10th International Conference on Short and Medium Span Bridges Quebec City, Quebec, Canada.DOC

10th International Conference on Short and Medium Span Bridges Quebec City, Quebec, Canada, July 31 – August 3, 2018 GROAT ROAD BRIDGE REHABILITATION Oosterhof, Steven A.1,3, Garay, Juan1, Robson, Neil1, Schettler, Caroline2 and Montgomery, Jim1 DIALOG, Canada City of Edmonton, Canada SOosterhof@dialogdesign.ca Abstract: The Groat Road Bridge over the North Saskatchewan River provides an important connection for vehicles, pedestrians, and cyclists between downtown Edmonton, the University of Alberta, and the river valley To extend the life of this seven-span bridge by 75 years, an intensive rehabilitation has been undertaken Several unique challenges were addressed through the design of this rehabilitation, including: the atypical design of the existing structure, the need to keep the bridge open to traffic during construction, the mandate to respect existing bridge aesthetics, the desire for a wider shared use path, and sensitivities related to the project’s location in the heart of the city’s river valley The rehabilitation includes a complete superstructure replacement, comprising haunched steel plate girders with a composite concrete deck and reconstructed abutment roof slabs The existing bridge was designed to accommodate longitudinal thermal movements through a series of expansion hinges in the concrete girders working in concert with several rocking piers designed to rotate about their bases The rehabilitation addressed the lack of redundancy and poor performance of the existing system by eliminating the expansion hinges and fixing the pinned bases of the rocking piers, which required the specification of a critical construction sequence to address pier stability and prevent damage to the structure during construction As part of the rehabilitation project, the bridge deck was widened to increase the sidewalk width to provide a 4.2 m wide shared-use path To avoid widening the existing piers or abutment, the new superstructure introduces substantial cantilevers that were a significant consideration in the bridge’s design, aesthetics, and temporary stability under construction loading Additionally, bridge demolition and construction will be completed in halves to keep the bridge open to traffic during construction Introduction The Groat Road Bridge over the North Saskatchewan River is a unique and elegant structure that is located in an important parks area in the heart of Edmonton’s river valley, to the west of downtown Edmonton, Alberta on the north bank and the University of Alberta on the south bank (see Figure 1) Given its age and current condition, the bridge is in need of a major rehabilitation to meet the requirements of the current Canadian Highway Bridge Design Code, CSA S6-14 A complete superstructure replacement has been designed to extend the life of the bridge by 75 years Construction is scheduled to begin in 2018 267-1 Figure 1: Groat Road Bridge over the North Saskatchewan River 2.1 Existing Bridge History The Groat Road Bridge over the North Saskatchewan River carries two northbound lanes and two southbound lanes of traffic, with a sidewalk on the east side The bridge was designed for the Bridge Branch of the Government of the Province of Alberta by Structural Engineering Services Ltd (a firm that later became Lamb McManus Associates Ltd.) and constructed between 1953 and 1955 The bridge was originally known as the West End Bridge, as it was near the west city limits at that time The bridge comprises a cast-in-place concrete deck supported on six reinforced cast-in-place concrete girder lines, with seven spans (as shown in Figure 2): a 33.53 m end span, five spans at 44.50 m, and a 33.53 m end span for a total length between abutment bearings of 289.56 m The bridge is on a skew of degrees LHF The haunched concrete girders vary in depth from 1.67 m near mid span to 3.35 m over the piers The bridge drawings note an initial design loading in 1953 for the “AASHO H20-S16-44” truck (equivalent to the AASHTO HS-20 truck), which weighs 320 kN 267-2 Figure 2: Groat Road Bridge general arrangement When originally built, the deck and girders were cast monolithically in forms supported by falsework on berms in the river The river flooded during construction, causing portions of the falsework to collapse before the concrete had cured fully Consequently, long-term deflections of the girders were excessive, resulting in an uneven ride for vehicles travelling across the bridge This was addressed as part of a major rehabilitation completed in 1990, when a complete deck replacement was performed During the deck replacement, the girder webs acting alone were not able to support the weight of fresh concrete, so the deck was replaced in patches 2.2 Structural System By today’s standards, the structural arrangement of the bridge is unconventional Figure indicates that longitudinal expansion and contraction of the girders is accommodated by three piers that are designed to rock; that is, they are hinged at the top and bottom The other three piers have no hinge at the bottom, and are thus fixed against rocking The piers are reinforced concrete with spread footings that are founded on clay shale bedrock Figure shows the concrete hinge that is located at the top of each pier to allow rotation of the girders relative to the pier A concrete shear key extending from the soffit of the girder is cast on a thin layer of lead in a groove on top of the pier At the bottoms of the expansion piers, loads are transferred through the web of a W610 x 195 (24” WF @ 130#) structural steel member that is cast into the shaft and the top of the footing below (Figure 4) Because