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Connection Details for PBES
Appendix D: Case Studies
The following pages contain brief cast studies of prefabricated bridge projects. The first three are actual projects. The third is a hypothetical bridge project that demonstrates the use of this document.
|Case Study Number||Title|
|1||New Hampshire DOT
Mill Street over the Lamprey River
Epping, New Hampshire
State Highway 36 over Lake Belton
Short Span Local Bridges in Texas
|4||Hypothetical Bridge Replacement Project Using All Prefabricated Components
Details taken from this document
Case Study 1:
Mill Street Bridge over the Lamprey River
Epping, New Hampshire
The New Hampshire Department of Transportation has been a lead state in the use of high performance concrete for bridges. To further extend the capabilities of high performance concrete, the state undertook the development of details for an all precast concrete bridge. The intent was to expedite construction using prefabricated components with simple yet durable connections.
A trial demonstration project was selected in the Town of Epping, New Hampshire. The project scope included the removal of two short span bridges that span spilt channels of the Lamprey River and combine them into one single span bridge that would span both channels. The project site was chosen for the demonstration project because there was a short detour available that would allow for full closure of the roadway.
The bridge is located in the Town of Epping New Hampshire. Epping is typical of many towns in New England that are home to short span bridges that are often over 100 years old. The existing river crossing consisted of two bridges that carried Mill Street over two channels of the Lamprey River. The site also included a historic mill dam that needed to be kept in place in order to minimize changes to the river and to satisfy historic preservation criteria at the site. The new bridge was designed to span both channels and the dam. The resulting span length is 115 feet. The alignment of the road is straight and there is an intersection just beyond the south end of the bridge.
The New Hampshire DOT was interested in developing a rapid bridge replacement system for use on state highway projects. The Epping site was chosen as a demonstration project for several reasons. There was a reasonable detour around the site; therefore a temporary bridge could be avoided. The site had low traffic volume, therefore if problems arose on this demonstration project, there would not be significant risk associated with not completing the project on schedule.
The intent was to develop a bridge design that could be constructed in less than 2 weeks using all precast concrete elements. In order to simplify the contracting for this trial project, the existing bridge removal and excavation were excluded from the 2 week timeframe. This was done because there was bedrock at the site; which could complicate the site preparation work and lead to contracting problems with unknown foundation conditions. On future accelerated construction projects, the state would include the site preparation within the accelerated construction timeframe.
A decision was made in the early design phase of the project to build a bridge with traditional abutments and walls that used connections that have been used in the past on other structures. By using traditional foundations, the details could easily be transferred to other accelerated construction projects.
The state wanted to use a proven system of connecting precast elements whose behavior is known. The system chosen was taken from the vertical construction industries (parking garages, stadiums, and buildings). The connections are made using grouted reinforcing splice couplers. The use of these connectors allowed for a simplified emulation design procedure for the substructure elements (see Section 1.4.2 for more information on these connectors and emulation design).
The substructure is a simple cantilever abutment and cantilever wingwall system that is supported on reinforced concrete spread footings. The site constraints, hydraulics, and maximum beam span dictated that the abutments be full height abutments with the base located at the edge of the river channel. The design of the connections emulated a typical cast-in-place concrete construction joint at the base of the wall element, except the lap splices were replaced with the grouted reinforcing splice couplers. All other aspects of the design were based on conventional reinforced concrete methods.
One of the most unique aspects of this project was the use of precast concrete footing elements. At the time of the design, the state was not aware of any other bridge projects that had used precast concrete footings; therefore new details had to be developed. Conventional footings are designed using one-way slab action; which means that the transverse connection between footing elements need not transmit moment. Instead, a simple grouted shear connection was used that would transfer shear between the footing elements in case there was minor differential settlement across the length of the wall section.
Proper seating of the footings over the substrate was a concern. In order to provide uniform bearing, a grouted void was used. The footings were also cast with leveling bolts that allowed for adjustment of grade in the field. The intent was to support the footings on the leveling bolts until the grout could be placed under the footings. Grout placement was done through pipe blockouts in the footing. A flowable grout was placed from the center and allowed to flow to the edges of the footings where it was contained with a simple dam system. The strength and curing of the grout did not hinder the continuation of the construction because the bearing pressures on the grout are very low during construction. These pressures are even very low after the bridge is in service. Typical bearing pressures on spread footings are in the range of 50 pounds per square inch; therefore a high strength grout is not needed under the footings.
