The American Association of State Highway and Transportation Officials recently adopted a new strut-and-tie modeling (STM) specification and is strongly encouraging bridge engineers to adopt the STM method. Unfortunately, guidance in deploying STM technology, and more specifically, guidance that follows the AASHTO LRFD [Load and Resistance Factor Design] Bridge Design Specifications, can be difficult for bridge practitioners to come by. Additionally, very few U.S. colleges include STM in their structural engineering curriculum, further widening the knowledge gap. In response, the National Highway Institute created Strut-and-Tie (STM) Modeling for Concrete Structures (course number130126).

What is Strut-and-Tie Modeling?

STM is a technique that is commonly used to reduce complex states of stress in reinforced and prestressed concrete structures into a simplified truss model. STMs are made up of elements loaded uniaxially in tension (referred to as ties) or compression (referred to as struts). The intersection points of the struts and ties are called nodes. Engineers can then determine the forces on this streamlined truss model using basic statics and truss analysis. The calculated forces in each strut, tie, and node element are then checked against the permissible values in the AASHTO LRFD bridge specifications. STM analysis offers a unified approach that considers all force effects (moment, shear, axial) simultaneously, which can reduce the analysis effort compared to traditional methods.

STM provides engineers with a simplified analysis and design tool for deep concrete bridge elements and disturbed regions that would otherwise require a rigorous refined analysis. STM has long been established as a practical analysis tool for disturbed regions and deep beams. However, this modeling tool has not been smoothly integrated into state-of-the-practice bridge design, which has resulted in inappropriate use. In some cases, it has resulted in poor inservice performance.

NHI’s STM for Concrete Structures course aims to address the uncertainties behind using STM and act as a primary source of reference material for STM applications. The 1.5-day instructor-led training serves as a significant step in providing the knowledge transfer necessary for STM to be used more frequently and more effectively.

Bridging the Knowledge Gap

During this training, participants learn the fundamentals of STM and its application in bridge design, including identification of regions and specific applications to bridge superstructures and substructures. They also learn the required procedures for developing and designing an STM model. The course covers element-level considerations including struts, ties, and nodal zones, and serviceability considerations including crack control, shear stress check, and sizing of members to minimize diagonal cracking. The course reviews the STM provisions presented in AASHTO’s LRFD specifications and teaches participants to apply STM fundamentals and procedures through four comprehensive design examples that progress in complexity. Course developers selected the design examples to cover as many real-life STM bridge modeling variations as possible.

“The topic and course [are] a high-value opportunity,” says attendee William Nickas, managing director of transportation systems for the Precast/Prestressed Concrete Institute (PCI). “Congratulations to NHI and FHWA for creating a curriculum that will certainly help bridge engineers get up to speed with this concrete topic.”

This course is ideal for practitioners at State departments of transportation, including bridge and structures engineers and practicing bridge engineers who are responsible for concrete bridge design and evaluation. This includes engineers of all levels, as well as designers, consultants, reviewers, maintenance engineers, management engineers, and load rating engineers. Participants should have a bachelor of science degree in civil engineering, a working knowledge of AASHTO LRFD specifications, and relevant design experience using the current AASHTO LRFD specifications on at least one concrete bridge project.

For more information on this course, to register for a session, or for information on how to bring this training to your agency, visit www.nhi.fhwa.dot.gov.

Reggie Holt is a structural engineer for FHWA and technical lead for course 130126.

Judy Francis is a contracted marketing analyst for NHI.