Method to Compress Prefabricated Deck Units by Tensioning Elements at Intermediate Supports

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/449,713, filed Mar. 6, 2011 by the present inventor.

FEDERALLY SPONSORED RESEARCH

Not Applicable

SEQUENCE LISTING OR PROGRAM

Not Applicable

BACKGROUND OF THE INVENTION

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 deck units supported by longitudinal load-carrying members. This member or members is usually a single girder or multiple girders.

This member or members can be comprised of various materials including steel, concrete, wood or fiber-reinforced plastic.

To improve deck durability, it is important to have a pre-compression force across deck joints to minimize the propensity of the deck to crack under loading.

Currently, such pre-compression force is supplied via standard post-tensioning systems, which utilize post-tensioning tendons or bars within ducts. US Federal Highway Administration technical report (#FHWA-IF-09-010) and Precast/Prestressed Concrete institute State-of-the-Art Report on Full-Depth Precast Concrete Bridge Deck Panels (SOA-01-1911) provide a comprehensive summary of current engineering practice using precast deck units, showing that all current precast deck systems with longitudinal compression utilize post-tensioning systems in the deck. Other patent references, such as U.S. Pat. No. 7,475,446, U.S. Pat. No. 7,461,427, and U.S. Pat. No. 5,457,839, teach various methods of using post-tensioning systems to provide deck compression. However, using standard post-tensioning details carries with it the disadvantage of requiring additional cost and time to construct. This invention provides a more economical solution.

U.S. Pat. No. 7,475,446 B1, by the present inventor, teaches a solution to introduce pre-compression force across deck joints via post-tensioning external to the deck, using a method to transfer longitudinal compression to the deck units under the following conditions:

• • a. All deck units are non-composite with the longitudinal load-carrying member when the longitudinal compression transfer occurs.
  • b. Longitudinal tensioning elements are required to be anchored at one more specially designed deck end units.

The proposed method discussed herein also provides a solution to introduce pre-compression force across deck joints via post-tensioning external to the deck, but 65 applies the following differing conditions from those presented in U.S. Pat. No. 7,475,446:

• • a. Composite deck connection units may be used in the transfer of longitudinal compression to the deck units.
  • b. Anchorage of the tensioning elements directly into the deck units is not required, as the tensioning elements are anchored to attachments to the longitudinal load-carrying members themselves.

Therefore, the method proposed herein eliminates the need for special end deck units to be fabricated and allows for a substantially reduced quantity of tensioned structural elements to be employed versus that presented in U.S. Pat. No. 7,475,446.

U.S. application Ser. No. 12/857,713, by the present inventor, teaches a solution to introduce pre-compression force across deck joints via post-tensioning external to the deck, using a method to transfer longitudinal compression to the deck units under the following condition:

• • a. The longitudinal axial pre-compression of the deck is reacted by the longitudinal tensile component of the post-tensioning, wherein the longitudinal axial tensile component is not transferred via the supporting girders.

The proposed method discussed herein also provides a solution to introduce precompression force across deck joints via post-tensioning external to the deck, but applies the following differing condition from those presented in U.S. application Ser. No. 12/857,713:

• • a. The longitudinal axial pre-compression of the deck is reacted by a longitudinal tension component in the supporting girders, wherein this reaction is facilitated by anchor assemblies at the supporting girder ends at intermediate supports that allow for tension to be transferred between girders in the same line but in different spans.

Therefore, the method proposed herein does not require post-tensioning tendons running along or within the majority of the length of the supporting girders to provide longitudinal compression to the deck panels, as presented in U.S. application Ser. No. 12/857,713.

U.S. application Ser. No. 12/963,233, by the present inventor, teaches a solution to introduce pre-compression force across deck joints via tensioning the supporting girders themselves under the following condition:

• • a. Tension is induced in the supporting girders through the use of compression jacks placed within gaps provided in the deck system to jack against a jacking frame apparatus or composite deck panel.

The proposed method discussed herein also provides a solution to introduce pre-compression force across deck joints via tensioning the supporting girders themselves, but applies the following differing condition from those presented in U.S. application Ser. No. 12/963,233:

• • a. Tension is induced in the supporting girders through the stressing of tensioned structural elements installed in anchor assemblies at the ends of the girders supported on intermediate supports.

Therefore, the method proposed herein does not require the use of jacks placed within gaps in the deck as presented in U.S. application Ser. No. 12/963,233.

OBJECTS AND ADVANTAGES

Accordingly, several objects and advantages of the present invention are to provide a structural system that:

• • a. facilitates rapid construction of a structure consisting of prefabricated deck units, wherein increasingly tight construction schedules and/or site constraints can be accommodated;
  • b. provides compression across joints between deck units to improve deck durability by tensioning the bridge girder;
  • c. typically increases the overall load resistance of the structure by tensioning the girder, whereby reducing the amount of material required in the girders;
  • d. mitigates the need for deck closure pours, whereby reducing the cost and time of construction and potential deck durability compromises
  • e. facilitates the use of an deck system with post-tensioning external to the deck for multi-span concrete girder bridges without requiring the use of a substantial length of post-tensioning tendons, whereby reducing the amount of materials required for construction and whereby construction schedules can be further accelerated.

