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Commentary on CSA S6:19, Canadian Highway Bridge Design Code
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This is the fifth edition of CSA S6.1, Commentary on CSA S6:19, Canadian Highway Bridge Design Code. It supersedes the previous editions, published in 2014, 2006, 2000, and 1990.
Throughout this Commentary, CSA S6:19 is referred to as the Code. Other Codes are always identified in a manner that allows them to be readily distinguished from the Code.
The purpose of this Commentary is to provide background on the design provisions of the Code and thereby to help designers deal with issues not explicitly addressed in the Code.
Each section and clause in this Commentary bears the number of its corresponding section or clause in the Code, with the addition of the prefix C. For example, Section C1 provides commentary on Section 1 of the Code, and within Section C1, Clause C1.1.1 provides commentary on Clause 1.1.1 of the Code. The same approach is used in the numbering of annexes. Tables and figures are numbered sequentially (for example, the first table in Section C3 is Table C3.1, which is followed by Table C3.2, etc.). However, they do not correspond to the tables and figures bearing the same numbers (minus the C) in the Code.
The Code contains many clauses dealing with approval by owner or regulatory authority (see the definitions in Clause 1.3.2 of the Code). Where possible, this Commentary provides guidance for owners or regulatory authorities consulting such clauses.
Section C1 - General
C1.1.1 Scope of Code
The OHBDC (MTO 1991) was written for application within Ontario. CAN/CSA-S6-88 was generated with interprovincial co-operation for use in the other provinces of Canada and was largely derived from the preceding OHBDC edition. The provinces and CSA then agreed that the successor edition to both codes would be the Code, published by CSA.
The scope of the Code is a little broader than that of the third and last edition of the OHBDC (MTO 1991). Long span bridges and movable bridges are included. Over the years, new sections have been added to the Code, namely Section 16 on Fibre-reinforced Structures and Section 17 on Aluminum Structures. The 2019 edition of the Code includes a new annex in Section 8 on fibre-reinforced concrete. In addition to incorporating newer technology, more emphasis is placed on criteria related to seismic design, durability, sustainability, and access for inspection and maintenance. Also new in the 2019 edition of the Code are limited climate change requirements and guidance provided for the design of structures in impacted regions of Canada.
The scope statement lists types of structures to which the Code is not intended to apply. The list is not exhaustive. The application of the Code to the types of structures listed is not precluded where the owner of the structure has designated all or part of the Code as being applicable.
C1.1.2 Scope of this Section
Geometrical provisions have been minimized by referring to the Geometric Design Guide for Canadian Roads (TAC 2017).
Many catastrophic failures have been caused by scour at bridge piers and abutments. Good hydraulic design is a fundamental requirement for bridges. Basic hydraulic requirements are specified in the Code, and reference is made to the Guide to Bridge Hydraulics (TAC 2004) for guidance concerning good hydraulic design and detailing.
Section C2 - Durability and sustainability
The scope of this Section has been expanded to include design requirements for durability and sustainability as well as special considerations for climate and exposure considerations. It has been recognized that the ultimate and serviceability limit states requirements of the Code cannot be met without proper integration with design for service life, quality management of design and construction, and life cycle maintenance program.
The high cost of repairs and replacements, as well as the high costs to users due to traffic delays are compelling reasons to focus attention on durability and sustainability. On heavily travelled highways, it is increasingly difficult to obtain access to carry out the necessary repairs or replacement. The safety risk to workers and to the travelling public is another major consideration, as is the increasing cost of delays and detours.
The sustainability of highway bridges and other structures covered by the Code, has been given widespread attention, as the consideration of environmental impacts as well as the importance of social and economic well-being of the neighboring communities and society at large have been recognized. The focus on sustainability in making decisions regarding the design, construction, and maintenance of bridges is sought by stakeholders and the public.
Bridges and other structures covered by the Code might be subjected to a variety of natural and manmade hazards, including traffic overload, vehicle collision, aging and deterioration, and extreme events (including earthquake, extreme weather events, flood/scour, fire/heat, and blast). Regional and local climatic characteristics influencing durability and service life should be identified and included in the design and construction of new bridges and in the maintenance of existing bridges and structures covered by the Code.
Section C3 - Loads
The Code is applicable to all highway bridges in Canada, including those with long spans.
