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
1

DIALOG, Canada
City of Edmonton, Canada
3

2

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.
1

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.

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Figure 1: Groat Road Bridge over the North Saskatchewan River
2
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 9
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.

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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 2 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 3 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

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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 5 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 6 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.

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Figure 6: Girder expansion and contraction hinge
3
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 7.

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 8. The new superstructure is designed to carry CL-800 loading (as
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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, 4 and 5; refer to
Figures Figure 2 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.

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The rocking condition will be eliminated from the structural system as part of the rehabilitation. Figure 9
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 3 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.

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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
4

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.

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