RP Pipe

Figure 4.2 : Loading (26 psi) rigid concrete pipe (left) and flexible plastic pipe

26 psi Computed

26 psi Computed

12 psi

41 psi 14PSI

41 psi KPSI

gradients, and pipe bridging.

When compared to steel pipes there are similarities and dissimilarities. They both differ from concrete (includes asbestos filled type) pipe which is a rigid pipe that cannot tolerate bending or deflection to the same extent as RTR and steel pipe. The following review provides information on the design approach and results of tests conducted on these type pipes (rigid RTR, rigid steel, and flexible concrete). They were buried in trenches under 25 ft of the same dirt and subjected to actual load testing. Specific pressures varied from installation to installation, but the relationship in the way these pipes react to the same burial condition generally remains constants.

As shown in Fig. 4.23 the load on the surface of the (a) rigid pipe (concrete) is higher at the crown and is transmitted through the pipe direcdy to the bed of the trench in which the pipe rests and using some side support. The RTR or steel flexible conduit (b) deflects under covering load of earth, this deflection transfers portions of the load to the surrounding envelope of soil that increases the strength of the flexible conduit. Analyzing the type and consolidation of backfill materials is to be considered an integral part of the design process. Because less of a load on the trench bed occurs the trench requires less bedding bearing strength reducing the installed cost.

Steel pipe is considered a homogeneous isotropic material (equally strong in both hoop and longitudinal directions) where RTR is an anisotropic material (different strength in both the hoop and longitudinal directions). These directional behavior results in the

Cross-section of filament wound layup of the RTR pipe

Cross-section of filament wound layup of the RTR pipe

Strength longitiudinal V Strength hoop modulus of elasticity (E) to be equal in all directions for steel and not equal for RTR.

The RTR pipe structure is shown in Fig. 4.24 where the glass fibers are filament wounded at a helix angle that is at 55 to 65° to the horizontal to maximize hoop and longitudinal stress efficiency. Glass fiber content is at a minimum of 45wt%. An internal corrosion resistant leak proof barrier liner is usually included that is not included in the stress analysis.

Required is to design a pipe wall structure of sufficient stiffness and strength to meet the combined loads that the pipe will experience in long time service. One design is a straight wall pipe in which the wall thickness controls the stiffness of the pipe. Another way is to design a rib-wall pipe on which reinforcement ribs of a specific shape and dimension are wound around the circumference of the pipe at precisely designed intervals. As previously reviewed (RIB section) the advantage of a rib wall pipe is that the wall thickness of the pipe can be reduced (also reducing costs) while maintaining or even increasing its overall strength-to-weight ratio. Where burial conditions are extreme or difficult underwater installations exist a rib-wall pipe design should be considered.

Maximum allowable pipe deflection should be no more than 5% using the Fig. 4.25 equation where AX = deflection. This value is the standard

Pipe deflection equation

Pipe deflection equation

of the pipe industry for steel conduit and pipe (AWWA M-ll, ASTM, and ASME). Deflection relates to pipe stiffness (El), pipe radius, external loads that will be imposed on the pipe, both the dead load of the dirt overburden as well as the live loads such as wheel and rail traffic, modulus of soil reaction, differential soil stress, bedding shape, and type of backfill.

To meet the designed deflection of no more than 5% the pipe wall structure could be either a straight wall pipe with a thickness of about 1.3 cm (0.50 in.) or a rib wall pipe that provides the same stiffness. It has to be determined if the wall structure selected is of sufficient stiffness to resist the buckling pressures of burial or superimposed longitudinal loads. The ASME Standard of a four-to-one safety factor on critical buckling is used based on many years of field experience. To calculate the stiffness or wall thickness capable of meeting that design criterion one must know what anticipated external loads will occur (Fig.

