Not Applicable
Not Applicable
1. Field of Invention
This invention relates to the design and construction of structures, specifically to structures with prefabricated deck units.
2. Prior Art
Full-depth precast concrete deck has gained popularity as an accelerated construction method. Use of full-depth precast concrete deck allows for the deck concrete and reinforcement to be placed in a controlled environment, improving the quality of the deck. Since the units are prefabricated, they can be delivered to a site and erected quickly.
Structures using full-depth precast concrete deck typically consist of a plurality of longitudinally spaced concrete units supported by longitudinal load-carrying members. These members are usually a single girder or multiple girders or beams. This member or members can be comprised of various materials including steel, concrete or fiber-reinforced plastic.
When no longitudinal post-tensioning is used in conjunction with a precast concrete slab deck, the use of cast-in-place joints between precast deck units is required. The cast-in-place joint requires extensive fieldwork and the uncompressed joint typically exhibits long-term maintenance and durability problems.
An improvement that has been made to precast concrete decks is to introduce longitudinal post-tensioning. The post-tensioning can provide a compression force across the deck joints, whereby improving the durability of cast-in-place joints. With the exception of the technology proposed in U.S. Pat. No. 7,475,446 B1, all current precast deck construction employs internal post-tensioning, wherein post-tensioning ducts or sheaths are embedded inside the concrete deck. The current practice of using internal post-tensioning has several disadvantages, including:
U.S. Pat. No. 7,475,446 B1 provides a solution to introduce post-tensioning external to the deck, using a method to transfer longitudinal compression to the deck units when all deck units are non-composite with the longitudinal load-carrying members and with longitudinal tensioning elements anchored at one more specially designed deck end units. The proposed method discussed herein also provides a solution to introduce post-tensioning external to the deck, but utilizes composite deck connection units in the transfer of longitudinal compression to the deck units and does not necessarily require anchorage of the tensioning elements into the deck units, as the tensioning elements can also be anchored in the longitudinal load-carrying members themselves or other locations.
Accordingly, several objects and advantages of the present invention are to provide a structural system that:
Further objects and advantages will become apparent from a consideration of the ensuing description and drawings.
In accordance with the present invention a structural construction system comprises prefabricated deck units spaced along longitudinal load-carrying members with tensioned structural elements. Axial compression of these prefabricated deck units is produced through the use of composite deck connection units by tensioned elements typically anchored in the deck units or in longitudinal load-carrying members.
A preferred embodiment of the bridge construction system of the present invention is illustrated in
However, those features comprising the structural construction system mentioned in the preferred embodiment and the substructure and span arrangement mentioned above can have various embodiments not mentioned in the preferred embodiment, as discussed in detail hereinafter and as will become apparent from a consideration of the ensuing description and drawings.
Concrete girders 21 are placed on and supported by abutments 25 and pier 23. Girder post-tensioning tendons 52 are anchored at the end of concrete girders 21 next to abutments. Concrete girders 21 are of bulb-T beams, but may be of any suitable structural shape, such as U-beams, box beams, etc. On top of concrete girders 21, a plurality of leveling devices is placed that allow for relative longitudinal motion between concrete girders 21 and the precast concrete deck units 38 or 26. In the preferred embodiment, the leveling devices are comprised of shims 27, however leveling bolts or other devices that can provide support for the deck and allow for relative longitudinal motion between concrete girders 21 and the precast concrete deck units 38 or 26 can be used. As will be evident from the description hereinafter, this allowance for relative motion will allow for the precast concrete deck units to be compressed by the tensioning of post-tensioning tendons 52. Shims 27 may be of steel, plastic, elastomeric materials, teflon-based or teflon-impregnated materials, etc.
A plurality of voids 28, similar to those used in conventional precast deck placement, are provided in deck units 38 or 26 above concrete girders 21 to allow for mechanical connection of deck units to concrete girders 21 by means of shear connectors. The voids 28 will be grouted in two different stages, first for the deck connection units 26 and the second for all other deck units 38, as hereinafter described in detail. Deck connection units 26 in typical situations are defined as the last deck unit at each end of the bridge, and in the typical embodiment consist of precast concrete deck units, but may consist of slabs, panels, brackets, blocks or corbels, etc. Haunches 30 will also be grouted at the same time as the shear connectors. Shear connectors shall be detailed to allow relative motion between precast concrete deck units and concrete girders 21 during the precast concrete deck unit erection process, as hereinafter described. In the preferred embodiment, shear connectors are shear studs 50 and shear stud base 58. Shear stud base 58 is comprised of steel plates embedded in concrete girders 21. Shear studs 50 are welded to shear stud base 58 after precast concrete deck units are in place. Other types of shear connectors can be used, such as reinforced bars protruding from girders 21 or other devices that can transfer the horizontal shear force between the precast concrete deck units and concrete girders 21 after voids 28 and haunches 30 are grouted.
Joints between adjacent precast concrete deck units can be of the match-cast type, with or without epoxy, as shown in
In the preferred embodiment, post-tensioning tendons 52 are anchored at the girder ends as shown in
Alternate embodiments for the present invention are described hereinafter:
The preferred embodiment in the context of the example bridge is illustrated hereinafter.
Abutments 25 and pier 23 are constructed. Concrete girders 21 are fabricated with post-tensioning ducts, post-tensioning anchors and shear connectors 50. A plurality of precast concrete deck units, comprising deck connection units 26 and typical units 38 are fabricated at a precast concrete facility and transported to the bridge site.
