ERECTION PROCEDURE FOR GLUED-LAMINATED TIMBER BRIDGE DECKS WITH DOWEL CONNECTORS

Transcription

1 ERECTION PROCEDURE FOR GLUED-LAMINATED TIMBER BRIDGE DECKS WITH DOWEL CONNECTORS USDA FOREST SERVICE RESEARCH PAPER FPL U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WIS.

2 ERECTION PROCEDURE FOR GLUED-LAMINATED TIMBER BRIDGE DECKS WITH DOWEL CONNECTORS 1 By ROGER L. TUOMI, Engineer Forest Products Laboratory, 2 Forest Service U.S. Department of Agriculture INTRODUCTION The versatility and the durability of wood contribute to its importance as a material for bridge construction. Timber bridges are attractive both for economy of cost of material and ease of erection. Because of low weightto-strength ratio, relatively light lifting equipment can be used. Erection requires a minimum of skilled labor, and can proceed even under adverse weather conditions. Prefabricated assemblies can be quickly installed at a site with a minimal disruption of traffic. Wood is inherently durable; records establish that timber bridges have provided service for more than 500 years. Many covered bridges built more than a century ago are still in service, although their construction preceded the development of preservative treatments. The secret of their longevity has been a design which provides for protection of the superstructure. Timber bridges are probably best suited for rural areas and secondary roads for spans up to 50 feet or so. They are esthetically pleasing in these settings (fig. 1). These are also areas where plant-mixed concrete and structural steel are not readily available. Local contractors generally can handle timber bridge jobs since specialized labor and equipment are not essential. An added benefit is that wood is relatively inert chemically and is unaffected by salt or other de-icing agents, a serious problem with other materials. Experience has shown that the bridge deck is the component most vulnerable to decay. For many years the nailed-laminated deck was the standard of construction. Recently, the glued-laminated deck system was developed, 3 and a design procedure was adopted. 4 This system provides protection to the superstructure in a manner much like the covered bridge. A tight deck with waterresistant wearing surface, coupled with preservative treatment, will ensure the service life of the modern timber bridge to 50 or more years. The success of any construction job depends on two major factors: Accurately fabricated materials and correct erection procedure. The guidelines presented here are based on the experiences gained on eight bridge jobs. Each was by a different crew with no past experience in this type of work. Forest Products Laboratory engineers witnessed a number of these jobs, and made suggestions, based on prior observations, to new work crews. All crews tended initially to make the same mistakes, but after efficient procedures were worked out, construction progressed very well. 1 Presented at Northwest Bridge Engineers Conference, Boise, Ida., Sept Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3 McCutcheon. W. J. and R. L. Tuomi Procedures for design of glued-laminated orthotropcc bridge decks, USDA For. Serv. Res. Pap. FPL 210. For. Prod. Lab., Madison, Wis. 4 American Association of State Highway and Transportation Officials lnterim specifications for bridges. -1-

3 Six of the bridges were erected by Forest Service Construction and Maintenance crews and two by contractors. Three bridges erected by the Forest Service were completed in winter in near-blizzard conditions. Concrete work or field welding would have been impractical at these times. Winter construction was elected because it was the only practical season to close the roads for a short period; workloads were down and manpower was available. Winter construction provided an added advantage: Rivers were frozen, and workmen had access to the underside of the bridge without the need to erect scaffolds or platforms (fig. 2). Probably the best example of a smooth erection job was one of the two contractorbuilt bridges. The bridge was 72 feet long by 26 feet wide. A crane was at the site 1 day, set the stringers, and placed the deck panels loosely on top of the stringers. The crane was then removed. A crew of from three to four then completed all deck work, installed diaphragms, and set guardrails in only 1-1/2 days (44 man-hours). Obviously, all went well on this job. The procedures presented here for bridge construction should both prevent common mistakes and expedite efficient construction. Undoubtedly, with added experience further improvements will evolve. Figure 1. Timber bridge complements natural setting. (M ) -2-

