Amerykańsko-Polski warsztat na temat struktur i materiałów betonowych

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(1)Maher K. Tadros Charles J. Vranek Amgad F. Morgan Girgis. Amerykańsko-Polski warsztat na temat struktur i materiałów betonowych OPTIMIZATION OF PRESTRESSED CONCRETE GIRDER EFFICIENCY. Streszczenie Niniejszy referat dotyczy optymalizacji prefabrykowanych dźwigarów z betonu sprężonego i innych elementów wzdłużnych usztywniających stosowanych w budowach mostów i innych budowlach. Metody poprawy strukturalnej i ekonomicznej wydajności dźwigarów i podobnego kształtu elementów nośnych obejmują zwiększenie powierzchni kołnierza dolnego, stosowanie zbrojenia spawanego, stosowanie betonu wysokowartościowego oraz układanie dźwigarów w sposób ciągły w przypadku dużych obciążeń. Wszystkie te rozwiązania można stosować łącznie w celu zapewnienia znacznie lepszych osiągów i większej powierzchni przęseł. Niniejszy referat dotyczy systemów ciągłych. Większość dźwigarów ze strunobetonu konstruuje się jako pojedyncze przęsła. Chociaż jest to najprostsza metoda budowy budynków i mostów, nie zapewnia ona najlepszych osiągów pod względem wydajności wykorzystania materiałów ani funkcjonalności. Rozwarcie spoiny zwykle przyciąga brud i wilgoć, co wymaga stałej konserwacji i skraca żywotność konstrukcji. Istnieją cztery różne metody zapewnienia ciągłości dźwigarów betonowych, a mianowicie: 1) Dźwigary tworzy się jako proste przęsło niosące ciężar własny i ciężar pomostu. W pomoście umieszcza się zbrojenie dla zapewnienia ciągłości systemu przenoszącego obciążenia statyczne i dynamiczne. 2) Dźwigary tworzy się jako proste przęsło niosące ciężar własny i w formie ciągłej dla wszystkich dodatkowych obciążeń. Ciągłość zapewnia się dla ciężaru pomostu poprzez połączenie dźwigarów prętami gwintowanymi o dużej wytrzymałości przed nałożeniem pomostu. Zbrojenie to przyczynia się do zachowania ciągłości ze względu na wszystkie dodatkowe obciążenia. Maher K. Tadros, PhD, PE – University of Nebraska-Lincoln, Omaha, Nebraska Charles J. Vranek Professor of Civil Engineering – University of Nebraska-Lincoln, Omaha, Nebraska Amgad F. Morgan Girgis, PhD – Research Assistant Professor – University of Nebraska-Lincoln, Omaha, Nebraska.

(2) 3) System ten działa tak samo jak system nr 2) z tą różnicą, że ciągłość zapewnia się poprzez napięcie na pełnej długości. 4) Gdy długość dźwigara jest mniejsza niż pełna długość przęsła ze względów transportowych, dźwigary można łączyć przy pomocy techniki post-tensioning na pełnej długości. Metoda post-tensioning na pełnej długości jest stosowana w przypadku dwóch z tych systemów. Pozostałe dwa nie stosują jej, oferując tym samym oszczędności przy budowie. W szczególności, nowy system wypracowany przez University of Nebraska, system zapewnienia ciągłości przy pomocy prętów gwintowanych (TRCS) będzie tu omawiany szczegółowo. System TRCS charakteryzuje się większą wytrzymałością konstrukcji niż inne systemy, w tym post-tensioning. Zaprezentowana zostanie analiza oraz pełne testy systemu. Przedstawione też zostaną możliwości zwiększenia powierzchni przęseł.. Abstract This paper focuses on optimizing precast prestressed concrete Igirders and other stringers used in bridge and building applications. Methods to improve the structural and economical efficiency of Igirders and similar stringer shapes include increasing the area of the bottom flange, using welded wire reinforcement, using high performance concrete, and making the girders continuous for superimposed loads. All these improvements can be combined to give substantially improved performance and increased span capacity. This paper focuses on the continuity systems. Most pretensioned concrete bridge girders are constructed as simple spans. While this is the simplest method of construction for buildings and bridges, it does not result in the best performance in terms of materials usage efficiency or functionality. Open joint usually attract debris and moisture, creating continuous maintenance attention and relatively short life. There are four different types of creating continuity in concrete girders. They are 1. Girders are simple span for their weight and for deck weight. Deck reinforcement is placed to render the system continuous for superimposed dead load and live load. 2. Girders are made simple span for their weight and continuous for all additional loads. Continuity is achieved for deck weight by coupling the girders with high strength threaded rods before the deck is placed. The deck reinforcement contributes to the continuity due to all subsequent loads. 3. This system is the same as system (2) except that continuity is achieved through full length pottensioning. 4. When the girder length is smaller than the full span length due to shipping constraints, the girders may be spliced with full length posttensioning. Full length posttensioning is utilized for two of the systems. The other two involve no forms of field posttensioning, and thus offer construction economies. In particular, a new University of Nebraska developed system, the threaded rod continuity system (TRCS), will be discussed is detail. The TRCS has more structural efficiency than any of the other systems, including posttensioning. Analysis and full scale testing of the system will be presented. Span capacity improvements will be shown. Keywords: Concrete, Bridges, optimization, Precast, IGirder, Posttensioning, Continuity.

