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2016 Prize Winner: Long Span ~ Brazos River

TEXAS, LIKE MANY OTHER SOUTHERN STATES, has seen substantial population growth recently.

This growth is one of the key drivers for expanding Inter- state capacity, particularly in cities like Waco. I-35 meanders through the east Texas city, home of Baylor University, and the Waco District of the Texas Department of Transportation (Tx- DOT) wanted to do more than just add capacity with the $17.3 million IH-35 frontage road bridge project. It wanted to make a statement. By pioneering the application of extradosed bridges in the U.S., the city of Waco and TxDOT did just that (see side- bar for a description of extradosed bridge design).

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Spanning the Brazos River and parallel to the existing main- line I-35 bridges, the new IH-35 frontage road extradosed bridges are 620 ft long and are the first extradosed cable-stayed bridges in the U.S. to use a steel superstructure. Traffic on each bridge is one way, with the new bridges placed to the outside of the main-line bridges. The existing mainline bridges will soon be replaced with new steel box-girder bridges as part of the IH-35 corridor improvements. In the final configuration, both the northbound and southbound frontage road bridges will be separated horizontally some 60 ft from the corresponding mainline bridges.

The roadway for each frontage road bridge carries three traffic lanes with shoulders, as well as a 10-ft, 6-in.-wide sidewalk for pedestrians and cyclists. In addition, scenic overlooks providing unobstructed river views are incorporated into the pylons to enhance the bridge experience for pedestrians. Each of the new twin structures is a three-span bridge with a 250-ft center span and 185-ft side spans. Matching the span configuration of the proposed new mainline bridges, this configuration aligns the piers within the river for all the bridges in their final condition.

Each bridge’s superstructure consists of 6-ft, 6-in.-deep steel trapezoidal box edge girders, 3-ft, 6-in.-deep steel-plate I-girder floor beams and 10.5-in. cast-in-place concrete deck. Transverse floor beam spacing varies from 13 ft, 3 in. in the end region of the side spans to 15 ft in the regions near the pylons. The trapezoidal box edge girders are composed of 3/4 inch web plates, with 2 inch wide top flange plates and a 5-ft-wide bottom flange plates. Top flange plate thickness varies from a typical 1 in. up to 3 in. for the regions over the bearings at the pylons, and the bottom flange plate varies from a typical 1-1/4 in. to 3 in. over the pylons. The box edge girders are continuous for the entire length of the bridge, supported on single disc bearings at the abutments and pylons. The transverse I-girder floor beams consist of 1/2 in. web plates with 1-ft, 6-in.-wide by 1-in.-thick top and bottom flange plates. Each H-shaped pylon consists of two 9-ft, 3-in. by 9-ft, 3-in. rectangular towers with a haunched 5-ft, 3-in.-wide crossbeam that supports the superstructure.

The project team chose a steel-girder composite cross section for the design. While a concrete cross section is typical for extradosed bridges, TxDOT preferred steel girders with concrete decks because of its familiarity with this superstructure scheme. In addition, structural engineer AECOM evaluated the use of a cast-in-place concrete box girder superstructure but determined it to be economically untenable. In addition, the project team used a steel trapezoidal box section for the edge girders, rather than a steel-plate I-girder section more commonly used on composite cable-stayed bridges, in order to provide greater superstructure stiffness and less reliance on the cable stays. The team also wanted to visually match the new mainline bridges, which will be steel box-girder bridges. The cable system consists of a single plane of five cable stays at each pylon supporting the edge girders (a total of 20 stays for each bridge). The cable stays are anchored at the deck level to the web of the steel box girder and pass through cable saddles in the pylons. With 12 strands per cable, the stays are composed of 0.62-in.-diameter, seven-wire, low-relaxation strands. For improved corrosion resistance, each strand is coated with wax and encapsulated inside high-density polyethylene (HDPE) sheathing. The strand-bundled stays are further protected by an outside HDPE pipe.

Since an extradosed bridge has two load-carrying systems, cable support can be provided for only a portion of the span. Consistent with the geometry of many extradosed bridges, the final stay for the IH-35 frontage road bridges is offset from the pylon by approximately 20% of the main span. From this first stay, the cable support points are spaced 14 ft, 9 in. along the edge girder, for a total of 59 ft. This results in approximately 50% of the main span being cable supported, which is consistent with most existing extradosed bridges that have cables distributed across approximately 60% of the span.

