ERECTION PROCEDURE FORGLUED-LAMINATEDTIMBER BRIDGE DECKSWITH DOWEL CONNECTORS

USDA FOREST SERVICERESEARCH PAPERFPL 2631976

U.S. DEPARTMENT OF AGRICULTUREFOREST SERVICEFOREST PRODUCTS LABORATORYMADISON, WIS.

ERECTION PROCEDUREFOR GLUED-LAMINATEDTIMBER BRIDGE DECKSWITH DOWEL CONNECTORS

1

ByROGER L. TUOMI, EngineerForest Products Laboratory,2 Forest ServiceU.S. Department of Agriculture

INTRODUCTION

The versatility and the durability of woodcontribute to its importance as a material forbridge construction. Timber bridges are at-tractive both for economy of cost of materialand ease of erection. Because of low weight-to-strength rat io, relat ively l ight l i f t ingequipment can be used. Erection requires aminimum of skilled labor, and can proceedeven under adverse weather conditions. Pre-fabricated assemblies can be quickly in-stalled at a site with a minimal disruptionof traffic.

Wood is inherently durable; records es-tablish that timber bridges have providedservice for more than 500 years. Manycovered bridges built more than a centuryago are still in service, although their con-struction preceded the development of pre-servative treatments. The secret of theirlongevity has been a design which providesfor protection of the superstructure.

Timber bridges are probably best suitedfor rural areas and secondary roads for spansup to 50 feet or so. They are estheticallypleasing in these settings (fig. 1). These arealso areas where plant-mixed concrete andstructural steel are not readily available.Local contractors generally can handle tim-ber bridge jobs since specialized labor andequipment are not essential. An added bene-fit is that wood is relatively inert chemicallyand is unaffected by salt or other de-icingagents, a s e r i o u s p r o b l e m w i t h o t h e rmaterials.

Experience has shown that the bridgedeck is the component most vulnerable todecay. For many years the nailed-laminateddeck was the standard of construction. Re-cently, the glued-laminated deck system wasdeveloped,3 and a design procedure wasadopted.4 This system provides protectionto the superstructure in a manner much likethe covered bridge. A tight deck with water-resistant wearing surface, coupled with pre-servative treatment, will ensure the servicelife of the modern timber bridge to 50 ormore years.

The success of any construction job de-pends on two major factors: Accurately fab-ricated materials and correct erection pro-cedure. The guidelines presented here arebased on the experiences gained on eightbridge jobs. Each was by a different crewwith no past experience in this type of work.Forest Products Laboratory engineers wit-nessed a number of these jobs, and madesuggestions, based on prior observations, tonew work crews. All crews tended initiallyto make the same mistakes, but after efficientprocedures were worked out, constructionprogressed very well.

3 McCutcheon. W. J. and R. L. Tuomi. 1973. Proceduresfor design of glued-laminated orthotropcc bridgedecks, USDA For. Serv. Res. Pap. FPL 210. For. Prod.Lab., Madison, Wis.

1 Presented at Northwest Bridge Engineers Conference,Boise, Ida., Sept. 16-18. 1975.

2 Maintained at Madison, Wis., in cooperation with theUniversity of Wisconsin.

4 American Association of State Highway and Trans-portation Officials. 1974. lnterim specifications forbridges.

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Six of the bridges were erected by ForestService Construction and Maintenance crewsand two by contractors. Three bridges erectedby the Forest Service were completed inwinter in near-blizzard conditions. Concretework or field welding would have been im-practical at these times. Winter constructionwas elected because it was the only practicalseason to close the roads for a short period;workloads were down and manpower wasavailable. Winter construction provided anadded advantage: Rivers were frozen, andworkmen had access to the underside of thebridge without the need to erect scaffolds orplatforms (fig. 2).

Probably the best example of a smootherection job was one of the two contractor-built bridges. The bridge was 72 feet long by26 feet wide. A crane was at the site 1 day,set the stringers, and placed the deck panelsloosely on top of the stringers. The crane wasthen removed. A crew of from three to fourthen completed all deck work, installed dia-phragms, and set guardrails in only 1-1/2days (44 man-hours). Obviously, all went wellon this job.

The procedures presented here for bridgeconstruction should both prevent commonmistakes and expedite efficient construction.Undoubtedly, with added experience furtherimprovements will evolve.

Figure 1.—Timber bridge complements natural setting.

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Figure 2.—Because erection of timber bridges requires minimum laborand equipment, work can progress under winter and other ad-verse weather conditions.