4 Figure 2. Because erection of timber bridges requires minimum labor and equipment, work can progress under winter and other adverse weather conditions. (M ) FABRICATION In general, our experience with eight laminated Forest Service bridges indicates that the laminators have done a satisfactory job in fabricating members to the requirements of contract plans and specifications. These jobs have demonstrated that the tolerances possible in glulam (glued-laminated timber) plants will result in a minimum of erection problems through misfabrications if materials are specified as meeting the requirements of PS 56-73, Products Standard of the Department of Commerce. Some minor problems have been experienced on other jobs in the overall length of bridges. The final width of a panel tends to be slightly wider than the nominal width. For example, a 32-lam panel often ends up slightly wider than 48 inches. Also, a slight amount of sweep may occur in the panels if the clamps are not perfectly alined, and make getting a tight field joint difficult. Finally, the mastic sealer in field joints adds slightly to the effective width. These factors are accumulative, and the completed deck tends to grow in overall dimension. An experienced laminator overcomes these potential problems by trimming both edges of all panels. The centerline is first established. Then reference lines are laid out on each side of the centerline so that the finished panel will be 1/8 inch less than the nominal width. The edges are then trimmed to square the panels or to remove any possible sweep. -3-

5 One bridge engineer designs decks to be slightly shorter than the overall bridge length. He makes up the closing dimensions with treated pieces of nominal 2-inch lumber. He reasons that the leading corners of the first panels at the approach ends of the bridge are subjected to mechanical damage when a vehicle first contacts the bridge. If damaged, these end pieces can easily be replaced without disturbing the deck. Laminators must know exactly what is needed so that the components will fit together in the field. The precise dimension of components, hole sizes, and hole locations must be clearly defined. The quality of material must be specified. A specification and a sample shop drawing are provided in the appendix. All final cuts and hole drilling should be completed at the fabrication plant before preservative treatment to ensure that potential decay areas are protected. Grades of materials will vary across the depth of stringers, with the high quality material placed in the outer surfaces where stresses are greatest. This is not possible for deck panels; therefore the same grade should be used throughout. Low-grade material is generally suitable for deck panels. Savings can at times be realized by specifying a onesize greater thickness in a lower grade material. This depends on material availability, grade demands, and inventory. Precise drilling of dowel holes is essential. The holes must be spaced equally along the panel edge centerlines, and the axes of the holes must be perpendicular to the panel edge, so that in the field the dowels will aline. A drilling jig or template should be used. The necessary precision cannot be achieved with hand-held drills. Some fabricators have used gang drills so that several holes can be drilled at one time. Others have devised a jig whereby a dowel is affixed to the drilling apparatus. The dowel is inserted into the previously drilled hole as a guide to ensure exact spacing between holes. Holes should be drilled to a depth 1/4 inch deeper than one-half the dowel length. The question of hole diameter tolerance often arises. For optimum performance, the dowels must fit fairly snugly into the holes. Most designers specify galvanized dowels. Measurements have shown that galvanizing adds about inch to the nominal dowel diameter. Also preservative treatment causes a small degree of swelling which reduces the hole diameter slightly. A method that has worked well is to drill the holes first with a wood drill equal to or slightly smaller than the nominal dowel diameter, next ream the holes with a steel drill 1/32 inch larger than the dowel diameter. This results in a good fit in the field. Wood drills become dull rather quickly when used for drilling the finished hole, and the holes tend to become slightly oblong, and have a fuzzy texture which impedes insertion of the dowel. Steel drills, by contrast, produce a smooth perimeter surface. An actual job dowel should be used at the plant to check the fit prior to production drilling. Although accuracy in drilling is necessary, such precise fabrication is not difficult; fabricators can meet the requirements with a reasonable amount of care. TRANSPORTATION Timber bridge components are usually shipped by truck or by rail or by combinations of the two. Treatment plants are generally located in areas separate from the fabrication plant so that an intermediate shipment is involved. In many cases, all of the material for a complete bridge can be carried by a single truck. Most bridge materials are treated either with creosote or pentachlorophenol in petroleum oil so that protective wrapping is not needed for shipment. Wrapping is necessary only for architectural-or appearance-grade material if esthetic appeal is of major importance. It is common and acceptable practice to nest or to bundle a number of pieces. However, banding can damage the edges of a member if it is not properly protected (fig. 3). A nominal 2-inch piece of lumber should be placed across the width of the panels in line with the bands. This also provides an access area for insertion of lifting equipment. Some procurement people have the misconception they should seek bids only from fabricators near the bridge site. Apparently they believe that transportation is the controlling cost factor. Although important, transportation is not the major cost. The -4-