Due to the use of the relatively more flexible steel superstructure (supported at the pylons by bearings), the resulting liveload stress range in the stays was greater than the Post-Tensioning Institute’s (PTI) limit that would allow the stays to be designated as low-fatigue. The stress variation caused by live loads (AASHTO HL-93 live load with no pedestrians) varied up to approximately 15 ksi versus the 6.75 ksi limit for the stays to be considered extradosed. So the stays were designed accordingly, using the same provisions in the PTI manual as for conventional cable-stayed bridges.

A complete erection scheme was also developed during the design of the bridge to inhibit both cracking of the concrete deck during construction and slippage of the stay strand through the saddles. Further, the cable-stay and saddle system chosen was specified to allow for the installation and replacement of stay strands on an individual strand-by-strand basis. This was no small consideration since the ability to replace strands on an individual basis will allow future bridge maintenance workers to pull and inspect strands without needing to replace the entire stay. Stay strands will be placed within individual holes in the cable saddle, significantly reducing the risk of fretting corrosion and facilitating strand-by-strand replacement. Although using saddles is a common practice elsewhere in the world, it is relatively new in the U.S. and only a few bridges have been designed with this system.

Not only was this the first use of a steel extradosed bridge in the U.S., but the project also had to content with heavier than usual deadline pressure thanks to Baylor announcing that it would be constructing its new Lane Stadium football facility adjacent to  the bridges, with an opening date of August 31, 2014. Nevertheless, the bridges were delivered 4.5 months ahead of the original schedule and opened to traffic that July.

Owner
Texas Department of Transportation, Austin
Designer
AECOM, Glen Allen, Va.
Contractor
The Lane Construction Corporation, Lorena, Texas
Steel Fabricator
Hirschfeld Industries, San Angelo, Texas
Detailer
abs Structural Corporation

Full-Speed Ahead

The new Blue Line extension to the Washington, D.C.-area Metrorail system features steel bridges for cost savings and construction speed.

The first ever large-scale design-build project awarded by Washington Metropolitan Area Transit Authority (WMATA) is a 3.1 mile-long extension of the Blue Line on Washington D.C.’s Metro rail system. This extension is located in Prince George’s County, MD, east of the capital, and is the first Metrorail line in the county to extend past the Capital Beltway (I-495). The Largo Extension connects two new passenger stations to the Metrorail system—one at Morgan Boulevard and the other at Largo Town Center. The project also includes 2.2 miles of cut-and-cover tunnel construction, 0.6 miles of dual aerial structures, and the operations building. Only about 2.5% of the entire alignment (0.1 miles) is built at grade.

Full Speed

Going Design-Build
The Blue Line extension program was split into three major construction contracts: site preparation and beltway crossing; line, trackwork and systems; and stations and parking facilities. The first contract was advertised as a conventional design-bid-build project; the two remaining contracts were awarded as designbuild projects. Revenue operations of Metrorail trains are expected to begin in December 2004, less than three years after awarding the design-build contracts.

Design-build and steel were chosen to meet the project’s tight schedule. As the designer for the joint venture of Lane Construction, Granite Construction and Slattery Construction (LGS), Jacobs Civil provided construction documents for all structures, electrical facilities, mechanical facilities and track, and coordinated the installation of the automatic train control, traction power and communication systems to provide an operating segment in less than three years. This very tight schedule required that WMATA, LGS and Jacobs work as a team to agree on solutions that were cost effective and could be constructed efficiently. Based on these goals, the team selected steel structures.

By bringing the fabricator on-board at an early date, during the bidding process, the designer and installer were able to work together to develop a product that would permit accelerated delivery and erection of the aerial structures. During the final design stage, open communication between the designer, fabricator and erector made it possible to iron out steel details, plate sizes and job-specific quality issues months before the design drawings were released for construction. This helped the fabricator with the procurement and planning of the work and minimized RFIs and change orders.

