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Transverse Design of Adjacent Precast Prestressed Concrete Box Girder Bridges

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Transverse Design of Adjacent Precast Prestressed Concrete Box Girder Bridges Transverse Design of Adjacent Precast Prestressed Concrete Box Girder Bridges Ahmed EI-Remaily American and japanese design and detailing Research Assistant University of Nebraska-Lincoln practices of prestressed concrete box girder Omaha, Nebraska bridges are compared. A new precast, prestressed box girder bridge design suitable to U.S. practice is proposed. The design is based on requiring the deck to act as a rigid assembly of longitudinal and transverse members. This is consistent with japanese practice and fulfills the intent of several state DOT initiatives. The proposal advocates quarter-point diaphragms Maher K. Tadros, Ph.D., P.E. with relatively large amounts of transverse post­ Cheryl Prewett Professor tensioning. A design chart and recommended of Civil Engineering University of Nebraska-Lincoln details are provided for bridges up to 80 ft (24 Omaha, Nebras ka m) long. The amount of post-tensioning was found to be unaffected by bridge span length. Also, a comparison with the requirements of the Takashi Yamane AASHTO LRFO provisions is given. A fully Chief En gineer worked numerical design example is included to Kyokuto Corporation demonstrate the proposed design procedure. Hiroshima, Japan recast, prestressed concrete box girders are widely used in short and medium span bridges in North America. PBased on the National Bridge Inventory, Dunker and Rabbat showed the change in percentage of the eight most common prestressed concrete bridge types built in the Uruted States during the period 1950 to 1989 (see Fig. 1).' Gary Krause, Ph.D., P.E. Stringer and multiple box sections are the most prevalent Associate Professor of Civil Engineering types of prestressed concrete bridges. Each system ac­ Universi ty of Nebraska-Lincoln counted for about one-third of all prestressed concrete Omaha, Nebraska bridges constructed in the United States during 1979 to 1989. 96 PCI JOURNAL en Q) Q ·.:'a 40 m Q) -...Q) CJc 0 0 30 'a Q) en en ...Q) en -! a. 20 -0 fie 10 om-~~~~----~~~~9------9----==~=-----~----~ 50-54 55-59 60-64 65-69 70-74 75-79 80-84 85-89 Year Built -Stringer/multi-beam or girder __._Box beam or girders - multiple ---tr-Siab ~Tee beam """"*--Continuous stringer/multi-beam or girder ~Box beam or girders - single or spread -...-Continuous box beam or girders - multiple ~Cont i nuous box beam or girders - single or spread Fig. 1. Percentages of prestressed concrete bridge types built during 1950 to 1989 (Ref.1 ). Compared with other types of pre­ category of prestressed concrete grouted shear keys and, in some states, stressed concrete highway bridges, tee bridge. 1 are provided with a nominal amount of and single/spread box structures have In adjacent box girder bridges, full-width transverse post-tensioning as the highest deficiency percentages. boxes are placed butted against each shown in Fig. 2. In most applications, However, since their introduction, no other as shown in Fig. 2. Adjacent box a 2 in. (50.8 mm) non-structural wear­ major structural improvements have girder bridges are widely used in most ing surface is added. In a few cases, been made to the system. Therefore, parts of the United States for spans up however, a 5 to 6 in. (127 to 152 mm) there is definitely good reason to im­ to 100 ft (30.5 m) due to ease of erec­ structurally composite concrete over­ prove these bridge types and thus con­ tion, shallow superstructure depth, and lay is used. tinue and let grow the already excel­ aesthetic appeal. The girders are gen­ Recent surveys of adjacent box lent reputation and performance of this erally connected at their interfaces by girder bridges have revealed frequent July-August 1996 97 ~ Girder length: 81' - 6" : Elevation ~ I " " " " I ~-- ------ ---- ---- --- ----------- --- - - - - -- --- --J r- -- - .., I I Girder center - -r- ------------ - -- - - - · - · I I - - --- -- -- -- ----------------------------------- -- - --,~ Transv:rse J Plan post-tensioning Section between diaphragm .....e---.--____, ..~ Section through diaphragm 52' -2" 1'-2" 24'- 11" 24' -11" 1'-2" 270k - ~" dia. post-tensioning strand 12@ 48" = 48'-0" (576") Typical Cross Section Fi g. 2. Elevation, plan and typ ical cross section of precast box girder bri dges in the United States. Note: 1 ft = 0.305 m; 1 in . = 25.4 mm. longitudinal cracking in the grout keys ment corrosion. This is particularly cient considerati on of the structural and reflective cracking in the overlay prevalent in bridges without relatively behavior of a bridge. 2 Both shear and over these keys. In some cases, water thick composite concrete overlays or bending must be transferred at the and deicing chemicals have penetrated inadequate transverse post-tensioning. transverse joint between girders in through the cracks, causing concrete Martin and Osborn related the prob­ order to control both translational and staining and spalling and reinforce- lem of reflective cracking to insuffi- rotational deformation. 