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Bridge Design Practice in North America – An Overview

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By
Isidro P. Buquiron

Introduction
The primary objective of code specifications in bridge design is public safety. Thus, in the United States, the American Association of State Highway Officials (AASHO) was formed in 1914 which later on issued the first edition of the Standard Specifications for Highway Bridges and Incidental Structures in 1931 [1]. The concept of safety provided in this document is to guarantee that all structural element in the system in part and as a whole must have minimum resistance that will exceed the load and demand applied to the structure during its specified years of service. In Canada, specifically in Ontario, the American Association of State Highway and Transportation Officials (AASHTO) specification was widely used, however unofficially before the first edition of the Ontario Highway Bridge Design Code (OHBDC) was issued in 1978 [2].

Bridge design method incorporated in the bridge specifications progressed with the development in the science, technology and research. Working stress design (WSD) or allowable stress design (ASD) which was based on elastic theory was the design philosophy behind the early bridge specifications. New knowledge in material science and the advent of structural reliability theory initiated gradual changes in the code design methods into what is now termed as Limit State Design (LSD) in Canada, or Load Resistance Factor Design (LRFD) in the United States. The first edition of the Ontario Highway Bridge Design Code (OHBDC) was a milestone in the use of limit state design for bridges in North America [3]. This specification was based on probability models of loads and resistances where corresponding factors were calibrated using second moment reliability method. A document published in 1977 by the Ontario Ministry of Transportation (MTO) titled “The Development of the Ontario Bridge Code” provides very significant information about this milestone.

The new design philosophy in the OHBDC paved the way for the limit state design to be used in the United States [4]. Initiatives were then undertaken to adopt the limit state design approach for bridges starting within the Subcommittee on Bridges and Structures (SCOBS) in 1986. The American Association of State Highway and Transportation Officials (AASHTO) subsequently issued the first edition of AASHTO LRFD Specifications for Bridges in 1994 [5]. In 2000, the Canadian Standards Association (CSA) issued the first Canadian Highway Bridge Design Code (CHBDC) S6-00 which supersedes the previous CSA S6-88 and that of Ontario’s OHBDC-91 [6]. Subsequently, CSA issued its eleventh edition of the “Canadian Highway Bridge Design Code” (CHBDC) S6-14 in 2014 and likewise, AASHTO released its seventh edition of the AASHTO LRFD Bridge Design Specifications also in 2014.

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Limit State Design
The core concept of probability based structural analysis may be conveyed by this quote;

“the art of formulating a mathematical model within which one can ask and get answer to the question: “What is the probability that a structure behaves in a specified way when given that one or more of its material properties or geometric dimensions and properties are of a random or incompletely known nature, and/or that the actions on the structure in some respects have random or incompletely known properties? [7]”.

Limit state is regarded as the condition of the bridge structure and its loadings at the verge of structural failure, and not able to function as designed. The early concept of limit state design philosophy with a probability based safety methodology where presented in a thesis published in 1926 [8]. The notions presented were persuasive but this proposal was not adequate at that time to demonstrate that the probabilistic method was far better than the conventional safety factor approach. However, the former USSR started to incorporate limit state design philosophy in its building code in 1955 and about a decade later, initiatives were undertaken to incorporate limit state principles into its bridge and culvert specifications [9]. This undertaking in the former Soviet Union was the first significant effort to incorporate structural analysis, load requirements, and analysis of safety in a code specification.

In the European Union, effort of consolidating design rules for different types of structural materials was initiated by the Joint Committee on Structural Safety (JCSS) in the 1970s [10]. It also prepared the General Principles on Reliability for Structural Design which then serve as the basis for the Eurocode 0 (EN 1990- Basis of Design). EN 1990 specifies the requirements on structural reliability concerning to safety, serviceability as well as durability for general cases and other cases not covered by other Eurocode standards [11].

The essential feature of bridge code specification using limit state method, is its capability to incorporate variability in materials resistances and structure loadings, as well as achieving a comparatively uniform levels of safety for various limit states and bridge type without the need to perform probability or statistical analysis every time [12]. A significant disadvantage is the vast change in design concept and philosophy which could be a challenge in its implementation.

