Executive Summary "Les Ponts Jacques Cartier et Champlain Incorporée" (PJCCI) requested that Buckland & Taylor (B&T) study the overall condition of the approach span edge girders of the Champlain Bridge to understand how the corrosion that is evident in the edge girders may affect the overall behaviour of the bridge and how various strengthening techniques installed on the girders can benefit their capacity. Documentation was provided by PJCCI to B&T which described the condition of the edge girders; B&T did not perform on-site detailed inspection to verify this data. Following our preliminary assessment of the girder demands in comparison to their capacity, concerns were brought to the attention of PJCCI. Based on these concerns, six critical girders were recommended to be strengthened, with the strengthening to be completed before the end of September 2013. In order to allow traffic to continue to use the bridge while the repairs are being implemented, the six girders must be inspected two times per week for any signs of distress. At the time of writing this report, this strengthening is underway, and the twice per week inspections have not identified any increased structural distress in the six girders. In order to ensure that the safety of bridge users is not compromised, many factors were taken into account during our analysis of the edge girders. This report details how we verified that the bridge, in its current condition, provides an acceptable level of safety for bridge users, consistent with the requirements of the Canadian Highway Bridge Design Code. Based on our final review of the demands, existing strengthening, capacities and observed deterioration on the girders, mitigation measures are presented in this report to address the risks and maintain a consistent level of safety in the coming years. These measures include strengthening of the girders by fiber-reinforced plastic, queen-posts, and/or modular trusses. Supplementary to the girder strengthening, additional recommendations to address the risk environment include annual detailed inspections, and continued detailed structural assessments of all the edge girders, centre girders and pier caps. Additional strengthening requirements may result from the continued inspections and assessment activities. PJCCI s dedication to inspection, maintenance and strengthening programs developed in their long-term planning initiatives has provided significant durability benefits for the structure to date and will continue to define the path forward in safely managing the ongoing deterioration of the bridge. Due to the nature of the deterioration, we believe it is critical to complete the strengthening work in a short timeframe; this places an immediate need to begin the inspection and assessment work as soon as reasonably possible while still maintaining a strategic and planned approach to risk mitigation. A preliminary budget estimate of $400 - $500M is proposed to address all the future work to be done in the next 5 years for inspection and rehabilitation. Champlain Bridge Approach Spans
Table of Contents 1 Introduction... 1 2 Description of Structure... 3 2.1 Structural Components... 4 2.1.1 Main Girders... 4 2.1.2 Diaphragms... 6 2.1.3 Deck Slab... 6 2.2 Early Deep Post-Tensioned Girder Design... 6 2.3 Design Impacts... 7 3 Observed Condition of the Girders... 8 4 Engineering Assessment of Current Condition...14 4.1 Assessment of Bending Capacity...14 4.2 Assessment of Shear Capacity...15 4.2.1 Spacing of Transverse Reinforcement...15 4.2.2 Determination of Effective Internal Post-Tensioning Tendons...16 4.3 Remedies Installed to Date for Shear Deficiencies...17 4.3.1 Installation of Fiber-Reinforced Plastic (FRP)...17 4.3.2 Installation of Queen-Posts...18 5 How Consistent Safety is Maintained for Bridge Users...20 5.1 How the Level of Safety is Determined...20 5.1.1 Demands...20 5.1.2 Capacity...21 5.1.3 Risk Factors...21 5.2 Current Level of Safety...23 5.3 Maintaining an Acceptable Level of Safety...24 6 Summary of Evaluation...26 7 Rehabilitation Measures Required to Manage Risk...28 7.1 Immediate Actions Required - Emergency Strengthening Measures...28 7.2 Short-Term Actions Required (Immediately to end of 2014)...29 7.3 Five-Year Action Plan (2014-2018)...32 7.4 Budget Estimate...33 8 Conclusions...34 Champlain Bridge Approach Spans i
1 Introduction This report investigates the structural capacity of the post-tensioned concrete girders of the Champlain Bridge approach spans, accounting for the deterioration identified in previous inspection reports, known repair and strengthening projects, and recent exploratory openings performed by Les Ponts Jacques Cartier et Champlain Incorporée (PJCCI). Buckland & Taylor (B&T) did not perform on-site inspection or detailed review of components other than the approach span edge girders, adjoining diaphragms and transverse deck post-tensioning. While B&T was on site for the opening of 19 exterior girder soffits as part of our Exploratory Openings Mandate, this work was primarily performed at night and observations were localized to the condition of the post-tensioning strands being uncovered. Detailed structural analysis and conclusions were developed based on the documents provided by PJCCI representing the condition of the structure. Throughout this report, critical girders are identified based on demand-to-capacity ratios for shear, flexure and tension in the reinforcement. Presence of severe corrosion is identified through broken strands, reflective cracking and significant changes in the observed girder conditions. Additionally, the presence/absence of external strengthening (such as queen-posts) is accounted for within the analysis. Recommendations regarding strengthening options are then presented for immediate and short-term actions. Recommendations for a 5-year plan are also presented. This report is organized in the following sections: Section 2 gives a brief description of the layout of the approach spans and explains how the original allowable stress design approach to shear and bending differs from the strut and tie and limit state design methods used today; Section 3 summarizes the observed condition of the edge girders; Section 4 presents an engineering assessment of the edge girders based on their current condition, including existing strengthening such as external longitudinal post-tensioning, queen-posts and also new remedies now being utilized such as fibre reinforced plastics (FRP); Champlain Bridge Approach Spans 1