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F A B R I C A T I O NIn general, our experience with eight

laminated Forest Service bridges indicatesthat the laminators have done a satisfactoryjob in fabricating members to the require-ments of contract plans and specifications.These jobs have demonstrated that the toler-ances possible in glulam (glued-laminatedtimber) plants will result in a minimum oferection problems through misfabrications ifmaterials are specified as meeting the re-quirements of PS 56-73, Products Standardof the Department of Commerce.

Some minor problems have been experi-enced on other jobs in the overall length ofbridges. The final width of a panel tends tobe slightly wider than the nominal width.

For example, a 32-lam panel often ends upslightly wider than 48 inches. Also, a slightamount of sweep may occur in the panels ifthe clamps are not perfectly alined, and makegetting a tight field joint difficult. Finally,the mastic sealer in field joints adds slightlyto the effective width. These factors are ac-cumulative, and the completed deck tends to“grow” in overall dimension.

An experienced laminator overcomesthese potential problems by trimming bothedges of all panels. The centerline is firstestablished. Then reference lines are laid outon each side of the centerline so that thefinished panel will be 1/8 inch less than thenominal width. The edges are then trimmedto square the panels or to remove any possi-ble sweep.

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One bridge engineer designs decks to besl ight ly shorter than the overal l br idgelength. He makes up the closing dimensionswith treated pieces of nominal 2-inch lumber.He reasons that the leading corners of thefirst panels at the approach ends of thebridge are subjected to mechanical damagewhen a vehicle first contacts the bridge. Ifdamaged, these end pieces can easily be re-placed without disturbing the deck.

Laminators must know exactly what isneeded so that the components will fit to-gether in the field. The precise dimension ofcomponents, hole sizes, and hole locationsmust be clearly defined. The quality of ma-terial must be specified. A specification anda sample shop drawing are provided in theappendix.

All final cuts and hole drilling shouldbe completed at the fabrication plant beforepreservative treatment to ensure that po-tential decay areas are protected.

Grades of materials will vary across thedepth of stringers, with the high qualitymaterial placed in the outer surfaces wherestresses are greatest. This is not possible fordeck panels; therefore the same grade shouldbe used throughout. Low-grade material isgenerally suitable for deck panels. Savingscan at times be realized by specifying a one-size greater thickness in a lower grade ma-terial. This depends on material availability,grade demands, and inventory.

Precise drilling of dowel holes is essen-tial. The holes must be spaced equally alongthe panel edge centerlines, and the axes ofthe holes must be perpendicular to the paneledge, so that in the field the dowels will

aline. A drilling jig or template should beused. The necessary precision cannot beachieved with hand-held drills. Some fabri-cators have used gang drills so that severalholes can be drilled at one time. Others havedevised a jig whereby a dowel is affixed tothe drilling apparatus. The dowel is insertedinto the previously drilled hole as a guideto ensure exact spacing between holes. Holesshould be drilled to a depth 1/4 inch deeperthan one-half the dowel length.

The question of hole diameter toleranceoften arises. For optimum performance, thedowels must fit fairly snugly into the holes.Most designers specify galvanized dowels.Measurements have shown that galvanizingadds about 0.015 inch to the nominal doweldiameter. Also preservative treatment causesa small degree of swelling which reduces thehole diameter slightly.

A method that has worked well is to drillthe holes first with a wood drill equal to orslightly smaller than the nominal dowel diam-eter, next ream the holes with a steel drill1/32 inch larger than the dowel diameter.This results in a good fit in the field. Wooddrills become dull rather quickly when usedfor drilling the finished hole, and the holestend to become slightly oblong, and have afuzzy texture which impedes insertion of thedowel. Steel drills, by contrast, produce asmooth perimeter surface. An actual jobdowel should be used at the plant to checkthe fit prior to production drilling.

Although accuracy in drilling is neces-sary, such precise fabrication is not difficult;fabricators can meet the requirements witha reasonable amount of care.

T R A N S P O R T A T I O NTimber bridge components are usually

shipped by truck or by rail or by combina-tions of the two. Treatment plants are gen-erally located in areas separate from thefabrication plant so that an intermediateshipment is involved. In many cases, all ofthe material for a complete bridge can becarried by a single truck.

Most bridge materials are treated eitherwith creosote or pentachlorophenol in petro-leum oil so that protective wrapping is notneeded for shipment. Wrapping is necessaryonly for architectural-or appearance-grade

mater ia l i f esthet ic appeal is of majorimportance.