Current bridge code specifications in North America ascertain that the probability of exceeding a specified limit state is statistically and appropriately small. This is achieved by providing minimum loads, and load factors together with minimum resistance values and resistance factors which should be coherent with a given design condition [13].

The Canadian Standards Association (CSA) CHBDC S6-14 specifies three limit states [14];

1.    Fatigue Limit State: A requirement that may lead to the formation of cracks as a result of the accumulated cyclical application of load. Specified truck loadings and dynamic effects were given to prevent excessive fracture throughout its designed service life.

2.    Serviceability limit states: Relates to the deformation specifically deflection  and crack width of structural members subjected to normal loading conditions. Bridge serviceability limit state may come through different ways and might show confined deterioration for instance cracks, spalls, vibration, and undue deflection.

3.     Ultimate limit state: Define the condition that pertains to the maximum load carrying capacity of the bridge or any of its component along with extreme load cases. The ultimate limit state loadings may create adverse conditions such as structure instability, section fracture, undue plastic hinge formation and buckling.

The Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specification has four limit states in its 2012 edition [15];

1.    Fatigue and Fracture Limit State: Limit state that defines constraint on stress range as a result of a single design truck occurring at the number of expected stress range cycles. This limit state can also be considered as the materials toughness requirements from AASHTO Materials Specifications.

2.    Serviceability limit states: Limit state that pertains to restrictions on stress, deformation, and crack width under regular service load conditions.

3.    Strength Limit State: Limit state taken to ensure that strength and stability, both local and global, are provided to resist the specified statistically significant load combinations that a bridge is expected to experience in its designed lifetime.

4.    Extreme Event Limit States: limit state shall be taken to ensure the structural survival of a bridge during a major earthquake or flood, or when collided by a vessel, vehicle, or ice flow, possibly under scoured conditions.

Implementations 
There has been earnest effort in the part of industry regulators in Canada and the United States to update and implement the limit state design method. Significant new provisions were being added in the code specifications to reflect the current development in the technology. In the CHBDC S6-14 specification issued in December 2014, it included important new provisions in Section 6-Foundation and Geo-technical Systems [16] that is consistent with the code design objectives. The American Association of State and Transportation Officials ceased updating the AASHTO Standard Specification which includes both Allowable Stress Design (ASD) and Load Factor Design (LFD) methods in 2003 to enable the LRFD specification gain considerable acceptance and use [17]. The Federal Highway Administration (FHWA) of the United States together with various states have established a goal that LRFD standards be incorporated in all new bridge designs after 2007 [18].

In the 1990s, the International Standard Organization (ISO) published the ISO:2394 – General Principle on Reliability for Structures which mandate for the adaptation of the reliability based limit state design for all ISO member countries [19]. Some unique challenges remain for the implementations of this design philosophy not only within North America but also in other parts of the world. Reliability based limit state design codes in geotechnical engineering is still a work in progress “for the simple fact that the ground is by far the most variable and thus uncertain of all engineering materials” [20]. Nevertheless, it is interesting to note that geotechnical limit state code started in Denmark in 1956 and Japan implemented its “Principles of foundation Design Grounded on Performance Based Design Concept” (JGS4001-2004) in 2004 [21].  The goal within the industry is to harmonize sub-structure design into the same level as that of structural engineering into this new design framework. The task can be a tall order.

In the United States, some code specifications still utilize the conventional Allowable Stress Design (ASD) as a design option from that of limit state design method, which indicates the slow adaptation to this method [22]. Notwithstanding the hurdles of the new limit state design code specification, the concept of performance based design is currently gaining ground to be included into the current codes particularly in seismic analysis and design. New initiatives are being put in place towards a performance based regulatory code and standards where code provisions are being tasked to include evaluation tools or compliance methods to support specific code requirements [23]. Thus, performance based design provisions are already incorporated into the current bridge codes in Canada and the U.S. albeit mainly with seismic design.

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Conclusion
Bridge design practice which apply limit state design approach results in a much more economical bridge structure without forfeiting safety [24]. This concept allows engineers to be more aware of the uncertainty and the inherent risk in bridge structural design. However, the design philosophy behind this technique also brings some challenges to engineers whose training and practice were mainly from the previous working stress or allowable stress design procedures. The bridge engineering community may be in for another milestone in innovation with the trend towards a “risk-based design” paradigm, aptly called performance based engineering [25].