The wall stems were placed in a shallow trough cast in the top of the footing to facilitate grouting and to improve shear resistance in the connection. This trough could have been eliminated by checking the shear resistance of the connection using the shear friction provisions in AASHTO; however the state preferred to use the trough to facilitate the grouting and to provide extra shear resistance.
The vertical joints between wall panels were detailed with simple grouted shear keys that are similar to a typical contraction joint in New Hampshire standards for reinforced concrete cantilever walls. The original contract plans had a layout of both horizontal and vertical joints in the wall stems. The layout was shown as schematic, and the contractor was allowed to modify the joint layout within certain parameters that were defined on the contract documents. The contractor chose to use only one horizontal joint at the wall to footing interface and run the wall elements full height. This approach is recommended because it limits the wall construction to only one moment connection over the entire height. Additional horizontal joints would require the use of more grouted reinforcing splice couplers, which would lead to higher costs.
All other connections including the abutment cheekwalls were made using the grouted reinforcing splice couplers or grouted shear keys.
The New Hampshire DOT has been using precast prestressed adjacent box beam bridges for many years. These beams allow for a simple superstructure that can be built very quickly because there is no need for a reinforced concrete deck. Typical details in the northeast historically have not included a concrete over-pour on these beams. The system is overlaid with bituminous pavement after the installation of a membrane waterproofing system.
The connection between beams is made using a grouted shear key combined with an unbonded lateral post-tensioning tie. The lateral ties used in the northeast for the last ten years are monostrand post tensioning system that is commonly used in post tensioned parking garage structures. Each strand is delivered to the job site in a grease filled sheath that is run through a transverse void in the beams. The ends are anchored to an epoxy coated anchor plate and sealed with a grease filled cap. This unbonded system does not use grouted ducts; therefore the installation can be completed very quickly. A typical installation of all lateral ties in a span is less than 1 hour.
To expedite construction of the railings, the state detailed a reinforced concrete curb that was installed at the precast plant. This curb included anchor bolts for a standard New Hampshire aluminum rail system that was installed after erection of the beams.
The contractor was given two weeks to complete the bridge installation after the existing bridges were removed and the excavation was completed. The existence of bedrock required that the bottom be over excavated so that the precast footings would fit within the excavation. A thin layer of low-density concrete fill was placed to facilitate the placement of the footings on the uneven bedrock (a layer of structural granular fill could have been used also). The timeline for construction is shown in Figure CS1-1:
Figure CS1-1: Construction Timeline
The contractor was given options for backfilling the walls and abutments that included the use of flowable fill or standard compacted gravel fill. Flowable fill is a mixture of sand, water, and a small amount of cement that has the consistency of flowable concrete. The contractor chose the compacted gravel option due to difficulties with containing the flowable fill during placement. The area for backfill was relatively small and was accomplished with small vibratory compaction equipment.
The construction of the Mill Street Bridge was a great success. The construction time of eight days was faster than the specified 14 day construction window. Construction costs for the foundations were higher than conventional construction; however when factoring in the elimination of a temporary bridge crossing, the overall cost of the project was only 8% higher than conventional construction.
The following items contain concepts that should be followed on future projects using these methods and lessons learned on the Mill Street Bridge:
- The tolerances on the placement of the grouted reinforcing splice couplers are critical, but not unattainable. The couplers have some tolerance and it is possible to oversize the couplers by two bar sizes to give even larger tolerances. For instance, a number 11 coupler can be used with a number 9 bar. The use of narrower wall elements will also help to alleviate fit-up problems with the couplers. This requires fewer couplers per piece, which which less chance for dimensional problems.
- Specifications should indicate that the tolerable distance to each coupler be measured from a common working point as opposed to center-to-center spacing. This way there will be no build up of placement tolerances in the form.
- Simple geometric layouts of abutments and walls are preferred over complex geometry. The Mill Street Bridge was designed with walls that were either 0 degrees or 90 degrees when compared to the abutment wall face. It is possible to build a precast substructure with complex geometry; however more care may need to be taken during fabrication. It may be desirable to have the structure dry fit in the shop prior to shipment to ensure the geometries are correct.
- The shape of pieces should be kept simple, so that basic flat slab precasting can be used.
- Allowing the contractor to adjust the size and number of pieces is beneficial so that the contractor can bid the project based on the available equipment.
- Maximum element dimensions and weights need to be specified so that shipping can be made without special permits.
