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54 CHAPTER FOUR DESIGN FOR LATERAL LOADING SCOPE that are then used by GD to perform revised lateral load analyses. In addition, the Bridge Office may conduct their Twenty-two states indicated in the survey that lateral own analyses using soil and rock-structure interaction mod- loading considerations govern the design of drilled shafts els with soil and rock properties provided by GD. The in rock or IGM on at least some of their projects. Several Bridge Office uses the modeling results to establish design states responded that lateral loading governs 100% of parameters for drilled shaft reinforced concrete. According their designs. The survey also demonstrated that the most to the 2006 Interim AASHTO LRFD Bridge Design Speci- widely used analysis is the p-y method, although other fications, the strength limit state for lateral resistance of methods are also used. In this chapter, analytical models deep foundations is structural only. The basic assumption identified by the literature search and by the survey are re- is that "failure" of the soil/rock does not occur; instead, the viewed and evaluated for their applicability to rock sock- geomaterials continue to deform at constant or slightly ets. Structural issues associated with rock-socketed shafts increasing resistance. Failure occurs when the foundation are considered. reaches the structural limit state, defined as the loading at which the nominal combined bending and axial resistance is reached. DESIGN PROCESS Axial loading in both compression and uplift requires struc- Deep foundations supporting bridge structures may be sub- tural analysis and reinforced-concrete design for drilled shafts. jected to lateral loading from a variety of sources, including When lateral loading is not significant, structural design is rea- earth pressures, centrifugal forces from moving vehicles, sonably straightforward. When lateral loading is significant, braking forces, wind loading, flowing water, waves, ice, seis- the combined effects of lateral and axial loading are analyzed mic forces, and impact. Reese (1984) describes numerous using models described in this chapter, which account for the examples of bridges, overhead sign structures, and retaining effect of axial load by treating the shaft as a beam column. For structures as typical examples of transportation facilities that this reason, structural design issues associated with rock- must sustain significant lateral loading. Drilled shafts are of- socketed shafts are addressed in this chapter. ten selected for such structures because they can be designed to sustain lateral loading by proper sizing of the shaft and by providing a sufficient amount of reinforcing steel to resist the ROCK-SOCKETED FOUNDATIONS resulting bending moments. FOR LATERAL LOADING Design for lateral loading of drilled shafts requires sig- Rock-socketed shafts provide significant benefits for car- nificant interaction between geotechnical and structural rying lateral loads. Embedment into rock, in most cases, engineers. As described in chapter one, the Bridge Office reduces the lateral displacements substantially compared (structural) is responsible for structural analysis and design with a deep foundation in soil. To take full advantage of of the superstructure and the foundations. However, to model rock-socketed drilled shafts, designers must have confi- foundation response to lateral loading it is necessary to ac- dence in the analytical tools used for design. The survey count for soil/rock-structure interaction. The Geotechnical questionnaire shows that traditional methods of analysis Division (GD) normally conducts foundation analysis using for lateral loading of piles and drilled shafts in soil are the the models described in this chapter. For preliminary foun- most widely used methods currently employed for rock dation design, geotechnical modeling of foundation re- sockets. Recent research has led to some advancements for sponse by the GD is used to provide the Bridge Office with applying these methods to rock. In addition, several re- information such as depth of fixity, trial designs (diameter searchers have proposed new analytical methods that provide and depth) of drilled shafts, and equivalent lateral spring designers with useful tools for predicting load-displacement values for use in seismic analysis of the superstructure. The response and/or structural response of the reinforced- Bridge Office conducts analyses of the superstructure based concrete shaft. Each method has advantages and disadvantages on models that include fixed-end columns at the depth of for design purposes and these are discussed in the following fixity. The structural analysis may result in revised loads sections.