Why Steel?
At the conceptual stage of the project, the aerial structures were envisioned as prestressed-concrete trapezoidal box girders supported over a concrete sub-structure. However, during the proposal process, the design-build team determined that steel superstructures would reduce the mobilization cost and enhance the construction schedule. The aerial portion of the project was not large enough to overcome the substantial initial investment necessary for segmental construction, even if all segments were designed with the same cross section and geometry. Due to the terrain and the presence of the features such as crossovers and turnouts, it was necessary to vary the deck geometry and span lengths.

Under the circumstances, a welded steel plate-girder superstructure was more suitable than a concrete alternative, since it was adaptable to the design conditions. In view of the potential cost savings and an accelerated construction schedule, WMATA agreed to material substitution as long as steel met all of the required project design criteria. The State of Maryland’s preference for steel structures over major highways also was taken into consideration.

Full Speed

The Blue Line extension includes 37 spans of steel aerial structures. Thirty of them are single-track bridges preferred by WMATA to permit bypassing during emergencies, and for ease of maintenance and repairs. Five spans carry turnouts for future extensions in addition to the main tracks. Another two spans support double crossovers near Largo Town Center Station. The basic configuration of the single-track bridge is a two-girder system with a composite concrete deck. The rails attached with direct fixation devices are supported by reinforced concrete plinths poured separately in a top-down method. The two girder cross section is optimal for the 16′-to-18′ deck width required to accommodate a single track, cable troughs, safety walks and handrails while maintaining the minimally required horizontal clearances. A three-girder cross section was less economical, but allowed reduction in the depth of the superstructure; this type of cross section was used only on one of the spans where limited vertical clearance under the bridge required a shallower superstructure. Straight girders were used predominantly for steel framing, even at locations with circular or spiral alignment. The girder spacing was kept at 8′, except on spiral or curved alignments, where it was sometimes increased to up to 10’ to avoid uplift. Also, it was increased to avoid placing excessively high live loads on the girders. Horizontally curved steel tub girders were used at one location, where, due to the sharp curvature, a straight girder arrangement was not feasible.

Besides the three-girder system, there were other variations from the basic twogirder arrangement. At the doublecrossover location, three additional girders were used between the inbound and outbound tracks to form a common 62′-wide deck. At the turnouts for future Metro extension, splaying three or four girders was necessary to support the decks with varying widths. Immediately west of the Capital Beltway (I-495), a through-girder bridge was designed to accommodate the severely restricted vertical clearance under the crossing.

Full Speed

The majority of the aerial structures within the project have span lengths in the 120′ to 150′ range. The structures that fall beyond this range are the 76′-long through-girder bridge and the 324′-long, two-span continuous bridge across the Beltway. Selection of span type (simple versus continuous) was influenced by the vibration design criteria. This is generally not the case for highway bridges of similar composition, where economy can be derived from making the spans continuous. The vibration requirements also reflect on the end/interior-span length ratio for continuous-span transit structures. For highway bridges, economic considerations traditionally dictate that this ratio be kept around 0.8. Usually, it is not so for transit bridges. To limit potential dynamic interaction between aerial structure and the transit vehicle, WMATA design criteria require the unloaded natural frequency of the first mode of vibration to be no less that 2.5 Hz for simple-span structures, and no less than 3 Hz for continuous span structures. (Similar or slightly different criteria also exist for other transit systems/agencies.) Vibration limitations also allow the transit-vehicle peak vertical accelerations to remain within the accepted guidelines, which are related to passengers’ comfort and ride quality.

Expansion and Contraction
Spherical bearings with high rotational capacities were chosen for the aerial structures. The bearings are arranged so that one pier has two fixed bearings followed by the next pier with two expansion bearings. Whenever possible, this kind of arrangement was selected to balance rail/structure interaction forces.

Rails are attached to the superstructure and to the subgrade or concrete track bed beyond the abutments by means of direct fixation fasteners. As a result of temperature variations (60°F rise or fall in temperature), continuously welded rails are assumed to develop thermal stresses under zero-strain conditions. The relatively massive girders, on the other hand, develop negligible thermal stresses, but are subject to thermal elongation/contraction.

The direct-fixation fasteners connecting the rails to the superstructure act as springs until they develop a force of 3 kips under a displacement of 0.4″. Upon reaching this limit, fasteners are designed to allow the rail to slip while maintaining the 3 kips force.