- Grouting of vertical joints in wall elements was difficult. The contractor tried to use backer rods; however the rods pushed out during grouting. More substantial forms are required for the vertical joints. Designer should add notes to the plans warning the contractor of the pressure head of the grout.
- The joint widths for the vertical wall elements were detailed as 1" wide, which made grout installation difficult. On future projects, these joints should be detailed as 1 ½" wide.
- Some states do not use shear keys between wall sections for cantilever walls. On future projects, the state will used non-keyed joints combined with expanding foam joint filler that can be injected into the gap between the wall elements after erection.
- The design of the footing leveling bolts should be based on the full weight of the footing element. It is inevitable that the footing will rest on just two of the bolts, which makes it difficult to adjust. One method of overcoming this is to adjust the elevation by turning the leveling bolts prior to release of the footing element from the crane. Greasing the bolts also improves this operation.
- The grouting of the reinforcing bar couplers went very fast (approximately 150 sleeves per hour). Cure time prior to backfilling is approximately 12 to 16 hours. It is recommended that the contractor's workers be trained by the manufacturer prior to installation.
Case Study 2:
State Highway 36 over Lake Belton
The Texas Department of Transportation undertook a project to replace an aging bridge that carries State Route 36 over Lake Belton in Belton, Texas. The existing bridge is multispan steel stinger bridge that is approximately 3700 feet long and is located approximately 35 feet above normal water levels. Figure CS2-1 shows the existing bridge.
Figure CS2-1 Existing Bridge
The new bridge is a 32 span prestressed concrete U-Beam bridge that is supported on twin column reinforced concrete pier bents with a hammerhead bent cap. The proposed bridge is a twin span with two side by side bridges that were nearly identical. There are 62 pier bents on the project. The superstructure is a standard precast prestressed U bean that is used in Texas. The beams support a standard 8" thick reinforced concrete deck. Figure CS2-2 is a computer rendering of the proposed structure.
Figure CS2-2 Proposed Bridge
The existing bridge had a low strength rating; therefore the delivery of materials via the existing bridge not possible. In order to facilitate construction, prefabricated pier bent caps were proposed. The bent caps offer the following advantages when compared to conventional cast-in-place construction.
- The use of a repetitive shape was ideal for prefabrication, which saved time. By using prefabricated elements, the state saved the time for setting the formwork, placing concrete and curing. Installation proceeded at a rate of two caps per week.
- There is a cost savings by minimizing the amount of labor on site and by reducing the overall project time.
- The quality of the caps was far superior to conventional cast-in-place concrete. High strength high quality concrete was specified that will be more durable than conventional construction. Quality control in a precast plant typically exceeds the quality that can be attained in the field.
- Safety was improved by reducing the amount of man-hours of labor and inspection personnel that need to be working on a high platform.
- The repetition of the pier caps made it cost effective to build custom forms for the bent cap shape. The cost of the forms was spread out among the 62 cap pieces that needed to be cast. Since a custom form needed to be made, designers chose to incorporate architectural details into the cap shape.
The bridge is located near Belton Texas. The bridge is very high above the normal water line because the water surface elevation on Lake Belton can fluctuate as much as 40 feet. There is barge access to most of the project site. The low load carrying capacity of the existing bridge (13 tons) greatly limited the delivery of materials for the new bridge. Concrete delivery had to be made via barges. Figure CS2-3 is a satellite photo of the project site.
Figure CS2-3 Project Site
The Texas Department of Transportation had completed several precast concrete bent cap project prior to this project. They had also complete several large research project that investigated the connection of the precast bent cap to concrete columns. The Connection details that were studied included grouted voids and grouted post tensioning duct with mild reinforcement dowels. Previous projects with grouted ducts had the ducts projecting to the top of the pier cap. The Lake Belton project did not have this detail. Instead, all but two of the ducts were capped in the core of the pier cap; thereby eliminating issues with exposure of the ducts to the environment and also eliminating interference issues between the ducts and the densely spaced reinforcement in the top of the pier cap. Grouting was done via standard grout ports and vents. The connection is enhanced by the addition of spiral reinforcement to help enhance the strength and service performance of the connection.
Figure CS2-4 shows an elevation view of a typical bent. The lower portion of the bent consists of two 5 foot diameter drilled shafts. There is a tie beam located approximately 12 to 15 feet above normal water levels. The tie beam was used to make up for tolerances in the drilled shaft locations. Constant height columns with reveals and column capitals were then cast above the tie beam that were aesthetically pleasing and allowed for repetitive formwork.