References:
[1]     R. Barker, J. Puckett. 2013 “Design of highway bridges”. John Wiley & Sons, Inc. Hoboken, New Jersey

[2]     R.A. Dorton, P.F. Csagoly. October 1977. “The Development of Ontario Bridge Code”. Ontario Ministry of Transportation and Communications, ON., Canada

[3]     J.M. Kulicki. July 2006. “Developing a probability based limit state bridge specification – U.S. experience, Third International Conference on Bridge Maintenance, Safety and Management, Porto, Portugal.

[4]     J.M. Kulicki. July 2006. “Developing a probability based limit state bridge specification – U.S. experience, Third International Conference on Bridge Maintenance, Safety and Management, Porto, Portugal.

[5]     R. Barker, J. Puckett. 2013 “Design of highway bridges”. John Wiley & Sons, Inc. Hoboken, New Jersey

[6]      B. Bakht, M. Randerson, August 2001. “Transportation: New Highway Bridge Design Code”, Canadian Consulting Engineer, Toronto, ON M3B 2S9

[7]      O. Ditlevsen, H.O. Madsen. July 2005 “Structural Reliability Methods”, Technical University of Denmark, Lyngby, Denmark.

[8]     Contract Research Report (CRR) 398. 2001. “Probabilistic Methods : Uses and abuses in structural integrity, Health and Safety Executive, Norwich NR3 1BQ

[9]      O. Popov, S. Vadim. 1999. “Development of Bridge Design Code in Russia”, IABSE Symposium, Rio de Janeiro

[10]    Contract Research Report (CRR) 398. 2001. “Probabilistic Methods: Uses and abuses in structural integrity, Health and Safety Executive, Norwich NR3 1BQ

[11]    Contract Research Report (CRR) 398. 2001. “Probabilistic Methods: Uses and abuses in structural integrity, Health and Safety Executive, Norwich NR3 1BQ

[12]    R. Barker, J. Puckett. 2013 “Design of highway bridges”. John Wiley & Sons, Inc. Hoboken, New Jersey

[13]    L. Duan. February 2015. “Bridge design specifications”. Bridge Design Practice, State of California Department of Transportation, California

[14]    Canadian Standard Association (CSA). December 2014. “Canadian  Highway Bridge Design Code”. CSA Group, Ontario, Canada

[15]     American Association of State Highway and Transportation Officials (AASHTO).  2012. “AASHTO LRFD Bridge Design Specifications, AASHTO, Washington DC.

[16]    Canadian Standard Association (CSA). December 2014. “Canadian Highway Bridge Design Code”. CSA Group, Ontario, Canada

[17]    L. Duan. February 2015. “Bridge design specifications”. Bridge Design  Practice, State of California Department of Transportation, California

[18]     American Association of State Highway and Transportation Officials (AASHTO).  2012. “AASHTO LRFD Bridge Design Specifications, AASHTO, Washington DC.

[19]     International Organization for Standardization (ISO). 1998. “ISO 2394 – General principles on reliability for structures”. International Organization for Standardization, Geneva, Switzerland.

[20]     G. Fenton, et al. July 2015 “Reliability based geotechnical design in the 2014 Canadian Highway Bridge Design Code”. Canadian Geotechnical Journal, NRC Research Press, Canada

[21]     Y. Honjo, et al. October 2010. “Development of the design codes grounded on the performance – based design concept in Japan” Gifu University, Gifu, Japan

[22]     ANSI/AISC 360-05. 2005 “Specifications for Structural Steel Buildings” American Institute of Steel Construction (AISC), Inc. Chicago, Illinois

[23]     Y. Honjo, et al. October 2010. “Development of the design codes grounded on the performance – based design concept in Japan” Gifu University, Gifu, Japan

[24]     D. Blockley. 1980. “The nature of structural design and safety” Ellis Horwood Limited, West Sussex, England

[25]     B.R. Ellingwood, et al. January 2014. “Eurocodes and Their Implications for Bridge Design: Background, Implementation, and Comparison to North American Practice”, Journal of Bridge Engineering, ASCE, Reston, Virginia

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We are thankful to Er. Isidro P. Buquiron for submitting this very useful paper to us.

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