Section C4 - Seismic design
This Commentary explains the rationale behind the seismic design provisions in the 2019 edition of the Code, including performance-based design, seismic hazard, time-history analysis, geotechnical aspects, concrete and steel design, base isolation and damping, and seismic evaluation of existing bridges. This Commentary also explains the background and implementation of performance-based seismic design of highway bridges. Section 4 is currently unique within this Code to have a performance-based design methodology, as opposed to the load and resistance factor approach to other environmental and internal structural design effects.
The 2014 edition of the Code introduced seismic hazard, and structural and geotechnical aspects of the seismic design of bridges. For the 2019 edition of the Code, the geotechnical aspects of seismic design have been moved to Section 6. The reader must refer to both Sections of the Code and Commentary for a complete seismic design approach.
Section C5 - Methods of analysis
This Section presents the background for the methods of analysis for the design and evaluation of bridge superstructures. It includes general requirements for specific bridge types and specific requirements for the use of the simplified method of analysis. Table C5.1 illustrates representative cross-sections and elevations of bridge types and applicable Clauses covered by Section 5.
Section C6 - Foundations and geotechnical systems
Limit states design philosophy was accepted in structural design prior to its application to foundations and geotechnical systems.
Previously, substructure design was based on allowable stress, even though much of geotechnical engineering has been based on concepts of limiting equilibrium — an ultimate limit state. The use of two design philosophies — one, a limit states design philosophy for superstructures; the second, an allowable stress design philosophy for substructures — led to complications. It became evident that a limit state approach was required for foundation design.
This evolution has not been without difficulties. Initially (OHBDC, 1979, 1983), the partial factor format was adopted following Danish practice (DGI, 1985 and earlier). Soil strength factors were applied to soil cohesion and angle of internal friction to account for uncertainties in material properties. However, the partial strength factor approach did not lead to design consistency with the allowable stress approach and the use of partial strength factors was not readily accepted by many geotechnical engineers. When strength factors were applied to cohesion and friction, well-proven empirical relationships for geotechnical design no longer applied. For example, many of the empirical relationships for bearing resistance of shallow and deep foundations are based on limiting vertical deflection. Thus, it was difficult for the geotechnical engineer to utilize earlier predictive methods and sources of data.
To address these concerns, the approach was modified in the third edition of the OHBDC (1991). Resistance factors were applied to the various ultimate limit states of geotechnical resistance rather than using partial strength factors applied to the soil strength parameters of friction and cohesion. The resistance factors were chosen to give a level of safety generally comparable to that obtained by the earlier allowable stress design procedures. Thus, traditional design aids were applicable, with some modifications.
The total resistance factor approach has been carried on in the Code. The ultimate and serviceability geotechnical resistances are computed using traditional methods with characteristic geotechnical parameters. This Code now applies both a resistance factor and a consequence factor to the geotechnical resistance to obtain the factored resistance used in design. The two factors are designed to keep the failure probability of the geotechnical system below acceptable levels, the latter of which are related to failure consequence.
Some key elements of the Code are as follows:
• As in the 2014 edition of the Code, foundation design considers structural loads applied against geotechnical resistances.
• In the Code, resistance refers to the load that the ground or geotechnical system can support at each limit state. At ultimate limit states, this load generally corresponds to a limit equilibrium capacity. At serviceability limit states, this load typically corresponds to that which yields less settlement or displacement than a predefined amount.
• In response to a desire in the geotechnical community to provide differing levels of safety for differing failure consequences (e.g., failure of a major multi-lane highway bridge versus failure of a minor secondary bridge), a consequence factor has been introduced in the Code. Three failure consequence levels are considered: high, typical (the default), and low. The consequence factor multiplies the characteristic resistance in the same way as does the resistance factor.
• Three levels of site understanding are considered in the determination of the resistance factor: high, typical (the default), and low. Site understanding reflects the level of confidence in geotechnical response predictions.
• In order to ensure the quality of geotechnical engineering, minimum requirements are given for site investigation for appropriate levels of site understanding and for reporting. Communication between the structural engineer and the geotechnical engineer is required throughout all phases of the project in order to ensure clear interpretation and application of the geotechnical information used in design. Specific reference is made to requirements for inspection and quality assurance.
A number of general papers discussing limit states design in geotechnical engineering are available, for example, Fenton (2013), Fenton, Naghibi, and Bathurst (2012), Fenton, Griffiths, and Ojomo (2012), Fenton and Griffiths (2010), Le and Honjo (2009), Honjo et al. (2009), Becker (2003), Becker (1996a and 1996b), Meyerhof (1995 and 1994), Green (1991), Barker et al. (1991), Christopher (1990), Baikie (1998 and 1985), Ovesen (1981), Bolton (1981), and Lumb (1970).