These fastener-imparted forces are accumulated in rails, creating stretches of tension and compression in addition to the rail thermal forces. The resulting maximum forces in rails depend on span lengths and bearing arrangement. If in a succession of simple-span bridges, every fixed pier has equal span lengths on both sides, then the rail/structure interaction forces will be balanced completely and locked in the rails.

Similar assumptions of symmetry also are made for the continuous structures to keep the rail-structure interaction forces balanced. If the condition of symmetry in the superstructure is not satisfied, the unbalanced forces are transferred to the substructure elements. For tall and slender piers, this could be a problem, but for the massive and inflexible piers in the Blue Line Extension, the effect was practically negligible.

In a properly designed structure, the rail/structure interaction should not generate excessive force in the substructure, and should not overstress the rails. Thus, the magnitude of the interaction force in rails is a good measurement of the appropriateness of the span lengths and bearing arrangement. Analysis indicated the maximum rail force to be around 80 kips; well within 132 kips allowed by the WMATA design criteria.

Alternating fixed piers with expansion piers in a succession of simple span structures, while beneficial from the rail/structure interaction point of view, results in deck joints over the expansion piers that also exhibit the maximum thermal movements. This was the dilemma for the double crossover at the Largo Center Station. The 320′-long crossover extended beyond two 140′-long spans, and included three deck joints. All of them had to be designed for the minimum possible thermal movements. Therefore the above-described optimal bearing arrangement was compromised. The pier at the middle of the crossover was designed as a fixed pier, and the other two piers had a combination of the fixed and expansion bearings. As mentioned before, the WMATA rail/structure interaction requirement was still satisfied.

The design also was checked for emergency conditions, such as derailment and rail-break loadings. A derailed train was assumed to shift laterally as far as 3′ from its normal position and impose a 100% impact on the structure for two axles with a normal impact of 30% for the rest of the train. For rail-break forces, only one rail at a time was assumed to be broken under extreme condition of a temperature drop of 100°F.

Fatigue Factor
One of WMATA’s main concerns in the selection of superstructure material was its ability to withstand fatigue. WMATA specifically emphasized that details used in the design of the aerial structures would not be susceptible to fatigue cracking. Since the majority of the bridges are designed as two-girder systems, all girders (main-load carrying members) and their fasteners are considered non-redundant load-path structures—that is, where failure of a single element could cause collapse of the structure. Accordingly, the allowable stress ranges indicated in AASHTO Table 10.3.1A for non-redundant load-path structures were used in proportioning the girders.

WMATA rapid-transit loading with full impact is used to determine the magnitude of stress range for 3 million cycles over the life of the structure in order to ensure that the maximum stress-range cycles will fall below the fatigue limit for all service conditions. The weld details for the diaphragm connection plates and transverse stiffeners are made in a way that no significant out-of-plane deformations will develop and their fatigue resistance would correspond to Category C. Bearing stiffeners are made out of 11/2”-thick or 1”-thick plates. Their connections to bottom-flange plates, where large, concentrated live load reaction forces are transferred, are made with partial-penetration welds.

Although the stiffeners are milled to bear, the concentrated cyclic compression occurring in a region of high-tensile residual stress makes this area vulnerable to fatigue cracking. There are also fabrication tolerances that can result in small gaps between the stiffeners and the bottom-flange plates. To avoid these situations, partial-penetration welds are used instead of fillet welds.

ASTM A709 grade 50W steel was used throughout the superstructure. The primary load-carrying components subjected to tensile stresses also were supplied to meet the additional requirements for the Charpy V notch testing requirements of AASHTO.

Aerial structures for WMATA’s Blue Line extension required some special design considerations that are not normally encountered/followed in typical highway- bridge design. By making a few prudent design decisions, composite steel superstructures proved to be a good choice for this design-build project.

Gautam Ghosh, P.E, S.E is a senior structural engineer/project manager with Jacobs Civil. Dan Korzym is a senior project manager with the Washington Metropolitan Area Transit Authority. Gregory Mester, P.E. is senior project manager and Alexander Rosenbaum, Ph.D., P.E. is chief structural engineer for Jacobs Civil’s Arlington, VA office.

