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10th International Conference on Short and Medium Span Bridges

Quebec City, Quebec, Canada, July 31 – August 3, 2018

DESIGN AND CONSTRUCTION OF INTEGRAL STEEL BOX PIER BEAM FOR STEEL BOX GIRDER BRIDGES

Botros, Mofeed1,2

1 WSP Canada Inc., Canada2 mofeed.botros@wsp.com

Abstract: The structure is located on highway 401 eastbound off ramp to Leslie Street (W-N/N Ramp) in the city of Toronto, province of Ontario, Canada. The bridge is a semi-integral abutment steel box girder type with a total width of 12.05m and consist of 3-spans of 20m, 30m and 20m on a curved alignment with radius of 130 m. girders are supported on Integral steel box-pier beam. The girder depth is 1200mm which integrated with the steel box pier beam to provide the railway clearance, minimize the grade raise of the ramp, and eliminate the skew at the piers. A single column for each pier is proposed to be rigidly connected to the piers beams. The pier columns will be continuous with concrete caisson foundations supported within the very dense sandy silt to silty till to support the structure. The girders will be connected to the sides of the beam, comprising a steel box beam, which in turn will be rigidly connected to the concrete pier columns which provide transferee of moment between the two systems. The inside of the steel box bent beam will be filled with concrete at the column connection. Suggested erection procedure is provided to ensure that the integrated pier beams and girders will be connected and in position before pouring concrete inside the pier beams with the means of temporary support of the integrated pier beams. The bridge was designed according to CSA S6-14. Analysis was performed to evaluate stresses and forces during various construction stages and final condition. Shear studs are provided inside integrated pier beams and at top/bottom flanges to sustain all horizontal forces due to longitudinal strain (Temperature), centrifugal forces, braking force, transverse wind load, vertical shear forces and bending moments (longitudinal, transverse and torsional). Top flange and web of integrated pier beam shall be fracture critical member as any loss of this member will cause fatal bridge failure. Pier columns were designed to sustain all loads from superstructure and crash load that apply to the railway crash wall.

1 INTRODUCTION

Highway 401 is a vital economic link connecting the State of Michigan (USA) and southern Ontario with eastern Ontario and Quebec (Canada). The structure will be located on Highway 401 eastbound off-ramp to Leslie Street (W-N/S Ramp) and carries traffic over railway tracks owned by GO Transit, (formerly owned by CNR) within the City of Toronto, Ontario, Canada. The location may be found on Figure 1.

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The existing Highway 401 eastbound off-ramp structure to Leslie Street N/S is a six-span simply supported bridge with reinforced concrete deck slab composite with steel I girders supported on conventional reinforced concrete piers and abutments (see Figure 2).

Ramp W-N/S

Figure 1: Key Plan

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The Preliminary Design Study concluded that the Highway 401 Overpass at Leslie warranted replacement. To maintain all mainline lanes during construction, the strategy for replacement included a 19m horizontal realignment of Highway 401 to the south. The new eastbound collectors will encroach on the space currently occupied by the existing W-N/S ramp due to the 19m horizontal shift.

2 COMPARISON OF ALTERNATIVES

During the initial review, a single span rigid frame option recommended in the preliminary design report (PDR) which have been made by other consultant firm in 2009, an issue regarding possible pile conflict between existing and new piles for mainline structures was identified. In addition, it was determined that the east leg of the recommended rigid frame alternative blocked passenger access to the GO Transit platforms. Furthermore, the rigid frame could only be backfilled on the west side, which was detrimental to the structural characteristics of a rigid frame structure. Additional concerns regarding available falsework clearance to construct rigid frame and maintain railway traffic during construction were also identified.

Irrespective of the structural, constructability, and station access concerns during and after construction, the most detrimental issue impacting the rigid frame alternative is new GO Transit requirement to maintain the ability for a future third track. The PDR option could easily accommodate the three tracks, but the future platforms would be narrow because of the span and rigid frame walls. Due to the limited space of PDR rigid frame option to accommodate the future tracks and platforms, the multi-span steel box girder option was recommended and approved by the client for the following advantages:

a) Less vertical clearance required during construction due to no falsework and formwork required;b) Abutments could be located outside GO Transit/Metrolinx right of way;c) Access to GO Transit platforms maintained during construction;d) Provides for future third track and future platforms;e) Shorter construction duration and Less construction cost;f) No conflict with existing pipeline utilities or existing foundations;g) No overbuilding required and less construction space.

Figure 2: Location of the Existing and New Structure

HWY 401 eastbound collector to be widened

Existing W-N/S ramp to be removed

New bridge

Future HWY 401 widening

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3 PROPOSED WORK

The proposed structure will be a 3-span integral steel box-pier structure carrying two 3.75 m lanes of Highway 401 eastbound ramp with a left shoulder of 1.0m and a right shoulder of 2.5m over GO Transit railway (see Figure 4). The new structure will be constructed on a new curved alignment with a radius of 130m located south of the existing structure and the existing 6-span steel girder structure will be removed (see Figure 3).

