 kip-in2. The spreaderbeams are attached with 3/4 inch diameter dome-head bolts through the deck. The bolts attachingthe spreader beam are placed in the pre-drilledholes which previously held eye-bolts used to liftthe panels.

The purpose of the spreader beam, forPanel-Lams, is to increase the lateral distribution ofwheel loads to a wider portion of the deck. It doesthis by increasing the transverse stiffness of thedeck system, requiring much larger size of thespreader beam, than the stiffener beams used inglulam deck system. The purpose of “stiffenerbeams” used in glulam deck systems is to resistinter-panel shear, hence the much smaller size witha minimum EI of 80,000 kip-in2. The stiffness of thespreader beams used for Wheeler’s Panel-LamDecks are about 19 times stiffer than the stiffenerbeams in the Glulam System.

Figure No. 4 - Shown are the details of the Ship-LapJoint that is used to connect adjacent deck panels. Thespacing of the vertical Drive Spikes is designed to createcomposite action of the two half-planks used in the Ship-Lap Joint. Single-Member bending resistance isachieved.

The design algorithm approved byAASHTO for this type of bridge has undergonesome changes over the years. Initially, whenWheeler first introduced its Dowel-LaminatedPanel-Lam Deck System, the deck was idealized asa beam the depth of which was equal to the deckthickness and the width of the beam was assumedto be the width of the tire plus one thickness of thedeck. This was the same distribution width usedfor Nail-Laminated decks. However, after theconstruction of a few of these new bridges it wasvery apparent that this deck system offered greaterload distribution over the old Nail-Laminateddesign.

Wheeler conducted full-scale tests of thissystem to validate the load sharing properties of

the deck and to test the Ship-Lap Joint. This testingwas conducted in the spring of 1978 at its CassLake Bridge Fabrication Plant. The testing wassupervised and observed by Twin City Testing,Inc., a independent material testing andengineering firm. Numerous load tests wereperformed on various aspects of a full-scale bridge.The analytical results of the tests were presented atthe 1980 meeting of the AASHTO Timber BridgeCommittee. That Committee accepted the resultsand presented it to the full AASHTO BridgeCommittee which approved a change in theirStandard Bridge Specifications to increase thedistribution width for timber decks asmanufactured by Wheeler. The change allowed thewidth of the idealized beam to the width of the tireand twice the thickness of the deck. See Figure No.5 for illustration of the Distribution Width used for

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

the design of Dowel-Laminated Decks. Thischange was predicated on the conditions of the full-scale tests conducted by Wheeler. Those conditionsincluded a 6 inch by 12 inch spreader beam at midspan and a Ship-Lap Joint between panels.

Thousands of bridges utilizing this designhave been built across thenation. See PhotographNo. 10 for a typical Dowel-Laminated LongitudinalSlab Span Bridge. These structures have a numberof very positive attributes that may be overlookedas they perform their function decade after decade.

Photograph No. 10 - This is a typical three-span Dowel-Laminated Longitudinal Slab-Span Bridge. This bridgehas a Crash-Tested Railing.

Some of these advantages are subtle and may beonly appreciated by bridge engineers. The entiresystem is very redundant as there are numerouslongitudinal members with massive load sharing,to the point where the weakest member no longerdefines the system’s strength. Coupled with theuse of many ductile fasteners creates a system withthousands of load paths, the weakest member doesnot determine the strength of the system.Additional, the fact that each of the individuallaminates are treated after all of the cutting anddrilling is done and prior to assembly into panels isa major advantage. The panels are somewhatanalogous to ships with many water-tightbulkheads, a breach in one of them does not imperilthe entire panel. The completed panels have amuch higher percentage of the entire volumeimpregnated by the preservative than most othertypes of timber deck panels.

The manner in which the panels areassembled creates an upper surface that is ideally

suited to receive a bituminous wearing surface. Bymanufacturing a panel with the bottom surfacesmooth, all of the variance in size in the depth ofthe individual laminates occurs in the top surface,thus creating many gripping surfaces for thebituminous mixture.

Another feature is the fact that the panelsare produced in widths almost twice as wide asother types of panels, producing decks with lesstotal linear feet of joints where possible reflectioncracking of the asphalt may occur.

An example would be a typical 100 footlong, 4 span bridge with a 24 foot roadway. TheDowel-Laminated bridge would be constructedwith 4 panel wide with three joints for a totallongitudinal joint length of 300 feet. While thesame example bridge, constructed with GlulamPanels, would be 6 panels wide with 5 joints for atotal longitudinal joint length of 500 feet.