The bent caps were 39 ft long and 5.5 ft wide. The bottom soffit of the cantilevers has a 17 foot radius. The depth of the cantilevers varies from 3 feet to 5.5 feet. The cross slope of the roadway was accomplished by varying height beam pedestals on top of the bent caps.
Figure CS2-4 Proposed Pier Bent
Figure CS2-5 shows the arrangement of the post tensioning ducts within the bent cap. There were a total of 14 ducts in the connection. Most were used to anchor #11 reinforcing bars that projected from the cast-in-place columns. Several were used for temporary threadbar connections. In later stages of construction the use of the temporary threadbar connections was waived because of the stability of the bent cap on the columns.
Grout tubes were used to deliver the grout to both the ducts and the interface gap between the columns and the bent cap. Elevations and the gap between the cap and the column were set by the installation of leveling shim packs (4 sets per column) that were placed prior to setting the cap. A for was used to contain the grout at the top of the column. After grouting (Figure CS2-6), the form was removed and replaced with dry pack grout (Figure CS2-7). Mock-ups of the grouting area were made and tested prior to production in order to ensure that the grout was filling all the voids.
Figure CS2-5 Grouted Reinforcement PT Duct Layout
The bent caps were fabricated off site at a precast fabrication plant. Trucking the elements required the use of non-standard trailers due to the high weight. Once on site, the caps were transferred to a work barge with a crane. Once in position, the crane lowered the bent cap on top of the cast-in-place concrete columns.
Figure CS2-6 Grouted Placement
Figure CS2-7 Dry Pack Grouted Placement
The Lake Belton project was very successful. The construction of the bent cap was much faster than conventional cast-in-place construction. The caps were installed at a rate of 2 per week. Construction costs were slightly higher than cast-in-place construction; however the high quality concrete and improvement of worker safety made the cost increase acceptable. The state anticipates that costs will decrease as contractors become familiar with the process.
The following items contain concepts should be followed on future projects using these methods and lessons learned on the Mill Street Bridge:
- Since a non-conventional grouted post tensioning tube connection was used, research was critical in order to determine the behavior of the connection. The research proved that the connection was viable and low cost. This connection is not recommended for high seismic zones; however there is on-going research in California to enhance this connection type for use in high seismic areas.
- The tolerances on the vertical reinforcing projecting from the cast-in-place columns was set at ½” This was found to be more than adequate to make the connection.
- The use of mock-ups for grouting procedures was a benefit for both the contractor and the inspection personnel.
- The quality of the concrete used in the precast bent caps far exceeded the quality of normal cast-in-place concrete. This was due to the use of better concrete and better curing methods in the fabrication plans.
- Through the use of prefabrication on several projects, the state has concluded tha prefabrication has several advantages over conventional construction:
- Reduced traffic disruption
- Increased work zone safety
- Reduction in environmental impacts
- Improved constructability
- Increased quality
- Lower life cycle costs
- The state plans on developing new prefabrication concepts for future projects.
Case Study 3:
Short-span Local Bridges in Texas
The Texas Department of Transportation has been developing standard designs for shortspan bridges in Texas. The state has numerous county bridges that are in need of replacement. Prefabrication of replacement bridges is a large part of this initiative.
Several different types of prefabricated bridge elements have been developed for use on this program. The intent is to minimize on-site casting and curing of concrete. A typical offsystem bridge construction contract duration in Texas is approximately 6 months. The goal is to reduce this duration to one month or less.
Local off-system bridges present a different set of constraints when compared to high volume highway bridges. The fact that a bridge has low traffic volume does not mean that prefabrication is not viable. Several factors can play into the need for prefabrication on local bridges:
- Detour lengths in rural area can be significant. The cost for detours or temporary structures can be significant. The detours are also disruptive to local residents and businesses.
- Remote bridge location can make the delivery of cast-in-place concrete difficult. The ability to precast concrete elements off site can lead to significant savings.
- By using prefabricated butted beam elements, a bridge can be built without deck forming and casting. The beams can be overlaid with asphalt pavement, thereby reducing construction time and cost.
- Prefabricated steel railings can be used on low volume roads, which eliminates the need for cast concrete parapets.
