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154 A p p e n d i x B problem Statement A large number of steel bridges have been constructed as a series of simple spans with deck expansion joints at each pier. This was a popular system concept because it was easy to design and construct. Leaking deck joints, however, have become a leading cause of deterioration and subsequent reduced service life for all types of bridges. Figure B.1 shows a typical example of a steel bridge simple-span system with a leaking deck joint over a pier. New bridge systems that eliminate deck joints as much as possible have now become the norm. Converting existing simple-span systems to continuous and eliminating deck joints have also been used as ways to extend the service life of existing bridges. Converting existing bridges to continuous has often been done as part of overall bridge rehabilitation when decks were being replaced. However, types of details and levels of achieved continuity have varied considerably. Some state departments of transportation (DOTs) have developed specific guidelines for splicing girders over piers, and others have addressed this on a case-by-case design basis. As a result, many variations exist in the industry. There is a need to provide more uniform and consistent guidelines regarding design criteria, details, and performance that will result in cost-effective service life extension. Objectives The objectives of the study presented in this appendix were to review current state DOT criteria and details for converting existing simple-span steel bridges to continuous and to develop recommendations for consistent cost-effective approaches. Scope of Work The scope of work for this study included ⢠Collecting, reviewing, and summarizing the state of cur- rent practice from various states; ⢠Developing recommended criteria for continuity retrofit that include guidelines for analysis, along with considering system behavior; and ⢠Developing recommended details. Results Review of Current Practice A brief survey of national HDR, Inc., offices was conducted to identify current state DOT practice and experience, if any, for converting existing simple-span steel bridges to continu- ous. Reference was made to three possible approaches: ⢠Converting girders to fully continuous for dead load and live load; ⢠Converting to continuous for live load only; and Converting Existing Simple-Span Steel Bridges to Continuous Figure B.1. Steel simple-span girders with deck joint over pier.
155 New York Experience The New York State DOT (NYSDOT) provides a good sum- mary of criteria for continuity retrofit in the NYSDOT Bridge Design Manual, which discusses feasibility, general design con- siderations, and design guidelines (NYSDOT 2008). The man- ual addresses full continuity versus continuous for live load and discusses advantages and disadvantages. Figure B.2 shows typical details from the NYSDOT manual. Fully continuous retrofits are advantageous because the combined girder behaves like a conventional continuous girder, and a single bearing can be used. Continuity is pro- vided for deck dead load and live load, with simple-span behavior for steel girder dead load. However, this concept requires more complex splice details for the web and flanges and may require long flange cover plates to accommodate the larger negative moment, which can make the retrofit costly. For continuous for live load retrofits, only the girder flanges are spliced, and two lines of bearings from the existing simple- span configuration are retained. The top flange detail uses a Figure B.2. Typical details for converting simple-span beams to continuous from the NYSDOT Bridge Design Manual (2008). ⢠Converting by using a continuous slab over the joint and not connecting the girders (link slab). Specific questions focused on the respondentâs local industry experience concerning steel girder continuity retrofit, such as ⢠What experience have you or your state had in developing design, procedures, and details for converting existing simple-span steel bridges to continuous? ⢠What standards, if any, does your state DOT have, either in a design manual or standard design drawings, for doing this? It was found that a number of states have performed such conversions, primarily during deck replacement rehabilita- tions, but few have specific design procedures or standard details. Some states have guidelines for new design, but not for retrofit of existing; however, these details for new design could also be adapted for continuity retrofit. The following subsections summarize various practices around the country.
156 conventional splice, but the bottom flange connection can be made using a compression block fitted and welded between the ends of the girders. This live load retrofit method also adapts well if the existing deck is not fully replaced, but only a short section over the pier. New York State Thruway Authority Experience From about 1988 to 1992, the New York State Thruway Authority (NYSTA) routinely converted many bridges to continuous for live load by simply joining the bottom flanges and making the deck continuous over the joint. This prac- tice was halted because of concerns of properly analyzing the resulting semirigid connection. Many of these bridges are still in service, usually with a crack over the joint, but are otherwise performing well. These were simple details with little cost. Around 1995, NYSTA developed a standard for the design and detailing of a fully continuous retrofit splice. After imple- menting on a few retrofits, primarily on grade separation structures, it became clear that they were not cost-effective, and the NYSTA stopped doing them. The cost of cleaning and painting the old steel, combined with the extensive field work required to remove the end diaphragms, field drill numerous bolt holes, rework the pedestals, and so on, started to approach the cost of new steel. This was also found on a retrofit in Pennsylvania reported later. Tennessee Experience The Tennessee DOT has experience with new design for bridges that have girders constructed as simple spans that are then con- verted to continuous. Over several years they developed and experimented with different concepts. The first was a detail for simple span for noncomposite dead loads and continuous for composite dead loads and live loads. Continuity over interior supports was achieved by a cast-in-place concrete diaphragm (see Figure B.3). The Tennessee DOT then thought that more overall econ- omy could be realized by a concept that made the beams also continuous for the dead load of the wet concrete slab. This modification was achieved by a splice detail, as shown in Fig- ure B.4. The bottom flange was connected by a welded cover plate and wedge kicker plates, and the top flange was spliced with a single-shear bolted cover plate. The top and bottom cover plates were extended to accommodate the additional negative moment bending stress due to continuity. This detail was also combined with a concrete diaphragm over the piers. Figure B.3. Tennessee continuity diaphragm detail.