In an attempt to minimize joints, some ofthe more recent Glulam Designs, have specifiedpanel lengths that span two spans. In the examplebridge above, a Glulam design would have onetransverse joint of 24 feet, while the Dowel-Laminated would still have three transverse jointsfor a total of 72 feet. Therefore, the combined jointlengths for the Dowel-Laminated bridge would be372 feet, while the Glulam bridge would be 524feet, this results in approximately 40% more thelength of joints than the Dowel-Laminated System.

Perhaps more importantly, is the fact thatthe Dowel-Laminated Panels have a directmechanical connection using a ship-lap joint withinthe panels themselves, whereas the Glulam Panelsare only supported at a couple of point along thelength of the span.

GLUED-LAMINATED PANELSLongitudinal timber bridge deck systems

utilizing Glued-Laminated Panels were developedin the 1970’s by U.S. Forest Service at the ForestProducts Laboratory at Madison, Wisconsin. Thepanels used were generally 4 feet in width andcould contain multiple piece laminates on thedeeper deck sections. The width of the panels isrestricted due to the limiting size of treatingcylinders. The major design problem to beovercome was found to be the transfer of shearbetween adjacent panels.

A number of different schemes for sheartransfer were investigated. They included tongue-

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Figure No. 6 - This is the most commonly usedanchorage system used for Stress-Laminated Bridges inthe U.S.

and-groove joints, splines, and steel dowels. Earlywork seemed to favor the use of steel dowels as themost effective vertical shear transfer mechanism.This is one example of an idea that looked good onpaper, but doesn’t work as intended in reality. Ifthe panels and the holes for the steel dowels arefabricated with tolerances necessary for the sheartransfer to occur without undo differential paneldeflection, the dowels cannot be installed. If theholes for the dowels are drilled with sufficientoversize to allow for construction, the sheartransfer is not achieved without differentialmovement. Other investigation were started tofind a substitute for the steel dowels.

Early in the 1980’s research was conductedby W.W. Sanders at Iowa State University at Ames,Iowa, investigating the use of transverse membersunder the deck, called “Stiffener Beams”, to resistinter panel shear and to replace these dowels.These stiffeners are relatively small members,typically about 4 inches by 6 inches and can beattached to the underside of the deck in a numberof ways. Normally there is one Stiffener Beam atmid-span, and others spaced not to exceed 10 feet.These Stiffener Beams normally have an EI of80,000 kip-in2, and must not be confused with themuch larger spreader beams used with the dowellaminated system.

The design of this type of deck system isbased on the determination of a Load Fraction,which is the portion of a wheelline load assumed tobe carried by an individual panel. The LoadFraction is a function of the width of the panel, thespan length and the number of lanes of traffic.

The Glulam Panels used for this type ofdeck are normally treated after gluing. Thepreservative envelope is therefore restricted to onlythe perimeter of the panel, unlike the Dowel-Laminated Panels where each individual laminateis treated prior to assembly into panels. As suchGlulam Panels have a much lower percentage oftheir total volume of material impregnated withpreservative. Long-term durability andserviceability are directly related to the amount ofpreservative in a component. The Glulam panelshave relatively smooth surfaces and do not providegood adhesion for the asphalt mixture.

Unfortunately, Glulam Deck Panels tendto assume an “hour-glass” shape due to greatermoisture absorption through the end grain at theends of the panels. This is in part due to the factthat the moisture content of the lumber material

going into the panel is at a much lower moisturecontent than the equilibrium moisture content ofthe completed panel prior to installation in thebridge.

Some timber bridges lend themselves tocontinuous spans. Having deck panels that arecontinuous over three or more supports may offersome advantages over simple spans. However, thesuggestion that Continuous Glulam Panelseliminate a transverse crack in the bituminouswearing surface over the intermediate support isnot always true. There may develop tension cracksover the intermediate support. Additionally, theconnections of the ends of continuous deck panelsmust be designed to withstand repetitive up-liftloads resulting from the negative moment over themiddle support.

STRESS-LAMINATED DECKSStress-Lamination is a construction

technique originally developed by the OntarioMinistry of Transportation and Communication forfabricating large structural components usingshorter-than-span-length timbers and high-strengthrods. The basic concept is to achieve load-transferbetween individual members by the friction forceinduced by normal pressure created by post-tensioning of transverse rods. See Figure No. 6 fora partial cross section of a Stress-Laminated Deck.

Wheeler was the very first Company orPublic Agency to investigate the application of thissystem to highway bridges in the United States.Starting in 1985, Wheeler funded research by M. G.Oliva at the University of Wisconsin, at Madison,Wisconsin on the application of this structuralsystem to aspects unique to the United State’sbridge market.

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Photograph No. 11 - A construction scene of a Stress-Laminated bridge. High-strength steel rods are placedtransverse to the laminates which have been predrilled.The rods are stressed with a hollow core hydraulic jack.

