After 40 years of heavy usage on the existing northbound structure (60 years on the bascule span) of the Whitestone Expressway, the steel superstructure and concrete deck were badly deteriorated.
The maintenance required for the grating deck on the bascule span became a recurring problem, and the fatigue evaluation of the existing bascule girders indicated that the girders were approaching the end of the remaining safe fatigue life.
The non factor
The objective of this project was to correct the structural and geometric deficiencies of the existing northbound Whitestone Expressway to provide a safer roadway with improved operational features. The replacement structures eliminated nonstandard and nonconforming features. The improvements consisted of eliminating the weaving condition by providing separate exits from Astoria Boulevard and the Van Wyck Expressway, standard lane and shoulder widths and longer vertical curves that provided adequate stopping sight distances for a 60-mph design speed. The complexity of this urban interchange required the vertical alignment of the Whitestone Expressway to provide a 14-ft 6-in. vertical clearance under the existing southbound Van Wyck Expressway and a 35-ft vertical clearance over the 140-ft-wide Flushing River within a 420-ft distance.
The structural configuration of the existing bascule span played a role in developing the replacement alternatives in that the bascule span consisted of nonredundant girders with a full-width counterweight. The structure could not be easily demolished in stages while utilizing the remaining sections to maintain traffic. The alternatives that were chosen for this project consisted of utilizing three high-level fixed structures to be constructed within the existing highway corridor of the Whitestone Expressway.
The new Astoria Boulevard approach structure carries two lanes of northbound traffic and utilizes a new structure for access to the northbound service road and Linden Place. The new Van Wyck Expressway structure carries two lanes of the northbound traffic and utilizes an on-grade Linden Place exit lane. The new Linden Place exit ramp structure provides direct access to the northbound service road and Linden Place from the Astoria Boulevard approach. Both the Astoria Boulevard approach and Van Wyck Expressway have separate dedicated off-ramps to Linden Place, eliminating the nonconforming weaving movement of the existing expressway alignment.
Implementation of the high-performance steel (HPS) grade 70 and grade 50 steel for the curved hybrid steel trapezoidal box girder superstructure is the first of this type in the U.S. The use of HPS began in the late 1990s and the extreme geometric constraint conditions prompted the Hardesty & Hanover design team to investigate a highly efficient design to accommodate the shallow superstructure over the waterway.
The innovative approach involved utilizing straight and curved continuous steel trapezoidal hybrid box girders with the grade 70 steel top and bottom flanges over the negative-moment region in order to achieve the 305-ft main span over the Flushing River. The use of shallow constant-depth trapezoidal box girders was necessary to satisfy the vertical clearance over the navigational channel of the Flushing River and provide the required minimum vertical clearance under the southbound Van Wyck Expressway elevated structure.
Approximately 10% of the steel weight used for the construction of the box girders was HPS steel. The effective use of HPS and trapezoidal girder configuration validate the efficiency and cost benefit of utilizing HPS and constant-depth trapezoidal girder configuration. The hybrid girder design demonstrated the greater strength characteristics of the grade 70 steel while maintaining economical use of grade 50 steel in the webs over the negative-moment region and the remaining portions of the girder. Of over 7,000 ft of new structure designed and constructed, approximately 4,500 ft consisted of trapezoidal box girders.
Design challenges
The continuous curved multibox girders of the Whitestone Expressway and the northbound Van Wyck Expressway are supported on two bearings at the intermediate supports and one bearing at the end supports. Results of the initial analysis utilizing two bearings at the end supports indicated high torsional stresses developing on the box girder due to the support restraints. However, when a single bearing centered under the box girder ends was utilized, the torsion was resolved into a single reaction acting on the bearing.
The Linden Place exit ramp consists of a three-span trapezoidal single box girder supported by two bearings at each support location. Since this box girder has a straight alignment, it is not subject to the high torsional forces as were noted for the curved box girders of the Whitestone Expressway and the northbound Van Wyck Expressway. However, the seismic analysis showed that one of the two bearings at the ends of the box girder was subjected to uplift. Therefore uplift-restraint bearings were specified for the ends of the box girders.
The torsional stiffness of the box girders was provided by the combination of the top lateral bracing and cross-bracing during the girder erection and concrete-deck placement.
The vertical stiffeners consisted of two pieces; the main (upper) stiffeners were welded to the box girder web but were clipped at the bottom above the bottom flange to provide clearance for automatic welding of the web plate with the flange plate. Once the web and flange were welded together, the lower sections of the vertical stiffeners were bolted to the upper portion of the vertical stiffener as well as the bottom flange in the tension zone and welding in the compression zone.