Further objects and advantages will become apparent from a consideration of the ensuing description and drawings.

SUMMARY

In accordance with the present invention a structural construction system comprises prefabricated deck units spaced along longitudinal load-carrying members. Axial compression of these prefabricated deck units is produced by external tensioned elements anchored in anchor assemblies integrated with the longitudinal load carrying members at intermediate support locations. The prefabricated deck panels are fitted with means to react against the longitudinal load-carrying members.

DRAWINGS

Figures

FIG. 1 shows the elevation view of an example bridge used to describe the present invention.

FIG. 2 shows the plan view of the example bridge.

FIG. 3 shows the general cross section of the example bridge.

FIG. 4 shows the plan view of a typical deck unit.

FIG. 4A shows the bulkhead view of a typical deck unit.

FIG. 4B shows the transverse cross-section of a typical deck unit.

FIG. 4C shows the section of a shear key at a typical deck joint.

FIG. 4D shows the detail of shear connectors and void for shear connectors.

FIG. 5 shows mechanism to apply deck compression force.

FIGS. 6A-6B show examples of girder connections at the median pier.

FIG. 7 shows an example girder connection at the median pier for an alternate embodiment using concrete girders.

REFERENCE NUMERALS

• 11 girder
• 12 abutment
• 13 pier
• 15 approach slab
• 18 precast deck unit
• 20 deck joint
• 22 gap at pier between girders
• 26 shear studs
• 28 shear connector voids
• 29 shear keys
• 30 haunch
• 32 deck connection unit
• 33 post-tensioning bar
• 34 anchor assembly
• 35 stiffeners
• 36 bearing plate
• 37 anchor plate
• 38 nut
• 51 bearing stiffener
• 52 bottom flange connection plate
• 54 high strength filler
• 56 anchor blocks

DETAILED DESCRIPTION

FIGS. 1 Through 7—Preferred Embodiment

A preferred embodiment of the bridge construction system of the present invention is illustrated in FIGS. 1 through 5 in the context of a two-span bridge, hereinafter referred to as “example bridge”. The example bridge has two abutments 12 and an intermediate pier 13 acting as substructure units. The preferred embodiment of the bridge construction system is comprised of steel girders 11 acting as longitudinal load-carrying members, precast concrete deck units 18 acting as prefabricated deck units. The precast concrete deck units can be constructed using long or short line match-casting or without match-casting.

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.

Steel girders 11 are placed on and supported by abutments 12 and pier 13. Steel girders 11 are of fabricated plate girders, but may be of any suitable structural shape, such as tub girders, rolled beams, trusses, etc. Bearing stiffeners 51 typical to such support configurations are provided. A gap 22 between the girder ends at the pier 13 is provided to allow for relative movement between the steel girders 11 of each span. On top of girders 11, a plurality of leveling devices is placed to support the precast deck units 18 that also allows for relative longitudinal motion between girders 11 and the precast concrete deck units 18. In the preferred embodiment, the leveling devices are comprised of shims, however leveling bolts or other devices that can provide support for the deck and allow for relative longitudinal motion between girders 11 and the precast concrete deck units 18 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 reacting to the tensioning of girders 11. Shims 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 18 above girders 11 to allow for mechanical connection of deck units 18 to girders 11 while shear connectors voids 28 are grouted. Haunches 30 will also be grouted at the same time as the shear connector voids 28. Shear connectors shall be detailed to allow relative motion between precast concrete deck units 18 and girders 11 during the precast concrete deck unit erection process and prior to grouting, as hereinafter described. In the preferred embodiment, shear connectors are shear studs 26 welded to the girders 11.

Deck joints 20 between adjacent precast concrete deck units can be of the match-cast type, with or without epoxy, or cast-in-place using concrete, grout or other suitable jointing materials. In the preferred embodiment, match-cast epoxy joints are used.

In the preferred embodiment, anchor assemblies 34 are fabricated with the girders at the ends of the girders supported on the pier 13, as shown in FIG. 5. Post-tensioning bars 33 acting as tensioned structural elements are placed within the anchor assemblies 34. The deck connection unit 32 at each end of the bridge 245 is made composite prior to any stressing of the post-tensioning bars 33 so that the deck connection unit 34 will act as a mounted element that can react the deck compression with girder tension that results from stressing post-tensioning bars 33. FIG. 6A and FIG. 6B show that the anchor assemblies 34 consist of stiffeners 35 and anchor plates 37.