Based on the results achieved by ProfessorOliva, Wheeler Consolidated designed andfurnished the first Stressed-Laminated TimberBridge in the U.S. The bridge was constructed inFebruary of 1986 in Cook County, Minnesota, onthe Gunflint Trail Road.

The basic design of this type of bridgeconsists of solid sawn members 2 to 4 inches inthickness and from 8 inches to 16 inches in depth,with lengths generally less than 20 feet. A typicallayout of the laminates has a pattern of butt jointsthat repeats itself every four laminates. Hole aredrilled in the laminates at some interval, generally2 or 4 feet. See Photograph No. 11 for a view of theconstruction of a Stress-Laminated Bridge. Highstrength steel rods (150 ksi) are placed in the holeswith suitable anchorage plates at each end of therods. The rods are then tensioned with a hollowcore hydraulic jack. Sufficient tension is impartedto each rod so as to create approximately 100 psipressure between all interfaces of all laminates.

Due to the large amount of creep in thewood under compression, the tension in the rodsmust be reestablished several times in order to

ensure sufficient long-tern friction forces existwithin the deck system. The level of long-termstress in the system is a function of a number offactors, some of which are not well understood.

The system does offer some advantagesover the other deck systems discussed. Forexample, it is very efficient in creating the loadsharing amongst the laminates. However it doesnot increase stiffness and consequently thecontrolling criteria for these decks is generally live

load deflection. Also, even with outstandingstrength characteristics, these systems have notproved to be very cost effective. This coupled withthe concern over the loss of stress over time, aremajor deterrences to the use of these systems.Wheeler is currently not recommending this typeof design to it’s clients. Research work on thissystem continues and perhaps someday this designwill be a viable alternative.

CONCLUSION SThis paper begain with the description of

four different timber deck systems. Experience hasindicated that of these four designs, only twosystems have the minimum attributes to rise to thelevel necessary for serious consideration for use intodays bridges. Those two systems are:

• Dowel-Laminated• Glued-Laminated.These two deck systems, while performing

the same basic function within the bridge, are quitedifferent in several important aspects that impacttheir performance. This discussion will not focuson those aspects of Slab Span Bridges in generalthat make them attractive for sites with certainrequirements, such as low head-room.

A very important difference between thesetwo systems is the amount of wood actuallyimpregnated with a preservative. As previouslystated, the Dowel-Laminated Deck Panels have amuch higher proportion of their volume actuallycontaining the wood preservative. Again,durability of treated timber components is directlyrelated to the amount of active preservative in thecomponent.

Following in importance, is the amountof both longitudinal and transverse joints betweenpanels. Again, the Dowel-Laminated Panels offersuperior performance in that they are are suppliedin much wider widths resulting in less total lengthof joints. This occurs even where there might be anopportunity for glulam panels to be continuousover a pier. Moreover, the Dowel-LaminatedPanels are directly connected to each other by theShip-Lap Joint.

Probably the next most significantcharacteristic is the adhesion of the bituminousmixture to the deck surface. Dowel-Laminateddeck panels offer an excellent surface for thewearing course to adhere. Obviously there areadditional areas that could be pointed to asdifferences favoring one or the other, but these are

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Wheeler Consolidated, Inc. Kenneth A. Johnson, P.E.St. Louis Park, Minnesota April 10, 2002

Photograph No. 12 - Here is a typical urban setting for aDowel-Laminated structure. This bridge has a sidewalkon one side that also contains a utility chase.

Photograph No. 13 -- This is a Three-Pin Arch with aspan of 120 feet. The deck system consists oftransverse floor beams spaced at 20 feet and Dowel-Laminated Longitudinal Deck Panels spanning betweenthe floor beams. This bridge carries two sidewalks andseveral utilities.

the differences that dictate long-term performanceand serviceability. In addition to physicaldifferences listed above, there is usually a costsavings of the dowel-laminated system over theglulam system.

I am confident that when the facts aboutTimber Deck Systems now available to bridgedesigners and owners are reviewed, the WheelerDowel-Laminated Panel-Lam Bridge Deck Systemwill be the Deck System of choice. See PhotographsNo. 12 - 15 for examples of the range of bridgesusing Wheeler Components. We also want ourclients to know that we are continuing to look forways to improve our bridges. Wheeler is currently

supporting research at the University of Minnesotaby H. Stolarski, who is currently developing ananalytical model of the Dowel-Laminated DeckSystem. Bridge engineers will be able to use theanalytical tools being developed by ProfessorStolarski to design for longitudinal stiffness of thedeck by changing the size and number oftransverse spreader beams. Additionally, we atWheeler are constantly in touch with both ownerand contractors to find better ways of designingand building bridges. We are concerned that thehigh quality and close tolerances in ourmanufacturering are reflected in the ease ofinstallation and the final product.