Section 5 discusses evaluation of the reliability index for the bridge and identifies the inspection, monitoring, strengthening and strategic planning activities necessary to maintain at all times a reliability index value which is consistent with the intent of the Canadian Highway Bridge Design Code (S6 Code); Section 6 provides a summary of our evaluation results; and Section 7 establishes a prioritization sequence for rehabilitation measures in order to systematically manage the risks to the structure as the severity of the various structural defects increase with time. 2 Champlain Bridge Approach Spans
2 Description of Structure This report studies the behaviour of the approach spans in Sections 5 and 7 of the Champlain Bridge. The spans in Section 5 are analyzed since their span is the longest (53.475 m) which results in the highest demands in the different components of the bridge. This is a slightly conservative approach; however, the shortest span in these sections is only 51.410 m so the difference is insignificant. A general view of the bridge is shown in Figure 1. Figure 1: General view of Champlain Bridge Section 5 and 7 Each span of the approaches is a simply supported system and has a cross-section of seven precast post-tensioned (PT) girders. The approaches accommodate six traffic lanes. The deck slab between the top flanges of the girders at deck level is made up of cast-in-place infill strips. Two diaphragms are present at the bearing centrelines and two intermediate diaphragms are located 17.984 m from either bearing centreline. The deck is post-tensioned in the transverse direction in the slab and the diaphragms. The top flanges of the girders together with the cast-in-place infill strips constitute the deck over which an asphalt riding surface is installed. This results in a structure that is highly integrated in both the longitudinal and transverse directions. The deck cross-section is shown in Figure 2. Champlain Bridge Approach Spans 3
Figure 2: Layout of a Typical Approach Span 2.1 Structural Components 2.1.1 Main Girders As shown in Figure 3, each girder has 24 inclined post-tension tendons anchored either at the ends or the top flange of the beam. Each tendon is made of 12 strands of 7 mm (0.276") in diameter. The transverse reinforcement of the girders consists of alternating #4 and #5 stirrups spaced at 800 mm (31.5") in the span and #5 stirrups spaced at 267 mm (10.5") near the supports. Considering the light amount of mild reinforcement in the girders, the tendons contribute significantly to the shear and bending capacity. The nominal material properties were considered as follows: Concrete compressive strength, f' c = 34.5 MPa Mild steel reinforcement yield strength, f y = 275 MPa PT tendons ultimate strength, f pu = 1,627 MPa The compressive strength was determined, as outlined by the S6 Code, and by the information provided on the as-built drawings. Data from cores previously taken by PJCCI has demonstrated that, in general, the concrete strength of the girders is higher than assumed. However, in consideration of the deterioration observed and 4 Champlain Bridge Approach Spans
since concrete compressive strength has limited influence on the shear capacity of a girder (which was determined to be the governing load condition), the as-designed compressive strength was used for the analysis. Edge Girder 3.72 m Infill Strip 3.07 m Figure 3: Girder Longitudinal Post-Tensioning and Deck Section The cross-section of a girder at mid-span is shown in Figure 4. A detail of the tendon layout at a cross-section at mid-span is also shown in Figure 4. Tendons 1 to 14 that are anchored at the face of the girder are on the exterior of the bottom flange and tendons 15 to 24 anchored at the top are located in the middle part of the bottom flange. PT Tendon Figure 4: Precast Girder Section Geometry PT Tendon Layout at Midspan Champlain Bridge Approach Spans 5
2.1.2 Diaphragms There are four diaphragms in each span: two end diaphragms and two intermediate diaphragms. Each post-tensioning tendon in the diaphragms consists of 12-7 mm (0.276") diameter strands. The end diaphragms are 1220 mm deep, 203 mm wide and contain two PT tendons; one in the deck slab and one at the bottom of the diaphragm. The intermediate diaphragms are 2840 mm deep, 203 mm wide. The diaphragm post-tensioning consists of six 12 strands tendons. 2.1.3 Deck Slab The deck slab between the top flanges of the girders at deck level is composed of cast-in-place infill strips. The slab and the girders are connected using transverse post-tensioning. The typical slab thickness is 216 mm. The transverse posttensioning tendons are 12-7 mm (0.276") diameter strands spaced at 1016 mm. 2.2 Early Deep Post-Tensioned Girder Design At the time when the Champlain Bridge was designed, the Codes were still using the Allowable Stress Design (ASD) approach, as opposed to the Load and Resistance Factor Design (LRFD) method used today. The understanding of shear behaviour, in particular, was not as developed as it is today. The original calculation notes from H.H.L Pratley (dated 1960) show that the shear design was based on the calculation of principal stresses in the web under service loads. Although the effects of the post-tensioning tendons were considered, the designer concluded that "principal stresses are acceptable with no reinforcement." Therefore, only the minimum amount of transverse shear reinforcement was specified in the girders (alternating #4 and #5 bars at 800 mm spacing). The design codes at the time did not account for durability, modes of failure or shear design of deep girders. All these considerations have since been included in the modern-day design codes; for example the use of factored loads and capacities, the limitations on the spacing of the stirrups, the minimum amount of shear reinforcement and the strut-and-tie methods developed for the calculation of the shear capacity in deep girders. Therefore, there are significant differences in how a prestressed girder would be designed now as opposed to the original design of the Champlain Bridge. 6 Champlain Bridge Approach Spans