there is a gap between the pier shaft concrete and the top of the footing, the flexible web of the structural steel member allows the expansion piers to rock as the supported girders expand and contract with changes in temperature Figure 3: Concrete hinge at top of piers 267-3 Figure 4: Hinge at bottom of expansion piers The open abutments have a concrete beam and slab roof system, rocking concrete columns supporting the deck girders, and concrete front walls, wing walls and pile caps that are founded on precast concrete piles Figure shows the detail for the girder support columns, with hinges at the top and bottom that allow the columns to rock to accommodate thermal movement of the girders Each hinge consists of a concrete shear key cast onto a thin layer of lead in a groove on top of the member below, similar to the detail at the piers The concrete hinges at the tops of the columns have deteriorated significantly Figure 5: Rocking columns at abutments (left) and hinge deterioration at top of rocking columns (right) As part of the structural system to allow longitudinal thermal expansion and contraction, each girder line has two hinges along its length Figure indicates that the hinges at the time of the original construction consisted of a vertical plate connected at the top and bottom by pins to channels cast into the girders Based on measurements of longitudinal bridge displacements taken over a range of service temperatures, it is suspected that these expansion joints have seized, preventing the bridge from expanding and contracting in the manner originally intended, and potentially exerting rotation onto the existing fixed pier bases Furthermore, the hinge is considered a poor detail for structural integrity; the inherent lack of redundancy creates the potential for the sudden and progressive failure of the system For these reasons, it was a priority to eliminate this system as part of the rehabilitation work 267-4 Figure 6: Girder expansion and contraction hinge 3.1 Rehabilitation Girder Replacement An assessment of the existing concrete girders has shown that their shear capacity is inadequate for the prescribed safety levels given in CSA S6-14 (CSA 2014) when loaded with a CS-615 vehicle Shear stirrups in the midspan region of typical girders are spaced at 1016 mm, which exceeds the maximum spacing allowed in the current bridge code for new design and the maximum spacing allowed to consider their contribution to shear strength for the evaluation of existing bridges Shear cracking has been observed on the surface of the existing girders, as can be seen in Figure Figure 7: Shear crack in existing concrete girder To address the strength deficiency of the existing girders, several options were developed to strengthen the girders, including externally-applied fibre reinforced polymer (FRP), external post-tensioning, and the use of extradosed cables supported on new concrete pylons constructed above existing piers A complete superstructure replacement was ultimately selected for the project by the client and consultant team based on a value assessment considering the durability, performance, functionality, constructability, schedule, aesthetics, life-cycle cost, and risk of the various options The new superstructure comprises five structural steel plate girders supporting a composite concrete bridge deck, as shown in Figure The new superstructure is designed to carry CL-800 loading (as 267-5 defined in CSA S6-14), the existing substructure is strengthened for the CL-800 loading, and deficiencies in the substructure are rectified The City of Edmonton established a mandate to respect the aesthetic of the existing haunched concrete girders of the historic bridge; consequently, the new steel girders are haunched to replicate the form of the existing concrete girders, with a parabolic profile varying from 2700 mm deep at the piers to 1350 mm deep at midspan Using haunched girders reduced the amount that the existing piers needed to be builtup compared to the use of shallower straight girders The existing bridge is also characterized by the continuous straight line of its slender cantilevered deck edge and unembellished pedestrian railing—elements which have been integrated into the new design As part of the rehabilitation project, the bridge deck is widened to increase the sidewalk width to provide a 4.2 m wide shared-use path To avoid widening the existing piers or abutment, the new superstructure introduces substantial cantilevers that were a significant consideration in the bridge’s design, aesthetics, and temporary stability under construction loading Bridge demolition and construction will be completed in halves to keep the bridge open to traffic during construction, which also contributed to the final arrangement of the new cross section Figure 8: Widened bridge deck section 3.2 Top of Pier Bearing Surface New concrete is installed at the top of existing piers to address the potential that existing concrete is deteriorated, in order to create a sound bearing surface for the new bearings, and to allow the installation of reinforcement to confine bursting stresses at the free edges of the piers 3.3 Pier Base Strengthening The rocking of the expansion piers relies on the bending of the webs of the W610x195 structural steel members that are cast into the pier shaft and the footing below Analysis of the bridge indicates that the webs of the wide flange beams above the footings of the expansion piers (Piers 2, and 5; refer to Figures Figure and Figure 4) are subjected to relatively high stresses under the combined action of gravity loads and the bending that occurs as the tops of the piers move when the girders expand and contract due to temperature