98 PCI JOURNAL 20 20 Girder length: 17 600 .. lr l~ Tran~verse post-tensioning t_A 4 300 4200 17 ()()() 300 Elevation 11200 600 10000 600 Curb (CIP) Wearing surface (50-80mm) Transverse post-tensioning Cast-in-place concrete I (at diaphragm location) I I 13 = 10 140 A-A B- B Cross Section Fig. 3. Precast, pretensioned concrete box girder bridge system for simple short span. Note: 1 m = 3.28 ft; 100 mm = 3.94 in . A few state DOTs, e.g., Michigan, are similar to those in the United combines the performance require­ use a combination of heavy struc­ States, except for size and shape of the ments for a Japanese bridge with the turally composite topping and a large longitudinal joint between the girders simplicity of American construction amount of transverse post-tensioning. and the amount of transverse post-ten­ practices. The proposed design in­ Composite topping is not a structurally sioning.3 Cast-in-place (CIP) concrete volves provision of post-tensioned efficient solution because it does not is placed in relatively wide and deep transverse diaphragms at quarter control differential rotation of the box, joints between girders, as shown in points of the bridge span. nor is it an economical solution be­ Figs. 3 and 4, as opposed to narrow The diaphragms would be made cause a composite concrete topping mortar-grouted joints in the United continuous in the space between the costs about four times as much as a States. Higher levels of post-tension­ boxes through deep blockouts filled thin layer of bituminous concrete. ing are used in Japan than is the gen­ with grout. Post-tensioning is provided In reviewing the practices in other eral practice in the United States. based on bridge width and loading, as­ countries, it was found that cases of In the following sections, various suming the bridge consists of an as­ longitudinal cracking are seldom re­ design approaches of typical precast, sembly of rigidly connected stringers ported in Japanese adjacent box girder prestressed concrete box girder and diaphragms. A preliminary design bridges. Cross-sectional shapes and bridges are discussed. A proposed de­ chart has been developed for simple design criteria for box girders in Japan sign is also presented. The design span bridges of common width and July-August 1996 99 170 560 Transverse 100 180 180 100 post-tensioning c - GlB m A :::::::: :;::::: ::::::: ::::: :::::: :::: :::: ::: :::::::: :::::: ::;::::: : I' ,,,, % ;::::' :,;:; :@ I ~u H :::::: ::::::: :::::: ::;::: ::::: ::: :::: k :::;:: :;4 :;:::: v :::::: ':':':: :::::l ::::: ::: }':: :::::: ::::::: ::::: :::::: :m ::::::: ::;::: :::::t \ :::::: :::::: ]~ s:~ }:; ::;:: t: :::;:: }{ :':} ::::::: :::: <I :;: :::::: ::::::: :::: ::::;:: ::::::: ::::;::::: ::::::: ::: :: :::::::: :;:: ::::::: ::::::: ::::::: }:': A - A B - B Cross Section Transverse post -tensioning Detail C Fi g. 4 . Transverse post-tensioning arrangement for Japanese box girder bridges . Dimens ions are in millimeters. Note: 100 mm = 3.94 in. material properties using AASHTO 2. Is a waterproofing material or terms of percentage of respondents. HS-25 live loading. membrane used over the longitudinal The survey data also indicated that 62 joints? percent of the respondents used 3. Is skew limited for the use of pre­ strands as the transverse tie material RECENT STUDIES stressed box beam bridges? while 38 percent used rods. The num­ In 1992, the PCI Committee on 4. Are shear keys grouted after ten­ ber of transverse ties varied largely Bridges formed a Subcommittee on sioning the ties? from state to state. Fig. 6 reveals that Reflective Cracking in Adjacent Box 5. Is there a problem with differen­ the number of transverse ties is se­ Beam Bridges to study the problem. tial camber between adjacent bands? lected quite arbitrarily. The subcommittee's report (1995) in­ 6. Are there any problems with un­ Case Western Reserve University dicated that at least two national sur­ even sealing of the beam ends for investigated the performance of shear veys had been conducted with the goal skewed bridges? keys in adjacent box beam bridges in of isolating the causes of reflective 7. What material is used for trans­ 1993.5 The five test bridges showed cracking.4 The following are specific verse ties? differential deflections between 0.08 questions the committee identified as 8. What spacing is specified for and 0.8 in. (2 and 20 mm), which indi­ most pertinent to its investigation: transverse ties? cated shear key fracture along part or 1. Are there any problems with The results of the survey for the first all of the bridge length. The large dif­ leakage at the joints between beams? six questions are shown in Fig. 5 in ferential deflection resulted in leakage. 100 PCI JOURNAL The test structures also showed a satisfactory load distribution among 80 beams after the shear key partiall y 70 fractured. Mild steel lateral ties were found to be ineffective in resisting dif­ 60 ferential deflections. The study sug­ ~ tiA) 50 gested either moving the shear key ...~ down to the neutral axis of the beam ~ 40 u= or using a stronger epoxy grout in the ... ~ 30 existing shear key. ~ West Virginia DOT investigated 20 several high volume, heavily loaded 10 bridges that had joint fai lure and top­ ping cracking.• The investigators con­ 0 .
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