The wide bottom flange plates of the box girder (up to 10 ft 9 in. wide) were subject to local buckling in the compression zones over the piers. Structural Ts were utilized as longitudinal stiffeners and were attached to the bottom flange in the compression zones and extended into the tension zones of the bottom flange. The ends of the longitudinal stiffener and welds were carefully designed to improve the fatigue category. The final 8 in. of the longitudinal weld attachment to the bottom flange consisted of a groove weld, and the web of the stiffener was cut back with a minimum of a 12-in. radius and ground smooth to the face of the bottom flange.
Since the demolition and the subsequent construction of the Whitestone Expressway was performed in stage construction, the design required additional analysis to evaluate the existing and proposed structure for each stage of construction. New piers 52 to 56 are multicolumn substructures that were evaluated as hammerhead piers when only one of the three columns would be available to carry traffic in Stage II. The hammerhead river piers 59 and 60 had to be designed to withstand the asymmetrical superstructure loads as well as associated live loads during Stage II construction carried by one of the two box girders that was installed on the pier and subject to live loads.
In addition to the safety and operational improvements on I-678, the local community requested improvement of the traffic congestion at the Linden Place intersections. Linden Place is a cross street that passes under the Whitestone Expressway with an intersection at both the northbound and southbound Whitestone service roads that run parallel on each side of the expressway. The service roads at Linden Place with the exiting Whitestone Expressway traffic was congested due to the heavy left-turn movement from the northbound and southbound service road turning onto Linden Place. In order to reduce the travel delay entering the intersections, a loop road was constructed south of Linden Place under the existing Whitestone Expressway overpass to allow the northbound service road traffic to bypass the intersections to go onto the southbound service road. This improvement reduced the ADT for the northbound service road by up to 39 seconds/vehicle depending on the time of day. Another loop ramp was constructed between the northbound and southbound service road north of College Point Boulevard to alleviate the traffic congestion caused by the unorthodox multilegged left-turn intersection at the northbound service road.
Going trapezoidal
The new Whitestone Expressway superstructure involves 24 spans of new structures consisting of continuous curved rolled multistringers, steel-curved multiplate girders, precast concrete multibox girders and continuous steel curved trapezoidal double-box girders. In addition, rehabilitation on eight spans of the approach elevated structure was required to accommodate the widened roadway.
The new northbound Van Wyck Expressway consisted of 24 spans of new continuous curved steel multirolled stringers, continuous curved steel multiplate girders and continuous curved and straight trapezoidal double-box girders.
The new southbound Van Wyck Expressway consists of three spans of steel multiplate girders structure that are made continuous by a special framed connection into the steel cap beams of the piers.
The new Linden Place exit ramp consists of five spans of multiplate girders and continuous steel single trapezoidal box girders.
The superstructure type used was dictated on its location on the profile and available headroom. The spans at the ends of the profile with limited headroom were shallow rolled stringers or precast concrete box beams. As the height of the profile increased, deeper plate girders with greater span lengths were utilized. At the crest of the profile over Flushing River, the greatest span lengths and trapezoidal box girders were utilized. Of over 7,500 ft of new structure designed and constructed (55 spans), approximately 4,500 ft, or 21 spans, consisted of trapezoidal box girders.
The sub plot
The substructure work for the entire project consisted of 53 new or modified existing piers and two abutments. There were nine different substructure configurations used on this project, three of which consisted of widening existing substructures that supported new superstructures or widened existing superstructures. There were three specially designed piers to support the rehabilitated southbound Van Wyck Expressway and for the precast concrete box girders of the Whitestone Expressway. The trapezoidal box girders were supported on two types of concrete piers, namely the hammerhead and the multicolumn with cap beam piers. Abutments were used at the ends of the Linden Place exit ramp structure and shared at the ends of the northbound Van Wyck Expressway and the Whitestone Expressway.
The mainline substructure types consisted of 19 new concrete hammerhead piers; 13 new concrete multicolumn and cap-beam piers; four new solid-stem piers or abutments; seven widened existing solid-stem piers; five widened existing steel-cap beams with concrete columns; four widened existing concrete cap-beams with concrete columns; one new steel hammerhead pier; one new concrete pile bent; one new multilevel pier consisting of a concrete stem pier on the lower level and steel frame for the upper level; and two new concrete abutments.