Several different prefabricated bridge elements have been developed for local bridges in Texas. The following are examples of these elements:
Precast concrete pier bent pile caps with steel piles:
Pile bents made with steel piles and concrete caps can be a very cost effective substructure for multi-span bridges. The piles can be driven quickly without significant disruption to the river. Upon completion of the driving operation, the piles are cut off to the proper elevation. The bent caps are made using precast concrete elements with embedded steel plates on the underside of the cap. The connection between the piles and the cap is made using overhead fillet welds. Figure CS3-1 shows a typical pile to cap connection.
Figure CS3-1 Steel Pile to Cap Connection
The detailing of the bent cap is rather simple; therefore it can be cast near site or in a local construction yard. The caps can be cast on the ground using standard concrete forms. Figure CS3-2 shows a typical cap after fabrication. Figure CS3-3 shows the welding procedure.
Figure CS3-2 Pile Bent Cap Fabrication
Figure CS3-3 Pile to Cap Connection
Precast abutment caps are similar to the pier bents. Abutment bent caps can be constructed with backwalls and integral "flying" wingwalls. Figure CS3-4 shows a typical abutment bent cap being installed.
Figure CS3-4 Abutment Bent Installation
Winged Slab and Double Tee Beams:
A family of winged slab and double tee beams has been developed for use in adjacent beam superstructures. These beams are use instead of traditional butted slab and box beam bridges due to leakage problems and joint failures. Figure CS3-5 shows a typical cross section of a winged slab bridge. Figure CS3-6 shows a typical cross section of a butted double tee bridge.
Figure CS3-5 Winged Slab Bridge Beams
Figure CS3-6 Butted Double Tee Bridge Beams
The beams are butted together and jointed using a welded rod and non-shrink grout. Upon completion of the grouting, the slabs are given a 2 course surface treatment and then covered with an asphalt overlay wearing surface. The bridge railings are then bolted directly to the winged slab. Figure CS3-7 show the details of the welded plate connection. This connection was studied in a Texas DOT Research Project Number 1856-2 to ensure that it would provide a durable connection. The research showed that the welded plate spaced at 5 feet on center combined with a grouted shear connection between the welded plates adequately provided load transfer to adjacent beams.
Figure CS3-7 Lateral Welded Plate Beam Connection Details
Conventional Butted Slab Beams:
Texas also uses butted slab beam bridges. These beams are connected by means of a grouted shear key combined with lateral post tensioning. The lateral post tensioning consists of a single ½" diameter prestressing strand in a grease filled sheath. The strands are spaced a 5 foot intervals and stressed to 31,000 pounds after the grouted keys have cured. Figure CS3-8 shows a typical cross section of a butted slab bridge. Figure CS3-9 shows typical post-tensioning details.
Figure CS3-8 Butted Slab Beam Bridge Beams
Figure CS3-9 Lateral Post-Tensioning Details
Flowable Fill Backfill:
Flowable fill is a mixture of sand water and fly ash. The fly ash content is kept low, so that the resulting material can be excavated in the future with relative ease. The intent of flowable fill is to provide a stable backfill material that does not need compaction. It can be driven on hours after placement. It has a final compressive strength of approximately 150 psi.
Figure CS3-10 Placement of Flowable Fill
- It is possible to build prefabricated bridges very quickly.
- Precast pile caps welded to steel piles is a viable option for accelerated bridge construction in Texas.
- Transverse connections for butted beam systems have been researched and a welded plate connector was developed. This connection can be used on various types of butted beam bridges.
- Flowable fill can be used to place backfill soils rapidly.
Case Study 4:
Hypothetical Bridge Replacement Project
Using All Prefabricated Components
Based on Details in this Document
This case study is for a hypothetical bridge replacement project. The intent of this study is to demonstrate the use of this manual and how details from several different sources can be combined into an actual bridge project. The bridge chosen is a typical expressway overpass. This example is for the development of a preliminary structure type study using prefabricated components.
The bridge is located in a northern climate. It is exposed to snow and de-icing salts. The bridge is located in a commercial/retail area. The bridge carries the local road over an expressway. There are four lanes of traffic on the bridge (2 in each direction) and four lanes beneath the bridge (2 in each direction). There are retail establishments near the bridge that would be affected by any type of construction. Traffic volume is high on both roadways. There is a detour available for the local road; however it is approximately 2 miles long.
The structure is a three span bridge (simple spans). The structure type is a composite deck supported by steel beams. The abutments are full height cantilever type and the piers are conventional concrete multi-column bents. A sketch of the existing bridge is shown in Figure CS4-1 on the following page.
