API 1102 Uncased Crossing 10 Foot Limit Understanding
- Joseph Pikas
Uncased carrier pipe is subjected to internal loading from pressure and external loading from earth loading (dead load) and live (cyclic) loading from highway or railroad traffic. Other loading due to special or temporary conditions should be evaluated on the specific situation in the field (See Knowledge Base Article for Temporary Conditions).
Highway Crossings: The pipeline is assumed to be loaded by two, adjacent trucks with single or tandem axles aligned. Railroad and highway crossings are assumed at 90 degrees to the pipeline.
Railroad Crossings: The pipeline is assumed to be loaded by a single train. For simultaneous loading from two adjacent tracks, stress increment factors are used for cyclic longitudinal and cyclic circumferential stress.
Soil properties, pipe, and operational characteristics must be known to determine the acceptable stress levels of the pipeline. API 1102 is primarily based design data that used to determine the graphs and tables.
Example: Live highway load, due to the wheel load at the surface.
· Maximum from a single axle, Ps. Recommended default is 24 Kips per axle, or 12 Kips per wheel set.
· Maximum from a tandem axle, Pt. Recommended default is 20 Kips per axle, or 10 Kips per wheel set.
· w (design surface pressure) = P/Ap, where
o P = Ps or Pt and Ap = 144 sq. inches (assumed contact area)
o Therefore, the recommended defaults are 83.3 psi for a single axle and 69.4 psi for a tandem axle.
Example: Live rail load, w, pounds per sq. inch, is the load applied at the surface of
the crossing.
· Assume that the load is evenly distributed over an area that is 8’ x 20’. The recommended default is 80,000 pounds (80 Kips) per axle.
· Therefore, at the junction of two rail cars (four axles), 320,000 pounds is distributed evenly over the 8' x 20' area. This default value reduces to 13.9 psi.
· If a different loading is known to be the maximum, then it should be used.
At a depth of greater than about 10’-15’, the controlling factor will be the hoop stress caused by internal pressure, basically Barlow’s analysis. Overburden stress from any surface load from highway or railroad will be very minimal. This is why the tables do not go beyond a certain depth, the stress on the pipeline from a surface load at a very deep installation will be very minimal to near zero.
Also, API 1102 program assumes that the pipeline is installed via trench-less method as it is a designed based analysis. The user can still check for the total effective stress caused by overburden soil loads and other design stresses using Spangler’s stress equation.