Concrete girders 21 are erected onto abutments 25 and pier 23. Concrete girders are supported by bearings or similar means, which can allow small movements of girder in the longitudinal direction of the bridge. A gap between girders, in the longitudinal direction of the bridge, is maintained at each pier location.
After concrete girders 21 are erected, the girder top elevation is surveyed and the shim thickness at each supporting point will be calculated so as to provide the correct setting elevations for deck units. A plurality of shims 27 is placed on top of the concrete girders.
Post-tensioning tendons 52 are run through post-tensioning ducts 22 and installed in post-tensioning anchorages 20. Post-tensioning ducts are coupled at pier locations; at this time, the couplers are loosely fit to allow for gap closing caused by future stressing.
Deck units are erected, placing one unit adjacent to the previously erected one and applying epoxy to the adjacent faces of the two units. High strength connection bolts 65 are then installed and tightened to ensure the gap between the adjacent units is sufficiently tight to allow the epoxy to set. This process is continued until both deck connection units 26 and all typical units 38 are installed.
After all deck units are erected, shear connector pockets and haunches of the deck connection units 26 are grouted. After grout reaches the design strength and the composite action between the deck connections units 26 and the girders is developed, post-tensioning tendons 52 are now stressed in what is hereinafter referred to as “Stage 1 Stressing”. Since at this time the girder can have longitudinal motion relative to the substructure and gaps between girders are left at pier locations, the girders do not resist the longitudinal components of post-tensioning force. Instead, the longitudinal component of the post-tensioning force is transferred through the deck connection units 26 and compresses all typical deck units 38 in between. Vertical deviation of the post-tensioning tendons 52 allows for the application of vertical forces to concrete girders 21.
These vertical forces significantly increase the load-carrying capacity of concrete girders 21.
After Stage 1 Stressing, voids 28 and haunches 30 of all remaining deck units are filled with grout, whereby making precast concrete deck units composite with concrete girders 21. Then, post-tensioning duct couplers at the pier are sealed.
Pier diaphragm 48 is poured using concrete, whereby making concrete girder 21 continuous between the two spans. Post-tensioning tendons 52 are then further stressed in what is hereinafter referred to as “Stage 2 Stressing”. Since the precast concrete deck units are now composite with concrete girders 21, Stage 2 Stressing engages the composite section similar to a typical post-tensioned set of girders. These increased vertical forces further increase the load-carrying capacity of concrete girders 21. Stage 2 Stressing has the added benefit of applying axial longitudinal compression forces to the composite section, both the precast concrete deck units and concrete girders 21, further increasing the durability and load-carrying capacity of the bridge.
After Stage 2 Stressing, post-tensioning tendons 52 are grouted, and other miscellaneous finishing details typical to bridge construction are accomplished, such as installation of cast-in-place or precast parapets, completion of bridge approaches, etc.
Post-tensioning tendons 52 stressed in Stage 1 will result in different stress distributions in the bridge than those resulting from Stage 2 Stressing. The amount of stressing force in each stage should be evaluated to achieve the most favorable outcome for the bridge. Post-tensioning tendons 52 can be stressed entirely in Stage 1, with no stressing in Stage 2, if desired.
Pier diaphragms, or other means to make the girder continuous over a pier, are optional. The girders can remain simple span when the bridge is in service. If girders remain simple span, Stage 2 Stressing is not applicable.
The operational description above is particular to the preferred embodiment of the present invention in the context of the two-span bridge heretofore defined. Alternate materials, member shapes, stressing stages, etc. can be used in employing the structural construction system of the present invention.
The present invention provides a structural system that eliminates many of the drawbacks found in current precast deck construction. Notably, it prevents potential duct conflicts and blockages by eliminating the need to couple deck post-tensioning ducts at deck joints. The durability of the deck and post-tensioning system is doubly enhanced by first, placing the post-tensioning system below the deck, whereby significantly reducing the susceptibility of the post-tensioning tendons to corrosion, and second, providing longitudinal compression in the deck, which greatly reduces cracking and subsequent intrusion of corrosive agents.
Beyond simply providing a system that eliminates drawbacks in current precast deck construction, the present invention, through the deviation of the post-tensioning tendons herein discussed, also can increase the load carrying capacity of longitudinal load-carrying members.
Another significant advantage of the present invention is its flexibility in providing the objects and advantages herein stated, all while accommodating a variety of girder shapes and materials, cast-in-place and match cast deck joints, and span configurations and lengths. In addition to this, the present invention does not require construction equipment not already common to precast deck construction and facilitates rapid construction.
Further, for multiple spans, the present invention does not necessarily require special deck end units to anchor the post-tensioning tendons, as contemplated in the invention of U.S. Pat. No. 7,475,446 B1, as post-tensioning tendons can be anchored solely in the longitudinal load-carrying members.
In conclusion, the present invention, through its use of innovative construction sequences, provides a structural construction system that is durable, easy to construct and cost effective. The present invention can accommodate a variety of structural configurations and can be rapidly constructed. All this while enhancing the load carrying capacity of the girders, and subsequently reducing required materials for these members.
Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, as illustrated and described herein, the present invention can accommodate a variety of lengths, shapes and materials for the prefabricated deck units, deck connection units, longitudinal load-carrying member and tensioned structural elements.
Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.
This application claims the benefit of U.S. Provisional Application No. 61/274,513, filed Aug. 18, 2009.
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