Design on Aboveground Supports

In some situations, it is necessary or desirable to use supports at designated intervals along pipelines. Aboveground, supported pipe is needed to transport water and other fluids within treatment plants and buildings. Also, pipe on piers is utilized to cross natural or man-made objects.

This section reviews the pertinent design considerations for aboveground ductile iron pipe-on-supports installations. Bridge-crossing installations, which are not specifically addressed, require special attention to their unique situations.

ABOVEGROUND INSTALLATIONS
For aboveground installations with one support per length of pipe (i.e., a span length of 6m), the minimum K-class of ductile iron pipe manufactured in all sizes is more than adequate to support the weight of the pipe and water it contains when analyzed in accordance with the suggestions of this procedure.

Other design considerations for pipes supported above ground may include the carrying capacity of the supports themselves, the strength of the structure from which a pipe may be suspended, and/or unusual or additional loads not in the scope of this section. Such loading may include seismic, frequency or resonance of vibrations, wind, water current, and other special design considerations.

It is also necessary to ensure a minimum of lateral and vertical stability at the supports for aboveground piping. Deflected pipe joints can result in thrust forces of hydrostatic or hydrodynamic origin, and, if not laterally and vertically restrained, unbalanced forces may result in additional joint deflection and possible failure of the pipeline.

Thermal expansion of ductile iron pipelines supported above ground is not usually of concern in correctly designed and installed systems because of the nature of the push-on joint. A 50° Celsius change in temperature results in expansion or contraction of a 6m length of ductile iron pipe of approximately 3.4mm. This is easily accommodated by correctly installed pipe and joints.

Occasionally, where support structures are expected to have significantly different behavior than the pipeline, special considerations for expansion, contraction, and supports may be necessary. For reference, the following are coefficients of thermal expansion for various materials:

Ductile iron: 11.2 x 10^-6mm/mm degree Celsius
Steel: 11.7 x 10^-6mm/mm degree Celsius
Concrete: 12.6 x 10^-6mm/mm degree Celsius

SUPPORT LOCATION
System security is maximized by positioning the supports immediately behind the pipe bells. When the support is placed near the bell, the bell section contributes beneficial ring stiffness where it is most needed. This ring stiffness, in turn, reduces the effect of support loads and localized stress. Supports should normally not be placed under spigots adjacent to bells, due to possible undesirable effects on joints.

SADDLE ANGLE AND SUPPORT WIDTH
Pipe supports should cradle the pipe in a saddle (see Figure 1). This cradling, which should follow the contour of the pipe, minimizes stress concentrations at the supports. It is recommended that the saddle angle (b) of the support be between 90° and 120°. Little or no benefit is gained by increasing the saddle angle more than 120°. With angles smaller than 90°, the maximum stress tends to increase rapidly with decreasing saddle angle.

There are some differences among published theories and data regarding the importance of axial support width for saddles. The most accepted formulas are found to be completely independent of saddle width. Some test data, however, show a decrease in measured stresses with an increase in saddle width. There is little effect on the maximum stress when saddle support width is increased more than . Therefore, for saddle supports, the minimum width (b) is determined by the following equation:

Where:

SUPPORT DESIGN
Additionally, supports, piles, and/or foundations should be adequately designed from a structural and soil-engineering standpoint to safely handle any loads transferred from the pipe.

FIGURE 1 -- SADDLE ANGLE AND WIDTH

BEAM SPAN FOR DUCTILE IRON PIPE ON SUPPORTS
Ductile iron pipe is normally manufactured in 6m nominal lengths, depending on the pipe manufacturer. The most common joint used with ductile iron pipe is the push-on-type joint. This rubber-gasketed joint allows a certain amount of deflection and longitudinal displacement while maintaining its hydrostatic seal. This makes these pipe joints ideally suited for normal underground and aboveground installation. The flexibility of the joints reduces the chance of excessive beam stresses occurring. For pipe supported at intervals, however, flexible joints usually require that at least one support be placed under each length of pipe for stability.

Various schemes have been successfully used to obtain longer spans where particular installation conditions presented the need, but these are special design situations and are not specifically addressed in this section. The design presented herein is based upon one support per length of pipe.

BEAM DEFLECTION AT CENTER OF SPAN
Computations for beam deflection are also based on the simply supported beam concept. This is likewise conservative due to the reality of offset joints. The maximum allowable deflection at mid-span to prevent damage to the cement-mortar lining is limited to:

Where:

Less deflection may be desired. The deflection of the beam may be significant for aesthetic reasons in aboveground installations or possibly for hydraulic reasons in gravity-flow pipelines. Limitations on the deflection, if any, should be determined by the designer as appropriate to a specific installation.

The beam deflection at center span for a uniformly loaded, simply supported beam can be calculated using the following formula:

Where:

INT Design Table For Pipe On Supports For Aboveground Piping

Notes:
Calculations for maximum support reaction stress, midspan deflection, and flexural stress are based on design principles from DIPRA's "Design of Ductile Iron Pipe On Supports." This analysis assumes a simply supported beam.

Weight calculations are based on ACIPCO Fastite® pipe full of water with standard ISO 4179 cement linings.