157 New Mexico Experience The New Mexico DOT developed a girder connection using a splice plate for the top flange and placed the connection bolts outside the poured concrete diaphragm at the piers (Figure B.5). Bolts placed heads-up and nuts-exposed-down permitted tightening after pouring of the deck and pier diaphragms. After deck pours, the bolt heads were locked into the con- crete deck. Bolts were then tightened by turning nuts from below. Final tightening came after all concrete had been poured for the deck and pier and abutment diaphragms, before opening for traffic. The design required adding reinforcing bars to normal longitudinal deck reinforcement over the piers. The additional reinforcement achieved the required negative moment capac- ity for the bridgeâs continuous live-load function. Under live loads the continuity connection plate would be lightly stressed. It was considered that the continuity plate connection would also add redundancy in the event that future deck deteriora- tion reduced the effectiveness of the deck reinforcement for negative moment capacity. Pennsylvania Experience In Pennsylvania, the simple-span approaches to the Fort Pitt Bridge in Pittsburgh were all converted to continuous. A series of spans in the 72- to 83-ft span-length range were retro fitted by adding top and bottom flange splices and web splices to achieve continuity for the deck dead load and live load (see Figure B.6). In a shorter span unit with a series of Figure B.4. Tennessee splice detail. Figure B.5. New Mexico top flange connection plate detail.
158 five 54-ft spans, alternate plans were prepared for stringer reha- bilitation with continuity splices versus full stringer replace- ment. For this unit, all contractors bid full stringer replacement. It was determined these short spans had a high number of continuity splices with respect to total steel weight, making stringer replacement more economical. West Virginia Experience In West Virginia, a continuity detail was used for new construc- tion on a two-span continuous overpass bridge. Figure B.7 shows the splice, which was designed for live load continuity. The top flange connection used a bolted splice, and the bottom flange connection used bearing plates and wedges for com- pression. The top flange splice and bottom flange continuity plates were not installed until after the deck was placed in posi- tive moment regions. Two Other Continuity Concepts: Elimination of Top Flange Splice and Link Slab Figure B.8 shows an economical and functional live load con- tinuity detail that has been used for spans up to about 150 ft. It uses bottom flange bearing plates and wedges for compression, but relies on a concrete diaphragm and top concrete slab for top flange continuity without a top flange splice. Elimi- nating the top flange splice creates a simpler construction sequencing. Another way of providing continuity and thereby elimi- nating joints over piers is the link slab concept, in which the Source: HDR, Inc. (a) (b) Figure B.6. Simple-span girders made continuous in Pittsburgh, Pennsylvania: (a) rolled I-beams and (b) riveted plate girders. Figure B.7. West Virginia live load continuity detail.
159 girders are not directly spliced; rather, the deck is made con- tinuous over the open gap between girder ends. See the link slab section in the main report for more detail. Design Considerations There are many benefits of converting simple spans to con- tinuous, including reducing the potential for continued deterioration due to leaking joints, increasing resistance to seismic displacements, and slightly improving the load- carrying capacity of the superstructure. The design for continuity retrofit needs to consider the system behavior of continuous girders as opposed to original simple-span behavior. For multispan continuous systems, larger movements may be realized at abutments or at interior piers if original fixed conditions are converted to full expansion conditions. Also, increased vulnerability to fatigue may result due to portions of the retrofitted beams being subjected to stress reversals and higher stress ranges than the original simple- span construction. The end regions of retrofitted girders origi- nally designed for small simple-span positive moments are subjected to larger magnitude negative moments. Although the deck joints over the piers are eliminated, the retrofitted deck in this area is subjected to tension under service loads, and crack control measures must be considered. Continuity can also increase seismic loads on individual piers depending on retrofitted bearing fixity configurations. Deck replacement projects provide excellent opportunities to include girder retro- fit because girders will be readily accessible and future costs of maintaining the joints will be eliminated (NYSDOT 2008). Conclusions and Recommendations There are several viable continuity retrofit options: ⢠All continuity retrofit options require a system evaluation for both girder stress and movement at bearings. ⢠A continuous for live load retrofit splice is typically more cost-effective than a full continuity retrofit splice. A splice detail that uses bottom flange butt plates and eliminates the top flange splice by using connection diaphragm and deck is recommended over details with a top flange splice. ⢠A link slab detail without stringer splice details is most economical if accompanying deck cracking is acceptable. ⢠Continuity splice details need to match existing as-built conditions. ⢠Full stringer replacement can be most economical for short spans accompanied with deck replacement, cleaning, and painting. Figure B.8. Continuity without top flange splice.
