Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
LRFD DESIGN REQUIREMENTS AND LIMIT STATES FOR LATERALLY
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| 4 LRFD DESIGN REQUIREMENTS AND LIMIT STATES FOR LATERALLY LOADED DEEP FOUNDATIONS 4.1 INTRODUCTION The design of laterally loaded deep foundations must address all applicable Limit States using appropriate load combinations, load factors, and resistance factors. The design of laterally loaded deep foundations requires interdisciplinary coordination between the structural engineer and the geotechnical engineer, especially for determining LRFD design requirements for laterally loaded deep foundations. 4.2 LOAD COMBINATIONS AND LOAD FACTORS The Limit State design approach in LRFD requires an identification of all potential failure modes, or Limit States. A Limit State is defined as a condition for which some component of the structure does not fulfill its design function. Four Limit States are identified in AASHTO 2014: Strength, Service, Extreme Events, and Fatigue. Limit States that typically govern deep foundation design include Strength I, Strength IV, Extreme Events I and II, and Service I. Service Limit States II, III, and IV and Fatigue Limit States I and II are typically associated with superstructure behavior and not generally not applicable to foundation design. The total factored load fora given Limit State is calculated as the sum of the individual load effects and corresponding load factors and load modifiers that apply to the given Limit State. For the applicable load, load combinations and load factors associated with each Limit State, refer to AASHTO 2014. Figures 4-1 through 4-3 present simplified illustrations of typical loads on deep foundations for transportation structures. Figure 4-1 illustrates typical loads on abridge abutment in the longitudinal direction (loads arising in the transverse direction are not included in this illustration. Typical loads for piers supported on deep foundations are shown in Figure 4-2. Design loads transmitted from the superstructure include dead and live loads, wind on structure, wind on live load, and temperature forces. Piers also resist loads from self-weight, wind on substructure, stream flow, buoyancy, ice flows, creep, shrinkage, and other loads. When piers are in a river, stream, or other navigable waterway, the deep foundations must be designed against factors such as scour, stream flow effects, temperature effects associated with the water stream, and potentially vessel collision. Horizontal forces and bending moments caused by lateral loads acting on noise walls and similar structures are illustrated in Figure 4-3. When deep foundations are used to stabilize landslides or slopes, the soil behind the foundation elements can generate very high lateral forces. The loads that need to be considered for such an application are illustrated in Figure 4-4. In general, the computation of lateral loads in slope stabilization cases is more complex than the load computation for retaining structures. Deep foundations for slope stabilization are discussed in Chapter 10. 40 As illustrated in Figure 4-5, the reactions at the column-foundation joint, or pile cap, computed by the structural analysis are taken as the force effects transmitted to the foundations. For deep foundations, the reactions are resolved into vertical, horizontal, and moment components, and these are taken as the factored values of axial, lateral, and moment force effects, respectively, at the top of the foundation or pile cap. Multiple iterations are typically performed to obtain agreement between deformations and forces at the structure/foundation interface as calculated by both the structural and geotechnical analysis. The resulting factored force effects are substituted into Equation 4-1. Although this is a somewhat oversimplified description of the actual process, it is the general procedure by which factored foundation force effects are determined for each applicable Limit State. Also, note that load factors for permanent loads are specified at maximum and minimum values. For foundation design, modeling of the structure while varying the load factors is necessary to determine the combination resulting in maximum force effect acting on the foundation, which are then used in Limit State checks. The loads in Figures 4-1 through 4-5 include the permanent and transient loads that should be considered Permanent Loads – CR = Force effects due to creep – DD = Downdrag force – DC = Dead load of structural components and nonstructural attachments – DW = Dead load of wearing surfaces and utilities – EH = Horizontal earth pressure load – EL = Miscellaneous locked-in force effects resulting from the construction process, including jacking apart of cantilevers in segmental construction – ES = Earth surcharge load – EV = Vertical pressure from dead load of earth fill – PS = Secondary forces from post-tensioning for Strength Limit States total prestress forces for Service Limit States – SH = Force effects due to shrinkage Transient Loads – BL = Blast loading – BR = Vehicular braking force – CE = Vehicular centrifugal force – CT = Vehicular collision force – CV = Vessel collision force – EQ = Earthquake load – FR = Friction load – IC = Ice load – IM = Vehicular dynamic load allowance – LL = Vehicular live load – LS = Live load surcharge – PL = Pedestrian live load – SE = Force effect due to settlement – TG = Force effect due to temperature gradient – TU = Force effect due to uniform temperature – WA = Water load and stream pressure – WL = Wind on live load – WS = Wind load on structure |