2.3 Design Impacts In managing the risk to the public, the original design of the Champlain Bridge plays a role in understanding its vulnerability to deterioration. It is through understanding these vulnerabilities that we can appropriately assign factors to relate the inherent risk in the structure to a level considered acceptable for continued use of the structure for the public. The deck system in its current condition is considered vulnerable to deterioration and potential failure for the following reasons: Reduced potential for load redistribution in the case of a girder failure; Possible water infiltration from the top of the deck into the inclined PT tendons, with water collecting at the low point near mid-span; Poor water drainage system allowing de-icing chemicals to deteriorate the edge girders for the first 30 years of the bridge life (until drains were added to the bridge in the 1990s); Low amount of longitudinal non-prestressed reinforcement in the girders, making them rely almost entirely on PT tendons for flexural capacity; Large spacing of the vertical stirrups (800 mm) in the girders, making them rely more on the concrete and PT tendons for shear capacity than is typically found in new design today; and Generally unknown precise condition of the internal PT tendons except at a few locations where exploratory openings have been performed. Instrumentation installed on the Champlain Bridge girders will provide details regarding their changing conditions over time, particularly for flexural failures. In the case of a shear failure, the instrumentation response time may limit its effectiveness. However, potential developments of structural cracking and the associated load redistribution will be captured and recorded to become part of the overall monitoring program to continue assessing the structure s condition. While the current risk environment is considered acceptable for continued use of the structure, it is dependent on a continued, up-to-date understanding of the structural condition (known through detailed inspections), on-going strengthening programs to address the girders with higher levels of deterioration, and a continued commitment to firm, strategic planning to ensure adequate strengthening is implemented to address the risk of further deterioration. Champlain Bridge Approach Spans 7
3 Observed Condition of the Girders The deterioration of girders, especially edge girders, has been documented in several inspection reports performed on behalf of PJCCI by various consulting engineers over the years. The critical component affecting the structural capacity of the girders is the longitudinal post-tensioning, which has to be effective and reliable in order for the structure to function as intended. Corrosion loss of the PT tendons directly impacts the girder strength. The first signs of corrosion showed up in the mid 1980's and, over the last 30 years increasing signs of corrosion have been observed on the edge girders. As shown in Figure 5 to Figure 8, corrosion signs are visible for inclined tendons in the web and for bottom tendons near the mid-span region. Strands loss and severe corrosion was also observed through exploratory openings located near mid-span. In most cases, it is difficult to assess the degree of corrosion because only localized openings or surface observations are possible. Therefore, there is a general uncertainty about the actual loss of tendon section and as a result, the shear and bending capacity of the girders. This progressive loss of reliability means that we have had to apply conservative assumptions concerning the condition of the PT strands in order to retain confidence in the structural integrity of the girders. Six edge girders in particular show signs of advanced deterioration and strand loss, but have not yet received any strengthening. These six critical girders require immediate action (at the time of this report, strengthening was underway): 1) 6W-7W P1 4) 27W-28W P7 2) 8E-9E P7 5) 32W-33W P7 3) 26W-27W P7 6) 42W-43W P1 Some corrosion of the vertical mild reinforcing has been noted around the regions of moisture at the girder soffits. However, there are no indications of reflective cracking in line with the vertical steel, nor do the most current inspection reports record any significant issues on the visual inspection drawings. It should be noted that, for the majority of these girders, structural cracking and indications of overloading have not been observed. In the few cases, the cracking observed has typically been hairline in width. These cracked girders were reinspected by PJCCI and found to have no evidence of change since the previous inspections. The absence of indications of overloading plays a significant role in the 8 Champlain Bridge Approach Spans
decision to allow the lanes to remain open while these short and medium term repairs are undertaken. Detailed visual inspections of all the edge girders are being undertaken by PJCCI yearly. Following each inspection, any changes to previously known conditions are reviewed and strengthening options reconsidered. Inclined tendon with signs of corrosion and reflective cracking Inclined tendon with reflective cracking Inclined tendon with signs of corrosion and reflective cracking Inclined tendon with reflective cracking and grout injected crack Inclined tendon with signs of corrosion and spalling Figure 5: Observed Signs of Deterioration on Web of Edge Girders Champlain Bridge Approach Spans 9
Bottom tendons with signs of corrosion and spalling Bottom tendons with signs of corrosion and spalling Bottom tendons with signs of corrosion and spalling Delamination of girder soffit Figure 6: Observed Signs of Deterioration on Soffit of Edge Girders 10 Champlain Bridge Approach Spans
Figure 7: Typical Damage Report for Edge Girder from Detailed Visual Inspection (42W- 43W P1 shown) Figure 8: Typical Damage Report for Edge Girder from Detailed Visual Inspection (27W-28W P7 shown) Champlain Bridge Approach Spans 11
Figure 9 shows typical observed signs of deterioration on the soffit of deck infill strips. At some locations, there is evidence of corrosion of the transverse posttensioning tendons in the deck. As described in Section 2 these tendons are essential to ensure the transverse integrity of the deck slab. Transverse post-tensioning tendon Transverse posttensioning tendon with signs of corrosion and spalling Deck infill strips Transverse posttensioning tendon with signs of corrosion and spalling Figure 9: Observed Signs of Deterioration on Soffit of Deck Slab 12 Champlain Bridge Approach Spans