(3) Optimization of Prestressed Concrete Girder Efficiency. 1. Introduction The use of precast prestressed girders in bridge construction began in the United States in the early 1950s. Until the 1960s, bridges built with pretensioned Igirders and castinplace (CIP) concrete decks were designed as simply supported spans due to all dead and live loads. In the early 1960s, a number of state agencies started to build continuous highway bridges with [1] prestressed girders . The deck slab was made continuous without any joints by adding longitudinal reinforcement in the deck slab over the pier. When the deck concrete cures and gains strength, additional loads from the superimposed dead load and live load can be resisted by the continuous composite girder/deck section. Thus, the prestressed girders are normally designed as simplespan beams due to their selfweight and deck weight, and as continuousspan beams due to the superimposed loads. This type of continuity has become the conventional method for highway bridge construction. However, the superstructure is continuous for only about onethird of the total load. The relatively high pretension force, compared to what required in the other methods described below, may cause creep growth of member camber which is restrained by the pier diaphragms. The lack of permanent negative moment in the piers may create a net positive restraint moment due to creep and result in bottom cracking at the piers. This “softening” of the negative moment region over the piers further reduces the continuity of the system. In highway bridges, the deck weight represents about onethird of the total load. If the precast girders are made continuous before casting the deck slab, it can largely reduce the positive bending moment, resulting in great saving in required prestressed force or significant increase in the span capacity that the girder can achieve. Currently, three methods of creating continuity for deck weight are available, i.e., longitudinal posttensioning, coupling girder top end strands, and coupling through high strength threaded rods [2, 3, 4]. In 1998, the researchers at the University of NebraskaLincoln (UNL) started to develop the concept of coupling precast girders for deck weight using high strength threaded rods. The high strength threaded rods are embedded in the girder top flange and protrude beyond the girder ends. They are coupled in the field at the pier diaphragms by the steel plates and the heavyduty nuts (see Fig. 1). After the diaphragm is placed and gains the specified strength, the deck slab is poured. Fullscale testing of the specimen has shown the precast girder with high strength threaded rods presented sufficient strength and ductility (see Fig. 2).. . . Keywords: Prestressedconcrete, Continuity, NUIGirder . $C. $C D. Fig. 1. Threaded Continuity System for Deck Weight Continuity. $C. $C. $C. $C. D. $C ;!C<,C<;C # * . =

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(8) . 2. System Implementation. . Up to date, several bridges have implemented the threaded rod continuity system including Clarks Viaduc, Platte River East Bridge, Wood River Overpass, and 262nd Stre * '/ ' * * * *#. '#* * et & I80 Bridge in Nebraska, USA and several bridges in Alberta, Canada. *# D* .* & /. * )$) " E 3(!This paper briefly discusses Clarks Viaduct and Platte River East Bridge. " * * *

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(10) . (%. 2.1. Clarks Viaduct. As the first bridge built in the United States implementing the threaded rod continuity [9] system, Clarks Viaduct in Nebraska has 4 spans of 100’ + 151’+ 148’+ 128.5’ (30.5 m 46.0 45.1 39.2 m). This viaduct was originally designed as a haunched steelplate girder bridge. After the construction drawings were released, the designers at Tadros Associates did the value engineering and proposed unique concrete alternate incorporating the threaded rod continuity system. Fig. 3 shows 50 in. (1,270 mm) deep girder in the precast yard, which was modified from the standard NU 1100 I-girder section (43.3 in. (1,100 mm)) deep to match the minimum steel section in the original design. A standard NU 1100 was revised ; with one foot of the top flange removed on both sides and 6.7 in. (170 mm) added to the top of the girder to form the girder section in this viaduct. The precast girders are seated over the CIP haunched beams and coupled by the bolted connection details (see Fig. 2). The CIP haunched beams are created to achieve the required flexural capacity at the negative moment zone, while keeping the geometry profile of original steel plate girders. Once the precast girders are coupled, the continuity blocks over the CIP haunched beams are poured. With the CIP haunched beams over the pier and the continuity for deck weight, an unprecedented spantodepth ratio of 36 was achieved. The value engineering of Clarks Viaduct has shown the threaded rod continuity system can be simply incorporated into the concrete superstructures as an alternative to compete with longspan steel bridges. It will not only result in longterm savings such as lower maintenance cost, but also will provide immediate benefits to the owner and the contractor in constructioncostsavings. Fig. 3. Precast girder of the Clarks Viaduct in the precast yard.. 4.

