LRFD for Geotech – Note 1

4.0 Role of Structural and Geotechnical Specialists in LRFD Framework

Since the substructure elements include both structural and geotechnical aspects it is imperative that the structural and geotechnical specialists work together during the design of substructures.   For example, in the design of a deep foundation it is necessary to consider limit states with respect to various factors such as structural axial strength, soil strength, lateral load behavior, structural lateral strength, settlement, scour, ship impact response and earthquake response.  This degree of complexity necessitates an interaction between the structural and geotechnical specialists.  During this interaction, it is important for both structural and geotechnical specialists to develop an understanding of each other’s work and refrain from adding “comfort” factors to the nominal load and resistance values to account for their discomfort or mistrust in each other’s preference.  Such “comfort” factors can lead to modification of the load and the resistance factors to an extent that they may not be applicable as developed for various limit states in the AASHTO-LRFD framework.  Such modifications can lead to spurious designs, which may be either overly conservative or unsafe.  Overly conservative designs misuse tax payer’s money while unsafe designs can lead to failures that may result in loss of life and/or potential litigation.

Acknowledgement:  The author wishes to acknowledge Dr. Edward A. Nowatzki, PE, Principal Engineer of NCS Consultants, LLC, for his effort in reviewing this article and providing comments.


1. AASHTO (2007), “AASHTO LRFD Bridge Design Specifications,” 4th Edition.  American Association of State Highway and Transportation Officials, Washington, D.C.

2. AASHTO (2007), “Standard Specifications for Highway Bridges,” 17th Edition.  American Association of State Highway and Transportation Officials, Washington, D.C.

3. Samtani, N. C. and Nowatzki, E.A. (2006). “Soils and Foundations,” Volume II, Report No. FHWA NHI-06-089, Federal Highway Administration, U. S. Department of Transportation [Available for free download at].

4. Turkstra, C. J. (1970). “Theory of Structural Design Decisions,” Solid Mechanics Study No. 2, University of Waterloo, Waterloo, Canada.

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