LRFD

State Department of Transportation
LRFD Implementation Plan Initial Draft

Background

Since the adoption of the first AASHTO Specifications in 1931, the body of knowledge in bridge research and design has been growing tremendously. A TRB research program (1987) concluded that the Standard Specifications include gaps and inconsistencies, and do not utilize the latest design philosophy and knowledge.

When AASHTO began publishing the Standard Specifications for Highway Bridges in the 1930s, one factor of safety was used. The methodology was called allowable stress design (ASD). In the 1970s, AASHTO began varying the factor of safety for each load in relation to the engineer's ability to predict the corresponding load. This bridge design methodology was named load factor design (LFD). AASHTO made this change from ASD to LFD in the form of interim revisions to the Standard Specifications for Highway Bridges. AASHTO had never totally rewritten its Standard Specifications.

Today, the bridge engineering profession is moving to Load and Resistance Factor Design (LRFD) and new specifications with a framework to implement new technology for decades to come. The new specifications utilize state-of-the-art analysis and design methodologies, and make use of load and resistance factors based on the known variability of applied loads and material properties. The load and resistance factors are calibrated from actual bridge statistics to ensure a uniform level of safety. The designer focuses on a design objective or limit state, which can lead to a similar probability of failure in each component. Bridges designed with the LRFD specifications should have more uniform levels of safety, which should lead to superior serviceability and long-term maintainability.

History of Major Events

• 1930s to 1999 - Publication of the Standard Specifications for Design of Highway Bridges. Changes were made annually and have resulted in a "patchwork document" which made it increasingly difficult to incorporate new innovations.

• In the late 1970s, the Ontario Ministry of Transportation published limit-state design bridge specifications with a side-by-side commentary, two features that pleased its users.

• In 1986, NCHRP Project 20-7/31 commenced to explore the feasibility of a new bridge design specification.

• In 1986, The American Institute of Steel Construction (AISC) published the LRFD Manual of Steel Construction, 1st Edition through the third edition that was released in 2001.

• In 1988, NCHRP 12-33 concerning the "Development of Comprehensive (limit-state) Specification and Commentary" started.

• In 1994, the AASHTO LRFD Specifications for Design of Highway Bridges, 1st Edition, was published in SI and Customary US units

• In the mid 1990s, structural steel design started to be taught using the LRFD Manual of Steel Construction, rather than ASD, at virtually numerous colleges and universities.

• In 1999, the AASHTO Subcommittee on Bridges voted to stop maintaining the (LFD) Standard Specifications for Highway Bridges.

• In 2000, the AASHTO/NCHRP favored research projects that improve or promote acceptance of the new LRFD Specifications.

Goal

In 2000, AASHTO and FHWA set a transition date of October 1, 2007 after which all new bridges on which states initiate preliminary engineering shall be designed by the LRFD Specifications.

The State DOT (DOT) will use LRFD 100% of the time in the design of all new and replacement structures by October 2006.

Strategy

The Bridge Office will adopt a two-pronged approach to the LRFD implementation:

• In FY 2003, the Bridge Production Office will use LRFD on pilot project(s) and will transition to 100% in FY 2006. The Bridge Office management will select the pilot project(s) targeted for LRFD criteria in assigning jobs to bridge designers. Or, in FY 2003, the Bridge Production Office will use LRFD for 25% of its total bridge designs and will increase that proportion by 25% each year, reaching 100% in FY 2006. Bridge Office management will refer to a growing list of bridge design types targeted for LRFD criteria in assigning jobs to bridge designers (simple-span, non-skew, uniform-width-deck projects first, and gradually add to the complexity)

• Starting in FY 2004, the Bridge Office will prepare a plan for promotion and support of LRFD implementation in DOT districts and among DOT consultants and fabricators. In support of LRFD implementation, the Bridge Office will prepare LRFD supporting materials that include design directives, manuals, standards, a web page, and an LRFD design manual. The Bridge Office will also facilitate training for DOT district staff, consultants, and fabricators.

The Bridge Office will also adopt an additional two-pronged approach to complement the LRFD implementation:

• Starting in FY 2003, the Bridge Office will participate in national LRFD forums. This participation is essential to learn of LRFD implementation in other states, to guide the evolution of LRFD requirements, and to ensure that LRFD tools meet DOT needs.

• Starting in FY 2003, the Bridge Office will facilitate research into major effects of LRFD implementation on DOT bridge design.

The Bridge Office will phase LRFD implementation on these four fronts to accommodate the time and resources required to:

• Help refine LRFD policy and guidelines.

• Develop supporting documents and training for DOT bridge designers and allow them to become efficient using new methods so that the letting schedule for bridges is met while designers are coming up to speed with the new methods.

• Adopt LRFD requirements to State bridge design.

Plan for Implementation

Schedule: LRFD design of new State bridges will be implemented in the two following phases:

Phase 1. FY 2003-2004: Perform initial assessment and implementation planning, and begin designing by LRFD

Phase 2. FY 2004-2006: Approve implementation plan, publish LRFD standards, and design all new and replacement DOT bridge structures using LRFD

Responsibilities:

Design Directives:
Meeting goals for percentages of bridges designed by LRFD:
Acquisition, development, and deployment of software:
Coordination of LRFD training:
LRFD Design Manual:
Standards:
Technical Support:

Resources Needed

Tools: The LRFD method is new with regard to the vehicular live loads, load combinations and resistance equations that must be considered, and the proposed implementation requires effective, accurate, and easy-to-use software. The proposed LRFD implementation requires the acquisition and/or development of the following:

• AASHTO LRFD Bridge Design Specifications, annual interims, and design examples resources.

• LRFD design software (multiple site-licensed packages for design of bridge elements), needed as follows:

Work remaining includes the following:

People: The proposed LRFD implementation requires staff time for the following tasks:

• Tools selection, testing, and deployment

• Development of a plan to promote use of LRFD in DOT districts and among DOT consultants and fabricators

• Preparation of LRFD implementation documentation (directives, manual, standards, and web page)

• Participation in national forums on LRFD design

Training: The proposed LRFD implementation requires the following types and amounts of training in:

• LRFD Design

• Tools

Summary of Costs

The following table summarizes costs expended and anticipated in support of LRFD implementation.

Concerns

The Bridge Office has the following concerns related to LRFD implementation:

List your concerns here and draft an action plan to resolve them.

Appendix A. National LRFD Forums with DOT Participation

Appendix B. DOT-Sponsored LRFD Research