Calculation of large-size polyethylene pipes with hollow walls
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CALCULATION OF LARGE-SIZE POLYETHYLENE PIPES WITH HOLLOW WALLS M. H. Stashchuk1,2 and M. I. Dorosh1
UDC 539.3
For polyethylene pipes with a hollow (cellular) wall, we have proposed a method for the estimation of the stress-strain state arising under the action of soils. We have considered two modes of the loading of a cellular pipe: with a rigid base and with subsidence into the soil. We have established which of the building standards for pipes with a solid wall should be used in the calculations of pipes with a hollow wall. We have calculated the critical deflections of pipes and determined the criteria that must be used for the assigned geometrical and mechanical parameters of pipes and soils. Keywords: polyethylene cellular pipes, minimal long-term strength, hoop stresses, annular rigidity, stress-strain state, pipe lifetime.
In recent years, metallic pipes have been widely replaced by polymeric ones. For this purpose, it is necessary to develop the corresponding methods for estimating the strength and reliability of substitutes themselves. At the same time, the replacement of large-size metallic pipes by polyethylene pipes with a hollow (cellular) wall structure becomes the most widespread [1–3]. Here, it is customary to use polyethylenes of grade PE-80 and PE-100 [3]. Constructions of such materials form the basis of building of low-head industrial, rainwater, and sewerage networks. Pipeline cellular constructions are manufactured by winding of the usual water polyethylene tubes of diameter 20 to 110 mm and welding of neighboring coils between themselves. Since the walls of such pipe elements are hollow, the expenditures for their manufacture and materials consumption decrease. The efficient selection of tube diameters optimizes the annular rigidity of pipes, necessary for ensuring the reliable operation of large-scale constructions. Formulation of the Problem To guarantee the required term of reliable operation of polymeric low-head pipelines of large diameter, including those with a hollow wall structure, the control of their initial deflections (shortening of the vertical diameter) becomes the first priority (Fig. 1а). It is customary to use here the Spangler empirical relation [4, 5]
Δ C1q = , D C 2 Sn + C 3E s′
(1)
where Δ is the shortening of the pipe vertical diameter, q is the intensity of vertical loading of the soil, D is the median pipe diameter, Sn is the annular rigidity of the pipe [5], E s′ is the secant modulus of the soil, and Ci (i = 1, 2, 3) are constants, a method for unambiguous choice of which has not been developed up to now, and they are different for different national building rules [5]. According to [6], in the designing of polym1 2
Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 48, No. 4, pp. 39–45, July–August, 2012. Original article submitted December 6, 2011. 456
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