In the preferred embodiment, the girder connection at the pier location is simply supported for dead load and continuous for live load. This is achieved by making the girder bottom flange connection at pier after all dead loads are applied to the structure. FIG. 6A and FIG. 6B show examples of the girder connection at the median pier. The tensile transfer connection near the top of the girder is made through the post-tensioning bars 33. When only the post-tensioning bar connection is made, the girder acts as simply supported in bending moment but can transfer axial tension. The girder becomes continuous in transferring moment when both bottom flange and the tensile transfer connections are made. FIG. 6A shows simply-supported connection configuration and FIG. 67B shows an alternate to connect bottom flanges with high strength filler material 54 placed between bottom flange connection plates 52. By connecting the bottom flanges, the girder system will act as continuous under subsequently applied loads.

Alternate embodiments for the present invention are described hereinafter:

• • a. The prefabricated deck units can be comprised of any other material that is suitable for supporting loads anticipated to be applied to the deck units, such as composite material, wood, steel-concrete composite units, etc.
  • b. The longitudinal load-carrying members can be comprised of any other material or cross-section suitable to support the loads applied to these members such as steel i-girders, precast prestressed concrete beams, composite material I-girders, single or multiple box girders of steel or concrete, trusses, wood beams, etc.
  • c. Though the preferred embodiment of the present invention is presented with respect to steel longitudinal load-carrying members, concrete girders, in particular, prestressed concrete girders can be readily used. The concrete girders may be cast with typical formwork with appropriate blockouts for the webs and flanges in order to provide anchor blocks 56 through secondary pours. Post-tensioning bars 33 can then be installed through holes provided in the anchor blocks 56. The tensioning bars can then be anchored via bearing plates 36 and stressed
  • d. The girder connection type at intermediate piers can be other types, such as simple support for both dead load and live load, or continuous for both dead load and live load.
  • e. Though the preferred embodiment of the present invention is presented in the context of bridges, it is not limited to bridge applications. Any structural application requiring decking support by longitudinal load-carrying members can utilize the present invention in alternate embodiments such as building floor systems and building roof systems.
  • f. In the preferred embodiment, the jacking is applied to the entire structure, from one end to the other. A structure can consist of more than one structural unit, where a structural unit is defined as that to which a jacking force can be applied from one end of the structural unit to the other, without applying force to other structural units.
  • g. Though the preferred embodiment of the present invention is presented with composite deck connection units 32 acting as mounted elements, cast end blocks, brackets or other elements affixed to the girders may be provided.

Operation

The preferred embodiment in the context of the example bridge is illustrated hereinafter.

Abutments 12 and pier 13 are constructed. Girders 11 are erected with anchor assemblies 34 pre-installed. A gap 22 between the girder ends at the pier 13 is provided to allow for relative movement between the steel girders 11 of each span.

The girder top elevation is then surveyed and the shim thickness at each supporting point calculated so as to provide the correct setting elevations for deck units. Shims are placed on top of the girders. Post-tensioning bars 33 are installed within the anchor assemblies 34 in conjunction with bearing plates 36 and nuts 38. Post-tensioning bars 33 are not stressed at this stage.

Precast deck units 18 are erected, placing one unit adjacent to the previously erected one and applying epoxy to the adjacent faces of the two units. Means is employed to provide a certain amount of compression over the epoxy joint (typically at 40 psi, similar to segmental bridge construction) to ensure the joint is properly set. This process is repeated until all deck units 18 are installed.

After all deck units are installed, deck connection units 32 are made composite with the supporting girders by grouting the shear connector voids 28 and the haunches 30 relative to the deck connection units. The post-tensioning bars 33 are then stressed. The nuts 38 are tightened against the bearing plates 36 and anchor plates 37 in conjunction with the stressing operations to lock in the stress in the post-tensioning bars 33.

After the stressing of the post-tensioning bars 33, shear connector voids 28 and haunches of all remaining deck units are grouted.

As an option, the girder bottom flange connection at the median pier 13 may then made so that the girder connection at pier location will function as continuous under subsequently applied loads. This is accomplished by placing high strength filler 54 between the bottom flange connection plates 52.

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, connection types at the median piers, means of stressing, etc. can be used in employing the structural construction system of the present invention.

Advantages

The present invention provides a structural system that eliminates many of the drawbacks found in current precast deck construction associated with standard longitudinal post-tensioning. Notably, it offers an alternate to provide pre-compression across joints of precast deck units without employment of post-tensioning tendons and associated ducts in the deck. This significantly reduces the cost and time of construction required.

Beyond simply providing a system that eliminates the drawbacks in current precast deck construction, the present invention can potentially increase the load carrying capacity of longitudinal load-carrying members by employing appropriate connection details at the median pier, and correct construction steps. In the preferred embodiment, the stressing of the post-tensioning bars introduces a negative moment at the midspan of the girders, which offsets part of the girder moment under service load.

CONCLUSION, RAMIFICATIONS, AND SCOPE

In conclusion, the present invention provides a structural construction system utilizing prefabricated deck units that is durable, easy to construct and cost-effective. The present invention can accommodate a variety of structural configurations and can be rapidly constructed.

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 jacking methods and details, a variety of girder connection methods, and a variety of shapes and materials for longitudinal load-carrying members.

Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.