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(13) Optimization of Prestressed Concrete Girder Efficiency. The value engineering of Clarks Viaduct has shown the threaded rod continuity 1 # *# 4 * **# ' system can be simply incorporated into the concrete superstructures as an alternative to '/ / * . #/ ## '/ 4 compete with longspan steel bridges. It will not only result in longterm savings such as /lower. *

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(19) 2.2. Platte River East Project. . ,. Platte River East, a bridge project including two structures at Omaha, Nebraska, is another

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(54) $. " D* . . -. The Platte River East bridge used the standard NU 1800 concrete girders spaced at 12’-6”. 1 * # * **(!! * / *)F$B (3.8 m). The bridge cross section is shown in Fig. 6. The 174’-0” (53.0 m) long girders in this bridge are the longest precast/prestressed concrete girders in the state’s history. The ;

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(61) 1 / system similar to that used in the Clarks Viaduct (see Fig. 7). * 4 '* ## * 4 # * **# Since there is no commercial software available for design, the spreadsheet programs were used to perform the working stress and the flexural strength design. The working ' '# * *#6 =

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(63) . stress design accounts for the transformed girder section and determines the amount of required prestressing strands. The ultimate flexural strength design adopts the approach " '' 4 * / * /' 4 of strain compatibility and Mast’s Unified Approach to include the high strength threaded # */ ' 4 . * <# *

(64) 1 4 . rods in addition to the mild reinforcement in the deck slab. As a result, a total of 42 - 0.6” diameter, Grade 270, low relaxation strands were used in the longer bridge. The shear * # ' ** ** ' '# :# * design follows the AASHTO LRFD Bridge Design Specifications, where a maximum shear / . *

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(71) Optimization of Prestressed Concrete Girder Efficiency. for an efficient and economical design and allows the concrete girder to compete with long-span steel plate girders without the use of post-tensioning.. Fig. 7. Threaded Rod Continuity Connection at the Piers. . . =

(72) %1 * **# . 3. Conclusions This paper describes a continuity system making the precast bridge girders continuous for. deck weight using high strength threaded rods. It presents the system implementations . including the Clarks Viaduct and the Platte River East Bridge. A number of significant advantages of the threaded rod continuity system over the conventional system are // * # ' ' / * * ## summarized as follows: 4 # . * * *

(73) 3 / ' '/ ' 1. Since the precast girders are made continuous for approximately twothirds of the total demand for force D*

(74) high #' . loads, *# it results in * reduced prestressing and for strength concrete at release. Utilizing the # proposed system, the same girder size can span about 10 to 15 * * *. ' . ' percent longer than that using the conventional system. 2 * 4 I 2. The overall structural performance is improved as the negative moment due to deck / * '* ## //<' 4* weight more than offsets the positive moments due to timedependent restraints without * # *# ** '*/ . * causing crack at the piers. 3. The threaded rod continuity system is believed to be an 2 efficient solution make deck

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(77) alternative for concrete superstructures to compete with longspan steel bridges and results in moderate cost savings. * **# ' * #' * 5. The proposed system can be incorporated into other types of girders such as inverted ## 4# /

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(82) Maher K. Tadros, Amgad F. Morgan Girgis. References [1]. Oesterle, R. G., Glikin, J. D., and Larson, S. C., Design of Precast Prestressed Bridge Girders Made Continuous, NCHRP Report 322, TRB, National Research Council, Washington, D.C., November 1989, pp. 15. [2] Tadros, M. K., Ficenec, Joseph A., Einea, Amin, and Holdsworth, Steve, A New Technique to Create Continuity in Prestressed Concrete Members, PCI Journal, V. 38, No. 5, SeptemberOctober 1993, pp. 3037. [3] AbdelKarim, A., and Tadros, M. K., StateoftheArt of Precast/Prestressed Concrete SplicedGirder Bridges, PCI Special Publication, PCI, Chicago, IL, October 1992. [4] Ma, Zhongguo (John), Huo, Xiaoming, Tadros, Maher K. and Baishya, Mantu, “Restrained Moments in Precast/Prestressed Concrete Continuous Bridges”. PCI Journal, V. 43, No. 6, NovemberDecember 1998, pp. 4057. [5] Sun, Chuanbing, Badie, Sameh S., and Tadros, Maher K., New stDetails for Precast Concrete Girders made Continuous for Deck Weight, Proceedings of the TRB 81 Annual Meeting, Washington, DC, January 2002. [6] Precast/Prestressed Concrete Institute Bridge Design Manual (PCIBDM), Precast/Prestressed Concrete Institute, Chicago, IL, October 1997. [7] Sun, Chuanbing and Tadros, Maher K., Discussion on Limits of Reinforcement in 2002 ACI Code: Transition, Flaws, and Solution, ACI Structural Journal, Vol. 102, no. 1, January 2005, pp 170172. [8] Sun, Chuanbing, Ph.D. Dissertation, University of NebraskaLincoln, May 2004. [9] Sun, Chuanbing, Tadros, Maher, and Girgis, Amgad, Threaded Rod Continuity System Making Precast Bridge Girders Continuous for Deck Weight, Submitted to ACI Structural Journal, July 2005. [10] Hennessey, Shane A., Bexten, Karen, Sun, Chuanbing, and Tadros, Maher K., Value Engineering of Clarks Viaduct in Nebraska, PCI National Bridge Conference, October 2002. [11] Hennessey, Shane A., Bexten, Karen A., Sun, Chuanbing, and Jaber, Fouad, Platte River East Sets Precast Record in Nebraska, PCI National Bridge Conference, October 2004.. 8.

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