Even though the deformation behavior of plastics is rather complicated, successful design can be simplified by using a creep or an apparent modulus that reasonably reflects response under the anticipated loads. For example, a buried pipe may be subjected to relatively short periods of externally induced high loads, followed by longer periods at lower loading. For such conditions, Boltzman superposition theory can be used to estimate an appropriate creep modulus. More often though, a simpler design check assuming a worse-case condition is all that is required. For example, the wall thickness of a buried pipe is first selected based on pressure rating considerations and then a check is made to ensure that this wall is sufficient to withstand a given combination of traffic (short-term) and soil (long-term) loads. To conduct the design check, it can be assumed that the two loads are simultaneously and continuously present; if the pipe is adequate to this task then no further calculation need be made. While procedures such as this are often used to simplify design, care must be exercised not to make unduly conservative assumptions which could rule out a fully acceptable construction that would be justified by a more refined evaluation.
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