The bridge structure has deteriorated to the point where replacement is the only option. The bridge has the following deficiencies:
- Minimal under-clearance (14’-5”)
- The piers and abutment are deteriorated due to close proximity to roadway (salt spray attack)
- The deck is severely deteriorated
- There are leaking deck joints at each pier and abutment
- The beams have peeling lead paint
- The beam ends are deteriorated due to deck joint leakage
Figure CS4-1: Existing Bridge
The state DOT undertook a public involvement process and presented several options to the local businesses. The primary concern of the businesses was the potential loss of income from the construction project.
The designers investigated and presented two options.
Option 1: Conventional construction using multiple stages
Under this option, both roadways would remain open to traffic for the entire duration of the project; however one lane of traffic would need to be taken out of service in each direction in order to make room for the construction activities. The bridge would need to be built in two stages, and it is estimated that construction would take approximately 18 months (with a 3 month winter shutdown).
- The roadways will remain open to traffic at all times
- Construction will take almost 2 years
- Traffic will be congested during construction.
Option 2: Use prefabrication, close the road and accelerate construction
Under this option, the traffic on the bridge will be detoured around the site. The entire bridge can then be built without traffic management on site. Virtually all components will be prefabricated off site prior to roadway closure. The goal would be to replace the bridge in 30 days.
- The overall project can be greatly reduced
- Quality can be improved by using plant produced concrete
- The road will be closed for 30 days, which will impact the businesses.
During the public hearing, the business owners expressed that they would prefer to live with a 30 day full closure as opposed to a protracted partial closure. This is not unusual. Businesses often see a marked drop-off in customers if they are near a construction site. Two years of a 20% reduction in business is much worse than 30 days of a more significant reduction. Based on this, the design team opted for the second option with the full closure and a detour.
Following the public involvement process, a formal type study was completed. The use of prefabricated elements does not necessarily correspond to a significant change in the structure type. It is possible to construct typical bridge structures with prefabricated elements. In this case, the designer opted for the following design after the type study was completed:
- Structure Layout: 2 span continuous (no deck joint at the pier)
- Place the pier in the center of the median between the existing piers
- Raise the grade of the bridge 3 feet
- Improve under-clearance and allow for deeper beams (longer spans)
- Integral abutment design (no deck joints at the abutments)
- Place the abutment on top of the slope to minimize the height of the stems
- Support the abutments on steel H-Piles
- Use a typical multi-column concrete pier bent
- Supported on spread footings
- Use a continuous steel plate girder with a composite concrete deck
- Unpainted weathering steel
- Use a bituminous concrete wearing surface
- Protect the deck with a waterproofing membrane
- Use open galvanized steel railings (aesthetics)
- Placed on top of a small curb section
The layout of the bridge is shown in Figure CS4-2. The new bridge offers significant improvements over the existing bridge. The under clearance has been increased to 16"- 5”. All deck joints have been eliminated. By pushing the substructure elements away from the roadway, there is an improvement in roadside safety and the potential for salt spray attack is virtually eliminated.
Figure CS4-2: Proposed Bridge
Use of this Manual:
The design of this bridge is no different than a typical highway overpass. The goal is to use connections that emulate traditional construction joints. The designers reviewed chapter one of this manual to become familiar with the options for connection types and the materials and tolerances that would be required. Following this, the designers searched through the applicable sections in chapters 2,3 and 4 for details that could be used on the structure type chosen.
The designers chose the following connection types for this bridge:
- Grouted Reinforcing Splice Couplers
- These couplers are very versatile because they simply replace a traditional reinforcing lap splice at a construction joint. They can be used for the footing to column connection, and the column to cap connection as well as other connections throughout the bridge. The design of the bridge is also simplified since traditional reinforced concrete design methods can be used.
- Grouted Voids
- Some of the potential connections on the bridge do not require large force resistance. In this case a simple grouted void with a pin will suffice. An example pf this is the connection of the approach slab to the abutment shelf.
- Cast-in-place Concrete Closure Pours
- There are a few connections that may require a concrete closure pour. This connection is normally limited to connections that require significant tolerance adjustment. This connection is anticipated for the connection of the superstructure to the substructure at the top of the integral abutment.
Schematic Pier Design:
The proposed pier design is a traditional multi-column concrete bent. In order to accelerate construction, all precast elements will be used (including the footings). The designer located details for this structure in chapter 4 of the manual (foundations) and chapter 3 (substructures).