As reviewed the strength of RTR pipe in its longitudinal and hoop directions are not equal. Before a final wall structure can be selected, it is necessary to conduct a combined strain analysis in both the longitudinal and hoop directions of the RTR pipe. This analysis will consider longitudinal direction and the hoop direction, material's allowable strain, thermal contraction strains, internal pressure, and pipe's ability to bridge soft spots in the trench's bedding. These values are determinable through standard ASTM tests such as hydrostatic testing, parallel plate loading, coupon test, and accelerated aging tests.

Stress-strain (S-S) analysis of the materials provides important information. The tensile S-S curve for steel-pipe material identifies its yield point that is used as the basis in their design. Beyond this static loaded yield point (Chapter 2) the steel will enter into the range of plastic deformation that would lead to a total collapse of the pipe. The allowable design strain used is about two thirds of the yield point.

Buckling analysis based on conditions such as dead loads, effects of possible flooding, and the vacuum load it is expected to carry

Buckling analysis based on conditions such as dead loads, effects of possible flooding, and the vacuum load it is expected to carry

RTR pipe designers also use a S-S curve but instead of a yield point, they use the point of first crack (empirical weep point). Either the ASTM hydrostatic or coupon test determines it. The weep point is the point at which the RTR matrix (plastic) becomes excessively strained so that minute fractures begin to appear in the structural wall. At this point it is probable that in time even a more elastic liner on the inner wall will be damaged and allow water or other liquid to weep through the wall. Even with this situation, as is the case with the yield point of steel pipe, reaching the weep point is not catastrophic. It will continue to withstand additional load before it reaches the point of ultimate strain and failure. By using a more substantial, stronger liner the weep point will be extended on the S-S curve.

The filament-wound pipe weep point is less than 0.009 in./in. The design is at a strain of 0.0018 in./in. providing a 5 to 1 safety factor. For transient design conditions a strain of 0.0030 in./in. is used providing a 3 to 1 safety factor.

Stress or strain analysis in the longitudinal and hoop directions is conducted with strain usually used, since it is easily and accurately measured using strain gauges, whereas stresses have to be calculated. From a practical standpoint both the longitudinal and the hoop analysis determine the minimum structural wall thickness of the pipe. However, since the longitudinal strength of RTR pipe is less than it is in the hoop direction, the longitudinal analysis is first conducted that considers the effects of internal pressure, expected temperature gradients, and ability of the pipe to bridge voids in the bedding. Analyzing these factors requires that several equations be superimposed, one on another. All these longitudinal design conditions can be solved simultaneously, the usual approach is to examine each individually.

Poisson's ratio (Chapter 3) can have an influence since a longitudinal load could exist. The Poisson's effect must be considered when designing long or short length of pipe. This effect occurs when an open-ended cylinder is subjected to internal pressure. As the diameter of the cylinder expands, it also shortens longitudinally. Since in a buried pipe movement is resisted by the surrounding soil, a tensile load is produced within the pipe. The internal longitudinal pressure load in the pipe is independent of the length of the pipe.

Several equations can be used to calculate the result of Poisson's effect on the pipe in the longitudinal direction in terms of stress or strain. Equation provides a solution for a straight run of pipe in terms of strain. However, where there is a change in direction by pipe bends and thrust blocks are eliminated through the use of harness welded joints, a different analysis is necessary. Longitudinal load imposed on either side of an elbow is high. This increased load is the result of internal pressure, temperature gradient, and/or change in momentum of the fluid. Because of this increased load, the pipe joint and elbow thickness may have to be increased to avoid overstraining.

The extent of the tensile forces imposed on the pipe because of cooling is to be determined. Temperature gradient produces the longitudinal tensile load. With an open-ended cylinder cooling, it attempts to shorten longitudinally. The resistance of the surrounding soil then imposes a tensile load. Any temperature change in the surrounding soil or medium that the pipe may be carrying also can produce a tensile load. Engineering-wise the effects of temperature gradient on a pipe can be determined in terms of strain.

Longitudinal analysis includes examining bridging if it occurs where the bedding grade's elevadon or the trench bed's bearing strength varies, when a pipe projects from a headwall, or in all subaqueous installations. Design of the pipe includes making it strong enough to support the weight of its contents, itself, and its overburden while spanning a void of two pipe diameters.

