Straight Pipe Forces and Moments: Local Option Each straight pipe has a local x-y-z coordinate system, with the positive x direction always along the pipe in the segment direction. For a pipe which is non-vertical, the y and z axes are as shown in the following Figure:

1. The y axis is in the vertical plane. The y axis will be parallel to the (global) vertical axis for a horizontal pipe. For an inclined pipe, the (local) y axis will be inclined, but still directed upwards.

2. The z axis is in the horizontal plane, forming a right handed coordinate system.

Figure H-6: Straight pipe local axes (non-vertical pipe)

For a pipe which is vertical, this procedure cannot be used because there is no unique vertical plane for locating the y axis. This axis is thus set parallel to the third global axis. Care must be taken, because the local x axis (the segment direction) may be either up or down, depending on how the segment was defined. The two possibilities are shown in the following table and in Figure H 7.

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Figure H-7: Straight vertical pipe local axes

As with the global option, the forces and moments in local coordinates act on the cross section exposed by removing all of the segment up to the point. Positive forces act in the positive x, y and z directions (a positive x force is compression), and positive moments follow the right-hand-rule convention.

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