Section 4.1.1 includes information on the connection of precast concrete footings to the subgrade soils. While this may not seem like a connection, the interface between the footing and the soil must transfer significant forces. The connection chosen was developed by the New Hampshire DOT (detail 4.1.1 A)
Section 22.214.171.124 includes information on the connection of precast concrete columns to precast concrete footings. The selected connection is a conceptual detail developed by the PCI Northeast Bridge Technical Committee. The detail is very similar to and abutment detail that was successfully built in Epping, New Hampshire. This connection is also used extensively in the building industry. The connection is made using grouted reinforcing splice couplers.
Section 126.96.36.199 includes information on the connection of precast concrete columns to precast concrete pier caps. The connection chosen has also been used by the states of Florida and Georgia (detail 188.8.131.52 A). The connection is made using grouted reinforcing splice couplers.
A double two column pier bent was chosen In order to simplify the connection of the pier cap to the columns and to reduce the shipping weight of the pier cap elements. Figure CS4-3 depicts a schematic exploded view rendering of the proposed pier.
Figure CS4-3: Proposed Pier
Schematic Abutment Design:
The proposed abutment is an integral abutment supported on steel H-piles. The majority of this structure will be precast. The connection to the superstructure will be a cast-inplace closure pour. The designer located details for this structure in chapter 3 of the manual (substructures).
Section 184.108.40.206 includes information on the connection of abutment caps and steel piles. The connection chosen was developed by the Maine DOT. Similar connections have also been used in other states (detail 220.127.116.11 B).
Splices in the abutment wall are required in order to keep the shipping weights reasonable. Flying wingwalls were also chosen for this design. A flying wingwall is a short wingwall that is cantilevered off the rear or side of the integral abutment. Sections 18.104.22.168 and 22.214.171.124 cover these connections. It involves match casting the abutment components and wingwall extensions. These details were also developed by Maine DOT (details 126.96.36.199. A and 188.8.131.52.A).
The connection of the precast approach slab to the integral abutment is a simple grouted void with a reinforcing bar pin. Section 184.108.40.206 includes information on approach slab connections. The detail chosen was also developed by New Hampshire DOT based on similar work on three Maine DOT projects (detail 220.127.116.11 A).
Figure CS4-4 depicts a schematic exploded view rendering of the proposed abutment.
Figure CS4-4: Proposed Integral Abutment
The connection of the integral abutment to the superstructure is one of the most complicated connections on the bridge. There is a need for significant tolerance adjustment at this connection (both vertically and horizontally). For this reason, a cast-inplace concrete closure pour is proposed. The Bridge Technical Committee of the Precast Prestressed Concrete Institute (PCI) Northeast Region has developed a detail for this connection. A precast concrete backwall is connected to the abutment stems using grouted reinforcing splice couplers. This backwall serves several purposes. First is acts as a form for the closure pour. It also supports the approach slab. It also provides the negative moment resistance for the superstructure connection. Section 2.5.2 covers this connection. The detail chosen is detail number 2.5.2 A. Figure CS4-5 depicts a schematic section of the connection.
Figure CS4-5: Schematic Superstructure to Substructure Connection
Schematic Precast Deck Design:
The proposed bridge deck is a full depth precast concrete system. Many states have built full depth precast concrete decks with similar details. The system chosen is prestressed in the transverse direction and post-tensioned in the longitudinal direction. The majority of the deck will be precast. A small closure pour is required to accommodate the roadway crown. The designers located details for this portion of the structure in chapter 2 of the manual (superstructures).
Section 18.104.22.168 covers the connection of the deck to the steel girders. This detail has been used in many different states. It consists of shear studs welded to the girder through a pocket cast into the deck (detail 22.214.171.124 A). The pockets are grouted to make the deck composite with the girders. Temporary support of the deck is required during construction. There is also a need to provide grade adjustment during construction. Detail 126.96.36.199 B is a detail that has been used many times in different states. The connection of the individual deck panels is a transverse connection that is perpendicular to the strength direction of the deck. The AASHTO LRFD Bridge Design Specifications  include requirements for this connection. They include the need for post-tensioning and grouted shear keys. Section 188.8.131.52 contains information on this connection. The designers chose details 184.108.40.206 B and 220.127.116.11 C for this connection.
