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10 th International Conference on Short and Medium Span Bridges Quebec City, Quebec, Canada, July 31 – August 3, 2018 DESIGN AND CONSTRUCTION OF THE BOYCHUK INTERCHANGE Dominic Daviau-Desnoyers, P.Eng, Ph.D. 1 and Munzer Hassan 1,2 , P.Eng, Ph.D. 1 CIMA+, Canada 2 [email protected] Abstract: The Boychuk grade separated interchange in Saskatoon, Saskatchewan provides a critical connection across a high demand intersection within the city. The aim of the project is to address the challenges the City, its citizens and businesses are facing with respect to traffic congestion, economic productivity and quality of life. The project includes the design and construction of a 4-lane bridge made of adjacent box girders with a cast-in- place structural slab in an area of weak and aggressive clay soil. Precast prestressed concrete adjacent box girders are widely used in short and medium span bridges. The rapid construction, the limited impact on traffic, the vertical clearance and the relatively high span-to-depth ratio are among the main advantages of this structural system. This paper will describe the design and technical challenges that were faced throughout the project. The key design and construction challenges include: the design of adjacent box girders using current Canadian design standards, the design of the longitudinal shear keys and the construction sequence of the superstructure with respect to durability, the foundation design challenges such as the poor and aggressive nature of the clay soil and the superstructure/substructure/soil interaction, the girders’ differential camber due to the construction staging and the lateral post-tension sequence. 1 INTRODUCTION The Boychuk grade separated interchange provides a critical connection at the high demand intersection of Boychuk Drive and Highway 16 in Saskatoon, Saskatchewan. The aim of the project is to address the challenges the City, its citizens and businesses are facing with respect to traffic congestion, 58-1

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The buildingSMART Canada BIM Strategy

10th International Conference on Short and Medium Span Bridges

Quebec City, Quebec, Canada,

July 31 – August 3, 2018

DESIGN AND CONSTRUCTION OF THE BOYCHUK INTERCHANGE

Dominic Daviau-Desnoyers, P.Eng, Ph.D.1 and Munzer Hassan1,2, P.Eng, Ph.D.

1 CIMA+, Canada

2 [email protected]

Abstract: The Boychuk grade separated interchange in Saskatoon, Saskatchewan provides a critical connection across a high demand intersection within the city. The aim of the project is to address the challenges the City, its citizens and businesses are facing with respect to traffic congestion, economic productivity and quality of life. The project includes the design and construction of a 4-lane bridge made of adjacent box girders with a cast-in-place structural slab in an area of weak and aggressive clay soil. Precast prestressed concrete adjacent box girders are widely used in short and medium span bridges. The rapid construction, the limited impact on traffic, the vertical clearance and the relatively high span-to-depth ratio are among the main advantages of this structural system. This paper will describe the design and technical challenges that were faced throughout the project. The key design and construction challenges include: the design of adjacent box girders using current Canadian design standards, the design of the longitudinal shear keys and the construction sequence of the superstructure with respect to durability, the foundation design challenges such as the poor and aggressive nature of the clay soil and the superstructure/substructure/soil interaction, the girders’ differential camber due to the construction staging and the lateral post-tension sequence.

1. INTRODUCTION

The Boychuk grade separated interchange provides a critical connection at the high demand intersection of Boychuk Drive and Highway 16 in Saskatoon, Saskatchewan. The aim of the project is to address the challenges the City, its citizens and businesses are facing with respect to traffic congestion, economic productivity and quality of life. The project includes the design and construction of a 4-lane bridge made of continuous adjacent box girders with a cast-in-place structural deck slab. The 65.4 m long and 23.1 m wide superstructure is supported on semi-integral abutments and a middle pier composed of five mechanically cleaned belled piles per foundation unit, which all are at a 9° skew angle. As the elevation of the bedrock at the site location is approximately at 426 m above sea level (or 84 m deep), deep foundations were designed to withstand the loads within the weak and aggressive clay and till soil layers. Figures 1 and 2 present the typical elevation and cross-section of the bridge respectively.

