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81 Figure 7-16. Simplified height-dependent scaling factor recommended for design. Recommendations for seismic coefficients to be used for when accelerations exceed the horizontal limiting equilibrium earth pressure evaluations based on the simplified straight line yield acceleration) was introduced by Richards and Elms functions shown can be expressed by the following equations: (1979). Based on this concept (as illustrated in Figure 7-17), Elms and Martin (1979) suggested that a design acceleration kav = kmax (7-1) coefficient of 0.5A in M-O analyses would be adequate for where limit equilibrium pseudo-static design, provided allowance kmax = peak seismic coefficient at the ground surface = Fpga be made for a horizontal wall displacement of 10A (in inches). PGA; and The design acceleration coefficient (A) is the peak ground acceleration at the base of the sliding wedge behind the wall = fill height-dependent reduction factor. in gravitational units (that is, g). This concept was adopted by For C, D, and E foundations soils AASHTO in 1992, and is reflected in following paragraph taken from Article 11.6.5 of the 2007 AASHTO LRFD Bridge Design = 1 + 0.01H [( 0.5 ) - 1] (7-2) Specifications. Where all of the following conditions are met, seismic lateral where loads may be reduced as provided in Article C11.6.5, as a result H = fill height in feet; and of lateral wall movement due to sliding, from values determined = FvS1/kmax. For Site Class A and B foundation conditions (that is, hard and soft rock conditions) the above values of should be increased by 20 percent. For wall heights greater than 100 feet, coefficients may be assumed to be the 100-foot value. Note also for practical purposes, walls less than say 20 feet in height and on very firm ground conditions (B/C founda- tions), kav kmax which has been the traditional assumption for design. 7.6 Displacement-Based Design for Gravity, Semi Gravity, and MSE Walls The concept of allowing walls to slide during earthquake loading and displacement-based design (that is, assuming a Figure 7-17. Concept of Newmark sliding block analysis Newmark sliding block analysis to compute displacements (AASHTO, 2007).