6 laminating industry, like all other businesses, is competitive. Often, the lowest bids ema- nate from plants 2,000 miles from the bridge site. Figure 3. Note improper banding of panels has caused corner damage. (M ) HANDLING The relatively light weight of timber components provides a wide latitude for choices in lifting equipment. The final choice usually depends on the availability of equipment near the bridge site. Cranes are probably the most desirable; the rubbermounted hydraulic units, the most mobile. Log buckers (fig. 4), end loaders (fig. 5), and forklifts have been used. In handling stringers and deck panels, do not lift by the edges parallel to the wide face of the member. This can induce high bending stresses across the wood grain, which can cause structural damage. The members should be supported across the wide face (fig. 4) or lifted at the ends (fig. 5) with C brackets. Fabric slings (fig. 6) work well as lifting members, but the loop should be positioned at a corner. At no time should the members be dragged or skidded about the work area. -5-

7 Figure 4. Large panels can be handled with light equipment. Proper support across wide face is required during handling. (M ) Figure 5. Small end loader used to position panels on stringer. Proper end support is shown for short panels. (M ) -6-

8 Figure 6. Here proper use of slings for lifting panels is shown. Loop should be positioned at a corner so that panels ride vertically. (M ) -7-

9 STORAGE Proper storage of materials is an important operation often overlooked. In one case materials were dumped in a pile on the ground, left for an extended period, and became warped; thus erection was made difficult. Problems with delivery schedules or work delays intensify these kinds of storage problems. Properly stored, the members will remain straight and dry. It is also much easier to get at the desired pieces with lifting equipment if there is some preplanning in storage locations. All material on the worksite should be stacked to prevent warping. The ground beneath the stacks should be cleared of weeds and rubbish, and leveled. The bottom panel should be about 12 inches above the ground, and supported on level blocks. Spacer pieces, approximately 2 inches thick, should be inserted between panels. They should extend across the wide face of the members; this allows for free circulation of air, and provides access for lifting equipment. The spacer blocks should be alined vertically and spaced at regular intervals (fig. 7); otherwise, the members will be subjected to bending stresses, and might warp during extended storage. When properly stacked the treated material does not have to be covered. Free circulation of air is all that is necessary. Impervious membranes, such as polyethylene film, only trap moisture that evaporates from below. Figure 7. Improper positioning of spacer blocks induces bending stresses during storage. Blocks should be alined vertically. (M ) ERECTION PROCEDURE Although this Paper relates basically to installation of the deck system, placement of stringers is important. The stringers might appear perfectly straight and symmetrical, but they do have top and bottom sides. A slight camber is generally built into them to account for deadweight deflection. Also, as mentioned, the stringers are not of a balanced design. Special materials are selected for the tension and the compression sides so they must be installed with the proper orientation. Check first for topside markings. After the stringers have been set, all panels should be loosely stacked upon the stringers. This frees the lifting equipment, and provides a good work platform for the crew. From this point, the panels can be slid into position by hand without undue effort. With proper placement, a panel need only be moved a few feet. A typical panel placement layout is presented in the appendix. The bridge builder should first check panels for sweep. If present, the effect can be minimized by placing all panels with the sweep in the same direction. (The problem is compounded if adjacent panels are posi- -8-