Design-Build Contractor/CM
Lane Construction Corporation, Meriden, CT; Granite Construction Company, Inc., Watsonville, CA; Slattery Skanska, Inc., Whitestone, NY—A joint venture (LGS)

Structural Engineer
Jacobs Civil, Arlington, VA

Beltway Crossing Bridge
CTC/Wallace Montgomery & Associates, Towson, MD

Dynamic Analysis
David N.Wormley, Penn State

Engineering Software
STAAD Pro
GT STRUDL
MERLIN-DASH
DESCUS

Engineering Subconsultant
Tuhin Basu and Associates, Inc., McLean, VA

Alternative Approach

The Lake of the Ozarks is one of Missouri’s most prominent recreation and tourism destinations.

MO Route 5 is a main access road into this region. The route crosses the Osage arm of the lake via the Hurricane Deck Bridge, originally built in 1936 as a 2,200-ft-long steel deck truss structure with 463-ft spans supported on dredged caissons in up to 85 ft of water. In 2009, it was determined that the truss was structurally deficient (due to section loss in the gusset plates) and had reached the end its useful life, and the Missouri Department of Transportation (MoDOT) began making plans to replace it.

The project site presented several challenges, including significant right-of-way restrictions, rock-bluff constraints in the approach roadway and environmental concerns that included nearby Native American burial grounds. On top of that, the closure of the bridge for the duration of the reconstruction was deemed unacceptable by the local stakeholders due to the 42-mile detour.

Procurement Approach
Engineering consultant Parsons was selected to provide the preliminary and final design services for the baseline design concept for the bridge replacement. To encourage contractor innovation, MoDOT elected to employ an alternative technical concept (ATC) procurement method for the project. Under the ATC process, contractors were invited to develop alternatives to, or modifications of, the baseline design with the intent of reducing costs without sacrificing MoDOT’s defined project objectives.

Alternative Approach

Two contractors submitted ATCs that represented significant departures from the baseline concept. The extensive nature of these two ATCs rendered the design cost, schedule duration and required resources prohibitive to performing a complete final design before the bid opening. Considering this limitation, these two contractors collaborated with MoDOT and Parsons to develop a conceptual design focused on defining the variables most crucial to the development of a detailed cost estimate and bid price for the project. Ultimately, these two ATC designs were advanced to only 30% completion before bid submittal, and pre-bid engineering deliverables were minimized. Contractors proposing ATCs that were significantly different from the baseline concept bid the project based on preliminary design quantities developed by Parsons, with quantity growth over 2% being the contractor’s risk.

In a nine-month period, Parsons mobilized four teams of designers, including one to perform preliminary design and prepare bid documents for the baseline approach and three to prepare bid packages for the confidential ATCs proposed by the contractors. These design teams were staffed from different offices, and administrative firewalls were designed to ensure complete confidentiality throughout the bid document preparation process.

Baseline Design
The concept for the baseline design was to reuse the existing caisson foundations by designing a new steel delta frame plate girder structure with matching span lengths. The delta frame structure was to be built immediately adjacent to the existing bridge, supported on 42-in. pipe piles, and tied to the existing foundations. Traffic was to be maintained on the existing bridge while the steel delta frame superstructure was built. Traffic would have then switched to the new bridge on the temporary alignment, and the existing deck truss superstructure and pier caps were then to be demolished. Once the new pier caps were complete, the new superstructure would have been moved laterally onto the rehabilitated permanent piers during a weekend closure.

The triangular-shaped delta frame, extending from the pier cap up to the bridge girders, was proposed for this project due to its ability to support long spans at a significant height with few piers. With the use of the delta frame, original bearing elevations were maintained, thus minimizing any necessary retrofit to the existing substructure.

Alternative Approach

The delta frame has a typical span of 462 ft, 10 in., matching the existing deck truss spans. Three delta frame girders spaced at 13 ft, 2 in. were used to support the 40-ft, 8-in. roadway cross section, which consists of one 12-ft lane in each direction with 5-ft shoulders and MoDOT Type B Safety Barrier Curbs supported on a 9.5-in. concrete deck cast on stay-in-place steel forms. The delta frame welded plate girders were 130 in. deep and the frame legs had a typical depth of 48 in. All structural steel was designed to be unpainted ASTM A709 Grade 50W weathering steel.

