Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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- 5.2.1 Fixity Depth
| 5.2 OTHER DESIGN TOPICS Other concepts and the number of DOTs mentioning each topic are presented in Table E, followed by a brief discussion regarding each. Table E DOTs referencing various design topics regarding lateral loading. Topic Number of DOTs Fixity Depth 16 Critical Depth/Reese (1984, 1985) 12 Group Multipliers 12 Head Fixity 4 Deflection Limits 22 Seismic 10 Design Procedure 6 Engineer Responsibilities 18 Resistance Factor 8 5.2.1 Fixity Depth The concept and definition of fixity depth, or point of fixity, is often interpreted differently between the structural and geotechnical disciplines and/or between different geotechnical practitioners. The structural definition for preliminary analysis per by Davisson and Robinson (1965) is where the fixity depth of a partially unsupported foundation element represents the point at which the element can be considered a cantilevered beam, with no lateral support and a fixed end condition. The Davisson and Robinson method is referenced in AASHTO 10.7.3.13 for preliminary design, and tends to provide a quick means to compare foundation alternatives with respect to the requirements to resist bending. For more rigorous strength analysis, AASHTO 6.15.2 defines fixity based upon the second point of zero deflection at the factored load based on a P analysis. The geotechnical discipline typically uses a definition based on the lateral head deflection at a defined depth of embedment based on a soil structure interaction analysis, whereby fixity is defined as the first or second point of zero deflection or the point of maximum negative deflection, but is dependent on the load case being analyzed (strength/service/extreme). Two of the investigated documents added 5 feet to the calculated point. The depth, as defined by the geotechnical discipline infers the minimum design length to safely resist lateral loads relative to deflection and soil strength (pushover analysis. The quantity in Table E makes no distinction between definitions, but simply presents the number of documents using the fixity depth terminology. Some variation of the geotechnical definitions described above is generally included in five of the documents, three of which use the second point of zero deflection. The other two instances reference the first point of zero deflection and the point of zero deflection, with no mention of first or second point. The load combinations employed for the analyses differ between the documents and do not employ strictly Strength or Service Limit States. Download 6.03 Mb. Share with your friends:
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