Section C7 - Buried structures
Projects using buried structures have grown in size and variety of use in recent years. Section 7 focuses on the design of structures which rely upon soil-structure interaction to achieve performance. In an effort to both bring consistency to how these structures are referenced and to emphasize the fact that these are structures in the same context as other bridge types in the Code, the term buried structures is used throughout in lieu of other commonly used terms such as culverts or pipes. Section 7 applies only to buried structures with a span greater than 3 m.
Unlike other sections of the Code, Section 7 specifies minimum standards of both design and construction. The performance of a buried metal or concrete structure is governed by the soil-structure interaction and depends as much on methods of construction as it does on design. Good construction practices for these structures are now established and it is the intent of this Commentary to emphasize the geotechnical requirements and recommended construction practice. Design provisions are also given for soil-metal structures with shallow, deep, and deeper corrugations. Concrete structures covered by Section 7 include round pipes, box structures, three-sided box structures, and arches with footings.
Section C8 - Concrete structures
Concrete structural components may be reinforced with non-prestressed or prestressed reinforcement or both, thus permitting partial prestressing. Although prestressing forces can be introduced in various ways, Section 8 is based on the use of high-strength steel prestressing tendons. Post-tensioning tendons may be internal or external but they must be protected by grouting. The use of low-density or semi-lowdensity concretes should be based on the availability of suitable low-density or semi-low-density aggregates.
This Section does not provide requirements for concrete poles. The requirements for concrete poles are given in CSA A14.
Section C9 - Wood structures
Section 9 applies to the types of wood structures and components likely to be required for highways, including glued-laminated girders, timber stringers, transversely and longitudinally laminated decks, laminated wood-concrete composite decks, prestressed laminated decks, and trusses. Section 9 does not apply to falsework or formwork.
Section C11 - Joints and bearings
The emphasized preferences are based largely on experience developed during the long-term operation and performance of many past installations. The joints and bearings need to allow movements to occur due to temperature changes, creep and shrinkage, elastic shortening due to prestressing, traffic loading, construction tolerances, or other effects. If these movements are restrained, large horizontal forces may be induced. In a cast-in-place concrete bridge deck, it may be unwise to fix or guide all of the bearings at a single support, because such fixity would prevent this transverse expansion and contraction. Externally applied horizontal loads such as wind, earthquake, or traffic braking forces may be carried either on a small number of bearings near the centreline of the bridge or by an independent guide system. The latter is likely to be needed if the horizontal forces are large.
The distribution of vertical load among bearings may also cause problems with individual bearings. This is particularly critical when the girders are stiff in bending and torsion and bearings are stiff in compression, and the construction method does not allow minor misalignments to be corrected.
Background information on requirements for joints and bearings can be found in Manning and Bassi (1986), Manning and Witechi (1981), and Mihaljevic et al. (2010).
Section C12 - Barriers and highway accessory supports
Section 12 specifies requirements for the design of permanent bridge barriers, including their transitions to approach roadway barriers. Requirements for temporary or movable barriers, retrofitting of barriers on existing bridges, barriers protecting bridge substructure elements, or barriers protecting other roadside hazards are not included.
Information on highway temporary barriers and some guidance on the design of existing bridge barrier upgrades may be found in AASHTO (2011).
Section 12 also specifies requirements for the design of highway accessory supports. Requirements concerning the performance of highway accessories, such as illumination levels and sign message presentation and legibility, are not included. The design of supports supporting small signs and traffic signals on cables spanning between supports is not covered by the Code. Guidance on the design of these structures can be found in AASHTO (2015).
Section C13 - Movable bridges
The history of movable bridge design specifications can be traced back at least as far back as 1908 to C.C. Schneider’s Paper No. 1071 in the June 1908 ASCE Transactions, Volume 60, page 258. The basic content of the Schneider specification appears to have been adopted by AREA, CESA, and AASHO Movable Bridge specifications in 1922, 1927, and 1938, respectively. Early movable bridges designed using the standards outlined in the Schneider paper have proved to be very durable. In contrast, early twentieth-century proprietary movable bridge designs using less stringent requirements have been more problematic. In 1970, the AASHO (now AASHTO) Movable Bridge Specifications were revised to more closely match the then current AREA specifications. Thus, it appears that the Schneider specification and the succeeding AREA and AASHO Movable Bridge Specifications have successfully defined adequate design standards for typical movable bridges.
