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2:18 Chapter 6 Design of PE Piping Systems crown may completely reverse its curvature inward and collapse.See Figure 3-1A, A deflection limit of 7.517o provides at least a 3 to 1 safety factor against reverse curvature. Bending strain occurs in the pipe wall as a result of ring deflection—outer-fiber tensile strain at the pipe springline and outer-fiber compressive strain at the crown and invert.While strain limits of 517o have been proposed,Jansen(t2)reported that,on tests of PE pipe manufactured from pressure-rated resins and subjected to soil pressure only,"no upper limit from a practical design point of view seems to exist for the bending strain. In other words,as deflection increases,the pipe's performance limit will not be overstraining but reverse curvature collapse. Thus,for non-pressure applications,a 7.5 percent deflection limit provides a large safety factor against ins a ility and strain and is considered a safe design deflection.Some engineers will design profile wail pipe and other non-pressure pipe applications to a 5% deflection limit,but allow spot deflections up to 7.59o' during field inspection. The deflection limits for pressurized pipe are generally lower than for non- pressurized pipe.This is primarily due to strain considerations.Hoop strain from pressurization adds to the outer-fiber tensile strain.But the internal pressure acts to reround the pipe and,therefore,Eq.3-10 overpredicts the actual long-term deflection for pressurized pipe.Safe allowable deflections for pressurized pipe are given in Table 3-11.Spangler and Handy t3)give equations for correcting deflection to account for rerounding. TABLE 3-11 Safe Deflection Limits for Pressurized Pipe DR or SDR Safe Deflection as%of Diameter 26 7.5 Sym: 17 6.0 11 5.0 7W— IV47737� 7.3 3.0 Based on Long Term Design Deflection of Buried Pressurized Pipe given in ASTM F1962.