A span configuration of 20m-30m-20m was established for the proposed structure in consultation with GO Transit to accommodate the existing and future tracks. The riding surface will comprise a 225mm thick concrete deck slab and 90 mm asphalt and waterproofing system. The deck will be super elevated 6.0% downward to the south side to accommodate the horizontal curved alignment for a 60 km/h ramp design speed.

The proposed steel box girder has a constant depth of 1200mm and will be integrated with the concrete pier bent to provide the railway clearance, minimize the grade raise of the ramp, and eliminate the skew at the piers (see Figure 5). A single column for each pier is proposed to be rigidly connected to the piers bents. The pier columns will be continuous with concrete caisson foundations supported within the very dense sandy silt to silty till to support the structure. The pier columns will be integrated with railway crash walls.

4 THE CHALLENGE

Figure 4: Proposed Structure Cross Section

Figure 3: Proposed Structure Plane and Elevation

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Conventional girder type superstructures are usually supported on bearings placed on the bridge substructure. The bearings are designed to support the vertical reaction of the bridge girders and may also be designed to restrain the horizontal movements of the bridge. The bearings are usually detailed to allow the superstructure to rotate at pier locations. The superstructures and substructures of the conventional bridges are essentially designed as separate systems.

For the integral steel box-pier structure, unlike conventional bridges, a rigid integral connection between the superstructure and substructure is constructed which provides transfer of moment between the two systems, in addition to the vertical and horizontal forces typically allowed for in conventional piers. The moments, in addition to vertical and horizontal forces, are transferred from the superstructure to the substructure through the integral pier beam.

For the Highway 401 Ramp W-N/S structure, this integral steel box-pier concept was adopted to satisfy the clearance requirement over the GO Transit railway (see Figure 5). Integral connections were used on this bridge as it is crossing over railway tracks at sharp skew angles. Pier cap bottom elevation will be the same as the elevation of the bottom of the girders. This will allow orienting the pier cap in the direction perpendicular to the girders without causing the pier cap to reduce the overhead clearance of the lower railway tracks. This orientation of the pier cap eliminated the problems associated with orienting the pier cap at a sharp skew, which would be required if a conventional pier cap were used.

5 THE DESIGN

The girders will be connected to the sides of the rectangular steel box beam, which in turn will be connected to the concrete pier column (see Figure 7). The beam will have a high torsional stiffness to effectively transfer girder bending moments to the concrete pier. Pier cap are accomplished by extending the column longitudinal reinforcement through holes in the bottom flange of the pier cap into the pier cap compartment directly above the column (see Figure 6). This compartment is bounded by the four sides of

Figure 5: Vertical Clearance Diagram

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the box-beam pier cap and two interior diaphragms of the box-beam. The compartment will be filled with concrete which transfers the load from the pier cap to the column reinforcement.

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Filling the pier cap compartment directly above the column with concrete and extending the column longitudinal reinforcement into the pier cap provides adequate anchorage to the column. Since the depth of the pier cap box is 1200mm which is less than the required development length of the longitudinal columns reinforcement (size 30M rebar), the use of headed rebar with threated mechanical connector are adopted to maintain bonding strength inside the concrete filled box (see Figure 7).

Extending the column spiral

Figure 7: Integral Pier Cap Beam Section

Figure 6: Integral Pier Cap Beam, Elevation and Bottom Flange

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reinforcement into the connection region is required to ensure adequate confinement of the connection region. The required spiral reinforcement in the connection region is taken as the greater of the minimum spiral reinforcement required by the design specifications and one-half the required column spiral reinforcement next to the integral connection region (see Figure 8).

Pier columns were designed also to sustain all loads from superstructure and crash load that apply to the railway crash wall. Column design moments shall be converted into shear force acting on the shear connectors. The magnitude of the shear force is determined by dividing the column top design moment by the distance between the planes of the shear connectors assumed to transfer the moment to the pier cap (see Figure 6). Column design moments and associated shears are taken as the largest calculated forces from all applicable ultimate limit states load cases that stated in CSA S6-14.

Shear connectors are provided inside the concrete filled compartment of the pier cap (see Figure 10). These shear connectors are required to be designed to transfer the column forces to the pier cap. Shear connectors will be sufficient to transfer the maximum shear in the column and installed on both side of the pier cap bottom flange (see Figure 8). Bottom and top flanges of pier cap beam are widened outside the box to allow for placing bolts connections to connect girder top and bottom flanges with pier cap beam flanges, also girder web plates are extended inside pier cap beam to provide continuity of girder through cap beam (See Figure 6, 9 and 13). Top flange and web of integrated pier beam shall be fracture critical member as any loss of this member will cause fatal bridge failure.

6 ERECTION PROCEDURE

The integral connection will be made after the superstructure girders are erected to avoid tight tolerances on the orientation and elevation of the pier cap and to allow the superstructure girders to be connected without undue difficulty.