Various changes and additions have been made over the years to these specifications.
The Canadian Engineering Standards Association (now CSA Group) published the first standard specification on movable bridges in 1927 as CESA A20-1927. An updated edition was prepared by CSA in 1960, designated CSA S20-1960, that incorporated the experience gained in the design of many movable bridges on the St. Lawrence Seaway. Updates to the Standard were issued up to 1969, after which it was withdrawn.
The void since 1969 was filled with the arrival of a new section in the the 2000 edition of the Code dedicated to movable bridges.
Section C14 - Evaluation
The cost of upgrading or replacing a bridge may be great; Section 14 offers a method of evaluation by setting safety levels that are consistent and appropriate for the bridge or bridge component being evaluated. The intention is to avoid some of the conservatism that, in the interests of simplicity, may have been incorporated into the design provisions. Section 14 is not to be used for design.
The philosophy behind Section 14 is to determine a suitable safety level for each element of the bridge under evaluation, which varies with the type of element failure to be expected: more safety is required for an element that fails abruptly; less safety is required for an element that will retain its capacity after failure and may shed its load to other members without collapse.
The main parameters in setting the required safety level, defined by the reliability index, β, are the behaviour of the element being considered, the behaviour of the structural system of which the element is a part, and the degree of inspection of the bridge. Inspection is important to ensure that the bridge is indeed in the condition the evaluator is assuming, and to verify that it has carried previous loads without distress.
Need for evaluation
The need for evaluation may be created by any of the following:
• observed or suspected defects, deterioration, or damage that may affect load capacity;
• an anticipated increase in actual or legally permitted traffic loading or loading effect;
• a change in road classification;
• a review of an existing load limit posting;
• any alteration to a bridge that may affect its live load carrying capacity;
• an application for a permit to allow a vehicle not conforming with legal limits to cross a bridge;
• an unsatisfactory performance of the bridge in terms of serviceability or fatigue; or
• a bridge, in a moderate or severe earthquake zone, not constructed to modern seismic standards, including detailing.
Section 14 applies to bridges where the level of load enforcement in place is typical for highways in Canada. It addresses the evaluation of an existing bridge for the purpose of
• establishing the legal load limit;
• establishing whether a bridge can safely carry trucks to the legal limit;
• establishing a restricted load limit for a bridge without sufficient capacity to carry legislated vehicle weights. The lack of sufficient capacity may occur because of deterioration or because the bridge was built for a lesser capacity or both;
• determining whether passage of an overload vehicle may be permitted;
• assisting in the development of programs for the repair, strengthening, and replacement of bridges; and
• assessing the adequacy of the bridge’s probable performance during an earthquake.
Section C15 - Rehabilitation and repair
Rehabilitation is the implementation of engineered design to address structural defects or deficiencies by modification, replacement, or repair of bridge elements with the intent of extending the service life of an existing structure, and often involves upgrading of elements to meet current standards. Repair, a subset of rehabilitation, is usually localized and may not involve upgrading of the element to meet the current standard.
Emphasis on remaining service life and assessment of ongoing deterioration in the rehabilitation design have been added in the 2019 edition of the Code.
Material specifications, rehabilitation procedures, and maintenance procedures are not covered in Section 15; however, they are important factors in ensuring the effectiveness of the rehabilitation design and achieving the service life objectives.
Section C16 - Fibre-reinforced structures
The scope of Section 16 has been enhanced in the 2019edition of the Code, as summarized below:
• durability, especially with reference to the 2019 edition of CSA S807 and the 2014 edition of CSA S808; and
• limit on bend-radius-to-bar-diameter ratio of bent FRP bars to guard against sustained load.
New provisions have been added on
• the development of bundled bars, bent bars, spliced bars, and headed bars;
• combined shear and torsion;
• compression members;
• deck slabs with FRP stay-in-place structural forms;
• strut and tie model for deep beams, corbels and short walls;
• barrier walls; and
• retrofit for confinement and lap splice clamping.
An Annex on GFRP composite bridges has also been added in Section 16.
Section C17 - Aluminum structures
Section 17 specifies requirements for the design, fabrication, and erection of aluminum highway and pedestrian bridges. See Section 4 for seismic design and Section 7 for aluminum soil-metal structures. Where permitted in Section 12, the contents of Section 17 may also be applied to highway accessory structures. Additional information on the design of highway accessory structures is provided in Section 12. The requirements of this Section are largely based on CSA S157 and Section 10 of this Code.