LRFD for Geotech – Note 1


LRFD for Bridge Substructure Design – A Note on Limit States and Interaction between Structural and Geotechnical Specialists

By
Naresh C. Samtani, PE, PhD
President, NCS Consultants, LLC (www.ncsconsultants.com)

Editor’s Note: Naresh is the employer of Randy Post, the owner and editor of GeoPrac.net.

[Editor May 22, 2007] The author has made changes to this article. It was updated May 8, 2007 on his website, NCS Consultants, LLC. My apologies for not publishing these changes sooner. [/Editor]

The Load and Resistance Factor Design (LRFD) approach is currently being implemented across the United States, particularly in the realm of federally funded transportation facilities.  The American Association of State Highway and Transportation Officials (AASHTO) recently released the 4th Edition of the Bridge Design Specifications based on the LRFD approach (AASHTO, 2007).  Starting October 1, 2007, the AASHTO-LRFD approach will have to be fully implemented by states seeking federal funding for new transportation projects.   It is important that the structural and geotechnical specialists involved in the design of such transportation facilities properly understand the basics of the LRFD approach as included in AASHTO’s specifications. 

In the context of bridge design, the substructure portion is considered to include all the elements below the level of the bridge deck.  The piers and abutments transfer the loads from the bridge deck to the foundations.  The way these loads are combined in the AASHTO-LRFD (AASHTO, 2007) approach and compared to resistances is significantly different from that in the Allowable Stress Design (ASD) approach in the 17th Edition of AASHTO’s standard specifications for highway bridges (AASHTO, 2002).  This note briefly presents the concept of limit states in the AASHTO-LRFD framework, identifies the common limit states, discusses the basic concept of load combinations and finally provides some thoughts on the interaction between structural and geotechnical specialists in the design of highway bridge substructures.