BRIDGE DESIGN: Three for the show

Nov. 4, 2011

Traditional, modern or monumental?

Traditional, modern or monumental?
Three distinctive designs were developed for the new Ohio River bridge to be constructed between Cincinnati, Ohio, and Covington, Ky., as part of the Brent Spence Bridge Replacement/Rehabilitation Project. Each design alternative—tied-arch, two-towered cable-stayed and one-towered cable-stayed—solves technical challenges for the bridge alignment, increased traffic volume and associated live load capacity, and minimum mainspan clearance. The cost-effective alternatives are intended to serve the Kentucky Transportation Cabinet’s (KYTC) needs and address the local communities’ desire for an improved crossing and distinctive aesthetics.
Locally, I-75 connects to I-71, I-74 and U.S. Rte. 50. The Brent Spence Bridge provides an interstate connection over the Ohio River carrying I-71 and I-75 traffic and provides access to downtown Cincinnati and Covington. Safety, congestion and geometric problems exist on the structure and its approaches. The bridge opened to traffic in 1963 and was designed to carry 80,000 vehicles per day. Currently, approximately 160,000 vehicles per day use it, and traffic volumes are projected to increase to approximately 233,000 vehicles per day in 2035.
KYTC and the Ohio Department of Transportation (ODOT) jointly decided to modernize the I-75 corridor from 3 miles north to 5 miles south of the bridge. A new bridge would be constructed in conjunction with rehabilitating the existing Brent Spence Bridge.
The bridge was rated as functionally obsolete in the National Bridge Inventory. The main portion consists of a double-decked steel cantilevered through-truss with an 831-ft-long main span flanked by two 453-ft anchor spans. The bridge is striped for four northbound lanes on the lower deck and four southbound lanes on the upper deck. It supports 46-ft-wide roadways on both the top and lower decks with a vertical clearance of 15 ft on the lower deck. All lanes are 11 ft wide with no shoulders. The narrowness of the travel lanes plus tight vertical clearances force the placement of wayfinding signage for the northbound lower deck within the floor beams of the upper deck.
In 2009, Parsons Brinckerhoff, working in conjunction with KYTC and ODOT, undertook a three-step bridge selection process to assist the agencies in selecting a bridge alternative. From 24 preliminary bridge concepts evaluated during step one, the final three bridge alternatives were identified in step three.   

Problem solver
The final design alternatives solve three main technical challenges: avoiding conflicts in the preferred alignment, carrying increasing traffic volume and associated live load on a double-decked superstructure and meeting a requirement for a minimum 1,000-ft-long main span.
The preferred alignment adjacent to, and just west of, the existing bridge would enable more tie-ins to existing interchanges and minimize the acquisition of new property. However, it required that the new bridge be designed to minimize the number and size of foundations to avoid conflicts with existing conditions both underground and aboveground.
The projected increase in average daily traffic from 160,000 vehicles per day to almost a quarter million required that the existing and new bridges carry a combination of 16 lanes of traffic on two or three 12-ft-wide lane alignments including the required safety shoulders. The new bridge must fit between the existing bridge and a historic landmark structure, making it impossible to fit the 16 lanes on a single deck. All bridge alternatives were required to use a double-decked superstructure that avoids the driving hazards on the lower deck of the existing bridge, including low clearances to the structure above, poor signage visibility and dim light.
The Army Corps of Engineers’ (USACE) requirement of a minimum 1,000-ft-long main span created another challenge. USACE mandated that the piers of the new bridge be placed outside those of the existing bridge to facilitate barge traffic and navigation and to protect the new bridge from vessel impact. The existing bridge, which will remain in service, is founded on caissons sunk deep into the riverbed. The new bridge is anticipated to be founded on drilled shafts, and the size of the footings must be balanced with the minimum span length to prevent interference with the existing bridge.
While the design alternatives must overcome these challenges, KYTC and ODOT also required all bridge alternatives to be constructible, nonobstructive to river traffic, aesthetically pleasing and cost-effective.  
In developing the final three design alternatives, great attention was given to the main members and major details to enable interior spaces to be accessible for inspection and future maintenance. The design also avoided locating deck joints directly above major components, such as bearings, to prevent corrosion.

