THRUST RESTRAINTS FAQS

Q: Do I need to be concerned with axial stresses in Ductile Iron pipe when using restrained joints?

A: No. Due to the conservatism of ANSI/AWWA C150/A21.50 and utilizing a conservative analysis, the maximum stress resulting from hoop stress due to internal pressure and the axial stress due to tension from restrained joints is always less than half the minimum yield strength of Ductile Iron pipe. For example, using the minimum manufacturing thickness (nominal thickness minus the casting tolerance), Table 1 is based on the maximum working pressure of the pipe plus 100 psi surge pressure.

Table 1
Pipe	Working Pressure + 100 psi Surge (psi)	Hoop Stress Due to Internal Pressure (psi)	Axial Stress Due to Restrained Joints (psi)	Maximum Stress (psi)	Safety Factor Based on Minimum Yield Stress of 42,000 psi
6-inch Diameter PC 350	450	7,763	3,997	8,732	4.81
12-inch Diameter PC 350	450	13,500	6,864	15,145	2.77
24-inch Diameter PC 200	300	14,885	7,518	16,676	2.54
64-inch Diameter PC 150	250	17,465	8,796	19,555	2.15

(Issue: Spring/Summer 2001)

Q: How do closed valves next to tees influence thrust restraint calculations?

A: The effect that a closed valve next to a tee will have on thrust restraint calculations depends on where the valve is located. The valve can be located either on the branch or on one side of the run of the pipeline. We will address both configurations.

First, however, let us address a tee that does not have an adjacent valve. With this configuration, the restraint that may be required is only along the branch pipeline. The forces countering the thrust force are the frictional resistance along the branch pipeline and the bearing resistance on the back of the tee and the lengths of pipe along the run immediately adjacent to the tee.

Next, consider a tee with an adjacent closed valve on the branch pipeline. If the valve is closed with the pressure on the "run" side of the valve, the run is unaffected. The thrust force on the "run" side of the valve will be balanced by the thrust force on the back of the tee. However, note that the valve and stub piece must be restrained to the tee. If the pressure is on the "branch" side of the valve, the thrust force will be balanced only by the frictional resistance along the branch. In other words, it should be treated as a dead end for calculation of the required length of restraint. In summary, for the case of the valve on the branch pipeline, the required length of restraint for the branch should be calculated based on the valve being a dead end. Additionally, the valve and stub piece must also be restrained to the tee. No restraint is required along the run.

Next, consider an equal outlet tee with an adjacent closed valve on the run pipeline. If the valve is closed with the pressure on the "tee" side of the valve, the configuration becomes very similar to a 90º bend. The required length of restraint for the branch and the run, on the side of the tee away from the valve, can therefore be calculated based on the procedures used for a 90º bend. However, note that the valve (and stub piece) must be restrained to the tee. If the pressure is on the side of the valve away from the tee, the thrust force will be balanced only by the frictional resistance along the run pipeline extending away from the tee/valve. In other words, it should be treated as a dead end for calculation of the required length of restraint. In summary, for the case of an equal outlet tee with an adjacent valve on the run pipeline, the required length of restraint for the branch and the "tee side" of the valve should be calculated based on the procedures used for a 90º bend. The restrained length of the branch should be the longer of that required for the tee itself or the 90º bend. The required length of restraint for the side of the run away from the tee/valve should be based on a dead end. Additionally, the valve and stub piece must also be restrained to the tee. In the case of reducing tees, it may be prudent to restrain the run on both sides of the valve based on a dead end.

In the above configurations where the situation is being treated as a dead end, consideration could be given to the use of the pipeline on the unpressurized side of the valve to act as a compression thrust block to restrain the thrust force against the valve. However, due to the expansion and contraction available with typical restrained joints, the possibility of excessive pipe movement might exist. Therefore, this method of restraint should be used with extreme caution.
(Issue: Spring/Summer 2001)

Q: Is there any difference in calculating the required thrust restraint length for horizontal, parallel Ductile Iron pipelines utilizing the same trench as compared to a single pipeline?

A: Yes. Thrust forces are restrained or balanced by the reaction of the restrained pipe unit with the surrounding soil. The source of the restraining forces is twofold. First, the static friction between the pipe unit and the soil, and second, the restraint provided by the pipe as it bears against the sidefill soil along each leg of the bend. Both of these forces are presumed to be functions of the restrained length on each side of the bend and they are presumed to act in the direction opposing the thrust force (i.e., directly opposing impending movement of the bend). When horizontal, parallel lines in the same trench make a horizontal bend, the restrained length calculation for the outer pipeline will be the same as for a single pipeline. The unit frictional force (F_s) is developed between the pipe and the backfill, and the unit bearing resistance (R_s) is developed and its pressure is transmitted to the backfill material which in turn is transmitted to the undisturbed trench wall. The restrained length calculation for the inside pipeline should normally only include the unit frictional force (F_s). If unit bearing resistance (R_s) is also assumed, the passive pressure would then be transmitted to the outside pipeline, its backfill material, and ultimately to the undisturbed trench wall, causing twice the pressure that R_s was intended to resist by design. The restrained length calculations for vertical up and vertical down bends of these horizontal pipelines would be the same as individual pipelines since forces are not transmitted from one pipeline to another for these configurations.
(Issue: Spring/Summer 1998)

Q: What wording can I incorporate into my specifications to ensure a qualified welder is used for field welding and field cutting restrained joints for Ductile Iron pipe?

A: The following wording may be incorporated: "Only qualified welders having adequate skill and experience in the practice of manual electric arc welding and cutting ferrous materials shall be allowed to field weld and field cut restrained joints for Ductile Iron pipe. Pipe manufacturers' recommended procedures shall be followed while exercising reasonable care.
(Issue: Fall/Winter 1997)

Q: What design soil parameters should I use if restrained joint pipe is laid in trench backfill with markedly different support characteristics than the native soil?

A: If restrained joint pipe is laid in trench backfill with markedly different support characteristics than the native soil, special considerations may be required. As the pipe is pressurized, it will transmit passive pressure to the backfill that will in turn transmit this pressure to the native soil. Therefore, the material that results in the smaller unit bearing resistance (R_s) should be used for the passive resistance and the unit frictional force (F_s) should be based on the backfill material surrounding the pipe.
(Issue: Spring/Summer 1996)

Q: Should I be concerned with thrust restraint of valves, and if so, why?

A: A thrust force will develop at a partially or fully closed valve if a differential pressure exists across the valve. The maximum thrust force will normally occur when the valve is closed and subjected to system pressure on one side and essentially no pressure on the other. In this case, the resulting thrust force would be the same as that created at a dead end (refer to DIPRA's brochure "Thrust Restraint Design For Ductile Iron Pipe," Third Edition). It is common practice, in the design of in-line valves in a piping system, to rely on the frictional forces that build due to the compression of unrestrained straight runs of piping on the low pressure side of the valve to resist the thrust forces created at such valves. This design practice has been successfully used for many years. For small-diameter pipe and/or low pressures, thrust restraint of valves normally is not a major concern because the required resisting force may build over only one or two lengths of pipe. For larger-diameter piping, thrust restraint of valves can be a greater concern due to the magnitude of the thrust forces involved. Additional thrust restraint measures may be required in some cases. Other installation measures, such as increased soil compaction around the valve, are good practices in all cases.
(Issue: Fall/Winter 1995)