Article
Bearing And Frictional Resistance: The Building Blocks Of A Restrained System
April 5, 2010
•Article: Bearing And Frictional Resistance: The Building Blocks Of A Restrained System
By EBAA Iron Sales, Inc.
Connections bulletins PD-1 through PD-4 discuss some of the specific applications of the design of a properly restrained pipeline. PD-2 specifically addresses horizontal bends and the various parameters involved in the design process. PD-3 applies those parameters to the design of vertical offsets and tees. Finally, the design of reducers, dead ends, and miscellaneous fittings is discussed in PD-4. This bulletin delves into a more detailed discussion of the bearing and frictional resistance forces at work to balance thrust forces generated in pressurized pipelines. To see how this information relates to the various pipeline elements, please refer to the other Connections bulletins.
The most significant factor used to counteract thrust in a pipeline at horizontal bends and tees is the bearing resistance of the soil. The bearing resistance of the soil is realized as passive pressure and is generated as the pipeline attempts to separate and move into the soil. This is the same resistance as that realized by the back side of a thrust block.
Additionally, there is some bearing resistance that results at the area formed by the diameter difference between the pipe bell and the pipe. In many cases, the calculated minimum restrained pipe length is less than a full length of pipe, and this may or may not include a pipe bell. As a result, it is not prudent for the design to rely on that interaction because the specific conditions that occur in the field are not always known. Therefore, this aspect of the pipeline is not included in the calculations and contributes a slight amount of conservatism to the design.
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