Figure 10 shows typical observed signs of deterioration on intermediate diaphragms. At some locations, corrosion of reinforcement and concrete spalling were observed. The diaphragms provide load sharing between girders under traffic, and deterioration to these components increases the exterior girder demands and, in turn, the risk. Intermediate Diaphragm Intermediate diaphragm with signs of corrosion and spalling Intermediate diaphragm with signs of corrosion and spalling Figure 10: Observed Signs of Deterioration on Intermediate Diaphragms Champlain Bridge Approach Spans 13
4 Engineering Assessment of Current Condition 4.1 Assessment of Bending Capacity The bending capacity of the girders depends on the amount of mild steel longitudinal reinforcement and internal post-tensioning tendons. The amount of mild steel longitudinal reinforcement in the girders is minimal and was not considered in the calculation of the bending capacity. Only the internal post-tensioning tendons were considered as contributing to the bending capacity of the girders. Any deterioration of the post-tensioning tendons therefore plays a major role in the bending capacity of the girders. The capacity of the girders was evaluated based on the observed deterioration of the post-tensioning tendons and the concrete as documented in previous inspection reports. The reports gave details on the exploratory openings at mid-span and the reflective cracking along the inclined tendons. For girder capacity calculations, tendons at the exploratory openings were considered fully lost if 50% or more of their sectional area was observed as missing due to corrosion. The extent of tendon loss along the length of the edge girders for calculation of the bending capacity was determined according to the S6 Code, Clause 14.14.3 which states that: "If a prestressing tendon is significantly corroded, the contribution of the entire tendon to the strength of a component shall be neglected." As a mitigation measure for the loss of section in the internal tendons, PJCCI has installed external post-tensioning tendons near the bottom flange of all the edge girders of the approach spans. The external tendons are located on each side of the bottom flange and anchored close to the bearings locations with prestressing bars. Installation of the external post-tensioning tendons started in 1986 on girders showing signs of corrosion based deterioration. To date, all of the 100 edge girders of the Champlain Bridge approaches have been retrofitted with external posttensioning tendons. The number of strands in each external tendon and their jacking stress were originally designed as a function of the level of deterioration in each edge girder, but this strategy quickly moved into preventive design where the maximum amount was added. The objective of adding the external post-tension was to compensate for any loss of bending capacity resulting from the corrosion of the internal post-tensioning tendons. 14 Champlain Bridge Approach Spans
The effects of the tendon deterioration and the presence of external post-tension were considered in the calculation of the bending capacity of the edge girders in their current condition. Our calculations show that the demand-to-capacity ratio in bending for the all the edge girders is below 1.0, with a maximum value around 0.95 in the most critical cases. Therefore, the edge girders in their current condition, as a consequence of the addition of the external post-tensioning tendons by PJCCI, have an adequate level of safety according to the Code with regard to their bending capacity. 4.2 Assessment of Shear Capacity The shear capacity was calculated according to the general method outlined in the S6 Code Clause 8.9.3 as the sum of the contributions from the concrete, the mild steel transverse reinforcement and the vertical component of the post-tensioning tendons. The following sections outline two situations where, based on sound engineering judgement and staying within the intention of the code requirements, the shear capacities were calculated using concepts not specifically described in the S6 Code. 4.2.1 Spacing of Transverse Reinforcement The transverse reinforcement of the girders consists of 16 mm (#5) stirrups spaced at 267 mm (10.5") from the bearings to 6.553 m into the span and alternating 12 mm (#4) and 16 mm (#5) stirrups spaced at 800 mm (31.5") in the rest of the span. The stirrup spacing of 800 mm away from the support zones does not meet the requirements of the S6 Code Clause 14.14.1.6.2 for a section with minimum reinforcement. This requirement is considered to be for evaluation purposes only and is intended to provide some amount of crack control at the service limit state. Based on our understanding of this requirement of Section 14, it was determined that the spacing requirement is appropriate for shallow girders, and not necessarily for the deep girders on Champlain Bridge. In a deep girder, more shear reinforcement would be available to intercept an inclined shear crack. Assuming a typical crack angle of 35 o, a potential shear crack in the 3 m deep girder would still be intercepted by at least 4 rows of stirrups spaced at 800 mm. Champlain Bridge Approach Spans 15
For the above reasons, and given that the girders have the minimum area of transverse reinforcement according to S6 Code Clause 8.9.1.3, the shear capacity was calculated assuming that the girders have the minimum amount of transverse shear reinforcement and the minimum 600 mm spacing requirement was waived. 