The Load Factor

by R. C. Prevost,

Document Type: Proceeding Paper

Part of: Pipeline Infrastructure II


Spangler's load-factor formula for projecting conduits is generalized to all cases of buried rigid pipe, whether in trenches or under embankments; this is in accordance with the analytical methods of structural design, and suggests a simple mathematical expression for the load factor (Lf). The discussion below of this formula provides evidence of the importance of lateral active soil pressures hence of adequate pipe-installation practices, setting lower and practical values to Lf. The calculated values and data from different sources are correlated, and are generally seen to fit together well: there are, however, values of Lf which are either overly-conservative or overestimated. The language used for specifications, as well as consistency among data, relating to bedding procedures could be improved. Lf values, and design and construction practices can be readily verified, even in the field, through strain measurements. The empirical external-load/internal-pressure parabolic interactions in rigid pipe are shown to be an effect of the stiffening of pipe by pressure, limited by the significantly different behaviour of some pipe materials when subjected either to bending stresses (external loads), or to uniform tensile stresses (internal pressure). The conventional, analytical ('stress-analysis') method, still favored in Continental Europe, makes a long detour around the Lf method, while aiming at the same results; it does not allow for the external-load/internal-pressure interaction, nor for the different bending/bursting behaviour of some pipe materials, and misses Lf's direct connection with the bedding and installation specifications and the actual pipe performance; the Lf method should prevail.

Subject Headings: Pipelines | Load factors | Pressure pipes | Lateral pressure | Pipe materials | Bending (structural) | Ultimate strength | Stress analysis

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