Figure 8: Integral Pier Cap Beam Section, Reinforcement

Figure 9: Integral Pier Cap Beam, Top Flange

Figure 10: Shear Stud Arrangement

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In order to achieve this, suggested erection procedure is provided to ensure that integrated pier beams and girders will be connected and in position (in term of vertical profile and horizontal alignment) before pouring concrete inside the pier beams with the means of temporary support of the integrated pier beams (see Figure 11). Analysis considered stability of girder during erection.

Jacking system will be placed under the pier cap beam at four locations on the temporary support tower and then connect all girders while the pier cap beam is on place. Jacking system shall be used to confirm the pier cap beam are in right location and elevation at all times during erection of girders and before pouring concrete in the pier cap compartment (see Figure 12).

The suggested erection procedure is as follow:

1. Design and construct temporary support at pier 1 and pier 2;

2. Place jacking equipment to align with cap beam web and centerline of girders – 4 minimum required at each pier;

3. Erect pier cap beams above columns; align holes in bottom plate with column dowels and seat cap beam on jacking equipment;

4. Use jacking equipment to maintain the required super elevation and road profile by maintaining the erection elevation that provided in the drawing;

5. Erect girders and connect them with pier cap beams at splice locations;6. Readjust jacking under pier cap beams if necessary to confirm that the pier cap beams are still at

right elevations;7. Fill hatched area under cap beam with 30 MPa non- shrinkable grout using form and pump

method;8. After grout reached its full strength, tie 30m headed rebar to 30m vertical dowels from column

using mechanical connector;9. Place stirrups, ties and all other reinforcements inside pier cap beam;10. Fill hatched area inside pier cap beam with 30 MPa concrete;11. After concrete reached its full-strength place cover plate over cap beam top flange and weld it all

around (see Figure 9);12. Release jacking equipment under pier cap beam and start deck form work.

Figure 12: Temporary Support Location

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7 MODELING

A 3-dimensional finite element model was considered the most appropriate for modelling the curved box-girder bridge. The analysis was performed using CSIBridge modeling software (see Figure 13). The concrete deck slab, steel webs and bottom flanges, are modelled using 3-node and 4- node shell elements incorporating both membrane (in-plane) and bending (out-of-plane) stiffness. Uncracked concrete is assumed for all slab elements. Integral pier cap beam, girder top flanges, horizontal bracing members, and cross frames are modeled using 2-node space frame elements. Crash walls and their caissons were also modeled integrated with the pier columns to account for their stiffness that added to the column.

A stage construction analysis load case was performed to represent the behavior of the bare steel girder during erection and before composite action as shown in Figure 14. It is assumed that the bridge remains non-composite throughout slab casting, such that the construction dead and live loads are applied on the bare steel girders (see Figures 15 and 16). Although parts of the girders may become composite in sequential stages of the slab casting, the degree

Figure 13: Three-Dimensional Finite Element Model

Figure 13: Stage Construction, Bare Girder

Figure 16: Slab Casting Sequence 2Figure 15: Slab Casting Sequence 1

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of composite action is time-dependent.

A second model was constructed to incorporate the concrete slab elements to represent composite behavior under long-term loading, based on a steel-concrete modular ratio of 3n, for the application of the superimposed dead load. By substituting a concrete modulus of elasticity of 24 800 MPa (modular ratio = 1n), a third model is obtained to represent composite behavior under short-term loading, for the application of traffic live loads. Although the horizontally curved geometry was represented, the super elevation of the bridge was incorporated in the model.

Vertically, the structural discretization features a single rectangular element over the depth of the web. The shell element used in the model can represent the in-plane stiffness of girder webs using only a single element over the depth of the section. Longitudinally, each span contains element subdivisions, which means that stresses and forces are computed at the cross-frame locations. This longitudinal subdivision was selected to model lateral bending of the top flanges (before composite action) with sufficient accuracy.

For simplicity, the elements representing the steel top flanges and horizontal bracing are all located in the plane defined by the middle surface of the slab. When calculating stresses in the composite member. The concrete barriers are not incorporated into the model; their contribution to the strength and stiffness of the girders is assumed negligible.

8 CONCLUSION

Integral pier connections are most likely to be used for bridges with high skew angles and are supported on single column piers. The use of the integral pier connections results in lower elevation of the bridge deck and the approaches without reducing the bridge under clearance. The cost of bridges with integral connections is expected to exceed conventional bridges; however, because of the lower approach elevation, the combined cost of the bridge and the approaches is expected to be lower than that of a conventional bridge.

Reference

Wagdy G. Wassef and Dustin Davis, Modjeski and Masters, Inc., Harrisburg, PA. National Cooperative Highway Research Program, NCHRP Report 527.

Sri Sritharan, Justin R. Vander Werff, Robert E. Abendroth, Juli Redmond, Lowell F. Greimann, Iowa State University, Ames, IA. 2004. National Cooperative Highway Research Program, NCHRP Report 527.

Canadian Institute of Steel Construction, CISC, Steel Bridges Design, Fabrication, Construction – Course Design Examples (CSA S6-14).

Figure 14: Model Component Description