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TRANSPORTATION: New Highway Bridge Design Code

A new Canadian code guides engineers for the first time in areas such as the loading of long-span bridges and the durability of construction materials.Canadian Standards Association International rece...



A new Canadian code guides engineers for the first time in areas such as the loading of long-span bridges and the durability of construction materials.

Canadian Standards Association International recently published the long-awaited Canadian Highway Bridge Design Code as a national standard for Canada. The code’s official designation is CAN/CSA-S6-00.

A team of more than 100 engineers in 16 technical subcommittees and five task forces has been working since 1992 on developing the new code under the general direction of the 31-member technical committee.

Divided into 16 sections, the new “S-6″ or “CHBDC” has many unique and first-time features. Published in English and French, it includes:

extensive commentary, explaining the basis of each clause

provisions for variable design live loads

the first loading provisions for long-span bridges

the first design provisions for fibre reinforced components

an extensive section on movable bridges

provisions related to durability

a section on seismic design

an up-to-date section on evaluation

The latest bridge design code supercedes both CSA S6-88 and Ontario’s OHBDC-91. It represents the current state of knowledge on the subject, and is expected to be adopted across Canada.

Its various sections are described briefly in the following.

Framework

Section 1 – General. Deals with the geometric and hydraulic provisions for highway bridges, and provides broad guidelines for economic, aesthetic and environmental considerations. Reference is made to the TAC (Transport Association of Canada) Geometric Design Guide for Canadian Roads and the TAC Guide to Bridge Hydraulics.

Section 2 – Durability. This section is new and consolidates the durability aspects of materials used in the construction of highway bridges, culverts and other structures in transportation corridors. Protective measures and detailing requirements are specified to prevent the deterioration of individual components with respect to the expected environmental conditions.

Section 3, Loads and Load Factors. Specifies design loads, load factors and combinations used for calculating the design load effects. The design live loading consists of a five-axle CL-W Truck, where W is the total truck weight in kilonewtons. The truck loading is complemented by a uniformly distributed load (UDL), with the combined truck and UDL being referred to as the CL-W Lane Load. A loading less than CL-625 could be adopted only after approval is obtained by the authority having jurisdiction over the bridge under consideration.

Similar to the immediately previous edition of S6, the design live loading of CHBDC represents maximum loads permitted by vehicle weight regulations. The section specifies ship collision loads as well as forces generated by ice. It is significant that the live loading provisions of the code are no longer dependent on an upper limit of the span length. The CHBDC is probably the only design code in the world, which explicitly provides loading specifications for long-span bridges.

Section 4, Seismic Design. This section is also new. Its provisions were adopted largely from those of the U.S. AASHTO LRFD Highway Bridge Specifications. However, CHBDC is more explicit in its treatment of the response modification factors. There are new provisions for seismic base isolation, as well as for the ductility of substructure elements made of steel. In addition, provisions are given for the seismic evaluation and rehabilitation of existing bridges.

Section 5, Methods of Analysis. Provides requirements for the analysis of load effects in various kinds of bridges. In addition, the section specifies a number of simplified methods, which could be used in lieu of the computer-based rigorous methods. Many of the simplified methods specified in the code were developed expressly for the CHBDC with the help of well-tested rigorous methods

Section 6, Foundations. Provides minimum requirements for the design of foundations and earth-retaining structures. Similar to the rest of the code, the foundation section is based on a limit states design concept, and uses global resistance factors. It emphasizes the importance of communication between the geotechnical and structural engineers during design and construction.

Section 7, Buried Structures. This is new to S6. Besides dealing with soil-steel structures, which were covered by the OHBDC, the new code includes design provisions for metal box structures and buried concrete structures. The method for designing soil-steel structures, introduced initially in the OHBDC, now includes the consideration of flexural load effects, as well as the conventionally recognized thrust in the metal plates. The section provides guidance for determining the properties and dimensions of the engineered soil envelope, and gives specifications for construction procedures for buried structures.

Section 8, Concrete Structures. Deals with the full spectrum of structural concrete, including reinforced concrete, partially prestressed concrete and fully prestressed concrete. The section contains design provisions for both cast-in-place and precast concrete deck slabs, the latter category including the full-depth and partial-depth slabs. The compression field theory is used as a method of proportioning for shear and for torsion combined with flexure. The section provides an exhaustive table in which different values of the cover to reinforcement are given for different components exposed to different environmental conditions.

Section 9, Wood Structures. New to this section of S6 is the incorporation of provisions for materials and fastenings from the CSA Standard 086.1, 1994 Edition. The section includes design provisions for the stress laminated wood deck, a relatively new invention.

Section 10, Steel Structures. Novelty on a global scale is found here, where the use of cables and multi-arches is presented for long span bridges. Durability is also addressed much more fully than heretofore. There are significantly revised requirements for the design of beams and girders, composite beams and girders, horizontally curved girders, and orthotropic decks.

Section 11, Joints and Bearings. Recent research results have been used to develop these design provisions, which amalgamate certain features of CSA S6, OHBDC-91 and the new AASHTO LRFD Highway Bridge Code.

Section 12, Barriers and Highway Accessory Supports. The barrier crash testing programs in Ontario and Texas have led to changes, and crash testing requirements for breakaway highway accessory supports have been added.

Section 13, Movable Bridges. Limit States Design philosophy has been applied to the structural aspects of movable bridges, although the design of mechanical systems continues to follow the working stress principle, still prevalent in North America. The section draws from the old CSA S20 (withdrawn in 1977), AASHTO, and the American Railway Engineering and Maintenance of Way Association (formerly AREA).

Section 14, Evaluation. Both OHBDC and Clause 12 of S6-1988 incorporated novel methods of evaluating the strength of existing bridges. The philosophies of both these documents have been combined in Section 14 of the new code, to lead to a method that explicitly takes into consideration the variability involved in both component and system behaviour, and gives various inspection methods. The section includes a chart, which permits the direct determination of posting loads for structurally deficient bridges.

Section 15, Rehabilitation. New to the CSA S6 Standard and based on the OHBDC-91, the section provides guidance on the selection of loads and load factors for rehabilitation.

Section 16 Fibre Reinforced Structures. It is believed that no other published bridge design code contains design requirements for such structures. The provisions apply to the following components containing fibre reinforcement: (a) fully or partially prestressed concrete beams and slabs, (b) non-prestressed concrete beams and slabs, (c) deck slabs, (d) stressed wood decks, and (e) barrier walls.

The Canadian Society for
Civil Engineering in conjunction with Randerson Consulting is holding training sessions across the country in the provisions of the new code. A fall program will be delivered in September for those who missed the spring lecture tour. These sessions will commence in St John’s, Newfoundland (September 20-21), moving to Dartmouth/Halifax, Nova Scotia (September 24-25), and then to Calgary, Alberta (October 11-12). Contact CSCE at (514) 933-2634 or visit www.csce.ca.CCE

Baidar Bakht, D.Sc., P.Eng., is chair of the S6 Technical Committee and president of JMBT Structures Research in Toronto. Michael Randerson is with Randerson Consulting of Victoria, B.C.


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