This paper will describe the design and technical challenges that were faced throughout the project. The key design and construction challenges include:

· the design of adjacent box girders using current Canadian design standards;

· the design of the longitudinal shear keys and the construction sequence of the superstructure with respect to durability;

· the foundation design challenges such as the poor and aggressive nature of the clay soil and the superstructure/substructure/soil interaction;

· the girders’ differential camber due to the construction staging;

· the lateral post-tension sequence.

Figure 1: Boychuk bridge elevation

Figure 2: Boychuk bridge cross-section

1 DESIGN CHALLENGES1.1 Longitudinal Shear Keys and Lateral Post-Tension Sequence

The main design challenge includes designing a durable connection between the beams. In the United States of America, older bridges made with shear connected concrete girders were built with grouted shear keys, with some lateral post-tension in some states, and generally with a 50 mm thick non-structural wearing surface (El-Remaily et al. 1996). Failure of the shear keys and the reflective cracking in the overlay over the keys in system that rely entirely on shear keys to transfer the loads are among the most common issues surveyed over the years. These issues are mostly prevalent in bridges without a thick composite structural deck slab or without sufficient lateral post-tension. Per AASHTO LRFD Bridge Design Specifications (AASHTO 2016) definition, precast longitudinal components joined together transversely by a shear key only are defined as shear joints.

To design a durable connection, both shear and bending must be transferred at the joint between the girders to control translational and rotational deformations, therefore ensuring that the structural system acts as a single unit, with the intent to provide full continuity and monolithic behaviour of the deck. Design guidelines such as the Ministry of Ontario structural manual and the Alberta Transportation bridge structure design criteria require that adjacent box beam bridges be provided with a minimum structural slab thickness of 150 mm and 225 mm respectively. Precast components joined together by sufficient transverse post-tensioning, cast-in-place closure joints, a structural overlay, or a combination thereof are defined as shear-flexure joints in AASHTO.

In this project, shear-flexure joints have been designed with a combination of grouted shear keys and a 225 mm structural slab. Shear keys surfaces have been sand-blasted to remove laitance and enhance bond characteristics. In addition, lateral post-tension was used at abutments, intermediate diaphragms and at the pier. The lateral post-tension was divided into two steps: initial and final. The initial post-tension was done prior to grouting the shear keys with the intent of repositioning the girders in their theoretical position as girders were erected along the cross slope of the road. The longitudinal shear keys were then grouted, and the final post-tension was applied to induce a compressive stress along the shear keys. However, the amount of lateral post-tension is not sufficient to satisfy the AASHTO’s shear-flexure joint definition. The requirements for a shear-flexure joint are therefore satisfied only by the 225 mm thick structural slab.

Per the project agreement, girders had to be designed with the most stringent live load distribution factor between the simplified method and refined models. In refined modeling, decks with shear-flexure joints should be modeled as continuous plates whereas shear joints should be modeled with hinges. A grillage model was built to simulate the deck behaviour. To be conservative, transverse elements were modeled with two rigid links hinged at the mid-point, which corresponds to the shear keys location. This modeling approach leads to a larger live load distribution factor in comparison to a deck modeled with continuous plates. This approach is also consistent with the old Ontario Highway Bridge Design Code (OHBDC 1991) in which the transfer of loads from one beam to another was assumed to take place mainly through the shear key and the transverse flexural rigidity was assumed to be equal to zero. Using this approach, the results have shown that the live load distribution factors obtained using refined methods are smaller than for the simplified method. The simplified method is therefore conservative as it leads to larger loads due to the truck loads.

