News Details
Bridge rehabilitation with “Schöck ComBAR”
GFRP rebar in use at the Weightman Bridge in Niagara Falls, Ontario
Weightman Bridge at onset of rehabilitation. (Source: Schöck Canada Inc.)
Detail of flexible link joints with the continuous reinforcement. (Source: Schöck Canada Inc.)
GFRP rebar grade I in top layer of deck. (Source: Schöck Canada Inc.)
In 2010 the City of Niagara Falls rehabilitated the Weightman Bridge which crosses the Welland River along Portage Road. The approximately 4 million dollar project comprised the replacement of the complete bridge deck, the side walks as well as the steel railings. The bridge consists of two approach spans made of concrete slabs on steel girders and a central pre-stressed concrete girder navigational span. The deck joints of the existing bridge were replaced by continuously reinforced flexible links. Several technical innovations regarding the use of glass fibre reinforced polymer (GFRP) were involved in the design, including special rebar splices. Because of its excellent material properties, the newest generation GFRP rebar “Schöck ComBAR” from Schöck Canada was selected for the reconstruction of the bridge deck and the sidewalk slabs.
The four-lane Weightman Bridge in Niagara Falls, Ontario, was built in 1967. Heavy daily traffic crossing the bridge and the numerous frost-thaw cycles in the winters since the opening of the bridge had taken their toll on the bridge deck and side walk slabs, which showed extensive damage due to corrosion of the steel rebar. After a detailed analysis of the structure, the City of Niagara Falls decided that it was more economical to replace the entire bridge deck than to rehabilitate it.
A net value analysis showed that the installation of glass fibre reinforced polymer (GFRP) rebar in the upper layers of the deck and sidewalk slabs as well as in the barrier walls was the most economical renovation concept. As a result, the city included this concept in the original contract specifications.
Structural details
The three-span Weightman Bridge is located along Portage Road in Niagara Falls, Ontario. It is 97.5 metres long and 18.8 metres wide. The bridge superstructure is a classical slab-on-girder system. The substructure is formed by two outer spans – each consisting of six large steel girders – which cantilever beyond the two piers in order to support a central pre-cast concrete segment. The steel girders are nearly 43 metres long, extending about 9 metres beyond the piers. The shallow central section is about 12.1 metres long. It allows for limited navigation underneath the bridge. Flexible links at each end of the pre-cast section allow for the transfer of vertical loads while rotation within the link is still possible. In comparison to the old expansion joints, these links are a technically sounder solution.
The original deck and sidewalk slabs of the bridge had been heavily damaged by rebar corrosion and needed to be replaced. While the steel girders were deemed to be structurally sound, the central pre-cast section was also removed in the course of the rehabilitation. It was replaced by fourteen pre-cast, pre-stressed hollow core panels oriented along the axis of the bridge. The panels are 1.2 metres wide and only 400 millimetres high. Steel stirrups were installed in the top of the panels for connection of the cast-in-place deck slab.
The new deck was cast onto the hollow core planks in the central section and onto the steel beams along the two outer spans. The sidewalk slabs were then cast directly onto the deck slab. Finally, the new PL-2 concrete barrier walls are cast onto the sidewalk slabs.
GFRP reinforcement
The original contract documents for the rehabilitation called for the installation of either epoxy-coated or GFRP rebar in the top layers of the deck and sidewalk slabs. An analysis was performed after the tender was closed showing that it was more economical to use the GFRP reinforcement. The bottom layers were reinforced using carbon steel rebar.
The contract for the bridge rehabilitation contained the installation of grade I (E ≥ 60 GPa) GFRP rebar in the top layers of the bridge and sidewalk slabs and grade II (E ≥ 50 GPa) GFRP rebar in the PL-2 type barrier walls. The final design of the bridge deck called for 16 millimetre diameter grade I GFRP rebars at a spacing of 250mm in the longitudinal direction and 16mm bars at 250mm in the transverse direction. Below the inner edge of the sidewalk slab, additional 25mm bars were placed at 300mm to transfer the tensile forces in the top of the cantilevering deck slab. The sidewalk slabs were reinforced with 16mm bars at 200mm in the longitudinal direction and 16mm bars at 125mm in the transverse directions in the top layer. Carbon steel rebar was placed in the bottom layer of the deck and the sidewalk slabs. Bent epoxy-coated steel bars were used in the bottom layer in selected locations prone to rebar corrosion.
Due to its superior material properties, the latest generation GFRP rebar Schöck ComBAR, distributed by Schöck Canada, was chosen for the upper layer of the bridge deck and the sidewalk slabs. Schöck not only supplied the material but also provided the installation, which was performed by Total Bridge Services Inc. (TBSI) – a long-term partner company of Schöck Canada. For economic reasons, the barrier walls were reinforced using the grade II GFRP reinforcement V-Rod supplied by Pultrall Inc.
Stages of the project
To minimise disruption of city traffic in Niagara Falls, the rehabilitation of the Weightman Bridge was executed in two stages. In stage one, the right, western half of the bridge was closed. Both directions of traffic were then redirected onto the left-hand side of the bridge. In stage two, traffic was diverted onto the new deck while the east side of the bridge was rehabilitated. This construction schedule required splicing the transverse reinforcement in the deck along the centre line of the bridge. In order to do this, a new technology using GFRP bars glued into corrugated polyester tube sleeves was developed by Schöck and TBSI. In stage one, pieces of 650mm long corrugated PE tube with an interior diameter of 18mm were installed in the deck along the border between the two phases. In stage two, ComBAR bars with a core diameter of 16mm were inserted and glued into these tubes using an epoxy-based adhesive.
Along the two abutments of the bridge, another special detail in the rebar lay-out had to be developed on account of the unique material properties of fibre reinforced polymer rebars (FRPs). Unlike steel, FRPs cannot be bent once they have hardened. Stirrups and bent bars have to be prefabricated at the shop. More significantly, fibrous materials are not isotropic. Their strength is substantially higher in the direction of the fibres than it is perpendicular to them. As with wood, for instance, any redirection of the fibres in FRPs results in a substantial loss of strength. This can be compared to a knothole in wood. The problem arises when a tensile load is applied on a bent portion of a FRP bar. As the fibres on the outside of the bend are extended much more than the fibres along the inner side, interlaminar shear stresses and transverse stresses are induced in the bar. These result in premature failure of the bar along the bent portion. To alleviate the resulting problems in the concrete structure, straight ComBAR bars with end heads were used wherever possible in the design of the Weightman Bridge when bent epoxy- coated bars were replaced.
At the abutments, the longitudinal bars at the top were thus provided with end heads, as were the vertical bars at the centre of the abutment end beam. To provide closure in the system of load vectors, a short section of bent bar was placed at each intersection of these headed bars.
Structural design
The structural design of the new bridge deck and sidewalk slabs was performed by ELLIS Engineering Inc. of St. Catharines, Ontario – in close cooperation with the Schöck engineers. Ellis Engineering is a well-known consulting engineering firm specialised in the design and rehabilitation of bridge structures. The design was performed according to the Canadian Highway Bridge Design Code CAN/CSA-S6-06. Reinforcement drawings were drawn up by TBSI and checked by Schöck.
The Weightman Bridge is the first project in which the newest generation grade I GFRP rebar Schöck ComBAR is being installed in a bridge deck in Canada. It is also the first project ever in which Schöck supplied the material and the placement of the bars in a combined package.
Project data
owner: City of Niagara Falls, Ontario
contractor: Rankin Construction Inc.
structural engineer: ELLIS Engineering Inc.
installation: May – December 2010
