Highway Research Engineer

Transcription

1 Highway Research Council V rginia Cooperative Organization Sponsored Jointly by Virginia (A Department of Highways and 1974 June 73-R63 VHRC FINAL REPORT INDUSTRIALIZED TIMBER STRUCTURES by Harry E. Brown Highway Research Engineer University of Virginia) Charlottesville, Virginia

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3 was recently learned that a number of innovations in structural timber It are available to construction industry, but that y were largely components to bridge designers. purpose of this study was to develop for unknown a feasibility document that presented latest technology in Department structural timber components. In addition, cost information on industrialized most feasible timber system was to be presented. This work was carried by American Institute of Timber Construction (AITC), and system out by AITC was one featuring glue-laminated timber deck panels on selected eir glue laminated timber girders or on glulam process embodies.a components can be prefabricated in a form ready All rapid on-site assembly, which is particularly for on-site assembly minimizes on-site labor costs rapid construction time, and and SUMMA RY steel girders where deck replacement would bring an in-service bridge up to acceptable standards. number of industrialized construction precepts, including following: advantageous for emergency repairs; of modular glulam units can be accomplished assembly construction site by semiskilled labor and with at unusually light construction equipment. Department's Bridge Division is currently considering several sites for field trial of glulam concepts iii

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5 period May 17-19, 1971, Transportation Research Board, During of Virginia, and Virginia Highway Research Council cosponsored University workshop directed to study of industrialized construction of bridges. In a which was held at Airlie House in Warrenton, Virginia, workshop, of industry, government and educational and research institutions representatives overall objective of developing means for reducing bridge construction pursued through use of an industrialized construction approach. It was brought time at this meeting that a number of fairly recent innovations in structural timber out study of se innovations and possible application of m to A need would normally be undertaken by a staff researcher or joint Department's however it was decided that this study should be conducted under a appointee, with American Institute of Timber Construction (AITC) because of contract members of AITC, necessary expertise to conduct study associate readily available. working plan outlining this study was issued in was 1972 (Brown 1972) and final report by AITC was submitted to September Council on July 1, 1974 (AITC 1973). principal purpose of this project was to develop a feasibility-type on use of timber bridge systems so as to aid in informing Department document latest technology in industrialized structural timber components. More of project was to: specifically, FINAL REPORT INDUSTRIA LIZ ED TIMBER STRUCTURES by Harry E. Brown Highway Research Engineer INTRODUCTION components are available, but that y are largely unknown to bridge designers. absence of any local expertise. Between members of AITC staff and PURPOSE Review alternate systems available in industrialized timber bridge construction,

6 preliminary structural design calculations develop selected bridge system, for benefits anticipated from use of timber bridge systems on low traffic roads are listed below: volume bridge systems should be competitive in first Timber and in maintenance costs to concrete and costs remoteness of some bridge sites sometimes make practically inaccessible to ready-mix concrete m Thus, labor costs should be less. This constructed. particularly important in cases where labor is paid is travel time. Also, expensive detour roads may for be eliminated because erection of industrialized possibly assembly of an industrialized timber bridge should relatively simple, and refore low skilled and be durability of timber decks on se l[.ghtly traveled may be better than that of concrete because structures aestics of a timber bridge in a rural setting should pleasing. be compile cost estimates for selected system, 3 and 4. assess timber decking on steel stringers. BENEFITS steel systems that are frequently used in bridges of type this trucks. timber bridge would help alleviate this problem. time for erection should be reduced relative to required for concrete and steel bridges now being that bridge systems can be carried out more briskly. employed. economical labor can be of timber's immunity to deicing chemicals remaining part of this report deals with a summary of AITC report, discussion of a design seminar presented by AITC, and status of efforts for a field implementation. -2-

7 on-site by nailing pieces of dimension lumber toger and toenailing constructed lamination to stringers. But this system, used in Department's each SS4 and SS5 bridges, has a drawback in that boards tend to work loose standard of shrinking and swelling stresses resulting from changes in moisture because and from load vibrations. result is a noisy bridge that allows water conditions penetrate into deck elements and onto stringers below. to research, principally by Forest Products Laboratory (FPL) Recent of USDA Forest Service, located at Madison, Wisconsin, has revealed Division concept of glue-laminating dimension lumber (glulam) into girder and deck panels Figure 1). Glulam refers to an engineered and stress rated product of a timber (see A SUMMARY OF AITC REPORT most common type of timber decking on timber bridges is an assembly plant. re are a number of laminating plants throughout country laminating thus re is a competitive market for ir products. and Figure 1. Glulam deck panels being placed into position on glulam girders