1.2 Adjacent box girders – Transverse live load distribution

First introduced in the 1950’s, precast prestressed concrete adjacent box girders are widely used in short and medium span bridges in North America. The rapid construction, the limited impact on traffic, the vertical clearance and the relatively high span-to-depth ratio are among the main advantages of this structural system. The high span-to-depth ratio results from the fact that the material near the centre of gravity contributes very little for flexure and hence can be removed. This leads to a box girder, in which the reinforcement in the top and bottom flanges provide capacity for compressive and tensile forces respectively, hence forming a couple to resist flexure. The capacity to resist shear is provided by the vertical walls. Box girders are fabricated with void material using preformed cellular polystyrene, which are watertight and resistant to breakage and deformation during concrete placement. Because the void is made of polystyrene, virtually any void size or shape can be accommodated to meet project-specific needs such as concrete covers.

When placed adjacent to each other, beams act individually as there is no continuity enabling them to work as a single structure. In contrast, creating a connection between the beams with shears keys, lateral post-tension and/or a structural slab results in better transverse load distribution as the structure acts as a single unit instead of individual beams. Per the Canadian highway bridge design code (CHBDC), the structural response of concrete shear connected beams shall be considered as for slab-on-girder bridges when continuity is provided at the top slab only and as voided-slab bridges when continuity is provided at top and bottom flanges (CAN/CSA S6-14 Clause 5.5.5 (a)). Following the deck girder analogy, the transverse load effects shall be assumed to be taken by the deck. Using the simplified method of analysis for longitudinal load effects, the equations to calculate the load distribution factor for the moment of slab-on-girder type bridges and voided-slab bridges would lead to the upper and lower boundary values respectively. Although the live load analysis of shear-connected girder bridges may be calculated for box girders without continuity of transverse flexural rigidity at top and bottom (CAN/CSA S6-14 Clause 5.6.9), box girders longitudinally connected and having a fully composite structural slab are not explicitly addressed in the current edition of the code. Table 1 presents a summary of the equations used to calculate the live load distribution factors for moment and shear as per CAN/CSA S6-14 Table 5.14). The value of β, required to calculate DT is calculated as follows:

[1]

Table 1: DT and λ for concrete shear connected box girders (CAN/CSA S6-14 Table 5.14)

Condition

Load effect

n

DT

λ

FT

ULS and SLS1

Moment

≥ 4

where

ULS and SLS1

Shear

All

4.0

0.0

AASHTO LRFD Bridge Design Specifications 2016 Table 4.6.2.2.1-1 presents the most common deck superstructures and their typical cross-sections. Figure 3 presents an extract of the aforementioned table for a typical cross-section made of cellular concrete boxes with shear keys and a cast-in-place concrete overlay. Table 2 presents a summary of the equations used to calculate the live load distribution factors for moment and shear as per AASHTO LRFD Bridge Design Specifications 2016. For consistency, U.S. customary units were converted into metric units.

Figure 3: Typical cross-section

Table 2: AASHTO Live Load Distribution Factor for Moment and Shear

Condition

Load effect

One Design Lane Loaded

Two or More Design Lanes Loaded

ULS and SLS1

Moment – Interior girder

ULS and SLS1

Moment – Exterior girder

ULS and SLS1

Shear – Interior girder

ULS and SLS1

Shear – Exterior girder

Tables 3 and 4 present a summary of the live load distribution factors for the interior girder moment and shear respectively. The results presented in Table 3 show that the live load distribution factor for the interior girder moment calculated either with the simplified method for concrete shear connected girders provided in CHBDC and AASHTO are within the upper and lower boundaries corresponding to the slab-on-girder and voided slab respectively. The results presented in Table 3 also show the live load distribution factor for the moment in concrete shear connected box interior girders is conservative in comparison with AASHTO, which can be explained by the fact that the CHBDC does not consider shear-flexure continuity at the top of the section.

The results presented in Table 4 show that the live load distribution factor for shear in concrete shear connected box interior girders calculated using the CHBDC simplified method is within the higher and lower boundaries, which corresponds to the slab-on-girder and voided slab respectively. The results also show that AASHTO’s simplified method for shear-flexure connection leads to higher live load distribution factor for shear, therefore being more conservative.