ATC Design
General Contractor American Bridge Company’s ATC involved a total redesign of the baseline concept with a new permanent structure on a new parallel alignment, leaving just 2 ft between the new structure and the old structure. The new structure comprises two plate girder units with six typical spans of 265 ft, 210-ft end spans and an in-span hinge connecting the two units. The steel superstructure is founded on twin 8.5-ft diameter, steel-cased drilled shafts. An 8-ft barbell strut that sits between the drilled shaft and the 8-ft-diameter column ties the
columns together.

The slender substructures are up to 120 ft tall and are braced against sway by the steel superstructure. Instead of proposing a three-girder bridge, as in the baseline design, a four-girder, unpainted ASTM 50W steel bridge with 93-in. web depth was used, reducing deck thickness (8.5 in.), reinforcement, forming and future replacement expenses.

Five contractor bids were received on this project. Two contractors bid the baseline design with no modifications, one elected to bid the baseline design with minor ATCs proposed and two proposed major ATCs to the baseline design. American Bridge’s proposed major ATC was the lowest bid, at $32,303,295, closely followed by the contractor that bid the baseline delta frame design with a minor ATC, at $45,765 higher.

Alternative Approach

The bridge had to be open to traffic no later than the date established in the base design, so the project team had to compress both the design and construction of the ATC into the same schedule allotted by MoDOT for only the construction of the baseline design. This required an accelerated project mobilization. The project was awarded to American Bridge on January 4, 2012, and individual package productions were scheduled carefully so that the release-for construction drawings were available in time to begin each successive work activity. This integrated design-build-style project management approach facilitated successful design and construction in less than the time provided for only the construction of the baseline bid.

Demolition on the existing bridge began this past December and was expected to be completed in March, at the time of publication. The new bridge, which uses approximately 2,100 tons of structural steel, opened to traffic on September 9, 2013, three months ahead of schedule.

This project was featured in Session B6 at the World Steel Bridge Symposium in Toronto in March. Go to www.aisc.org/nascc to view the presentation.

Owner
Missouri Department of Transportation

General Contractor
American Bridge Company, Midwest District, Overland
Park, Kan.

Structural Engineer
Parsons Corporation, Chicago and St. Louis offices

Steel Team
Fabricator
W&W/AFCO Steel, Little Rock, Ark. (AISC Member/N SBA Member/AISC Certified Fabricator)
Detailer
ABS Structural Corporation, Melbourne, Fla. (AISC Member)

Ramping Up

Every Metro Area includes intersections that drivers dread.

For Twin Cities drivers, navigating the intersection of TH169 and I-494 meant traveling through three sets of traffic signals in short succession and dealing with frequent bottlenecks. (TH169 is a U.S. Highway; “TH” stands for “Trunk Highway.”) In addition to delays, the congestion in the interchange often resulted in traffic accidents.

Improvements to this interchange had been in the planning stage for many years, but lack of funding held up the project. In 2009, however, the Minnesota Department of Transportation (MnDOT) obtained approval from the FHWA to use performance-based design to improve the interchange. This allowed MnDOT to limit the scope of the project to provide the traffic movements needed at the time of construction, which reduced the projected cost by as much as $40 million; the final cost was $128 million. The resulting project eliminated at-grade crossings and included construction of six of the eight system-to-system interchange ramps, which used 2,500 tons of structural steel in all. Preliminary plans were required for the two remaining ramps to ensure that there will be adequate space to construct them in the future if and when they are warranted.

Ramping Up

MnDOT elected to use the design-build project delivery system to complete the project, which is located within the municipalities of Bloomington, Edina and Eden Prairie, Minn. The final design and construction of the project was awarded to the team lead by a joint venture of C. S. McCrossan/Edward Kraemer & Sons (CSM/EKS). Jacobs was the lead designer with TKDA playing a significant design role as well. The solution provided by the CSM/EKS team included 12 new or widened bridges, six roundabouts, 11,000 ft of retaining wall and seven noise/visual barrier walls. The plan maintained continuous access for through and local traffic during construction.

