Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Table 4-1: Resistance factors for lateral Geotechnical and Strength Limit State (from Brown et alb b)
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- SERVICE LIMIT STATE FOR LATERAL DISPLACEMENTS
| Table 4-1: Resistance factors for lateral Geotechnical and Strength Limit State (from Brown et alb b) Application Resistance Factor, φ r Pushover of individual deep foundation element head free to rotate 0.67 Pushover of single row of deep foundation elements, retaining wall or abutment head free to rotate Pushover of deep foundation element within multiple-row group, with moment connection to cap 0.80 The state of the practice for analysis of laterally loaded deep foundations is evolving as additional research and lateral load testing results are obtained. For example, local DOTs have developed locally calibrated resistance factors based on substantial successful experience and load tests (Abu-Hejleh et al. 2011). Additional references discussing local resistance factors include the Idaho Transportation Department Bridge Design LRFD Manual (2008), a study by Robinson et al. (2006) for NCDOT, and a study by MoDOT (Boeckmann et alb SERVICE LIMIT STATE FOR LATERAL DISPLACEMENTS Service Limit State failure modes are related to performance problems (e.g., deformation) fora structure under regular operating conditions. For laterally loaded deep foundations, Service Limit States relate to lateral displacement of foundation elements considering interaction with the structure. Laterally loaded deep foundations must have adequate structural and geotechnical resistance to keep bridge or structure displacements within established tolerable levels. Tolerable lateral settlement is project specific and should be determined by the project structural engineer. A resistance factor of 1.0 is applicable to Service Limit States. Soil-structure interaction analyses methods are to be used for Service Limit States and may include Broms method, strain wedge method, and p-y analyses depending on the type of structure and foundation (AASHTO 2014). Note that p-y analyses are the recommended method. Lateral displacements of foundations elements are analyzed at Service Limit State load conditions to ensure that the lateral foundation displacements are below tolerable lateral structure displacements. The tolerable lateral displacements of foundation elements are often limited based on lateral displacements that will adversely impact the structure, such as closure of joints on bridge structures, excessive structure lean or rotation, displacement of one structural element into another, aesthetics, etc. The magnitude of tolerable lateral displacements should be determined by the structural engineer and maybe based on considerations other than the geotechnical resistance or the structural capacity of the foundation (these displacements are often less than the amount of displacement that the foundation elements themselves can withstand, especially for ductile elements such as steel piles. The limiting deflection of the structure, and therefore the limiting deflection of the foundation, are assessed using Service Limit State load combinations and load and resistance factors equal to 1.0. Analyses of foundations for deformation are generally performed using p-y methods, although alternative methods such as the strain wedge method or FEM can be used analysis methods are discussed in more detail in Chapter 6. Lateral displacements can also be determined from lateral load tests. 45 Each individual or group of foundation elements should be designed so that deformations do not exceed criteria established for the bridge or other structure. The deformations to be checked relative to lateral loads on deep foundations include Horizontal movement at the top of the foundation Rotation at the top of the foundation Horizontal movement under scour at the design flood Horizontal displacement at the superstructure level Lateral displacement may also be part of the criteria used to define Geotechnical Strength Limit States, as discussed in Chapter 5. Lateral displacements may also be a consideration for Extreme Event loading, to avoid adjacent structures from impacting each other or to avoid excessive damage to the structure. Earthquake loads can result in liquefaction in subsurface soil layers or lateral displacement of a slope (in combination with excess pore pressures and gravity loads. Lateral slope movement or movement of surficial soil blocks overlying a liquefied layer may result in lateral displacements to deep foundations, which can result in additional loads on the foundation elements and reduce the available deformation capacity for the service limit resistance. Much work has been done in the field testing of drilled shafts and piles to measure lateral displacements. Examples of these are included in Brown et al. (2010) and shown in Figure 4-6. Measurements of lateral deformations and displacements should be part of any lateral load testing program. Measurements of lateral displacements can also be included as part of a monitoring program to verify performance of a foundation system. For example, lateral deformation measurements, such as inclinometers, maybe included within drilled shafts used for slope stabilization. Deformations of such shafts can be monitored following installation to verify that the slope has been stabilized or to establish the need for the installation of additional elements if ongoing or future movements are observed. A more detailed discussion of field testing of deep foundations and associated instrumentation is included in Chapter 12. Download 6.03 Mb. Share with your friends:
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