It is common and acceptable practice tonest or to bundle a number of pieces. How-ever, banding can damage the edges of amember if it is not properly protected (fig. 3).A nominal 2-inch piece of lumber should beplaced across the width of the panels in linewith the bands. This also provides an accessarea for insertion of lifting equipment.

Some procurement people have the mis-conception they should seek bids only fromfabricators near the bridge site. Apparentlythey believe that transportation is the con-troll ing cost factor. Although important,transportation is not the major cost. The

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laminating industry, like all other businesses, nate from plants 2,000 miles from the bridgeis competitive. Often, the lowest bids ema- site.

Figure 3.—Note improper banding of panels has caused corner damage.

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H A N D L I N GThe relat ively l ight weight of t imber

components provides a wide latitude forchoices in lifting equipment. The final choiceusually depends on the availability of equip-ment near the bridge site. Cranes are prob-a b l y t h e m o s t d e s i r a b l e ; t h e r u b b e r -mounted hydraulic units, the most mobile.Log buckers (fig. 4), end loaders (fig. 5), andforklifts have been used.

In handling stringers and deck panels,do not lift by the edges parallel to the wideface of the member. This can induce highbending stresses across the wood grain,which can cause structural damage. Themembers should be supported across thewide face (fig. 4) or lifted at the ends (fig. 5)with “C” brackets. Fabric slings (fig. 6) workwell as lifting members, but the loop shouldbe positioned at a corner.

At no t ime should the members bedragged or skidded about the work area.

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Figure 4.—Large panels can be handled with light equipment. Propersupport across wide face is required during handling.

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Figure 5.—Small end loader used to position panels on stringer. Properend support is shown for short panels.

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Figure 6.—Here proper use of slings for lifting panels is shown. Loopshould be positioned at a corner so that panels ride vertically.

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S T O R A G EProper storage of materials is an impor-

tant operation often overlooked. In one casematerials were dumped in a pile on theground, left for an extended period, and be-came warped; thus erection was made dif-ficult. Problems with delivery schedules orwork delays intensify these kinds of storageproblems.

Properly stored, the members will re-main straight and dry. It is also much easierto get at the desired pieces with lifting equip-ment if there is some preplanning in storagelocations.

All material on the worksite should bestacked to prevent warping. The ground be-neath the stacks should be cleared of weedsand rubbish, and leveled. The bottom panelshould be about 12 inches above the ground,and supported on level blocks. Spacer pieces,approximately 2 inches thick, should be in-serted between panels. They should extendacross the wide face of the members; thisallows for free circulation of air, and providesaccess for lifting equipment.

The spacer blocks should be alined ver-tically and spaced at regular intervals (fig. 7);otherwise, the members will be subjected tobending stresses, and might warp duringextended storage.

When properly stacked the treated ma-terial does not have to be covered. Free cir-culation of air is all that is necessary. Imper-vious membranes, such as polyethylene film,only trap moisture that evaporates frombelow.

Figure 7.—Improper positioning of spacer blocks induces bendingstresses during storage. Blocks should be alined vertically.

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E R E C T I O N P R O C E D U R EAlthough this Paper relates basically to

installation of the deck system, placement ofstringers is important. The stringers mightappear perfectly straight and symmetrical,but they do have top and bottom sides. Aslight camber is generally built into them toaccount for deadweight deflection. Also, asmentioned, the stringers are not of a bal-anced design. Special materials are selectedfor the tension and the compression sides sothey must be installed with the proper orien-tation. Check first for topside markings.

After the stringers have been set, allpanels should be loosely stacked upon thestringers. This frees the lifting equipment,and provides a good work platform for thecrew. From this point, the panels can be slidinto position by hand without undue effort.With proper placement, a panel need only bemoved a few feet. A typical panel placementlayout is presented in the appendix.

The bridge builder should first checkpanels for sweep. If present, the effect canbe minimized by placing all panels with thesweep in the same direction. (The problem iscompounded if adjacent panels are posi-

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tioned with sweep in opposite directions.)Thus to minimize the effect of sweep, simplyturn the involved panels end for end or flipthem.

The force for joining panels can be ap-plied either by pulling or by pushing. Forpulling, use either come-alongs or coffinghoists (fig. 8); for pushing, horizontal jackssuch as railroad jacks (fig. 9). Both methodshave been used successfully, but the hori-zontal jack is more positive and easier tocontrol.

In the pulling operation shown (fig. 8),a rakelike tool was fabricated to fit the lead-ing edge of the panel being pulled. Thehandle portion was about 6 feet long; thiskept the line of force low, and reduced thetendency to lift the edge of the panel. Acable or a chain was dead-ended at the start-ing end of the bridge. It was let out or ex-tended as work progressed toward the far endof the bridge.

