DESIGN INNOVATION: Tricky turn style

Aug. 2, 2010

Neither curve nor skew nor traffic could stop the Illinois Tollway’s new East-West Connector (EWC) Bridge in Oak Brook, Ill., from its appointed purpose—to carry two lanes of traffic from northbound I-294 to westbound I-88.

The EWC Bridge was recently reconstructed on a new curved alignment to separate electronic and cash transaction traffic as well as provide direct open-road tolling access.

Neither curve nor skew nor traffic could stop the Illinois Tollway’s new East-West Connector (EWC) Bridge in Oak Brook, Ill., from its appointed purpose—to carry two lanes of traffic from northbound I-294 to westbound I-88.

The EWC Bridge was recently reconstructed on a new curved alignment to separate electronic and cash transaction traffic as well as provide direct open-road tolling access.

The new bridge was one of several components of the 2.2-mile, $178 million Reagan Memorial Tollway (I-88) Reconstruction and Widening Project—part of the Illinois State Toll Highway Authority’s Congestion Relief Program. In addition to reconstructing and widening I-88, the project included work on four interchanges. The complexity of this venture increased substantially when combined with an accelerated design schedule, the presence of high-voltage transmission lines, a 20-ft shift of the I-88 alignment and the owner’s desire to minimize disruption to traffic flows.

Thrown for a curve

The EWC Bridge consists of two units separated by a modular deck joint. The first unit consists of two prestressed bulb-tee girder spans, and the second unit comprises three continuous steel I-girder spans. Both are on a curved alignment with a 1,400-ft centerline radius. The first unit of the bridge spans Salt Creek with two 140-ft spans. The second unit of the bridge spans over I-88 at a 59° skew and subsequently has two severely skewed supports and a maximum girder span length of 251 ft. Because of the curvature and skew, the span lengths for each girder vary significantly. For example, the Span 4 girder with the largest radius (G5) was approximately 76 ft longer than the Span 4 girder with the smallest radius (G1).

The analysis and design of horizontally curved and skewed steel I-girder superstructures is much more complex than the design of similar span straight-steel bridges. As such, these bridge types often employ higher level analysis techniques, such as 2-D grid finite-element models (grillage models) or 3-D finite-element models (FEM) using beam and shell elements, to determine the bridge’s behavior under dead and live loading conditions. In the case of the EWC Bridge steel I-girder unit, a 3-D FEM was used for final design and investigation of the conceptual erection sequence provided on the contract plans.

The combination of the horizontally curved alignment and the severely skewed supports of the EWC Bridge resulted in a unique behavior of the steel superstructure. In Spans 3 and 4, the horizontal curvature governs the behavior of the steel superstructure as the cross section rotates out of plane. In this case, the girder with the larger radius (G5) has a larger vertical displacement than the girder with the smaller radius (G1), because of the rigid body rotation of the structure. Furthermore, the out-of-plane rotation caused by the curvature in Span 4 is amplified by the skewed direction of Pier 4. However, the skew effects overcome the curvature effects in Span 5 as the cross section rotates out of plane, but in a direction opposite to that of Spans 3 and 4. In Span 5, the skew causes G1 to have a slightly longer span length than G5, contributing to this opposite out-of-plane rotation.

Initial analyses of the EWC Bridge predicted significant net uplift caused by live load at the bearing of girder G1 at the skewed Pier 4. This net uplift was a result of significant out-of-plane rotation within Span 4 under certain live-load conditions. The design solution to address this uplift effect was to take advantage of the superelevation of the structure and increase the girder depths of the steel girder unit uniformly across the bridge cross section, such that girder G1 has a web depth of 80 in., while girder G5 is 96 in. deep. Providing progressively deeper girders significantly increases the stiffness of the superstructure in Span 4 and combats this out-of-plane rotation resulting from the combined effects of curvature and skew.

Another unique design feature of the EWC Bridge is the cross-frame arrangement near the severely skewed Pier 4. In bridges with severely skewed supports, radial cross frames create an undesired load path when framing directly into skewed support. The undesirable transverse load path can lead to inordinately large cross-frame members. Select cross frames were omitted, thereby severing this transverse load path and redirecting the load through the girders to the pier.

The prestressed bulb-tee beams used on the EWC Bridge were 72 in. deep and employed 401?2-in.-diam., low-relaxation, grade 270-ksi strands. The 140-ft span length represented the upper limit for this section. Unique to the design of these beams was the issue of stresses and stability during transport and handling.