8 glulam process was selected by AITC as timber product most meeting needs of industrialized construction for rural secondary bridges nearly number of characteristics of glulam that make it a desirable material for Of design and construction of highway bridges, first is that almost any shape checking is greatly reduced and dimensional stability is vastly improved over to sawn members. And thirdly, re is an efficient placement of varying large grades in structural member with respect to stress requirements lamination upon timber. placed factor that has promoted use of glulam elements is advent of modern A impregnated preservative treatments It is reported that with modern pressure.treatment measures and a good design in which superstructure is preservative protected from elements, service life of timber bridges can be extended well fifty years or more. Although glulam process for girders has been around for a number of years, is only recently that concept of glulam deck panels has been explored. In order it fabricate glulam deck panels, same two-inch dimension material now used to nailed deck is glued with a wear-resistant glue into a flat slab. in panel is very stiff and sufficiently impervious when treated with a resulting been established by FPL, so that se criteria are now being effectively have used. When glulam girders and deck panels are combined into a summarized as Glulam lends itself to a systems approach because all i) can be prefabricated in a form ready for components on-site assembly refore, on-site labor costs rapid construction time can be minimized. and Timber is a very lightweight construction material and 2) allows for transportation of large, complete thus units such as girders with glulam panel structural that have been preassembled at manufacturing decking lightweight feature previously mentioned also permits 3) of lighter on-site construction equipment. This, plus use relatively low cost of glulam, leads to a lower total for initial structure. cost size structural member can be constructed with it. Secondly, since or pieces making up a member are dried prior to laminating, susceptibility individual to and covered with a wearing surface to resist penetration of surface preservative research, design, and construction criteria for.se deck panels water. system, bridge number of industrialized construction precepts become evident. Some of se are a follows: site. This again minimizes on-site labor construction time, and reduces shipping costs based on a weight basis. -4-

9 Aestically, timber best fits into many existing 5) particularly in rural and suburban areas environments, decks is. unlikely. Such is not always case with decks concrete Timber exhibits excellent reserve strength in dynamic 7) situations that occur in highway bridge use. loading spans of 50 years or more. use of a timber bridge system will greatly minimize 10) costs as it will not be affected by chemicals, maintenance would be made of selected timber system. Actually, preliminary calculations design calculations were made on two systems: (1) glulam deck panels structural gtulam girders, a.d (2) glu[am deck panels on steel girders. Calculations of on latter were undertaken in order to generate design parameters for redecking Design considerations were based upon HS or HS Dead loads AASHTO, a uniform three-inch asphalt wearing include (6 1 m) to 80' (24 4 m) in four-.foot (1o 2 m) 20' increments. Assembly of modular glulam units can be accomplished 4) construction site by semiskilled labor and field at modification is unlikely. where a natural appearance is desired. Glulam timber components are not affected by chemicals 6) corrosive materials and thus possiblecorrosion of or When considering construction materials, it is important 8) realize that timber is only natural resource that to can be replenished. Through use of modern pressure impregnated 9) treatments, timber becomes a durable preservative material with expected life elements, or or factors normally influencing and maintenance characteristics of. a bridge longevity structure One of aforementioned objectives was that preliminary structural in cases where present toenailed decks need replacing but steel operations are in satisfactory condition. g rders following conditions: i. Loading urfaceo 2o Spans -5--

10 (7.9 m) to 34' (10.4 m) in two-foot (0.6 m) 26' increments. Douglas fir or Number 2 MG, Sourn pine, L2 use stresses. wet dry use stresses. roadway width (26' 34') (7.9 10o 4 m) following design parameters are and girder size, girder spacing, deck thickness (5 1/8" or 6 3/4") (13o 0 or 17.1 cm) given: and total number of dowels, total board feet of girder (glulam), and total board size of deck Because material costs are variable, total board feet are given in feet as opposed to dollars With board footage for a particular layout known, tables cost quotations can be readily obtained from a fabricator. current was shown under design considerations, an asphalt wearing surface is called As on glulam deck panels wearing surface is needed to provide adequate for for vehicles and to provide a crown for drainage in cases where bridges are traction sloped. not is well to note again in se times when conservation efforts are popular It timber is only material used in construction that is a replenishable resource. that 3. Widths 4o Decks laminated deck panels, 5 1/8" (13.0 cm) Glued 6 3/4" (17. I cm) in thickness, 48" (121.9 cm) or in width, full length sections to span across bridge. 5. Materials Deck Glued Laminated Stringers 24F Combination, Douglas fir or Sourn pine, Sawn Timbers No. 1 grade, Douglas fir. 6. Structural Steel ASTM A-36 design information is listed in tabular form in preliminary plans. For particular loading (HS or HS ) span length (20' 80') ( m), a

11 part of contract package with AITC was a one-day design seminar A.interested designers which was held at Research Council on November 30, for seminar speakers were Thomas Williamson and Paul Beattie of Billy Bohannan of Forest Products Laboratory, and Ben Hurlbut, AITC, for Industrialized Construction, District Bridge Engineers, Bridge Committee of Central Office, representatives of FHWA and manufacturers, Engineers Field Erection Techniques and Cost Savings Using he Glulam Eo System Bridge meeting was highly informative, arid it was well received by those in attendance. Research Advisory Committee for Industrialized Construction, in late 1974, made formal recommendation that glulam timber deck panels February tried on Secondary Road System i. redecking of a present bridge and/or be new construction administration concurred with this recommendation and in subsequently preliminary sites were oi i ered by all eight construction districts to Secondary Roads Division, and se in turn were offered to Central Office's Division. se sites are presently under study by bridge engineers of Bridge Central Office If field implementation of glulam concepts result, work be covered under a new study and file number. will AITC SEMINAR engineer responsible for design aspects of report. Approximately consulting persons attended seminar, including members of Research Advisory 45 and or interested parties. outline of seminar consisted of following topics: A Glulam as a Basic Structural Material B. Research Leading to Design Criteria for Glulam Panel Decks Co Timber Bridge Design Criteria D. Preservative Treatments for Glulam CURRENT STA TUS

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13 Timber Highway Bridges --A State of Art Report", American "Modern of Timber Construction, July 1973o I s ifute REFERENCES H E., "Industrialized Timber Structures--Working Plan", Virginia Brown, Research Council, September Highway

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