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PART FOURThe Design ProcessTENSILE CAPACITYNote:Due to the spacingalgorithm used inKeyWall, some of thereinforcing layers maybe slightly over-stressedin the trial design section.It is our opinionthat a slight overstress isacceptable as long as thelayers above and belowthe layer in questionare below the allowablestress level.Tensile failure occurs when the long-term design strength of the reinforcement is exceeded and leads totensile failure of the elements. A factor of safety or load resistance factor is incorporated into the design tokeep the calculated applied stress safely below the rupture limit.A factor of safety is typically applied as a reduction to the Long-Term Design Strength (LTDS) of thereinforcement in allowable stress design. In Limit State Design, load factors increase the applied loads andmasses above their actual values and compare to the LTDS, similar to AASHTO LRFD. Both methodsachieve essentially similar results. The load factor design method allows factoring of various applied loads(i.e., live load versus dead load) and materials by factors, depending on their variability and potential effecton the design.Using the concept of “the sum of the parts equals the whole,” theoretical earth pressure stresses in eachelement can be isolated and calculated as an applied load. The “whole”, in this case, is the total internalstress within the reinforced zone. The internal pressure for earth pressure and surcharge are superimposedon the reinforcement levels as shown in Figure 4:15 using similar Coulomb and Rankine earth pressures ascalculated for the reinforced soil type:Earth Pressure Resistedby Top ReinforcementEarth Pressure Resistedby Second ReinforcementEarth Pressure Resistedby Third ReinforcementEarth Pressure Resistedby Forth Reinforcementσqσ aBase Shear at BoundaryFigure 4:15 Internal Stress Distribution4.18