The precast, prestressed beams were closely examined for these conditions, as lateral bending stresses about the weak axis can develop and be significant with great span lengths. Deep prestressed beams are very strong under vertical loading and relatively weak under lateral bending conditions. The lateral bending conditions occur when a beam hangs from crane lifting loops and during transportation of the beams to the bridge site. Transportation to the bridge site is accomplished by tractors with steerable trailers, and the degree of lateral bending is dependent on the suspension system of the hauling rig and the transverse slope of the roadway. To ensure an adequate capacity for these conditions, one permanent prestressing strand was added to each end of the top flange. The strong axis force effects of these added strands were then balanced with two additional draped strands through the web and bottom flange bulb.

Compatibility is key

Extra care was taken during the steel design to ensure compatibility with the subsequent construction contract for the remainder of the bridge. Several key areas were carefully analyzed for compatibility. The first area was compatibility with localized traffic control on I-88. With average daily traffic of approximately 160,000 vehicles, the Illinois Tollway wanted to limit inconvenience to their customers during construction. The maintenance-of-traffic criterion was to maintain three lanes of traffic in each direction throughout construction. HDR developed maintenance of traffic/staging plans that included lane splits, reverse curves and counterflow lanes to navigate through the project work zone. The design of the structure had to be compatible with this constraint such that the field splice locations in Span 4 were located based on the maintenance-of-traffic plan and not placed near points of contraflexure.

Once the geometry was established in concert with the maintenance-of-traffic plan, it became readily apparent that the presence of a series of high-voltage power lines immediately adjacent to the structure posed another significant challenge. The high-voltage lines, when energized, transmitted a zone of influence that severely limited crane placement. The project team initiated early coordination with the local utility company and maintained communication throughout design and construction.

A temporary power outage was scheduled to allow a crane to erect the steel adjacent to the power lines. The programmed outage was only permitted during off-peak usage months of the spring or fall. The erection of the EWC Bridge was planned for late October and into early November, which was compatible with the utility power outage and the I-88 corridor traffic-staging plan. Another key compatibility issue was the constructability of the design. With a heightened awareness of erection safety issues and a limited window for erection right before impending inclement winter weather conditions, a conceptual erection analysis was initiated during the final design phase after the girders were proportioned for the final loading conditions. The conceptual erection analysis was conducted based on a reasonable set of assumptions regarding a conventional erection sequence.

A suggested erection sequence was depicted in graphical and narrative form in the contract plans. Locations of temporary supports were shown, as space was extremely limited. A temporary support structure that was employed during steel erection in Span 5 was positioned to allow temporary traffic lanes on both sides of the support. Even though this conceptual erection sequence was provided in contract plans, the contractor was still required to submit its own erection plan and calculations, sealed by a licensed structural engineer.

Going down a critical path

Early in the design process, it was determined that construction of the EWC Bridge would be a critical-path activity within the corridor-improvement program. This posed yet another unique design challenge, as the planned construction of the EWC Bridge had to be coordinated with the overall I-88 corridor reconstruction sequence.

Completion of the EWC Bridge was a milestone for a major traffic-control stage that extended into the adjacent projects along I-88. To meet this milestone, it was decided to let an early steel-fabrication contract. While providing the Tollway with some assurance that critical milestones would be met on time, this did add significant coordination complexities for all involved, particularly during delivery and erection times.

Separate challenges

The EWC Bridge had many unique design and construction challenges stemming from the severe support skew, combined with the horizontally curved alignment. Additionally, the planned construction of the EWC Bridge had to be carefully coordinated with the master I-88 corridor reconstruction program. Several key issues became apparent as the design progressed from the initial design phase through bridge construction, but thorough planning and coordination permitted the EWC Bridge to open to traffic on schedule. The bridge construction contract awarded to F.H. Paschen had a value of $12.7 million, equating to approximately $300 per square foot.

The bridge comprises two separate units: a two-span prestressed concrete unit and a three-span continuous steel I-girder unit. Two separate superstructure types were used in an effort to minimize construction cost, factoring in escalating prices for steel at the time of construction. The resulting success of the I-88 reconstruction project earned recognition as one of the Top 10 Road Projects for 2009 by Roads and Bridges magazine.

About The Author: Chavel is a structural project engineer with HDR, Chicago. Peterman is a senior structural project manager at HDR, Chicago. McAtee is a project manager with the Illinois Tollway, Downers Grove, Ill.

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