When a pipe provides a support the normal pracdce is to solve all equadons simultaneously, then determine the minimum wall thickness that has strains equal to or less than the allowable design strain. The result is obtaining the minimum structural wall thickness. This approach provides the designer with a minimum wall thickness on which to base the ultimate choice of pipe configuration. As an example, diere is the situadon of the combined longitudinal analysis requiring a minimum of in. (1.59 cm) wall thickness when the deflection analysis requires a J/2 in. (1.27 cm) wall, and the buckling analysis requires a fa in. (1.9 cm) wall. As reviewed the thickness was the fa in. wall. However with the longitudinal analysis a in. wall is enough to handle the longitudinal strains likely to be encountered.

In deciding which wall thickness, or what pipe configuration (straight wall or ribbed wall) is to be used, economic considerations are involved. The designer would most likely choose the fa in. straight wall pipe if the design analysis was complete, but it is not since there still remains strain analysis in the hoop direction. Required is to determine if the combined loads of internal pressure and diametrical bending deflection will excccd the allowable design strain.

There was a tendency in the past to overlook designing of joints. The performance of the whole piping system is directly related to the performance of the joints rather than just as an internal pressure-seal pipe. Examples of joints are bell-and-spigot joints with an elastomeric seal or weld overlay joints designed with the required stiffness and longitudinal strength. The bell type permits rapid assembly of a piping system offering an installation cost advantage. It should be able to rotate at least two degrees without a loss of flexibility. The weld type is used to eliminate the need for costly thrust blocks.


There is a difference when comparing the plasdc to metal spring shape designs. With metals shape opdons are the usual torsion bar, helical coil, and flat-shaped leaf spring. The TPs and TSs can be fabricated into a variety of shapes to meet different product requirements. An example is TP spring actions with a dual action shape (Fig. 4.27) that is injection molded. This stapler illustrates a spring design with the body and curved spring secdon molded in a single part. When the stapler is depressed, the outer curved shape is in tension and the ribbed center section is put into compression. When the pressure is released, the tension and compression forces are in turn released and the stapler returns to its original position.

Other thermoplasdcs are used to fabricate springs. Acetal plastic has been used as a direct replacement for convendonal metal springs as well providing the capability to use different spring designs such as in zigzag springs, un coil springs, cord locks with molded-in springs, snap fits, etc. A special application is where TP replaced a metal pump in a PVC plasdc bag containing blood. The plasdc spring hand-operadng pump (as well as other plastic spring designs) did not contaminate the blood.

RP leaf springs have the potential iii the replacements for steel springs. These unidirectional fiber RPs have been used in trucks and automotive suspension applicadons. Their use in aircraft landing systems dates back to the early 1940s taking advantage of weight savings and

. TP Delrin acetal plastic molded stapler (Courtesy of DuPont)

. TP Delrin acetal plastic molded stapler (Courtesy of DuPont)

performances. Because of the material's high specific strain energy storage capability as compared to steel, a direct replacement of multileaf steel springs by monoleaf composite springs can be justified on a weight-saving basis.

The design advantages of these springs is to fabricate spring leaves having continuously variable widths and thicknesses along their length. These leaf springs serve multiple functions, thereby providing a consolidation of parts and simplification of suspension systems. One distinction between steel and plastic is that complete knowledge of shear stresses is not important in a steel part undergoing flexure, whereas with RP design shear stresses, rather than normal stress components, usually control the design.

Design of spring has been documented in various SAE and ASTM-STP design manuals. They provide the equations for evaluating design parameters that are derived from geometric and material considerations. However, none of this currendy available literature is direcdy relevant to the problem of design and design evaluation regarding RP structures. The design of any RP product is unique because the stress conditions within a given structure depend on its manufacturing methods, not just its shape. Programs have therefore been developed on the basis of the strain balance within the spring to enable suitable design criteria to be met. Stress levels were then calculated, after which the design and manufacture of RP springs became feasible.

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