Table 3: Live Load Distribution Factors for Interior Girder Moment

Interior Girder Moment (M)

Long Span (M+)

At Pier(M-)

Short Span (M+)

S6-14 – Voided slab

0.187

0.195

0.190

S6-14 – Slab-on-girder

0.308

0.329

0.317

S6-14 – Shear connected

0.255

0.297

0.270

AASHTO – Shear-flexure

0.245

0.250

0.257

Table 4: Live Load Distribution Factors for Interior Girder Shear

Interior Girder Shear (V)

Long Span(Abutment)

At Pier

Short Span(Abutment)

S6-14 – Voided slab

0.286

0.286

0.286

S6-14 – Slab-on-girder

0.403

0.403

0.403

S6-14 – Shear connected

0.302

0.302

0.302

AASHTO – Shear-flexure

0.433

0.438

0.443

To account for the 9° skew angle of the structure, the live load distribution factors for shear calculated with the simplified methods were amplified by the skew factor. The skew factor calculated according to the simplified method were 1.07 per the CHBDC and varied between 1.11 and 1.14 per the AASHTO for the long and short span respectively.

1.3 Abutment Design1.3.1 Deep Foundations

Boreholes drilled on-site have shown that the subsurface conditions consist of variable fills, clays, silts and sands in the upper 3 meter overlying silty clay, silt to firm clay till, fine to medium grained sand and very stiff to hard clay till. Two foundation options were considered in the design phase: cast-in-place concrete belled piles end bearing on very stiff clay till and steel piles driven into very stiff to hard clay till. Several soil samples were tested to measure soil properties such as resistivity, pH, chlorides and sulphates. The results were used to determine the degree of exposure for concrete subjected to sulphate attack and deterioration potential of steel piles. Due to the high potential for soil corrosion, the application of protective coatings though the fill soils or increasing the cross-sectional area of the steel piles were not sufficient to guaranty a 75-year service life. The poor soil capacity and the high potential for soil corrosion made impossible the design of fully integral abutments.

Although the conditions for drilling the belled piles are not ideal due to the sand layer, belled piles were judged as the best foundation option. Temporary steel casings were required through the depth of the sand layer to prevent the shaft walls from collapsing. The concrete mix design was also altered to meet the requirements of CAN/CSA A23.1-14 for concrete with severe sulphate exposure. In addition to using sulfate resistant cement, the concrete mix was designed to have a minimum specified 56 day compressive strength of 32 MPa and a maximum water to cement ratio of 0.45. Finally, the concrete clear cover was increased from 75 mm to 100 mm.

As mentioned above, the design of fully integral abutments was not possible. Semi-integral abutments were therefore designed to withstand the bridge loads and earth fills. In semi-integral abutments, expansion joints are eliminated at the end of the deck. However, the superstructure is not continuous with the abutments as conventional bearings are used to allow horizontal movements between the deck and the abutments. A control joint is provided at the end of the approach slab to allow horizontal movements such as thermic movements. Figure 4 shows a typical elevation of the bridge with semi-integral abutments each made of an abutment seat supported by five 1050 mm in diameter concrete cast-in-place caissons with 3050 mm in diameter bells.

Figure 4: Semi-integral abutment typical elevation view

1.3.2 Mechanically Stabilised Earth Walls

Several issues were raised during the geotechnical analysis of the global stability of the embankments. The analysis suggested that the mechanically stabilised earth (MSE) wall would not meet the performance requirements without any additional soil stabilization. As the bearing capacity of the natural soil was too weak to support the gravity loads of the embankment fill and the overturning loads generated by the earth pressure, stone columns were constructed to improve the slope stability and the load bearing capacity, reduce settlement magnitude and increase settlement rates. A bridging layer consisting of a high strength geogrid and a 1 meter thick layer of granular fill was constructed over the stone columns.