With the jacking procedure (fig. 9), thebridge was erected on a 30° skew so thejoints were not perpendicular to the stringers.Stop blocks were inserted at the far end of thebridge, and the jacking force was appliedagainst the loose panels, which were buttedtightly together. With one jack near each endof the panel it was easy to control the force. Itis extremely important that the panel edgesremain parallel during the jacking or thepulling operation.

The most serious problem that can de-velop during erection occurs during insertionof the dowels. Each new crew has a tendencyto insert the dowels fully into one panel,then try to aline the second panel. This pro-cedure will cause problems since all thetolerances are used up at the outset. Do notinsert dowels fully into one panel.The dowels should first be fed into thesecured panel by hand. They should bepushed in far enough so that they will stay

Figure 8.—Panels are pulled together with come-alongs or coffinghoists. Here rakelike tool is shown that is used to grip edge ofpanels.

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Figure 9.—Horizontal railroad jacks provide positive method of joiningpanels. This bridge was erected on a 30° skew.

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in place temporarily. Next the panel to bejoined should be slid forward until the gapbetween panels is about 1 inch less than thetotal dowel length. The dowels should thenbe pulled out of the first panel, and only thetips should be started into the holes of thesecond panel (fig. 10). With only the tipsstarted into the holes of the two panels beingjoined, the dowels will follow the line of leastresistance, and are essentially self-alining.The force required for joining panels with thistechnique is minimal.

Should one panel end get very far aheadof the other, the dowels will tend to bind.They can be freed with an impact blow froma sledge hammer to the trailing edge, but firstinsert a wood block against the edge being

struck. Do not strike directly against the edgeof the panels. If care is taken to keep theedges of the two panels parallel, this prob-lem should not occur.

The first, or starter, panel should besecured at the end of the bridge. The two endpanels (first and last) should have dowelholes drilled only on one edge. Check over-hang periodically to assure that the panelsare centered. Otherwise, the panels may driftoff line; it is difficult to make corrections ifthe drift is not discovered early. Anothermethod is to mark reference points on thepanels and stringers to check alinement.Except for the starter panel, the panelsshould not be spiked until they are all inplace. A couple of spikes driven only par-

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highest quality contain asbestos fiber fillersfor reinforcement. Mastics should be gooeyenough to adhere to a vertical surface.

The final operation is spiking the deckin place. Before driving the spikes, be sureto drill lead holes. This is the only time inthe field that there should be any drilling.For maximum withdrawal resistance, thehole should be approximately 70 percent ofthe spike diameter.

Use the factory-drilled holes in the deckfor guides when drilling lead holes into thetop of the stringers. The depth of the holesshould be equal to or slightly deeper thanthe penetration depth of the spikes.

Before the spikes are driven, the leadholes must be treated in the field with pre-servative (fig. 11). The same type of preser-vative used for treating the deck shouldbe used. An oil can or a grease gun with anozzle works well for directing the flow ofpreservative into the hole. When the spikeis driven, the preservative is forced into thesurrounding wood.

The spikes used for these bridges haveeither annularly or helically threaded shanks,and have a withdrawal resistance approach-ing that of lag screws. Generally, the tendencyis to specify longer lengths than necessary.The withdrawal resistance should equal theyield stress of the spike based on its netroot area. This occurs in Douglas-fir and

Figure 10.—Workman starts hand-feeding dowel tips into second panel.In alining dowels and holes, a gap should be approximately 1 inchless than dowel length. Only tips of dowels should be fed intopanels prior to jacking.

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tially down will hold the panels in place.The vertical edges of the panels should

be coated liberally with a heavy mastic be-fore they are joined. This provides an effec-tive seal to prevent water from penetratingthe deck. A slight extrusion of mastic at thetop of the deck indicates a tight fit.

The mastic can be either a coal tar baseor an asphalt cement. Coal tar base masticsmust be used with creosote preservatives,whereas asphalt base mastics are not com-patible with creosote. Either base can beused with pentachlorophenol treatments.

Both types of mastic are used as roofingcements, pipeline mastics, bedding com-pounds, or tie sealers, and can be purchasedeconomically in 5-gallon containers. The

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southern pine when penetration depth isabout 10 to 12 times the diameter of theshank. The total spike length is then the deckthickness plus about 12 times the spikediameter.

Spikes with low-profile dome heads areideal for decks. The large heads provide anadequate bearing area, and do not produceobjectionable projections at the deck surface.Some have notched heads, and can be re-moved with a spanner wrench if necessary.