4.2.2 Determination of Effective Internal Post-Tensioning Tendons The shear capacity of the girders relies heavily on the vertical component of the parabolic post-tensioning tendons. Consequently, deterioration of the post-tensioning tendons has a major impact. The capacity of the girders was evaluated based on the observed deterioration of the post-tensioning tendons and concrete as documented in previous inspection reports. The reports provided details on the findings at the exploratory openings at mid-span and the reflective cracking along the inclined tendons. For locations where the strands were individually observed through exploratory openings, a strand was considered being ineffective if 50% or more of its sectional area was noted as missing due to corrosion. The extent of tendon loss along the edge girders was initially determined according to the S6 Code Clause 14.14.3 which states that: "If a prestressing tendon is significantly corroded, the contribution of the entire tendon to the strength of a component shall be neglected." Following these criteria as defined in the S6 Code, the demand-to-capacity ratio in shear for some girders was found to be greater than 1.0, up to a maximum value of 1.6 in the most critical case (additional information on how the demand-to-capacity ratios were determined is outlined in Section 5.2). However, detailed visual observations did not support this finding as the minor amount of structural based cracking did not correlate to these high demand-to-capacity ratios. It was evident that the actual shear strength of the girders was much higher than shear strength values calculated based on the strict requirements of the S6 Code and further assessment was needed to appropriately define the capacity of the girders. Tendon losses due to corrosion appeared to be most serious at their low point at mid-span where water collects. As the strand parabola climbs away from the midspan low point, the corrosion is less severe and after a short bond length the grouted strand is able to retain its full originally installed tension. The grouted strand tension in the upwardly curving parabola provides the major part of the girder shear capacity. 16 Champlain Bridge Approach Spans
The tendon losses along the length of the edge girders were determined as follows: Tendons with 50% or more section loss at exploratory opening = Loss of tendon over the observed broken length + bonding length (1.5 m on each side of opening); and Reflective cracking along tendon = Loss of tendon over its full length. Reflective cracking along the tendon resulted in assuming a loss of the tendon over its full length as the cracking indicates the corrosion and presence of moisture is not confined to the low-point of the strands at the girder mid-points. Using these criteria, the shear demand-to-capacity ratios for most edge girders were found to be lower than 1.0, with maximum values around 1.1 for the most critical girder. 4.3 Remedies Installed to Date for Shear Deficiencies In this section, shear strengthening systems installed to date on the girders are presented. The effects of already installed strengthening programs were taken into account when assessing the capacities of each girder. These solutions include installation of fiber-reinforced plastics (FRP), and installation of queen-posts. 4.3.1 Installation of Fiber-Reinforced Plastic (FRP) FRP consist generally of carbon or glass fibers embedded in an epoxy matrix. As shown in Figure 11, some edge girders on the bridge have already been reinforced for shear by installing vertical strips of carbon FRP glued to the concrete faces of the web. These strips act as passive shear reinforcement in the same way as the steel stirrups inside the concrete girders. A minimum clear spacing of 100 mm was specified between the FRP strips, to allow for inspection of the concrete underneath for signs of cracks or other deterioration. Horizontal FRP strips running along the top and bottom of the vertical FRP strips were also added on some of the strengthened girders to serve as anchorages for the vertical strips. Vertical FRP strips alternating with bare concrete sections of girder web will permit observation of any development of shear cracks. FRP strips will control the width of such shear cracks. Champlain Bridge Approach Spans 17
The installation of FRP strips is an efficient way to provide emergency shear strengthening to girders, to address both original design deficiencies and deterioration issues. However, it is considered more as a short-to-medium term solution until the long-term durability of the FRP attached to the girder webs (which may still have active corrosion) can be confirmed by future inspection. FRP strip Figure 11: Installation of FRP Strips on Web of an Edge Girder 4.3.2 Installation of Queen-Posts As illustrated in Figure 12, queen-post strengthening is already underway at several locations and installation of queen-posts on 17 of the 50 spans is expected to be completed prior by the end of 2013. Two types of queen-posts are currently installed on the bridge. Type QP 1.0 as shown in Figure 12 was designed by B&T and has been used where there is sufficient vertical clearance underneath the girders, usually over water. For spans over land and underpasses where clearance is an issue, shallower Type QP 2.0 has been installed. The latter system was designed by AECOM. 18 Champlain Bridge Approach Spans
The queen-posts (Q.P. 1.0) consist of four 46 mm diameter prestressing bars each. The nominal jacking load in the PT bars is 1,900 kn for each queen-post and the vertical posts are located below the intermediate diaphragms. At mid-span, the horizontal PT bars are located below the bottom flange of the exterior girder. The anchorage working point of the prestressing bars is located near the bearing along the girder (inside the span). Bracings are located between the queen-post legs and the adjacent girder. As the prestressing bars are tensioned, the system acts to lift the girders at the diaphragm/vertical post locations. This counteracts the shear load effects and reduces the overall demand-to-capacity ratio in much of the shear zone. The effect of the queen-posts on the shear was determined by superposing the shear demands from the queen-posts to the other dead and live load demands. This resulted in a reduction of the net shear demands on the edge girders. The queen-posts also contribute to improve the bending behaviour of the girders and this effect was taken into consideration in the calculations as a reduction in the net bending moments applied to the girders. Edge Girder Queen-Posts Figure 12: Queen-Posts Installed Underneath the Edge Girders Champlain Bridge Approach Spans 19