This bridge as with most bridges requires a crown at the centerline of the deck. It is very difficult to cast a precast deck with a crown because the deck normally follows the roadway crown. Most deck panels are cast on a flat form. In order to build a crowned panel, custom forms would be required. Even if this was possible, it is impractical to cast, handle and ship a deck slab element that is as wide as this bridge deck (approximately 60 feet). For these reasons, the designers chose to use a small closure pour at the roadway crown. This connection needs to develop the full strength of the deck section. Section 18.104.22.168 has information on this connection. The designers chose detail 22.214.171.124 D for this connection.
The connection of the curb to the deck is somewhat problematic. All curbs and railings on federally funded project are required to be crash tested. There are very few crash tested prefabricated parapets or curbs. For this reason, the designers chose to use a cast-in-place concrete curb for this bridge. It should not effect the project schedule because it can be cast at the same time as the roadway crown and integral abutment connection.
Figure CS4-6 depicts the schematic deck design.
Figure CS4-6: Proposed Precast Concrete Deck
Figure CS4-7 depicts a partial view of the complete bridge structure.
Figure CS4-7: Partial View of the Proposed Bridge
The goal of this project is to accelerate the construction of this bridge less than 30 days. The timeline shown in Figure CS4-8 was developed based on actual projects around the country and are therefore considered to be reasonable.
Figure CS4-8: Construction Timeline
Notes on construction timeline:
- Utah DOT has demolished superstructures in less than 8 hours (without traffic below); therefore an estimated time of 5 days is reasonable (with traffic below).
- The construction of the new substructures can begin on day one. This is because the new substructures are proposed to be built in a different footprint than the old substructures. This is a recommended practice for construction of accelerated bridge projects.
- Several states have installed full depth precast concrete decks with weekend closures (including removal of the old deck); therefore 3 days for an installation is very reasonable.
- The closure pours and curb pour are shown in series (6 days total). It will be possible to construct these in parallel.
- The approach roadway work begins as soon as the substructures are set. The fact that the roadway is closed greatly facilitates raising the grade 3 feet for this project.
Quality and Durability:
The details chosen have a proven record of durability. The piers and pier connections are very similar to piers that were built for the Edison Bridge in Florida. Figure CS4-9 shows the completed Edison Bridge. This bridge was built in a highly corrosive environment. It has been in service since 1992 and is still in good condition.
Figure CS4-9: Edison Bridge
Full Depth Precast decks that are prestressed transversely and post tensioned longitudinally have proven to virtually crack free. History has proven that this cannot be said for cast-in-place concrete decks, which routinely have significant cracking due to shrinkage and other effects. The Connecticut DOT built a precast deck system in 1990. This deck is still in excellent condition.
The design of this hypothetical bridge also incorporated a jointless design. The bridge is continuous at the pier and the abutments are integral. This type of design eliminates potential leaking joints, which is the most problematic corrosion problem on bridges.
This hypothetical bridge project was based on the premise of eliminating multi-stage construction. The exact cost for stage construction is difficult to estimate, but anecdotal evidence has shown that this can be a significant cost. By closing the roadway and accelerating the bridge, no costly stage construction schemes are required. The contractor will have the entire site for construction instead of constantly fighting with traffic and exposing workers to a dangerous environment. There are other ways to reduce construction costs using accelerated bridge construction, such as:
- Standardization of prefabricated components and systematic use of accelerated bridge construction can reduce costs by building repetitive components. Contractors will become familiar with the processes and products, which normally leads to lower bid prices. The Utah DOT has seen steady decreases in bid prices as they systematically bid more accelerated bridge projects.
- By decreasing project construction schedules, the following cost savings can be realized:
- Reduces rental costs of worksites (trailers, land rental, etc.)
- Reduction in construction inspection costs. This is not a cost that is carried in the bid, but is a cost that is realized by the agency.
- Reduced costs of police details and flagging.
- By reducing overall project time, the effects of inflation on construction are reduced. Contractors estimate inflationary costs in bids. Faster projects equate to lower inflation estimates.
- This hypothetical bridge demonstrates that it is possible to build a typical bridge structure using all prefabricated elements.
- Designers do not need to change the way they layout and design prefabricated bridges. The bridge chosen is a typical highway overpass with up-to-date amenities such as continuous girders, integral abutments and weathering steel.
- There is not sacrifice of quality on bridges built with prefabricated elements. In fact, it is possible to improve quality using this approach. Precast elements have very few shrinkage cracks because they are allowed to shrink and cure with restraint from adjoining members. This is especially true for precast decks.
- By standardizing elements, eliminating construction staging, and reducing overall project time, it may be possible to reduce overall project costs using accelerated bridge construction.
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