As shown in Figure 4, the MSE wall leveling pad was placed directly above the bridging layer. In addition to resisting the horizontal loads generated by the soil pressure, MSE walls were designed to resist the horizontal loads induced by the bridge deck. In fact, the horizontal loads resulting from the horizontal movements of the bridge deck generated an additional horizontal pressure near the top of the MSE walls. Inputs such as the lateral stiffness of the bearings were required from the manufacturer to evaluate the horizontal loads transferred from the deck to the abutment seat and subsequently the piles.

To calculate those loads, a frame model was built to simulate the soil-structure interaction, in which the soil was modelled with linear springs. Figure 5 presents the additional loads that were considered in the model and for the MSE wall design. The results have shown that additional reinforcing strips anchored to the top panels were required to resist the horizontal loads from the deck.

Figure 5: Additional loads on MSE walls

2 CONSTRUCTION CHALLENGES2.1 Box Girder Differential Camber

Due to the eccentric nature of prestressing, prestressed concrete girders typically have an initial upward camber after releasing the strands (at transfer). The camber may either increase or decrease with time depending on the stress distribution across the member under sustained loads. Creep and shrinkage effects shall be included in the calculations of the camber, in addition to the staging loads such as the dead load of the slab and the superimposed dead loads applied on the composite section (barrier, median, wearing surface).

Beams are generally built sequentially at the precast yard. Therefore, several days will pass between the casting of the girders. The loading history will thus differ from one girder to another. As creep and shrinkage are time-dependent effects, the construction stages need to be estimated in the design phase to verify that the long-term camber will be positive (upward) and that a negative haunch will not occur at any point. In addition, post-tensioning tendons will need to be placed within ducts (corrugated steel pipes) as shown in Figure 6. The ducts will therefore need to be almost perfectly aligned both horizontally and vertically, the later being more complicated to control. With the rather complex nature of time-dependent effects and the number of assumptions needed to evaluate the camber, differential camber between beams may occur. Ducts were oversized to facilitate placing the tendons through the ducts from one side of the bridge to the other. This mitigation measure only allows a tolerance of about 10 mm in differential camber. In the event of larger differential camber, girders can be pre-loaded at the manufacturer’s yard with concrete blocks equivalent to the load of the tributary area of the slab until transport and erection. Monitoring of the camber was therefore planned as a preventing measure.

Figure 6: Lateral post-tension duct sealing

2.2 Box Girder Erection and Lateral Post-Tension

Although the design of adjacent girders is relatively simple, the erection procedure is rather complex. Transverse movement of the bearing and construction tolerances may lead to construction issues such as different design (theoretical) and as-built girder positions.

During the erection of the girders, girders were unloaded on their respective pair of bearings, which were placed along the cross-slope of the bridge deck. As the girders are not perfectly vertical but rather positioned perpendicularly to the cross slope, bearings are expected to deform transversely upon girder unloading due the transverse component of the girder’s dead load. Hence, a small transverse movement occurs at each girder resulting in the girder shifting from the theoretical position.

Due to unbalanced prestressing loads, girders may bend out-of-plane upon strand release. Experience on previous projects has shown that the out-of-plane deflection can be as large as 10 mm. In this project, girders were positioned intentionally 10 mm apart from each other to allow for construction tolerance.

As mentioned in Section 2.2, the lateral post-tension sequence was divided into two steps: initial and final. The intent of the initial lateral post-tension was to reposition the girders to their theoretical position, if needed. Therefore, the initial prestressing load corresponding to the transverse component of the girder’s dead load was calculated and applied in the initial post-tension stage. Once in their theoretical position, clear gaps may still be present between some girders. Therefore, excessive transverse movement of the outer bearings could occur during the final post-tensioning stage if the girders are perfectly straight. To mitigate this risk, the top and bottom flanges of the girders were shimmed with hard wood forms. Once the shear keys were sealed and grouted, the final post-tension was applied, and the ducts were grouted. The hard wood forms were then removed where possible. Per design, half of the tendons were prestressed from the east side of the bridge whereas the other half were prestressed from the west side. This procedure was elaborated to equilibrate the prestressing losses due to the cross-slope of the bridge.