Countersinking spike heads should beavoided if possible. However, if it is neces-sary, a washer should be used inside the holeunder the spike head. The hole should thenbe completely filled with mastic to preventthe entry of water.

Figure 11.—Lead holes, drilled through panel and into stringers afterpanels are positioned, are being field-treated with preservativeprior to driving spikes.

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S U M M A R Y O F E R E C T I O N P R O C E D U R EF O R G L U L A M D E C K P A N E L S

Erecting timber bridges with glued-lami-nated decking and dowel connectors canproceed smoothly if sound workable proce-dures are followed. Sound procedures anderection sequence based on the experienceof eight bridge jobs are summarized (fig. 12)in the following:1. Place all but one panel on the stringers

while lifting equipment is on the site.This can be accomplished with a crane,a forklift, a log bucker, or similar liftingequipment. Stack the remaining panelon the last panel at far end.

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2. Insert stop blocks against the far abut-ment in a manner so that the end panelwill bear against the blocks when jackingstarts.

3. Position and aline starter panel. (Starterpanel has holes on only one edge.) Checkoverhang from outside stringers.

4. Spike starter panel securely in place.5. Slide the first panel to be joined forward

until the gap is slightly greater than thedowel length. Insert all dowels into thestarter panel by hand.

6. Slide the panel toward the starter paneluntil the gap is approximately 1 inch lessthan the dowel length. Pull all dowels outof the first panel and insert the two tipsabout 1/2 inch into each panel. Do notinsert dowels fully into one panel.

7. Butter the mating vertical edges with coaltar mastic or similar material compatiblewith preservative being used.

8. Insert horizontal jacks into the voidcreated by the missing panel. Installkicker blocks if necessary to adjust forjack stroke or travel.

9. Jack carefully so that the edges of the twopanels remain parallel at all times. If onejack gets very farad, the dowels willtend to bind.

10. If the dowels should bind, an impact witha sledge hammer will usually free them.

11. Jack until the mastic extrudes at the topof the joint: this indicates a good fit. Driveenough spikes partially down to securepanel.

However, insert a wood block to absorbthe impact. Do not hit directly againstthe pane l .

12. Remove jacks, and slide next panel for-ward. Repeat until all panels are in place.The final panel will have to be jackedagainst a piece of heavy equipment orkicker blocks staked into the roadway.

Figure 12.—Erection sequence and panel placement for glulam deckpanels.

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APPENDIX

SAMPLE MATERIAL SPECIFICATIONFOR GLULAM DECK PANELS

Glulam Deck Panelsa.

b.

C.

d.

e.

f.

g.

The manufacture and quality control ofglulam deck panels shall be in accordancewith requirements of U.S. Department ofCommerce Product Standard PS 56-73,“Structural Glued Laminated Timber,”current edition, and the American Insti-tute of Timber Construction AITC 117-74,“Standard Specifications for StructuralG lued Lamina ted T imber , ” cu r ren tedition.

In lieu of dimensional tolerances given inPS 56-73, paragraph 4.2.1, the followingshall apply.

Thickness (parallel to gluelines):±1/16 inch

Width (perpendicular to gluelines):+0, - 1 /8 inch per panel

Length: ±1/8 inchSquareness of cross section: per

PS 56-73Crook: maximum of 1/8 inch for

single crook. The sum of maxi-mum deviations shall not exceedthis value for double crook.

Cup: maximum of 1/32 inch per footof width

All material used in deck panels will beof a uniform grade. Design stresses inbending shall be for loads applied parallelto g luel ine ( tables 2 and 4 of AITC

117-74).

Dowel connector holes and any othershop-drilled holes shown on the drawingsshall be completed prior to preservativetreatment.

A template or drilling jig shall be usedto insure that dowel holes are preciselyspaced along the panel centerline. Holesshall be a depth 1/4 inch greater thanone-half the dowel length.

A job dowel shall be used as a check forfit prior to production drilling. When gal-vanized dowels are used, the hole dia-meter can be 1/32 inch greater than thenominal dowel diameter.

Dowels shall be of size and grade speci-fied on the drawings with the tips slightlytapered or rounded to facilitate insertion.

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SAMPLE SHOP DRAWINGAn example of a shop drawing necessary forthe plant fabrication of the glulam deckpanels (fig. 13).

Figure 13.—Sample shop drawing to show details of a glued-laminatedbridge deck.

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* U.S. GOVERNMENT PRINTING OFFICE 1975-650-517/31 -15- 4.5 -16-2-76