changes There is concern that corrosion or fatigue of the steel wide flange member may have reduced the capacity of these piers to carry load over time Because of access difficulties, the condition of the wide flange beams has not been thoroughly assessed since the bridge was constructed Furthermore, the replacement of the superstructure and the concurrent elimination of the girder expansion hinge (shown in Figure 6) would increase the rotational demand on these webs if they continued to act as rocking piers supporting the new girders 267-6 The rocking condition will be eliminated from the structural system as part of the rehabilitation Figure schematically illustrates the approach to apply fixity to the base of the rocking piers by locking their rotation with a new concrete collar tied into the existing concrete pier and footing across the joint New bearings are required under the new girders at all piers and abutments The girders are attached to the central fixed piers (Piers and 4) with bearings that allow rotation of the girder relative to the top of the pier but prevent longitudinal and transverse movement The girders are attached to the tops of the remaining piers and abutments using conventional bearings that allow longitudinal movement but prevent transverse movement To reduce the overturning moment on the relatively narrow pier foundations (which were not originally designed to resist significant overturning), the sliding bearings have been designed with a relatively low coefficient of friction—a maximum of 4%—by using a lubricated dimpled sheet of unfilled PTFE and the contact area has been minimized Figure 9: Pier base rehabilitation The construction sequencing for the pier base strengthening work represents a distinct technical challenge of this rehabilitation project The pier base must become fixed against rotation before the girders at the top of the pier are free to translate in the longitudinal direction Otherwise, the pier would become a column pinned at the base and free at the top, which is inherently unstable However, in the permanent condition, the girders must be free to translate longitudinally to prevent a fixed-fixed column condition, which would overload the new concrete collar and exceed the overturning capacity of the footing under thermal loading To address these conditions, the pot bearings used to support the new steel plate girders are installed with temporary attachment plates that prevent longitudinal translation, thus mimicking the behaviour of the existing concrete girder bearings (shown in Figure 3) and stabilizing the rocking pier Existing girders are demolished and replaced half at a time so that at least three girders are always present to stabilize the top of the pier The concrete collar is then cast around pier base using a high-early-strength concrete mix, and the temporary attachment plates on the bearings are removed as soon as the new concrete reaches a minimum strength level within 24 hours 267-7 3.4 Abutment Reconstruction The existing rocking columns supporting the girder ends at the abutments (shown in Figure 5) are severely deteriorated, and are removed and replaced with a new continuous concrete abutment seat wall on the existing foundations as part of the rehabilitation Expansion and contraction in the new system is accommodated by conventional sliding bearings supporting the steel girders rather than by rocking action of the supports The existing concrete roof slabs at the abutments are similarly deteriorated, and will also be removed and replaced The lateral stability of the existing bridge abutment during construction is an important consideration to this project Because the bridge demolition and construction will be done in halves to maintain two lanes open to traffic, the existing bridge abutment must be braced for overall lateral stability as it continues to bear traffic after the roof slab is demolished, and no longer acts as a rigid concrete diaphragm connecting it to the opposite wing wall, as shown in Figure 10 TEMPORARY BRACING COLUMNS SUPPORTING CONCRETE GIRDERS NOT SHOWN FOR CLARITY Figure 10: Abutment with temporary bracing after demolition of first half Conclusion The rehabilitation of the Groat Road Bridge over the North Saskatchewan River presents a series of unique challenges as a consequence of the atypical structural design of the existing structure, the need to keep the bridge open to traffic during construction, the mandate to respect existing bridge aesthetics, the desire for a wider bridge deck, and sensitivities related to the project’s location n the heart of the city’s river valley These challenges required an innovative approach to the design and construction of the superstructure replacement With construction scheduled to begin in 2018, the project will extend the life of this important bridge structure in Edmonton by 75 years or more Acknowledgements The engineering design of this rehabilitation was completed by DIALOG for the City of Edmonton References CSA 2014 Canadian Highway Bridge Design Code, CAN/CSA S6-14 Canadian Standard Association, Toronto, Ontario, Canada 267-8 ... cantilevers that were a significant consideration in the bridge’s design, aesthetics, and temporary stability under construction loading Bridge demolition and construction will be completed in halves... continuous concrete abutment seat wall on the existing foundations as part of the rehabilitation Expansion and contraction in the new system is accommodated by conventional sliding bearings supporting... of the existing bridge abutment during construction is an important consideration to this project Because the bridge demolition and construction will be done in halves to maintain two lanes open