5 How Consistent Safety is Maintained for Bridge Users The Canadian Highway Bridge Design Code (S6 Code) provides a framework in Section 14 for evaluating the required level of safety for bridges. The main parameters in setting this level, often referred to as the reliability index, are the behavior of the element being considered, the behavior of the structural system of which the element is a part, and the level of inspection of the bridge. Through understanding and using sound engineering judgment in our evaluation of these parameters, we have shown that the current reliability index of the Champlain Bridge approach spans is consistent with the target reliability indices defined in the S6 Code. The details of how a consistent and acceptable level of safety is currently in place on the Champlain Bridge and recommendations for how to maintain it, with consideration to the anticipated future deterioration, are described in the following sections. 5.1 How the Level of Safety is Determined Through the use of engineering principles, a commonly acceptable means of describing the level of safety is to discuss demand-to-capacity ratios. What these are is a ratio between the imposed loads on a structure (the Demands) and the ability of the structure to withstand those loads (Capacity). When the demand-to-capacity ratio is one or less, this signifies that the capacity can meet the requirements of the demands, based on agreed upon load factors. The load factors used to calculate demand-to-capacity ratios presented in this report are calculated according to S6 Code Clause 14.15, in the Evaluation section of the Code. The load factors used for Evaluation in S6 Code Section 14 are typically lower than the load factors used for a new bridge because Section 14 takes into account the actual state of the bridge. Many factors must be considered when evaluating the risk environment for each bridge. It is through the application of the S6 Code and using sound engineering judgment that these factors are determined. 5.1.1 Demands The demands on a structure are determined by an understanding of the types and number of highway trucks and passenger vehicles crossing the structure, the natural wind and snow environment, and the inherent dead load (self-weight) of the various 20 Champlain Bridge Approach Spans
components. Factors are then applied to each of these demands based on how confident we are in our understanding of these loads. Both the loads and the factors applied to them are defined and supported by the S6 Code and are considered appropriate within Canada. 5.1.2 Capacity The capacity of the structure is determined primarily through the application of the S6 Code engineering equations; taking into account size of components, material strengths, geometry and an overall consideration for how confident we are in the available information. For example, variability in the strength of concrete is taken into account by applying a factor which reduces its effective strength. These factors (called Resistance Factors) are determined and applied as per the code requirements, developed through years of testing and S6 Code developments. The capacity of items, such as the PT strands, is also defined in the code. However, the S6 Code permits engineers to use sound engineering judgment to justify alternate means of determining the capacity if supported by acceptable engineering practices and the observed condition of the structure. 5.1.3 Risk Factors Many of the load and capacity factors referenced in the previous two sections are determined, in part, based on an understanding how the structure works and where its vulnerabilities may be. This understanding is based on the following four criteria. System Behaviour (S6 Code Clause 14.12.2) The system behaviour accounts for the effect of a failure of an element on the overall integrity of the structure. For evaluation of shear and bending effects, Category S2 was considered where element failure is not expected to lead to a total collapse. If an edge girder were to fail in shear, our review of the structural system indicated that the rest of the span would likely not collapse because of bonded transverse tendons in the deck slab, insuring partial structural integrity of the remaining girders. However, it is important to note that the original design, as discussed in Section 2.3, leaves the structure more vulnerable than is typically desired. Champlain Bridge Approach Spans 21
Element Behaviour (S6 Code Clause 14.12.3) Within this criterion, the composition of the structure is considered and its vulnerability to element failure based on demands is taken into account. For evaluation of shear effects, Category E2 was considered where the element is subject to sudden failure with little or no warning but will retain post-failure capacity. The shear ductility is guaranteed by the fact that girders have the minimum amount of stirrups and, in some cases, external FRP. While only the minimum amount of transverse shear reinforcement was specified in the girders (alternating #4 and #5 bars at 800 mm spacing), due to their large depth (more than 3 m), a sufficient number of them will cross a potential crack, reducing the risk of brittle failure. For evaluation of bending effects, Category E3 was considered where the element is subject to gradual damage with warning of probable failure. There would be warning signs of a bending collapse: increase in deflection in the beam, sag in the deck, extensive cracking, etc. Inspection Level (S6 Code Clause 14.12.4) A benefit is realized from the detailed visual inspections performed for PJCCI in recent years. Based on these inspections (and their continuation), a greater confidence in the performance of the structure to date is achieved. Without these inspections, the assumptions on the contribution of corroded tendons to the girder shear capacity would be more difficult to justify as there would be limited physical evidence that the concrete was not cracked along the entire length of the tendon. For evaluation of shear and bending effects, Category INSP2 was considered where inspection is to the satisfaction of the evaluator (B&T). B&T used inspection reports prepared by others to assess deterioration in the girders. In the future, the inspection category could be increased to INSP3 if the evaluator becomes more closely involved in the inspection process. Increasing the inspection category does not reduce the D/C ratio based on reducing the demands or capacities; rather it assumes a higher level of confidence in what the information assessment is based on, so slightly lower load factors may be used. 22 Champlain Bridge Approach Spans