2.3 Double Casing for Belled Piles at Pier

When excavating though soft, weak or unstable ground such as a sand layer, a means of temporary stabilisation of the soils is needed. Steel casings are often used for borehole support. Where substantial lengths of casing are needed, or where the depth requiring casing varies significantly in any one area, the use of double walled temporary segmental casings provides an economical solution. In this project, 16 meter long casings were required to support the boreholes as the sand layer was found at depths of about 12 meter to 15 meter. One of the problem that might arise from driving or drilling casings into the ground is the risk of not being able to remove it as skin friction increases with length. To prevent such problem, the contractor proposed using double walled temporary segmental casings in the top 5 meter of the pier piles. In this way, 5 meter long and 1800 mm in diameter temporary casings where driven into the ground prior to drilling the pile shaft casings (Figure 7). This mitigation technique allows the pier pile shafts and the enlarged area (bell) and the reinforcement details to remain as per design with the exception that additional concrete cover will be placed within the first 5 meter to fill up the larger temporary casing area.

Figure 7: Double casing elevation view at pier

A numerical model (3D frame model with soil-structure interaction) was used to verify the impact of the larger pier pile diameter in the first 5 meter below the original ground. The results have shown that the behaviour of the bridge does not change significantly. In fact, the soil stiffness is generally neglected within the frost depth and is small for clay soils.

3 CONCLUSIONS

This paper presented the design and technical challenges that were faced throughout the design and construction of the Boychuk interchange. Precast prestressed concrete adjacent box girders joined transversely by shear-flexure keys were selected to minimize the depth of the deck and for the limited impact on traffic. Although their design is simple, proper planning must be done to bridge the gap between the theoretical design and constructability.

As the live load distribution factor is critical in the design of bridges, this paper presented a comparison of the simplified methods presented in the latest edition of the CHBDC and AASHTO for the live load distribution factors of precast longitudinal components joined together transversely by shear-flexure keys. The results show that the live load distribution factors are within the lower and upper boundaries defined by slab-on-girder type bridges and voided-slab bridges.

This paper also presented solutions to some of the challenges that were faced during the project such as the construction of a durable connection between the girders by longitudinal shear keys, the post-tension sequence, the differential camber between the girders and the erection procedure. Moreover, the design requirements for MSE walls and the soil improvement method that was used to increase the soil’s load bearing capacity and the global stability of the embankment were briefly presented. Finally, a mitigation technique was presented to allow using temporary casings to support the unstable ground around the boreholes of the deep foundations. All solutions resulted from fruitful exchanges between the engineers of record and the contractor, hence bridging the gap between design and constructability.

Acknowledgment

The authors gratefully acknowledge and thank the City of Saskatoon and PCL Construction for giving us the opportunity to publish and present this paper. The authors would also like to acknowledge the design teams at CIMA+, TREK Geotechnical, Catterall and Wright and Pinchin for their contribution in providing the engineering support and project experience. Proactive/lean planning and communication of all parties contributed greatly to the success and delivery of this project.

References

AASHTO. 2016. AASHTO-LRFD Bridge Design Specifications. Third Edition, American Association of State Highway and Transportation Officials, Washington, DC.

CSA. 2014. Concrete materials and methods of concrete construction / Test methods and standard practices for concrete, CAN/CSA-A23.1/2-14, Canadian Standard Association, Toronto, Ontario, Canada.

CSA. 2014. Canadian Highway Bridge Design Code, CAN/CSA-S6-14. Canadian Standard Association, Toronto, Ontario, Canada.

CSA. 2014. Commentary on CSA S6-14, CAN/CSA-S6.1-14. Canadian Standard Association, Toronto, Ontario, Canada.

El-Remaily, A., Tadros, M.K., Yamane, T. and Krause, G. 1996. Transverse Design of Adjacent Precast Prestressed Concrete Box Girder Bridges, July-August, 96-113.

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