10 tioned with sweep in opposite directions.) Thus to minimize the effect of sweep, simply turn the involved panels end for end or flip them. The force for joining panels can be applied either by pulling or by pushing. For pulling, use either come-alongs or coffing hoists (fig. 8); for pushing, horizontal jacks such as railroad jacks (fig. 9). Both methods have been used successfully, but the horizontal jack is more positive and easier to control. In the pulling operation shown (fig. 8), a rakelike tool was fabricated to fit the leading edge of the panel being pulled. The handle portion was about 6 feet long; this kept the line of force low, and reduced the tendency to lift the edge of the panel. A cable or a chain was dead-ended at the starting end of the bridge. It was let out or extended as work progressed toward the far end of the bridge. With the jacking procedure (fig. 9), the bridge was erected on a 30 skew so the joints were not perpendicular to the stringers. Stop blocks were inserted at the far end of the bridge, and the jacking force was applied against the loose panels, which were butted tightly together. With one jack near each end of the panel it was easy to control the force. It is extremely important that the panel edges remain parallel during the jacking or the pulling operation. The most serious problem that can develop during erection occurs during insertion of the dowels. Each new crew has a tendency to insert the dowels fully into one panel, then try to aline the second panel. This procedure will cause problems since all the tolerances are used up at the outset. Do not insert dowels fully into one panel. The dowels should first be fed into the secured panel by hand. They should be pushed in far enough so that they will stay Figure 8. Panels are pulled together with come-alongs or coffing hoists. Here rakelike tool is shown that is used to grip edge of panels. (M ) -9-

11 Figure 9. Horizontal railroad jacks provide positive method of joining panels. This bridge was erected on a 30 skew. (M ) in place temporarily. Next the panel to be joined should be slid forward until the gap between panels is about 1 inch less than the total dowel length. The dowels should then be pulled out of the first panel, and only the tips should be started into the holes of the second panel (fig. 10). With only the tips started into the holes of the two panels being joined, the dowels will follow the line of least resistance, and are essentially self-alining. The force required for joining panels with this technique is minimal. Should one panel end get very far ahead of the other, the dowels will tend to bind. They can be freed with an impact blow from a sledge hammer to the trailing edge, but first insert a wood block against the edge being struck. Do not strike directly against the edge of the panels. If care is taken to keep the edges of the two panels parallel, this problem should not occur. The first, or starter, panel should be secured at the end of the bridge. The two end panels (first and last) should have dowel holes drilled only on one edge. Check overhang periodically to assure that the panels are centered. Otherwise, the panels may drift off line; it is difficult to make corrections if the drift is not discovered early. Another method is to mark reference points on the panels and stringers to check alinement. Except for the starter panel, the panels should not be spiked until they are all in place. A couple of spikes driven only par- -10-

12 highest quality contain asbestos fiber fillers for reinforcement. Mastics should be gooey enough to adhere to a vertical surface. The final operation is spiking the deck in place. Before driving the spikes, be sure to drill lead holes. This is the only time in the field that there should be any drilling. For maximum withdrawal resistance, the hole should be approximately 70 percent of the spike diameter. Use the factory-drilled holes in the deck for guides when drilling lead holes into the top of the stringers. The depth of the holes should be equal to or slightly deeper than the penetration depth of the spikes. Before the spikes are driven, the lead holes must be treated in the field with preservative (fig. 11). The same type of preservative used for treating the deck should be used. An oil can or a grease gun with a nozzle works well for directing the flow of preservative into the hole. When the spike is driven, the preservative is forced into the surrounding wood. The spikes used for these bridges have either annularly or helically threaded shanks, and have a withdrawal resistance approaching that of lag screws. Generally, the tendency is to specify longer lengths than necessary. The withdrawal resistance should equal the yield stress of the spike based on its net root 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 inch less than dowel length. Only tips of dowels should be fed into panels prior to jacking. (M ) tially down will hold the panels in place. The vertical edges of the panels should be coated liberally with a heavy mastic before they are joined. This provides an effective seal to prevent water from penetrating the deck. A slight extrusion of mastic at the top of the deck indicates a tight fit. The mastic can be either a coal tar base or an asphalt cement. Coal tar base mastics must be used with creosote preservatives, whereas asphalt base mastics are not compatible with creosote. Either base can be used with pentachlorophenol treatments. Both types of mastic are used as roofing cements, pipeline mastics, bedding compounds, or tie sealers, and can be purchased economically in 5-gallon containers. The southern pine when penetration depth is about 10 to 12 times the diameter of the shank. The total spike length is then the deck thickness plus about 12 times the spike diameter. Spikes with low-profile dome heads are ideal for decks. The large heads provide an adequate bearing area, and do not produce objectionable projections at the deck surface. Some have notched heads, and can be removed with a spanner wrench if necessary. Countersinking spike heads should be avoided if possible. However, if it is necessary, a washer should be used inside the hole under the spike head. The hole should then be completely filled with mastic to prevent the entry of water. -11-