Different from one another
Each of the final three bridge alternatives combines a distinctive design aesthetic with common elements. In each alternative, the bridge alignment runs adjacent to, and just west of, the existing Brent Spence Bridge. In each case, a double-decked truss superstructure carries two roadways on each deck, with each roadway composed of two or three 12-ft-wide travel lanes and two 14-ft-wide shoulders. Each meets the USACE requirements. The river-to-superstructure clearance of each alternative is no lower than that of the existing bridge.
The three-rib, steel tied-arch bridge design alternative is a traditional design aesthetic, which would stand like a bookend to the existing Daniel Carter Beard Bridge (I-471), a tied-arch bridge 1.7 miles upstream.  
The tied-arch design is 226 ft high at the crown, with a double-decked truss system composed of a 166-ft-wide top deck and a 178-ft-wide lower deck. The outer two arch ribs and trusses are inclined toward the center rib 10° from the vertical, with a pleasing “basket-handle” effect.
The arch ties are three 38-ft-deep trusses each located at the base of the arch ribs. The tied-arch hangers are connected to the arch ribs at the top and anchored into the truss top chords at the bottom. The abrupt angle change in each arch rib as it meets the truss top chord at the “knuckle” causes dead load tie forces to be shared equally by the top and lower chords of the trusses.
The floor beams supporting the deck are designed to act as continuous beams over the center truss plane. The floor beams are cambered to share the vertical dead load equally between the three truss and arch-rib planes. Under live loads, the center truss, arch rib and hangers carry more load than those on the outside; these are sized accordingly.
The deck trusses serving as the arch ties are continuous over the main span river piers, eliminating a deck joint at the spring points of the arch to reduce corrosion.
During construction, the steel portion of the superstructure could be erected on a temporary pile-supported platform and transported by barge to the bridge site. After its placement on the piers, the concrete deck could be constructed and approach trusses erected.

Towering influence
One of two cable-stayed alternatives consists of a two-towered cable-stayed bridge, with each tower composed of three 403-ft-tall “needles.” This bridge would present a modern aesthetic with an unobstructed overhead view that creates a quality of openness on the upper deck.  
Each tower needle carries a plane of stay cables that support a truss at the top deck level. The deck system consists of a 172-ft-wide, double-decked, triple-truss superstructure. The cables and truss diagonals are inclined at the same angle, which provides a smooth visual transition from the light cables to the relatively bulkier truss. The diagonals also distribute the horizontal force of the cables into the top and lower chords of the trusses, where that load can then be carried in part by the concrete deck, maximizing the efficiency of the superstructure.
At the towers, the trusses are integrally connected to the concrete needles, which minimizes the overall width of the bridge and eliminates the need for costly tower bearings. This alternative uses continuous floor beams over the center truss to divide the dead load evenly between the three truss planes, which is similar to the tied-arch design.
The deck trusses, floor beams and stringers could be erected piece by piece using floating cranes and/or deck gantries. Although erection of the superstructure would require more time than the tied arch, the construction operations would not block river traffic.
The single-tower cable-stayed alternative is composed of two 510-ft-tall needles supporting a 155-ft-wide double-decked truss superstructure with two planes of doubled cables connecting to the top chord of the edge trusses. The main span is 1,023 ft 4 in. long with a total back-span length of 671 ft, and three anchor piers on the back span.
The tower of the bridge would be one of the tallest structures in Cincinnati and Covington, visible from eastern vantages on both sides of the river. Each tower needle features a vertical inside face and a tapering outside face. The vertical cable planes frame into cable saddles, which are placed within the tower.
The superstructure trusses distribute the horizontal cable load evenly to the top and lower decks. The trusses are integral with the towers, which eliminate the necessity for a truss bearing at the tower and minimizes the width of the bridge. The integral tower/truss connection does not introduce longitudinal thermal loads to the towers or foundations because the superstructure is free to expand or contract at the pier on the southern end of the main span.
The floor beams span the full 155 ft width of the superstructure ranging in depth from 6 ft to 12 ft deep at their mid-span. The 12-ft floor beam depth required an additional 7-ft 6-in. separation between the top and lower decks compared with the other two alternatives to provide the same vertical clearance for signage below the bottom of the floor beam flange.
This alternative uses an orthotropic deck for the main span supported on stringers between the floor beams, helping to carry the longitudinal cable force to lighten its dead load. On the back span, the weight of the main span is balanced by a 16-in.-thick concrete top and lower decks supported on stringers spanning the floor beams to support the horizontal cable loads.
Floating cranes and/or deck gantries would be used to erect the deck trusses, floor beams and stringers piece by piece. Back-span construction and tower erection could be performed from land simultaneously, reducing the time required for main-span construction.
The location, span and load requirements for the new Ohio River bridge posed a number of challenges to the design team. The final three bridge alternatives cost-effectively solve these technical problems. Whichever alternative is selected, the new bridge will stand as a prominent landmark on the Cincinnati-Covington riverfront and improve travelers’ experience of the crossing.

About The Author: Phelps is a project manager and senior supervising engineer with Parsons Brinckerhoff in the firm’s Cincinnati office. Hsu is a senior supervising engineer in New York City.

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