Important Structures (S6 Clause 14.12.5) Where structures are located such that they can affect the life or safety of people under the bridge, are essential to the local economy, or are designated as emergency response routes, a higher level of safety is expected of them. Therefore, the S6 Code builds additional conservatism into the analysis for these critical structures. In this particular case, the Champlain Bridge qualifies as an Important Structure because of the high-traffic volume and the socio-economic importance of the bridge to the Montreal area. To take into account the importance of the bridge, the Target Reliability Index (β factor) was increased by 0.25. Target Reliability Index β (S6 Code Clause 14.12.1) The target reliability index β was calculated from S6 Code Table 14.5 for normal traffic. For evaluation of bending effects, the β factor was calculated as 3.25. For evaluation of shear effects, the β factor was calculated as 3.50. These values are slightly less than the β factor of 3.75 used in the design of a new bridge with a 75 year service life. It was discussed internally within B&T and agreed that a higher probability of failure (higher D/C ratio resulting in a lower effective reliability index) would be acceptable considering the ongoing inspection and monitoring programs on the bridge and the ongoing strengthening of the deteriorated girders. 5.2 Current Level of Safety The current level of safety has been verified as being acceptable for maintaining traffic on the bridge based on the following criteria: Confidence in Visual Inspections The absence of indications of overloading plays a significant role in the decision to allow the traffic lanes to remain open while short and medium term strengthening projects are undertaken, additional details on the strengthening projects is found in Section 7. Detailed visual inspections of the edge girders are being undertaken by PJCCI yearly. Following each inspection, any changes to previously known conditions are reviewed. Champlain Bridge Approach Spans 23
To date, only limited, hairline shear cracking has been observed on the girders and the evaluation of past inspection reports indicates limited significant changes in these cracks during the last five years. Until the FRP is fully installed on the six critical girders, the frequency of inspection for these locations has been increased to twice per week. Based on our experience, we believe this frequency is acceptable. On-going Application of Strengthening to Girders Strengthening is an on-going task for the exterior girders on this bridge, identified in Section 3. PJCCI s commitment to continue inspecting, strategically planning and implementing the strengthening options demonstrates that, as the deterioration continues, so does the strengthening, meaning that consistent safety can be achieved, making it acceptable for traffic to remain in place. 5.3 Maintaining an Acceptable Level of Safety In looking to the future deterioration of the structure, the following criteria need to be met for an acceptable level of safety to be maintained: Detailed Visual Inspections As discussed in Section 5.2, detailed visual inspections play a key role in the understanding of the structural condition. PJCCI s commitment to inspect the exterior girders at least once a year, as a detailed visual inspection, permits the engineering judgment utilized in our current assessment (described above in Section 5.1.3 Inspection Level) to remain appropriate. Additional Exploratory Openings Further certainty regarding the condition of the PT strands can be determined through additional exploratory openings, similar to the 19 performed in 2013. Some girders have never been opened to determine the condition of the strands, while others were opened more than 30 years ago. This form of investigation will help determine the actual condition of the bottom layer of PT strands and will enable engineers to better define each girder s unique condition for selection of strengthening or monitoring. 24 Champlain Bridge Approach Spans
Application of Strengthening to the Girders As deterioration continues, it can be expected that the assumed shear contribution of the PT strands may become less reliable. As noted in Section 7, we are recommending that some strengthening be planned and implemented prior to the end of 2014 and significant strengthening be planned for the next five years. While the risk associated with the observed conditions of the girders is currently acceptable to maintain traffic on the structure, deterioration and degradation will continue with time. The short time frame recommended for budgeting for the strengthening projects is based on the uncertainty that can surround concrete corrosion. A detailed load evaluation and engineering assessment is also recommended to determine where PJCCI should focus the strengthening projects in order to best manage the risks. Real-Time Monitoring Systems PJCCI has implemented real-time monitoring systems on many of the more deteriorated girders to monitor for signs of overload. This information, along with the detailed visual inspections allows engineers to maintain a higher degree of understanding of the behavior and condition of structure than is typically found on older bridges across Canada. Continued Strategic Planning Over and above the shear strengthening and detailed inspections, PJCCI s continued commitment for high level strategic planning and conservative budgeting (as described later in Section 7) for ongoing strengthening results in a confidence that, if new issues or concerns arise, appropriate programs and personnel are in place and available to address the issues. Champlain Bridge Approach Spans 25
