Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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- 5.1 DESIGN METHODOLOGIES
- Figure E- 2: State DOTs referencing various design methodologies.
| 5. SUMMARY OF STATE DOT RESEARCH Within the manuals and design memoranda/directives investigated the approaches to design vary considerably, from no guidance to detailed design procedures. The division of responsibility between the geotechnical engineer and structural engineer also varies considerably among the available manuals and published procedures. Some manuals limit the geotechnical engineer’s responsibility to supplying p-y curves or soil parameters, whereas other manuals direct the geotechnical engineer to perform the majority of the design, with the structural engineer only responsible for verifying structural capacity. Deflection limits, when supplied, ranged considerably and varied between service and Strength Limit States. Given the foregoing variation instate DOT practices, differences are summarized in the following sections. 5.1 DESIGN METHODOLOGIES Figure E- 2 depicts the design methodologies referenced within the DOT manuals. Note that some DOT’s provide guidelines for or permit more than one methodology for various reasons including Preliminary versus final design Projects with low versus high anticipated lateral loads • Piles/shafts classified as short versus long Size and complexity of the project 267 Figure E- 2: State DOTs referencing various design methodologies. In the more thorough design manuals, Broms method is normally only used for preliminary design purposes. However, in some cases, such as projects with relatively low lateral loads, small scale, or low complexity it maybe the only method employed. Other DOTs simply list the methods, inclusive of Broms method, but do not offer written guidance on which maybe appropriate for various design stages or project complexity. The most prevalent design methodology involves the use of p-y curves. It is listed in nearly all the manuals that include sections related to resistance to lateral loading. In the vast majority of manuals, there is no specific guidance beyond mentioning the analysis method. There is often no discussion regarding what defines the design length, head fixity condition, service or strength state, or deflection criteria. A few of the manuals include empirical charts or tables based on general rules of thumb or analyses conducted by the DOT or a third party related to common pile/shaft sizes and loading conditions. The use of these items is limited to specific design cases and is indicated as such. No similarities could be ascertained based on a cursory review of the information obtained. The strain wedge model is only specifically mentioned by six DOTs. It is recognized that this method is another version of Soil Structure Interaction (SSI), similar to the p-y method, such that the methods could be used interchangeably. However, because the strain wedge model is not as widely used as the p-y method, this document only includes cases where the specific method or software is mentioned. Deep Foundation System Analysis Program (DFSAP), a strain wedge program, is no longer recommended or supported by the Washington State DOT due to lack of usage by the greater bridge community, other DOTs and consultants, compatibility with software upgrades, lack of agreement with AASHTO LRFD revisions and lack of conformity with WSDOT IT system and maintenance support (WSDOT Memorandum dated March 26, 2014). Other software packages still exist and AASHTO LRFD 2014 does allow use of the strain wedge model for the Strength Limit State in the comments section. Additionally, Report S2-R19B-RW-1 by the Transportation Research Board includes mention of strain wedge model in the main text for consideration in future AASHTO LRFD revisions. |