13 Figure 11. Lead holes, drilled through panel and into stringers after panels are positioned, are being field-treated with preservative prior to driving spikes. (M ) SUMMARY OF ERECTION PROCEDURE FOR GLULAM DECK PANELS Erecting timber bridges with glued-laminated decking and dowel connectors can proceed smoothly if sound workable procedures are followed. Sound procedures and erection sequence based on the experience of 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 lifting equipment. Stack the remaining panel on the last panel at far end. 2. Insert stop blocks against the far abutment in a manner so that the end panel will bear against the blocks when jacking starts. 3. Position and aline starter panel. (Starter panel has holes on only one edge.) Check overhang 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 the dowel length. Insert all dowels into the starter panel by hand. 6. Slide the panel toward the starter panel until the gap is approximately 1 inch less than the dowel length. Pull all dowels out of the first panel and insert the two tips about 1/2 inch into each panel. Do not insert dowels fully into one panel. -12-

14 7. Butter the mating vertical edges with coal tar mastic or similar material compatible with preservative being used. 8. Insert horizontal jacks into the void created by the missing panel. Install kicker blocks if necessary to adjust for jack stroke or travel. 9. Jack carefully so that the edges of the two panels remain parallel at all times. If one jack gets very farad, the dowels will tend to bind. 10. If the dowels should bind, an impact with a sledge hammer will usually free them. However, insert a wood block to absorb the impact. Do not hit directly against the panel. 11. Jack until the mastic extrudes at the top of the joint: this indicates a good fit. Drive enough spikes partially down to secure panel. 12. Remove jacks, and slide next panel forward. Repeat until all panels are in place. The final panel will have to be jacked against a piece of heavy equipment or kicker blocks staked into the roadway. Figure 12. Erection sequence and panel placement for glulam deck panels. (M ) -13-

15 APPENDIX SAMPLE MATERIAL SPECIFICATION FOR GLULAM DECK PANELS Glulam Deck Panels a. b. C. d. e. f. g. The manufacture and quality control of glulam deck panels shall be in accordance with requirements of U.S. Department of Commerce Product Standard PS 56-73, Structural Glued Laminated Timber, current edition, and the American Institute of Timber Construction AITC , Standard Specifications for Structural Glued Laminated Timber, current edition. In lieu of dimensional tolerances given in PS 56-73, paragraph 4.2.1, the following shall apply. Thickness (parallel to gluelines): ±1/16 inch Width (perpendicular to gluelines): +0, - 1 / 8 inch per panel Length: ±1/8 inch Squareness of cross section: per PS Crook: maximum of 1/8 inch for single crook. The sum of maximum deviations shall not exceed this value for double crook. Cup: maximum of 1/32 inch per foot of width All material used in deck panels will be of a uniform grade. Design stresses in bending shall be for loads applied parallel to glueline (tables 2 and 4 of AITC ). Dowel connector holes and any other shop-drilled holes shown on the drawings shall be completed prior to preservative treatment. A template or drilling jig shall be used to insure that dowel holes are precisely spaced along the panel centerline. Holes shall be a depth 1/4 inch greater than one-half the dowel length. A job dowel shall be used as a check for fit prior to production drilling. When galvanized dowels are used, the hole diameter can be 1/32 inch greater than the nominal dowel diameter. Dowels shall be of size and grade specified on the drawings with the tips slightly tapered or rounded to facilitate insertion. -14-

16 SAMPLE SHOP DRAWING An example of a shop drawing necessary for the plant fabrication of the glulam deck panels (fig. 13). Figure 13. Sample shop drawing to show details of a glued-laminated bridge deck. (M ) * U.S. GOVERNMENT PRINTING OFFICE /