6 Summary of Evaluation An evaluation was performed to determine the individual demand-to-capacity ratios (D/C) for each exterior girder; the results are presented in Table 1. The table shows the results for shear only, as this effect was found to be governing over the bending and reinforcement tension results. The results show the current D/C based on the condition of the structure, and does not include the effects of the queen-posts and FRP strengthening currently being applied to the six critical girders. This table demonstrates that the exterior girders have D/C ratios generally near or less than 1.0 based on our engineering assessment and evaluation following the S6 Code requirements (described in Section 4.2). For the purposes of our analysis, a D/C of 0.85 was set as the threshold, after which repairs would be recommended to take place prior to the end of 2014. We selected the threshold ratio of 0.85 based on our experience working with the code requirements, engineering judgment. It was set lower than 1.0 due to the significant amount of uncertainty regarding the extent of deterioration and its rate of change. The selection of the six critical girders was based on a review of the existing D/C ratios in combination with risks identified through significant deterioration effects noted in the previous visual inspection reports. While some of the six critical girders have D/C ratios higher than 0.85, the degree of over-stress is considered acceptable for the short-term based on the current visual observations and plans for continued, bi-weekly inspections, as discussed in Section 5.2 and the emergency strengthening measures being implemented. Additional girders were noted to also have D/C ratios higher than the 0.85 threshold but were judged to be acceptable since our review of the visual inspection reports confirmed there was a limited indication of deterioration which may affect the critical sections of the girder and minimal change in the deterioration in the past five years. The girders will be re-inspected during annual detailed visual inspections to confirm these findings and monitor for any change to the girders conditions. As the recommended strengthening programs (presented in Section 7) are implemented and new information from the exploratory openings and detailed visual inspections become available, the D/C of the girders will be updated. The changes in the D/C ratios based on the recommended immediate and short-term actions are presented in Sections 7.1 and 7.2 respectively. 26 Champlain Bridge Approach Spans
The installation of additional queen-posts is recommended to be based on continuous monitoring of the girders and if a girder strengthened with FRP shows signs of deterioration of the FRP, or a girder shows advanced signs of deterioration that requires both the FRP and a queen-post. Table 1: Demand-to-Capacity Ratios Prior to Emergency Strengthening Champlain Bridge Approach Spans 27
7 Rehabilitation Measures Required to Manage Risk The following sections describe B&T's recommendations to PJCCI in order to address the current condition of the bridge and maintain an acceptable level of risk to the structure. These actions include: Immediate actions, (already started), to install FRP on the six critical girders and to be completed before the end of September 2013; Short-term actions, starting immediately and completed prior to the end of 2014; and Actions of the five-year plan to be completed by the end of 2018. 7.1 Immediate Actions Required - Emergency Strengthening Measures As outlined in Section 3, B&T has identified the following six most critical edge girders that require immediate attention: 1) 6W-7W P1 4) 27W-28W P7 2) 8E-9E P7 5) 32W-33W P7 3) 26W-27W P7 6) 42W-43W P1 For these girders, the following actions must be taken immediately (this has been communicated to PJCCI and at the time of writing this report, the work is already underway): Implement emergency contracts to install carbon FRP shear strengthening on the six edge girders that have no strengthening. Assign crews to allow completion prior to the end of September 2013 (this installation is weather dependent and may not be practical after September); Visually inspect all six girders on a continuous basis (minimum twice per week) for signs of structural distress until all the FRP reinforcing is installed; and Install queen-posts on the six spans (twelve girders). On three spans, the queenpost installations were already prior to the emergency strengthening and work is underway. 28 Champlain Bridge Approach Spans
Table 2 shows the D/C ratios for each of the six critical girders and two additional spans following installation of the queen-post and the FRP, at the end of December 2013. Table 2: Demand-to-Capacity Ratios following Emergency FRP and Planned Queen-Posts 7.2 Short-Term Actions Required (Immediately to end of 2014) The short-term actions outlined in this section will require a significant engineering effort. It is recommended that this begin immediately to allow construction of the strengthening measures to begin in the spring of 2014. While the risk associated with the observed conditions of the girders is currently acceptable to maintain traffic on the structure, the deterioration and degradation will continue with time. To address this changing risk environment and maintain the level of safety required by the S6 Code, some actions are therefore required in the shortterm, these are: Assess the condition of all of the edge girders, centre girders, pier caps and foundations by visual inspections, paying particular attention to the durability of the FRP, reflective cracking and overall signs of corrosion; Start planning and engineering the on-going strengthening and monitoring program in 2013 and complete implementation of strengthening where the D/C ratio is more than 0.85 prior to the end of 2014, staging the most critical items first; Perform additional exploratory openings on girders with limited or obsolete data (approximately 20 locations); Champlain Bridge Approach Spans 29
Install FRP on 20 25 additional edge girders, over and above the six critical girders addressed in Section 7.1 and 7.2 (some of the girders to be reinforced with FRP are presented in Table 3, however we anticipate additional locations following the 2014 visual inspection results); and Add an asphalt-based waterproof membrane above the top flange of the edge girders to help prevent water from entering the post-tensioning anchors, and apply breathable waterproofing coatings to the outside faces of the exterior girder and at the girder ends beneath expansion joints; Based on our current understanding, we believe that all of this work can be performed while keeping full traffic running on the bridge. However, due to the nature of the deterioration, we believe it is critical to complete the strengthening work prior to the end of 2014. 30 Champlain Bridge Approach Spans
Table 3: Demand-to-Capacity Ratios following 2014 FRP Installations Champlain Bridge Approach Spans 31