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218 I Chapter 6 Design of PE Piping Systems crown may completely reverse its curvature inward and collapse.See Figure 3-1 A. A deflection limit of 7.5% 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 5%have been proposed,Jansen(12)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 instability and strain and is considered a safe design deflection.Some engineers will design profile wall pipe and other non-pressure pipe applications to a 5% deflection limit,but allow spot deflections up to 7.5% 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(13)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 �"�, .. s. IeM 26 7.5 17 6.0 ti 11 5.0 N ��r.�,�1 4 7.3 3.0 Based on Long-Term Design Deflection of Buried Pressurized Pipe given in ASTM F1962.