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PART FOURThe Design ProcessAASHTO LRFDLoad resistance factor design uses multiple variables to handle uncertainties. Load factors are applied toeach of the different types of loads: live, dead, horizontal and vertical. Resistance factors are applied tothe nominal resistance. The load factor and resistance factors are set based on statistical data andcan better represent the uncertainties compared to ASD/FS design methods. The general form ofLRFD is shown below.Equation (4c) CDR =φ Rγ t1 P t1 + γ t2 P t2> 1.0where:φ = resistance factorR = resistance (stabilizing forces)γ t1 , γ t2 = load factor for a certain load typeP t1 , P t2 = load of a certain type (destabilizing forces)CDR = capacity demand ratioGiven the nature of retaining wall design where certain loads contribute to the calculation of theresistance, R, load factors are also used to compute the nominal resistance. For example, in the slidingcalculation the weight of the reinforced zone contributes to the resisting force. A resisting load factoris applied to this load. AASHTO lists both driving load factors (maximum) and resisting load factors(minimum) in chapter 3 of the 2010 Standard Specifications for Highway Bridges. The applicable loadfactors are shown in the table below:Strength IDriving LoadFactorsResisting LoadFactorsEH d = 1.50 EH r = 0.90EV d = 1.35 EV r = 1.00ES d = 1.50 ES r = 0.75LL d = 1.75AASHTO LRFD Load FactorsExtreme I (Seismic)Driving LoadFactorsResisting LoadFactorsEH d = 1.50 EH r = 0.90EV d = 1.35 EV r = 1.00ES d = 1.50 ES r = 0.75LL d = 0.50 LL r = 0.00EQ = 1.00 n/aResistance factors for the external and internal failure mechanisms are listed below:Strength IExtreme ISliding RF sl = 1.00 Sliding RF sl = 1.00Overturning = NA Overturning = NABearing RF b = 0.65 Bearing RF b = 0.65Tension RF t = 0.90 Tension RF t = 1.20Pullout RF po = 0.90 Pullout RF po = 1.20AASHTO LRFD Resistance Factors4.10