Pier 57 is a double-level pier supporting the northbound Whitestone Expressway traffic on the lower level and the southbound Whitestone Expressway (I-678) overpass leading onto the southbound Van Wyck Expressway (I-678). Although the pier can be demolished and reconstructed in one stage through the use of a temporary structure and supports, replacement of the pier was extremely difficult due to the close proximity of two existing 24-in.-diam. high-pressure gas mains and two very closely adjacent abandoned footings. New pile layouts were developed to avoid the existing abandoned piles and footing of a nearby existing building. Most importantly, the Con Edison gas main coordination was a critical activity for the pier construction that affected the overall project schedule. The partnering effort involved utility participation during the design phase and coordination with the contractor to ensure the proposed foundations were perfectly fit between the existing underground obstructions identified in the design plans.
Life in stages
The reconstruction work was performed in four stages. One of the most difficult design challenges of this project was to maintain all travel lanes throughout all stages of construction by utilizing both the existing and proposed structures. The combination of onsite detours and temporary structures allowed the new construction to take place without diverting traffic onto the busy local streets and residential neighborhood. The improved structure alignment is developed within the existing highway corridor. The construction of these replacement structures caused no financial hardship to the adjacent industrial properties. The staged demolition scheme eliminated the need to utilize temporary supports to perform staged demolition for the nonredundant bascule span.
Preparation stage work involved construction of temporary on-grade pavements within the existing highway center median to carry the northbound Whitestone Expressway traffic andconstruction of a temporary Linden Place on-grade exit ramp.
Stage I work involved construction of the new northbound Van Wyck Expressway on the east side of the existing northbound Van Wyck and Whitestone expressways. Once the new structure was opened to traffic, the existing northbound structure then carried the southbound detour traffic from the existing southbound Van Wyck Expressway using a temporary crossover structure. This detour allowed the entire section of the existing structure to be closed and reconstructed in Stage II.
Stage II work involved construction of half of the new northbound Astoria Boulevard approach structure over the river along the west side of the existing bascule span and reconstruction of the southbound Van Wyck Expressway structure. The three lanes of traffic from the Astoria Boulevard approach continued onto the existing northbound bascule structure.
Stage III began once the western half of temporary single box girder of the Whitestone Expressway was completed and opened to two lanes of the Astoria Boulevard approach traffic. The bascule span was then demolished in the opened position to make way for the construction of the remaining eastern half of the structure and the new Linden Place exit ramp structure. During Stage III construction, the northbound Astoria Boulevard approach traffic accessed the Linden Place exit by means of a temporary structure connected to the new northbound Van Wyck Expressway leading to the new on-grade Linden Place exit.
Stage IV work involved removing the temporary structure and completing the remaining portion of the northbound Astoria Boulevard approach structure and the northbound Van Wyck Expressway that was utilized for the temporary structure.
The substructure utilized 1,100 concrete cast-in-place piles that varied in diameter from 14 to 24 in. The substructures were constructed with cast-in-place concrete that was poured in separate stages. The bar reinforcement for the pier columns and cap beams was preassembled on the ground and then lifted into place prior to the installation of the formwork.
The steel erection of the superstructure on land was performed using cranes and temporary support towers. The steel stringers and girders were transported to the site by trucks.
Due to the tight construction space and U.S. Coast Guard requirements over the navigable waterway, the river span construction utilized two methods to erect the steel box girders over the waterway. For both methods, the box girders were transported to the site by barge. The first method utilized temporary supports set up in the river and a barge-mounted crane to erect the trapezoidal box girders for the northbound Van Wyck Expressway. The erection consisted of lifting two girder segments at a time. The length of the lifted girders was approximately 160 ft and 165 tons.
The second method of girder erection was used on the Linden Place exit ramp structure and the northbound Whitestone Expressway/Astoria Boulevard structures. The girder erection for these structures was carried out between the existing southbound Whitestone Expressway and the new northbound Van Wyck Expressway structures. This procedure consisted of using a land-based crane and temporary support towers on one end of the girder and a beam and winch mounted on a previously erected longitudinal box girder on the other end of the girder. The girders were then brought to the site by barge. The barges were turned in the channel so that the box girders were transverse with respect to the channel and lifted in place. The beam and winch assembly were attached to the previously erected girder flanges to allow the box girder to be lifted and fastened to the previously erected girders at the field splice locations. The other ends of the girders were set on top of the temporary support towers by crane. The lifts for these structures consisted of three separate girder segments preassembled to form an approximate length of 202 ft and weight of 232 tons.
Shallow strength
In summary, the benefits of using HPS allowed the use of shallow, constant-depth girders, which have a lower fabrication cost than haunched girders, and mitigated the steep profile geometry to meet the navigational clearance requirements. The use of hybrid box girders resulted in a very economical and efficient design, which satisfied the difficult design criteria, and the well-planned staged construction allowed the project to be completed 3% under the contractor’s $178 million bid.