Curved steel plate girder superstructures were chosen for the three flyover ramps, which are elevated for a significant portion of their length. The use of structural steel was critical to meet the geometric requirements for these ramps, which feature tight radii, skewed substructures and spans of longer than 200 ft. The preliminary design of the two future flyover ramps used curved steel plate girders as well.

Ramp B Bridge
The longest and most challenging structure was the Ramp B bridge, which carries westbound I-494 traffic to southbound TH169. It has seven spans split into two units with a baseline radius of 654.8 ft. The three-span unit has span lengths of 125 ft, 204 ft and 159 ft. Spans of 159 ft, 160 ft, 160 ft and 125 ft make up the fourspan unit.

A four-girder cross section was used to support the 35-ft, 4-in.-wide deck. Because of the sharp curve, most of the bridge is in full superelevation with a 5.8% cross slope. Each girder has a constant depth web along its length, but the webs of the four girders vary in depth across the section from 61 in. at the inside girder to 73 in. at the outside girder. This configuration had geometric, structural, aesthetic and economic benefits. By making the low-side girders shallower, the structural depth was reduced 12 in.; this provided more vertical clearance to the roadways below. The bottoms of the girders were at similar elevations, which eliminated the need for large steps in the pier caps, and the visual quality of the bridge was improved by giving it a more balanced and less tilted look. The most tangible benefit was the reduction in steel quantity realized by using a more efficient depth for each girder.

The Ramp B bridge crosses over six through lanes of I-494, four lanes of TH169 and three lanes of Washington Avenue. Required horizontal clearances to the roadways below dictated that several of the piers be skewed. The largest skew is at Pier 5 and is greater than 54°. Two spans away, the north abutment has no skew. Pier 6, which is between the two, was set at a skew of about half that of Pier 5 to mitigate the effects of the drastic change in skew.

Ramping Up

At each bearing, a line of radial cross frames was provided. However, at the highly skewed Pier 5 the cross-frames tended to act as a lever such that live loads on the inside girders caused uplift at the bearing of the outside girder. The solution to this problem was to soften the structure by eliminating the cross-frame between the outside girder and the bearing location of the adjacent girder. The resulting spacing was less than the 30-ft maximum AASHTO requirement for cross frame spacing in curved girders, which made it a viable solution.

The combination of the large spans and skewed piers of the Ramp B bridge made the bearing design difficult. AASHTO Method A was initially used for the elastomeric bearing design, but at some locations there was no solution that could meet all of the requirements. Pot bearings were investigated but it was deemed that the lead time required for their fabrication would adversely impact the schedule. Approval was obtained from MnDOT to design elastomeric bearings using AASHTO Method B. Compared to Method A, Method B provides for increased bearing resistance in exchange for increased testing of the bearing components and the fabricated bearings. The elastomeric bearings designed by Method B met the requirements and were used in the bridge.

Anti-Icing System
The Ramp B bridge and Ramp K bridge (from westbound I-494 to northbound TH169) were among the first bridges in Minnesota to be constructed with a built-in anti-icing system. The system was required to apply anti-icing chemicals at rates of 5 to 87 gallons per lane-mile to the roadway at temperatures as low as -22 °F. Sprayer heads were located in the bridge deck and longitudinally spaced along the roadway alignment at 50-ft maximum spacing.

Ramping Up

Each sprayer is connected to a watertight stainless steel valve box placed in the barrier rail. Several safety considerations went into the barrier details. The deflection joints in the barriers were shifted away from the junction box locations, and additional reinforcing steel was placed adjacent to and around the boxes to aid in the resistance to vehicular impact. The access plates are set flush with the face of the barrier and have countersunk bolts to prevent vehicles from snagging on the boxes.

The project was finished on schedule, with substantial completion in November 2012 and final completion in June 2013. By all accounts the public is very happy with the completed project, as the system-to system movements that characterize the new interchange are much faster and safer than the previous at-grade crossings.

Owner
Minnesota Department of Transportation

Joint Venture Team
C.S. McCrossan, Maple Grove, Minn./ Edward Kraemer and Sons, Plain, Wisc.

Structural Engineers
Jacobs, Minneapolis
TKDA, St. Paul, Minn.

Steel Detailer
ABS Structural Corporation, Melbourne, Fla.
(AISC Member)